evolution of u.s. military aircraft structures technology

12
JOURNAL OF AIRCRAFT Vol. 39, No. 1, JanuaryFebruary 2002 Evolution of U.S. Military Aircraft Structures Technology D. Paul, ¤ L. Kelly, and V. Venkayya U.S. Air Force Research Laboratory, WrightPatterson Air Force Base, Ohio 45433-7542 and Thomas Hess § U.S. Naval Air Systems Command, Patuxent River, MD 20670-1906 A survey of the major structures technology developments that have in uenced modern aircraft design is pre- sented. The authors’ perspectives on the key materials and design concepts that are presently driving U.S. Air Force and Navy military airframes are presented. The current focus of research and development (R&D) struc- tural development resources and the reasons for this focus are addressed. Some thoughts on how to approach future designs are provided, and the structures technologies that are expected to be the focus of future R&D efforts are identi ed. Introduction D ESPITE the often made claims of revolutionary design, mil- itary airframe construction has historically been very evolu- tionary in nature and often a blend of traditional design and new technology that is emerging at the time of the system prototype def- inition. The new technology is incorporated because it is identi ed as having some potential improvement in airframe weight, perfor- mance, survivability, supportability, or cost. This evolution and technology insertion process will be discussed in terms of the key subareas of structural concepts, materials, design criteria, and analysis and design techniques. Structural Concepts The evolution of military aircraft structures from the aluminum alloy stressed skin semimonocoque construction of the C-47 (DC-3) in the 1930s to the 37% composite material dual structural load path ying wing of the B-2 and the 42% composite V-22 in the 1990s has been an interesting transition. The clich´ e “what goes around comes around” describes this evolution best. Hoff 1 traces semimonocoque composite (wood) fuselage struc- ture in the United States to the Vega, built by Lockheed in 1927. The Vega aircraft structure was based on U.S. Patent number 1,425,113 issued to Malcolm and Allan Loughead (Lockheed), J. Northrop, and T. Stadlman, with Northrop the principle designer. The patented design consisted of strips of spruce glued together in a concrete mold with a pressurized rubber bag. The 0.06-in.-thick middle layer ran circumferentially, and the two 0.04-in.-thick outer layers ran lengthwise. The structure was said to be so stiff that only moderately sized frames were required (semimonocoque ). Figure 1 shows a Vega semimonocoque wood fuselage. In production from 1927 to 1935, this aircraft made history. In 1930, Wiley Post won the Los Angeles to Chicago National Air Race and in 1933 made the rst around the world solo ight in a Vega. In 1932, Amelia Earhart, in a Vega, became the rst woman to y the Atlantic alone. The monocoque (derived from the Greek word monos, single, and the French word coque, shell 1 ) fuselage is actually attributed to a Received 6 March 1997; revision received 1 December 1999; accepted for publication 17 January 2000. This material is declared a work of the U.S. Government and is not subject to copyright protection in the United States. Copies of this paper may be made for personal or internal use, on condition that the copier pay the $10.00 per-copy fee to the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923; include the code 0021-8669/02 $10.00 in correspondenc e with the CCC. ¤ Chief Scientist, Air Vehicles Directorate. Branch Chief (retired); currently Senior Engineer, Titan Systems. Senior Scientist, Multidisciplinary Technology, Air Vehicles Directorate, Structures Division. § Staff Assistant for Business and Programs (retired), Aircraft Division. Frenchman, Louis B` echereau, who in 1912 built the fuselage for the Le Monocoque Deperdussin monoplane by gluing on a mold three layers of tulip wood, each about 0.06 in. thick. One layer ran fore and aft, the second in a right-hand spiral, and the third in a left-hand spiral. According to Ref. 2, a Swiss named Ruchonnet designed the rst wood monocoque structure in 1911 although it was not applied until the Deperdussin of 1912. Reference 3 indicates that Northrop had rst adopted the molded plywood fuselage approach for the Northrop designed Loughead Aircraft Corporation model S-1 sportplane in 1918. This was af- ter Stadlman (Loughead’s construction foreman) studied a German Albatros Company model D.Va biplane. The Albatros Company had been manufacturing three-ply, wood, monocoque fuselages as early as 1916. Wood continued to be used, off and on, up to the nonstrategic- material spruce and balsa sandwich structure of the Mosquitos pro- duced by de Havilland during World War II. De Havilland continued to produce wood fuselages into the postwar years. In 1930, Northrop introduced the use of riveted aluminum for the Alpha aircraft. The fuselage was ush-riveted aluminum semi- monocoque, and the wing box was aluminum-stiffened skin riveted to anged shear webs. Northrop’s riveted aluminum design utilized the wing covers to react bending, compression and tension loads. There were no spar caps per se (see Fig. 2a). This type of structure was also utilized for the Northrop Delta and Gamma aircraft. With slight modi cation, it was also employed by Douglas for the DC-1, DC-2, and DC-3 (the upper cover was stiffened by riveted corrugated aluminum, see Fig. 2b). (The concept of stressed-skin wing structure is attributed to Adolph Rohrbach of Rohrbach Metall ugzeugbau, GmBH, in 1920.) Howe’s comparison of the DC-3 to Abraham Lincoln in Ref. 4 is just one of many tributes to the dependable and versatile C-47 Skytrain version of the DC-3: “A plain honest background; a simple humble beginning; careful tempering and testing in early life; ca- pability and performance beyond the dreams of parents.” 4 Douglas DC-3/C-47 production from 1935 to 1946 totaled 10,654. It is esti- mated that 1000 are still ying today. With the success of the DC-3, riveted aluminum-stiffened skin stringer/frame structure became en- trenched as the preferred design concept. Northrop’s basic structural concept has been employed for over 60 years. However, the fuselage of a modern ghter can hardly be called semimonocoque in the strict sense of the term because of the many cutouts in the skin for access panels and the many highly loaded frames and bulkheads that react most of the aerodynamic loads. There have been many variations on the stiffened stressed skin/frame concept over the years, but for the most part stiffened 18

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Page 1: Evolution of U.S. Military Aircraft Structures Technology

JOURNAL OF AIRCRAFT

Vol 39 No 1 JanuaryndashFebruary 2002

Evolution of US Military Aircraft Structures Technology

D Paulcurren L Kellydagger and V VenkayyaDagger

US Air Force Research Laboratory WrightndashPatterson Air Force Base Ohio 45433-7542and

Thomas Hesssect

US Naval Air Systems Command Patuxent River MD 20670-1906

A survey of the major structures technology developments that have in uenced modern aircraft design is pre-sented The authorsrsquo perspectives on the key materials and design concepts that are presently driving US AirForce and Navy military airframes are presented The current focus of research and development (RampD) struc-tural development resources and the reasons for this focus are addressed Some thoughts on how to approach futuredesigns are provided and the structures technologies that are expected to be the focus of future RampD efforts areidenti ed

Introduction

D ESPITE the often made claims of revolutionary design mil-itary airframe construction has historically been very evolu-

tionary in nature and often a blend of traditional design and newtechnology that is emerging at the time of the system prototype def-inition The new technology is incorporated because it is identi edas having some potential improvement in airframe weight perfor-mance survivability supportability or cost

This evolution and technology insertion process will be discussedin terms of the key subareas of structural concepts materials designcriteria and analysis and design techniques

Structural ConceptsThe evolution of military aircraft structures from the aluminum

alloy stressed skin semimonocoque construction of the C-47 (DC-3)in the 1930s to the 37 composite material dual structural load path ying wing of the B-2 and the 42 composite V-22 in the 1990s hasbeen an interesting transition The cliche ldquowhat goes around comesaroundrdquo describes this evolution best

Hoff1 traces semimonocoque composite (wood) fuselage struc-ture in the United States to the Vega built by Lockheed in 1927 TheVega aircraft structure was based on US Patent number 1425113issued to Malcolm and Allan Loughead (Lockheed) J Northropand T Stadlman with Northrop the principle designer

The patented design consisted of strips of spruce glued togetherin a concrete mold with a pressurized rubber bag The 006-in-thickmiddle layer ran circumferentially and the two 004-in-thick outerlayers ran lengthwise The structure was said to be so stiff that onlymoderately sized frames were required (semimonocoque) Figure 1shows a Vega semimonocoque wood fuselage

In production from 1927 to 1935 this aircraft made history In1930 Wiley Post won the Los Angeles to Chicago National AirRace and in 1933 made the rst around the world solo ight in aVega In 1932 Amelia Earhart in a Vega became the rst womanto y the Atlantic alone

The monocoque (derived from the Greek word monos single andthe French word coque shell1) fuselage is actually attributed to a

Received 6 March 1997 revision received 1 December 1999 acceptedfor publication 17 January 2000 This material is declared a work of the USGovernment and is not subject to copyright protection in the United StatesCopies of this paper may be made for personal or internal use on conditionthat the copier pay the $1000 per-copy fee to the Copyright ClearanceCenter Inc 222 Rosewood Drive Danvers MA 01923 include the code0021-866902 $1000 in correspondence with the CCC

currenChief Scientist Air Vehicles DirectoratedaggerBranch Chief (retired) currently Senior Engineer Titan SystemsDaggerSenior Scientist Multidisciplinary Technology Air Vehicles Directorate

Structures DivisionsectStaff Assistant for Business and Programs (retired) Aircraft Division

Frenchman Louis Bechereau who in 1912 built the fuselage for theLe Monocoque Deperdussin monoplane by gluing on a mold threelayers of tulip wood each about 006 in thick One layer ran foreand aft the second in a right-hand spiral and the third in a left-handspiral

According to Ref 2 a Swiss named Ruchonnet designed the rstwood monocoque structure in 1911 although it was not applied untilthe Deperdussin of 1912

Reference 3 indicates that Northrop had rst adopted the moldedplywood fuselage approach for the Northrop designed LougheadAircraft Corporation model S-1 sportplane in 1918 This was af-ter Stadlman (Lougheadrsquos construction foreman) studied a GermanAlbatros Company model DVa biplane The Albatros Company hadbeen manufacturing three-ply wood monocoque fuselages as earlyas 1916

Wood continued to be used off and on up to the nonstrategic-material spruce and balsa sandwich structure of the Mosquitos pro-duced by de Havilland during World War II De Havilland continuedto produce wood fuselages into the postwar years

In 1930 Northrop introduced the use of riveted aluminum forthe Alpha aircraft The fuselage was ush-riveted aluminum semi-monocoque and the wing box was aluminum-stiffened skin rivetedto anged shear webs

Northroprsquos riveted aluminum design utilized the wing covers toreact bending compression and tension loads There were no sparcaps per se (see Fig 2a) This type of structure was also utilized forthe Northrop Delta and Gamma aircraft With slight modi cationit was also employed by Douglas for the DC-1 DC-2 and DC-3(the upper cover was stiffened by riveted corrugated aluminum seeFig 2b) (The concept of stressed-skin wing structure is attributedto Adolph Rohrbach of Rohrbach Metall ugzeugbau GmBH in1920)

Howersquos comparison of the DC-3 to Abraham Lincoln in Ref 4is just one of many tributes to the dependable and versatile C-47Skytrain version of the DC-3 ldquoA plain honest background a simplehumble beginning careful tempering and testing in early life ca-pability and performance beyond the dreams of parentsrdquo4 DouglasDC-3C-47 production from 1935 to 1946 totaled 10654 It is esti-mated that 1000 are still ying today With the success of the DC-3riveted aluminum-stiffened skin stringerframe structure became en-trenched as the preferred design concept

Northroprsquos basic structural concept has been employed for over60 years However the fuselage of a modern ghter can hardly becalled semimonocoque in the strict sense of the term because ofthe many cutouts in the skin for access panels and the many highlyloaded frames and bulkheads that react most of the aerodynamicloads

There have been many variations on the stiffened stressedskinframe concept over the years but for the most part stiffened

18

PAUL ET AL 19

Fig 1 Vega semimonocoque wood fuselage

Fig 2a Northrop wing design

Fig 2b Douglas D-C2D-C3C-47 wing

skin panels supported by frames and doublers around cutouts re-mains the most prevalent structural concept Some of the more note-worthy variations on this theme follow postbuckled panel tension eld design sandwich structure integrally stiffened structure andunitized construction Postbuckled panel design simply means thepanel is required to carry a higher load than that at which the panelskin buckles

The original application of postbuckled tension eld structure5 isattributed to Herbert Wagner at Rohrbauch Metall ugzeugbau in1925 Postbuckled structural members have been employed in manyaircraft since including the B-52 B-1B F5E and AV-8B

For the most part these concepts have been incorporated intothe same basic Northrop skinframe structural arrangement Onenoteworthy exception was the geodetic aircraft construction (calledgeodesic structure in the United States) employed by VickersArmstrong Ltd This was chie y Duralumin (see aluminum alloyssection) metal lattice with a fabric covering (see Figs 3a and 3b)

This type of construction was used for both the 1934 VickersWellesley and the 1936 Vickers Wellington Geodetic constructionwas described in Ref 6 as being ldquoimmensely strong and could takeany amount of punishment from ak and return home againrdquo

The 1938 British Empire Air Annual de nes geodetic constructionas an unbroken diagrid system in which torsion loads are taken bycontinuous members in tension and compression When reactingto wing torsion loads one diagonal system of bracing members is

Fig 3a Vickers geodetic fuselage production

Fig 3b Vickers geodetic wing construction

under compression while the other is under tension Because theyare attached at their crossing points they mutually restrain eachother against de ection Normal spars react bending loads

Although not adopted composite geodesic wing and fuse-lage designs were evaluated by Goldsworthy Engineering for theBeechcraft Starship in the early 1980s Geodesically stiffened l-ament composite panels are presently under assessment A limitedamount of test data7 indicates such construction to be very damagetolerant of low-velocity impacts

Sandwich construction replaces skin stiffeners with lightweighthoneycomb core and fasteners with adhesive bonding This type ofconstruction permits the use of very thin airframe skin operating athigh stress levels without buckling

The structural ef ciency of sandwich construction (lightweightand improved compression stability) has been exploited for manyyears in aircraft construction Hoff and Mautner in Ref 8 iden-tify some early applications of sandwich construction in airframesThey indicate that in 1924 von Karman and P Stock were granted aGerman patent for a sandwich glider fuselage and that a plywoodndash

cork sandwich wing monoplane was displayed at the French SalondrsquoAeronautique in 1938 They also indicate that the incentive forsandwich construction was the desire to build a true monocoqueairplane

There is some sandwich structure on virtually every aircraft Thede Havilland Mosquito bomber of World War II fame employedbonded wood sandwich structure for wing panels Wing skins con-sisted of discrete blocks of spruce core and cedar plywood facesheets The fuselage was a sandwich of spruce veneer on balsa corewith solid spruce core substituted where attachments (concentratedloads) existed

Although described in Ref 9 as being ldquorobust to battle dam-agerdquo maintenance problems arose with the bonded airframe in theSouth Paci c Tropical organisms and humidity were said to do moredamage than the Japanese (De Haviland rst utilized a plywoodndash

balsa sandwich for the Albatross fuselage a four-engined airlinerof 1938)

The more recent 1964 B-70 was constructed of brazed steel hon-eycomb sandwich (22000 ft2 68 of the airframe weight)

The C-5 contains 35000 ft2 of bonded sandwich including thefollowing wing leading edges slats and wing tips trailing-edge

20 PAUL ET AL

surfaces aps and nacelles troop deck oor pylons and fairingsand main landing gear pod

The General Dynamics B-58 which broke 12 world speedrecords was called the bonded bomber because of the extensiveuse of bonded aluminum sandwich construction Developed in thelate 1950s aluminum (2024-T867075-T6) sandwich panels cov-ered 90 of the wings and 80 of the total airframe The entireouter skin of the wing was made up of honeycomb core sandwichand the fuselage was a combination of beaded and honeycomb sand-wich panels Partly because of extremely lightweight sandwich de-sign and partly because of a large bombfuel pod that comprised thelower-half of the fuselage the structure made up only 165 of thetakeoff gross weight (TOGW) and only 14 of the maximum grossweight achieved later in ight refueling

This lightweight airframe design brought with it signi cant struc-tural maintenance problems The Strategic Air Command once es-timated the cost of operating two wings of supersonic B-58 aircraftequaled that of six wings of B-52s

Of course this was not all airframe maintenance The B-58 avion-ics system consisted of 5000 vacuum tubes and transistors The thin-sheet sandwich skin was however susceptible to in-service damagedue to handling walking and punctures and required periodic time-consuming costly inspections

According to Petrushka10 General Dynamics Director of Struc-tures in 1985 if the B-58 had not been retired early ldquoit is fair toconjecture that the extreme maintenance problems with honeycombsandwich could well have become the limiting life factorrdquo10

Some recent record setting aircraft that utilized compositesandwich airframes include the following 1) The graphite com-posite Hexcel honeycomb sandwich Voyager was employed byRichard Rutan and Jeana Yeager to accomplish their nine-daynonstop unrefueled 25000-mile ight around the world in 1987The Voyagerrsquos takeoff weight was more than 10 times the struc-tural weight 2) The high-altitude Boeing-built uncrewed graphiteKevlarreg epoxyNomex honeycomb Condor has a 200-ft wing spanwith a 367 aspect ratio that weighs only 2 lbft2 enabling it to setan altitude record of 66980 ft (for piston powered aircraft) and tostay aloft for 2 1

2days in 1989

Reference 11 indicates that a primary motivation for the develop-ment of the bonded honeycomb structure is the signi cant decreasein the number of parts with the associated decreased fabricationcost over mechanically fastened construction

Minimizing fastener installation time and cost has been a de-sire of manufacturing engineers ever since the rst all-metal air-frames Reference 12 indicates that the CurtisndashWright Corporationexplored stitching of metal aircraft parts during World War II to re-duce aircraft fabrication time associated with riveting and spotweld-ing An automotive industry metal stitcher (Fig 4) was adapted toairframe metal-to-metal and dissimilar material joining Stitchingand weaving is now being employed to create three-dimensional

Fig 4 CurtisndashWright metal stitching machine

Fig 5 Composite material stitching machine

Fig 6 Wright model A delivered 7 September 1908

carbon ber preforms [Such a stitching machine13 is funded bythe NASA Advanced Composites Technology (ACT) program (seeFig 5)] This machine developed by Ingersoll Milling Machine andPathe technologies has four high-speed stitching heads capable of800 stitchesmin

MaterialsThe rst US military aircraft (Fig 6) employed a composite

airframe (wood wire and fabric) The transition from wood andfabric composites to predominately metal construction in the UnitedStates took approximately 10 years This is considerably less timethan the present gradual transition to high modulus composites Thiscontention is supported by a review of the Aeronautical Chamberof Commerce of America Aircraft YearBook from 1921 to 1931

By 1931 the aircraft yearbook identi ed the following fourUS aircraft as employing stressed skin metal constructionBoeing Monomail Thaden T-4 Northrop Alpha and ConsolidatedFleetster(Although the Fleetster employed anall-metal monocoquefuselage initially the wing was of plywood construction Ref 14 in-dicates the aircraft was all metal by 1932)

The rst all-metal airplane an integrally braced monoplane wasdesigned and constructed by Hugo Junkers in 1915 Junkers obtaineda Deutsches Reich patent for an all-metal ying wing in 1910 InRef 15 he indicates that the wing for his initial sheet iron airplaneprototype was a ldquosupporting coverrdquo concept where all tensile com-pressive and shearing forces are taken up by the wing cover Be-cause the welded iron wing weight resulted in a poor airplane rate ofclimb he changed the wing design in subsequent models to ldquoreactbending moments with a more ef cient frame or trestle of tubesrdquo

The rst smooth skin metal monocoque fuselage patent num-ber 1557855 was issued to Flavius E Loudy in 1921 (see Fig 7from Ref 16)

In December 196917 a lecture to the Royal Aeronautical Soci-ety of England titled ldquoThe History of Metal Aircraft Constructionrdquowas given by Marcus Langley In the prolog to this lecture he indi-cated that his lecture was in some way an obituary of metal aircraftconstruction for the following reason ldquoAlthough we shall still have

PAUL ET AL 21

metal aircraft for many years to come quite new materials are be-ginning to appear and they have as many advantages over metal asmetal had over wood I am referring to such materials as carbon bers used in matrices of synthetic resinsrdquo17

Hindsight now shows that this pronouncement and many simi-lar since then were premature The airframe of todayrsquos aircraft 25years later is a hybrid of metal and composites A review of thecurrent status of airframe materials usage (Table 1) shows a fairlyequal balance in the use of aluminum titanium and compositesThis balance is a modern development driven primarily by perfor-mance requirements and affordability A discussion of each materialevolution follows

Aluminum Alloys

For 60 years aluminum alloys have been the primary materialsfor airframes No other single material has played as major a rolein aircraft production as aluminum speci cally the 2000 and 7000series ingot alloys in various heat treatments

In 1944 Alcoa developed 75S (AlndashZnndashMgndashCu) expressly to meetthe aircraft industryrsquos need for higher strength This alloy rst sawservice on the B-29 bomber and according to the 1945 aircraftyearbook ldquoPractically all the new war planes were utilizing highstrength 75Srdquo

The principal aluminum alloy since 1945 however has beenAlcoarsquos 24S (AlndashCundashMgndashMn) that contained the same alloyingelements as the older 17S but in different proportions for greaterstrength Produced in 1917 17S was Alcoarsquos version of Duralumin(from the French word dur meaning hard) patented by Alfred Wilmin Germany in 1908 Wilmrsquos Dural aluminum alloy was used for theZeppelin structure in 1911 Alcoarsquos 17S was used in constructionof the US Navy airship Shenandoah which rst ew in 1923 Thisalloy began the US stressed metal skin semimonocoque structurerevolution

Other countries were also quick to adopt Duralumin monocoquefuselage construction In England the Short Brothers Company builtthe metal Silver Streak biplane in 1919 which was all Duraluminexcept for the wing spars which were steel The wing consisted ofround steel tube spars and Duralumin ribs with right angle angesto which riveted Duralumin covers were joined18

Table 1 Materials use as a percentage of structural weight

Material B-2 FA-18EF F-22 V-22 C-17 AV-8B

Composite 37 22 25 42 08 26Aluminum 27 27 16 mdashmdash 70 47Titanium 23 23 39 mdashmdash 09 mdashmdash

Fig 7 F E Loudy patent 1557855 led 1921

Over the many years of utilizing aluminum materials several im-portant lessons have been learned Reference 19 cites the example ofthe KC-135 aircraft In 1954 the Boeing 367-80 became Americarsquos rst jet transport prototype The KC-135 was a derivative of theBoeing 367-80 (Dash 80) as was the 707 but the lower wing skinof the KC-135 was initially 7178-T6 Al A small critical crack lengthfor this material at the KC-135 wing limit load level led to casesof in-service rapid fracture

The concern about loss of an aircraft from degradation of failsafety from widespread fatigue cracking motivated the US AirForce to make a costly modi cation The lower wing skins werereplaced with 2024-T3 aluminum

In 1982 14 years of service experience with the C-5A resultedin greater emphasis being placed on fracture toughness corro-sion resistance and durability in selecting materials for the C-5B7075-T6 fuselage skin and cargo oor skins were changed to 7475-T761 where corrosion was a problem The wing plank material waschanged from 7075-T6 to 7175-T73 for increased toughness

Predictions as early as 198520 were that ldquolithium containing alu-minum alloys (AlLi) would nd signi cant use in both militaryand civil aircraftrdquo and that the ldquoultimate level of utilization of car-bon ber reinforced polymer matrix composites may be somewhatless than predicted in view of the potential of AlLi alloysrdquo

By the early 1990s however the realization set in that AlLiwas not ready to be a wide-scale direct substitute for conventionalaluminum aerospace alloys although it was being used in someproduction airframe applications in the United States and EuropeAt present it is the high-strength aluminum alloys such as 7150 and7055 with increased compression strength and balanced fracturetoughness and corrosion resistance that are enabling aluminum tomaintain its high level of use in the aerospace industry

Titanium

The rst major structural application of titanium was the DouglasX-3 that utilized 629 lb of Rem-Cru Inc titanium This was a com-mercially pure material used for the aft fuselage boom and stabilatorportions of the aircraft

The X-3 rst ew in 1952 and O A Wheelon a production de-sign engineer with the Douglas Aircraft Company was awarded theWright brothers medal for a published analysis of the use of titaniumin aircraft construction entitled ldquoDesign Methods and Manufactur-ing Techniques With Titaniumrdquo

One of the rst airframe production applications was the F-86Sabre Jet which is noted for a killloss ratio of at least 41 in theKorean War This aircraft which ew as the XP-86 in 1947 em-ployed 600 lb of titanium in the aft fuselage and engine areas (Fig 9)

22 PAUL ET AL

Fig 8 US production of titanium sponge

Fig 9 Titanium aft fuselage inproduction at North American Aviationin 1954

The history and early use of titanium was solidly wedded to theaerospace industry and the annealed alphandashbeta alloy Ti 6Al-4V hasbeen the workhorse of the industry

This alloy has been utilized on aircraft from the B-52 to theF-22 Reference 21 indicates this speci c alloy composition was rst melted and evaluated in 1953 at IIT Research Institute under aUS Air Force contract The Mallory Sharon Titanium Corporationversion of this alloy (MST 6Al-4V) was selected shortly thereafter(1954) for use in Pratt and Whitneyrsquos J-57 turbojet engine (disksblades and other parts) for the B-52

The rst commercial production of titanium named after theTitans of mythology began in 1946 Titanium was pushed into com-mercial production as a structural material in unprecedented timeby the combined efforts of industry and government to meet militaryrequirements

Figure 8 shows how rapidly titanium sponge (re ned metal beforemelting into usable form) production was scaled up In 1956 90 ofthe titanium market was for military aircraft production By 1980less than 20 of aerospace grade was used for military aircraftThe majority of titanium sponge is processed into titanium dioxidepigment for use as ller in paint paper plastics and rubber

The use of titanium has been increasing at the expense ofboth aluminum and composites and will continue to increase inmore unitized assemblies facilitated by combined superplastic-formingdiffusion-bonding welding and casting

The most famous US titanium airplane the YF-12ASR-71 rst ew as the A-12 in 1962 and was fabricated from primarily beta-120VCA alloy (Ti-13V-11Cr-3Al)

Johnson stated in Ref 22 that ldquoof the rst 6000 B-120 piecesfabricated we lost 95rdquo The material was described as being sobrittle that if it fell off your desk it would shatter on the oor

Fig 10a F-15CD conventional aft fuselage

Fig 10b F-15E SPFDB aft fuselage

The most widely used alloy is Ti-6Al-4V This alloy is particularlyamenable to fabrication by superplastic-formingdiffusion-bonding(SPFDB) processes and therefore was the subject of a systematicwell-planned research and development (RampD) thrust by the USAir Force and US Navy over a 15-year period from 1970 to 1985

Reference 23 indicates that the use of SPFDB titanium for theaft fuselage of the F-15E resulted in 726 fewer components and10000 fewer fasteners and achieved 15 weight savings over theCD models (see Figs 10a and 10b)

Actually it has been large forgings and castings that have facil-itated the creation of one-piece substructures Figure 11 shows aTi-6Al-4V F-22 aft-fuselage frame produced by WymanndashGordonThe F-22 wing carry-through bulkhead is fabricated from the largesttitanium forging by surface area to date (96 ft2 6560 lb) Four bulk-heads in the midfuselage are made of titanium The forward and aftboom structure on either side of the aircraftrsquos empennage sectionare formed from integrally stiffened titanium isogrid panels that areelectron beam welded to forged titanium frames The wing attach t-tings and rudder actuator support are hot isostatically pressed (HIP)titanium castings

The A-6E composite wing rear spar is a 27-ft titanium forgingand large one-piece titanium bulkhead forgings are employed by theFA-18 EF aircraft

The BellndashBoeing V-22 Osprey Engineering and ManufacturingDevelopment aircraft ( rst ight 5 February 1997) used a one-piecetitanium casting for the transmission adapter case stow ring that waspreviously 40 separate riveted components This part serves as theprimary support structure for the rotor system proprotor gearboxand engine and nacelle components The Howmet Corporation in-dicates a premium-quality consistently homogenous component is

PAUL ET AL 23

Fig 11 F-22 aft-fuselage frame forging produced on WymanndashGordon50000-ton hydraulic press

achieved by the HIP processing so that allowables do not have to bereduced by conventional casting factors

Recently RMI Titanium of Ohio developed a stronger more dam-age tolerant alloy (Ti-6-22-22S) that is nding application on theF-22 joint strike ghter and X-33 reusable launch vehicle

Timet Corporation alloy 10ndash2ndash3 is being used in the main landinggear and Timetal 21S for the nacelle cowl plug nozzle and Prattand Whitney Aircraft engine components of the Boeing 777 aircraftTitanium usage on Boeing aircraft has increased from 2 emptyweight of the 737 to 8 of the 777 The McDonnell Douglas C-17transport also utilizes alloy 10ndash2ndash3 for landing gear components

The focus of titanium alloy development however has shiftedfrom aerospace to industrial applications New product forms areemerging rapidly Titanium is growing in use for everything fromgolf clubs skis tennis rackets bicycles pots and pans and jewelryto even hip and knee replacements The kitchen sink cannot be faroff

Noting this trend toward increased titanium use a 1991 AdvancedMaterials and Processing magazine article was titled ldquoTitaniumStaves Off Compositesrdquo In reality titanium has grown in use partlybecause of its compatibility with composites both from a thermalexpansion characteristic and resistance to galvanic corrosion whenin contact with carbon epoxy

Advanced Composites

The present generation of composite structures technology ap-plied to the B-2 FA-18EF V-22 and RAH-66 has been in devel-opment for 30 years The present level of maturity is a result of acontinued investment by the RampD structures and materials commu-nities of all three services and NASA No other class of material hasreceived such high level of funding support for such a long time Nu-merous composite structures built from the 1960s to 1980s veri edcomposite structure advantages (primarily weight savings) leadingto more extensive application of composite airframes in the 1990swhen affordability became the major issue

Table 2 is a summary of some of the programs initiated to fos-ter the development and transition of composites Although theseinitiatives were seldom adequately funded to complete all initiallyplanned objectives they nonetheless were able to sustain substantialfunding for some limited period of time

Production in the United States of high modulus ber compositestructures commonly referred to as advanced composites beganwith boron epoxy F-14 and F-15 horizontal stabilizers in the early

Table 2 Advanced composite technology programs

Agency Acronym Program

US Army ACAP Advanced composite airframe programNASA ACEE Aircraft energy ef ciencyUS Air Force DMLCC Design and manufacture of low cost

compositesNASA ACT Advanced composites technologyUS Navy AV-8B Composite structure development programARPAa ACP Affordable composites for propulsionDARPAb APMC Affordable polymer matrix compositesDODcindustry CAI Composites affordability initiative

aAdvanced Research Projects AgencybDefense Advanced Research Projects AgencycDepartment of Defense

1970s The technology was fostered by the US Air Force BoronFilaments and Composites Advanced Development Program ini-tiated in 1964 as a result of a Project Forecast recommendationExperimental carbon epoxy structures were ying in 1970 and car-bon epoxy came to the forefront as the most cost-effective advancedcomposite material in the mid-1970s with production applicationson the F-15 B-1 and F-16 In 1978 carbon epoxy was accepted bythe Navy for F-18 and AV-8B primary wing box structures

Table 3 is a sample of some of the early composite structuredemonstrations Table 3 consists of a list of composite structuresfor which a metal baseline weight was available There have beenso many composite structures built since these and many more arecurrently in development that any list is out of date before it isprinted

There were of course many other composite structures some ofwhich did not have a metal baseline or for which a metal coun-terpart would not even be practical Some of those that come tomind are the Rockwell highly maneuverable advanced technologyremotely piloted research vehicle and the Grumman X-29 forwardswept wing demonstrator Both of these designs exploited the ad-vantageous aeroelastic characteristic of exible composite wings

The bulk of advanced composite material being used in ightin the United States today is fabricated from unidirectional tapeconsisting of AS-4 or IM7 carbon ber in 3501 thermoset epoxymatrix preimpregnated by Hercules Inc

Although developed some time ago the Slingsby T67 Fire ywas selected in 1992 for the US Air Force enhanced ight screener(T3A) The basic airframe started as a wooden construction thatSlingsby redesigned as primarily a wet layup glass ber polymercomposite airframe

Fiberglass-reinforced polymer construction has been employedfor many years in the glider industry It has been arguedpara that theHorton brothers of Germany created the worldrsquos rst compositeaircraft the Ho Va which began as a glider in 1936 The fuselageand wings for the rst Ho V were fabric-covered D-tubes moldedfrom paper- lled phenol resin R Horten noted that such moldedplastic construction promised to reduce production time and cost

The term molded reinforced plastic plane was being used by 1941(see Ref 24) Although nearly every form of material was tried as ller resin-coated wood veneer strips and sheets (usually mahoganyor spruce) were the most popular reinforcing agents Impregnationby liquid phenolic- or urea-type resins was common

Both Japan and Germany built highly successful berglass com-posite soaring planes in the 1950s Several of these are described inRef 1

The 42 by weight composite structure BellBoeing OspreyV-22 rst ew in 1989 The critical link between the blade andthe rotor hub (the yoke) of the V-22 is a composite structure Theyoke carries the blade centrifugal and lifting forces transmits enginetorque and permits lead-lag and apping motion Also each yokearm can accommodate pitch changes

The US Air Forcersquos latest ghter the F-22 Raptor ( rst ight7 September 1997) was the rst aircraft to employ resin transfer

paraData available online at httpwwwnur ugelcom

24 PAUL ET AL

Table 3 Composite structure weight savings

Baseline Composite WeightComponent weight lb weight lb savings

Flight service componentsEmpennage

F-111 horizontal stabilizer 1142 866 24F-14 horizontal stabilizer 1005 825 18F-15 horizontal stabilizer 527 395 25F-5 vertical stabilizer 119 83 30DC-10 vertical stabilizer 1005 834 17L-1011 vertical stabilizer 858 642 25B-737 horizontal stabilizer 262 204 22

WingsAV-8B wing 1143 949 17F-18 wing 1843 1641 11A-6 wing 6392 6400 0a

(selective reinforcement)C-130 wing box 4800 4200 125

FuselageUH-60 rear fuselage 444 399 10AV-8B forward fuselage 229 171 25

(selective reinforcement)CH-53 Aft fuselage 1159 947 18B-1 dorsal longeron 1485 829 44

OtherC-5 slat 241 190 21B-727 elevator 131 98 25DC-10 rudder 91 67 26L0-1011 aileron 140 107 23F-15 speed brake 112 89 21B-1 weapons bay door 147 129 12F-5 ap 34 25 26B-1 ap 87 73 16B-1 slat 74 61 18F-4 rudder 64 42 34F-11 spoiler 20 17 15Space shuttle doors 4150 3196 23DC-9 nose cowl 24 13 46F-16 landing gear door 53 42 20A-7 speed brake 123 74 40

RampD demonstration componentsWing

F-15 wing 2155 1787 17F-100 wing skin 616 475 23

FuselageF-111 fuselage 625 510 18CH-54 tail cone 464 397 14YF-16 fuselage 572 451 21F-5 fuselage 914 677 26

OtherB-1 vertical stabilizer 658 539 18B-1 horizontal stabilizer 3315 2844 14

aLoad capability 18 increase

molding (RTM) for composite part fabrication RTM is used forover 400 parts from inlet lip edges to sine wave wing spars Bothbismaleimide and epoxy parts are RTM processed The number ofparts made from thermoset composites is a 5050 split betweenepoxy resin and bismaleimide (BMI) Exterior skins are all BMIThermoplastics make up about 1 of the airframe by weight Theyare employed for the landing gear and weapons bay doors Auto-mated ber placement technology is utilized to fabricate the F-22composite horizontal stabilizersrsquo pivot shafts Overall 25 of theairframe structural material is composites

Helicopter rotor blade designers established an early position atthe forefront of the development and application of advanced com-posite structures and continue to design to maximize their bene ts

A Vertol CH-47 boronepoxy aft rotor blade was ight tested in1969 Of course glass ber-reinforced blades were in developmentand even production several years before that

Although weight reduction in helicopters is important it has beentheir adaptability to complex aerodynamic shapes that has enabledcomposites to play a major role in helicopter structures The con-cepts of hingeless and bearingless rotors were made practical by the

design exibility of composites Composite blades achieve virtuallyunlimited fatigue life

Most composite applications came about as a result of improvedperformance at an acceptable cost By the mid-1980s the struc-tural ef ciency of composites was fairly well recognized throughoutthe aerospace industry Emphasis then shifted to lowering develop-ment and production cost of composite assemblies the goal beingto achieve lower cost than competing aluminum construction Ex-panded use will require achieving this goal as well as projectedlong-term durability and supportability bene ts

The present emphasis on design to cost will continue facilitatedby integrated product teams (IPTs) and computer-aided design toolsHow well composite producibility and supportability issues are ad-dressed in the future will determine the extent and the rate at whichcomposite airframe applications will expand

Years of experience with composite materials have also resultedin lessons learned For example delaminations were found in thewing skins and substructure of US Marine Corps AV-8B and RAFUK-58 Harriers Investigations revealed that these delaminationswere caused by gaps between the skin and substructure that werenot properly shimmed prior to fastener torque-up during assemblyof the wings To correct this problem and prevent its occurrencein other composite aircraft production the US Navy now requiresthat liquid shimming be used to ll gaps between mating surfacesin composite structures Additionally closer dimensional tolerancesare placed on mating surfaces to ensure a better t

The one projection made by many was that 50or more of todayrsquosairframes would be made from composites This has not happenedfor two reasons 1) the cost delta for todayrsquos composites and 2) thepresent poor out-of-plane load reaction capability of laminated com-posite construction (and our limited ability to design to minimizethis shortcoming)

Design CriteriaUp to the mid-1950s the method for ensuring structural integrity

was essentially the same as that employed by the Wright brothersin 1903 A stress analysis was substantiated by a static test to a loadlevel greater than that expected in ight

The method consists of applying a factor of safety to the ightloads and verifying the structure could sustain this load by testing afull-scale airframe The factor of safety was developed to account forthe following sources of variability25 1)uncertainties in loads 2) in-accuracies in structural analysis 3) variations in material properties4) deterioration during service life and 5) variations in constructionquality

The aircraft yearbook in 1921 states ldquoThe fatigue of metals orprogressive failure from incipient cracking is the greatest bugbearof the designerrdquo The Bureau of Standards was reporting exu-ral fatigue data on duralumin sheets as early as 1922 HoweverRef 1 indicates that the rst mention of fatigue in government air-worthiness regulations did not appear until 1945 The concept ofmetal fatigue goes back a long way to the failures of railway carsand locomotive axles in the 1840s Reference 26 credits Wohler ofGermany in 1860 with the concept of designing for a nite fatiguelife In 1853 French engineers were inspecting axles of horse-drawnmail coaches for cracks and recommending their replacement after60000 km However Ref 27 indicates that it was not until 1914that suf cient test data were available to consider incorporating fa-tigue limits into aerospace structural steel speci cations In 1918the rst full-scale fatigue test of a large aircraft component wascarried out in the United Kingdom at the Royal Aircraft Establish-ment

Many papers were written in the 1940s predicting increases inairframe fatigue failures The authors of Ref 28 forecast increasesin fatigue troubles would stem from the following trends that ex-isted in 1943 1) a drive toward higher cruising speeds 2) the contin-uous increase in the ratio of useful load to gross weight and 3) theincrease in wing loading and gross weight plus an increase in servicelife necessitated by high initial cost

Reference 28 provides stressndashnumber of cycles (SndashN) curves onthe 17S and recommends designing on the basis of life expectancy

PAUL ET AL 25

The authors also recommend determining service loadings fromstatistical ight data recorded on existing aircraft but cautioned thatsuch data are applicable only to airplanes of speed wing loadingsize and mission similar to those in which the data were obtained

The author of Ref 26 indicates that Gassner of Germany hadwarned as early as 1941 that if the higher static strength of thenew aluminum alloy (7000-type) were taken advantage of a shorterfatigue life would result One goal of the researchers of that timewas to obtain suf cient data to be able to reduce factors of safetythat resulted from ignorance of the strength of materials

An International Committee on Aeronautical Fatigue wasfounded in 1951 at the suggestion of Plantema of The NetherlandsIt was two UK Comet crashes in 1954 and a series of US B-47catastrophic accidents in 1958 however that focused US Air Forceattention on structural fatigue and led to The US Air Force AircraftStructural Integrity Program (ASIP) This program adopted a fatiguemethodology approach commonly referred to as safe-life This ap-proach was used in the development of US Air Force aircraft inthe 1960s but was not adequate from the standpoint of addressingmanufacturing and service-induced damage Consequently aircraftdesigned both prior to and subsequent to the safe-life method contin-ued to experience premature failures The US Air Force ASIP wasrevised to include the incorporation of a damage tolerancedurabilityphilosophy into the design process Although initially applied tothe B-1A and modi cations of the F-111 C-5A and F-4 the dam-age tolerancedurability requirements were formally established inMIL-STD-1530A in 1975

The US Navy ASIP is similar It requires fatigue life of thestructure to be designed such that failure of a full-scale test articlewill not occur and that the structure will be free from any defectssuch as cracks deformation loss of modulus disbonding or fastenerhole deformation and delamination when subjected to a minimumof two lifetimes

Spurred on by joint service airframe requirements of the JointAdvanced Strike Technology program the services have developeda joint service guide speci cation for aircraft structures

All US Air Force aircraft since the F-16 have been subject todurabilitydamage tolerance requirements Each of the tests requiredby ASIP has served to identify de ciencies in structural design andanalysis resulting in modi cations to the structure that provided asafer and more supportable airframe

The full-scale airframe static fatigue durabilitydamage toler-ance tests are expensive and therefore there should be a reason-able expectation that they can with minor design modi cationsbe accomplished successfully This generally means conducting anextensive structural element and subcomponent test program in sup-port of the ASIP These tests (Table 4) address all of the critical loadpaths

This type of building block program that is testing large numbersof coupons elements and components prior to full-scale hardwarehas evolved as the most logical risk-reduction approach for newsystem airframe development The consequences of a failure late inthe development cycle of a system are too great to proceed any otherway

Table 4 Airframe design development tests

Aircraft Elements tested

B-2 160000 nonmetals 1450 metallic 6 leading edge74 wing panels 40 engine exhaust panels27 joints

C-17 6000 coupons 400 componentsF-22 Allowables

14500 composites 5300 metalsStructural integrity

4100 metals 8800 compositesReproducibility

80 full and subscale composite 15 titaniumstructures

FA-18 EF 10000 materials characterizationprocessing tests165 elements and components

V-22 9700 coupons 550 elements and components

The opposite is true for advanced technology demonstrators(ATDs) which introduce technology through prototyping thus ac-celerating transition of new technologies Failure in an ATD pro-gram is more accepted as a lesson learned rather than a failure ofthe contractor and procuring agency

Table 4 indicates that composite structures require more extensivedesign and certi cation testing than metals The additional testingrequirements add cost to the design allowables and design certi -cation efforts This was clearly identi ed in Ref 29 published inNovember 1994 as a lesson learned

In the case of composite structural assemblies however whenthe structure satis es all static load conditions fatigue is generallynot a problem The effects of moisture and temperature on designmargins can be established at the coupon and subcomponent levelwith the full-scale component margins adjusted appropriately

The one thing the described programs had in common was a sub-stantial nite element analysis effort (both global and local detailmodels) to support test hardware design and to certify the airframefor adequate design margins These programs demonstrate the im-portance of a thorough analysis effort coupled with a complete criti-cal area design development test program to substantiate the designIt would be unwise and uneconomical to dispense with either

Analysis and Design TechniquesThe introduction of digital computers in the 1950s paved the

way for the now emerging multidisciplinary design technologiesapplicable to the integrated design of a variety of engineering prod-ucts including air and space vehicles This section is a brief outlineof the developments that led to such software systems as ASKASTARDYNE NASTRAN ANSYS ABAQUS and ASTROS

Before discussing the chronology and signi cance of structuralanalysis developments it is appropriate to distinguish between twolevels of analysis 1) analysis of structural elements and 2) analysisof builtup structures

Analysis of Structural Elements

Structural elements or components can be categorized into threetypes line elements surface elements and solids The analysis ofsuch elements has been developed and documented over the last150 years by Euler Lagrange St Venant and others References 30and 31 document the chronology of these developments Over adozen solid mechanics books by Timoshenko constitute a rich com-pendium of analysis methods for structural elements The rangeand applicability of this continuum approach (the solution of dif-ferential equations) is however limited to very restricted cases ofloading boundary conditions and element shapes Such methods asRayleighndashRitz Galerkin and nite differences are approximationsThese methods replace a continuum model by a discretized equiva-lent The result is a set of algebraic equations that can be solved bydigital computers

Analysis of Builtup Structures

Builtup structures are simply constructed out of many structuralelements joined together at their boundaries The differential equa-tion approach for the solution of builtup structures is intractable Asan alternative a number of discretized procedures have been devel-oped since the early 1900s A structure is considered determinate ifits internal forces can be solved by the force and moment equilib-rium equations alone Otherwise it is considered an indeterminatesystem Analyzing complex indeterminate systems was a dauntingtask before the advent of digital computers

The force and displacement methods that emerged in the 1950swere a major breakthrough in structural analysis They refer to thediscretized model of the structure in which the continuum is re-placed by a nite number of grid points Each grid point is assumedto have six degrees of freedom (DOF) in space These DOFs areforces andor displacements In the force method the unknowns arethe set of forces released to make the indeterminate structure deter-minate Enforcement of compatibility conditions results in force-displacement relations connected by a exibility matrix In thedisplacement method the unknowns are the displacements of the

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 2: Evolution of U.S. Military Aircraft Structures Technology

PAUL ET AL 19

Fig 1 Vega semimonocoque wood fuselage

Fig 2a Northrop wing design

Fig 2b Douglas D-C2D-C3C-47 wing

skin panels supported by frames and doublers around cutouts re-mains the most prevalent structural concept Some of the more note-worthy variations on this theme follow postbuckled panel tension eld design sandwich structure integrally stiffened structure andunitized construction Postbuckled panel design simply means thepanel is required to carry a higher load than that at which the panelskin buckles

The original application of postbuckled tension eld structure5 isattributed to Herbert Wagner at Rohrbauch Metall ugzeugbau in1925 Postbuckled structural members have been employed in manyaircraft since including the B-52 B-1B F5E and AV-8B

For the most part these concepts have been incorporated intothe same basic Northrop skinframe structural arrangement Onenoteworthy exception was the geodetic aircraft construction (calledgeodesic structure in the United States) employed by VickersArmstrong Ltd This was chie y Duralumin (see aluminum alloyssection) metal lattice with a fabric covering (see Figs 3a and 3b)

This type of construction was used for both the 1934 VickersWellesley and the 1936 Vickers Wellington Geodetic constructionwas described in Ref 6 as being ldquoimmensely strong and could takeany amount of punishment from ak and return home againrdquo

The 1938 British Empire Air Annual de nes geodetic constructionas an unbroken diagrid system in which torsion loads are taken bycontinuous members in tension and compression When reactingto wing torsion loads one diagonal system of bracing members is

Fig 3a Vickers geodetic fuselage production

Fig 3b Vickers geodetic wing construction

under compression while the other is under tension Because theyare attached at their crossing points they mutually restrain eachother against de ection Normal spars react bending loads

Although not adopted composite geodesic wing and fuse-lage designs were evaluated by Goldsworthy Engineering for theBeechcraft Starship in the early 1980s Geodesically stiffened l-ament composite panels are presently under assessment A limitedamount of test data7 indicates such construction to be very damagetolerant of low-velocity impacts

Sandwich construction replaces skin stiffeners with lightweighthoneycomb core and fasteners with adhesive bonding This type ofconstruction permits the use of very thin airframe skin operating athigh stress levels without buckling

The structural ef ciency of sandwich construction (lightweightand improved compression stability) has been exploited for manyyears in aircraft construction Hoff and Mautner in Ref 8 iden-tify some early applications of sandwich construction in airframesThey indicate that in 1924 von Karman and P Stock were granted aGerman patent for a sandwich glider fuselage and that a plywoodndash

cork sandwich wing monoplane was displayed at the French SalondrsquoAeronautique in 1938 They also indicate that the incentive forsandwich construction was the desire to build a true monocoqueairplane

There is some sandwich structure on virtually every aircraft Thede Havilland Mosquito bomber of World War II fame employedbonded wood sandwich structure for wing panels Wing skins con-sisted of discrete blocks of spruce core and cedar plywood facesheets The fuselage was a sandwich of spruce veneer on balsa corewith solid spruce core substituted where attachments (concentratedloads) existed

Although described in Ref 9 as being ldquorobust to battle dam-agerdquo maintenance problems arose with the bonded airframe in theSouth Paci c Tropical organisms and humidity were said to do moredamage than the Japanese (De Haviland rst utilized a plywoodndash

balsa sandwich for the Albatross fuselage a four-engined airlinerof 1938)

The more recent 1964 B-70 was constructed of brazed steel hon-eycomb sandwich (22000 ft2 68 of the airframe weight)

The C-5 contains 35000 ft2 of bonded sandwich including thefollowing wing leading edges slats and wing tips trailing-edge

20 PAUL ET AL

surfaces aps and nacelles troop deck oor pylons and fairingsand main landing gear pod

The General Dynamics B-58 which broke 12 world speedrecords was called the bonded bomber because of the extensiveuse of bonded aluminum sandwich construction Developed in thelate 1950s aluminum (2024-T867075-T6) sandwich panels cov-ered 90 of the wings and 80 of the total airframe The entireouter skin of the wing was made up of honeycomb core sandwichand the fuselage was a combination of beaded and honeycomb sand-wich panels Partly because of extremely lightweight sandwich de-sign and partly because of a large bombfuel pod that comprised thelower-half of the fuselage the structure made up only 165 of thetakeoff gross weight (TOGW) and only 14 of the maximum grossweight achieved later in ight refueling

This lightweight airframe design brought with it signi cant struc-tural maintenance problems The Strategic Air Command once es-timated the cost of operating two wings of supersonic B-58 aircraftequaled that of six wings of B-52s

Of course this was not all airframe maintenance The B-58 avion-ics system consisted of 5000 vacuum tubes and transistors The thin-sheet sandwich skin was however susceptible to in-service damagedue to handling walking and punctures and required periodic time-consuming costly inspections

According to Petrushka10 General Dynamics Director of Struc-tures in 1985 if the B-58 had not been retired early ldquoit is fair toconjecture that the extreme maintenance problems with honeycombsandwich could well have become the limiting life factorrdquo10

Some recent record setting aircraft that utilized compositesandwich airframes include the following 1) The graphite com-posite Hexcel honeycomb sandwich Voyager was employed byRichard Rutan and Jeana Yeager to accomplish their nine-daynonstop unrefueled 25000-mile ight around the world in 1987The Voyagerrsquos takeoff weight was more than 10 times the struc-tural weight 2) The high-altitude Boeing-built uncrewed graphiteKevlarreg epoxyNomex honeycomb Condor has a 200-ft wing spanwith a 367 aspect ratio that weighs only 2 lbft2 enabling it to setan altitude record of 66980 ft (for piston powered aircraft) and tostay aloft for 2 1

2days in 1989

Reference 11 indicates that a primary motivation for the develop-ment of the bonded honeycomb structure is the signi cant decreasein the number of parts with the associated decreased fabricationcost over mechanically fastened construction

Minimizing fastener installation time and cost has been a de-sire of manufacturing engineers ever since the rst all-metal air-frames Reference 12 indicates that the CurtisndashWright Corporationexplored stitching of metal aircraft parts during World War II to re-duce aircraft fabrication time associated with riveting and spotweld-ing An automotive industry metal stitcher (Fig 4) was adapted toairframe metal-to-metal and dissimilar material joining Stitchingand weaving is now being employed to create three-dimensional

Fig 4 CurtisndashWright metal stitching machine

Fig 5 Composite material stitching machine

Fig 6 Wright model A delivered 7 September 1908

carbon ber preforms [Such a stitching machine13 is funded bythe NASA Advanced Composites Technology (ACT) program (seeFig 5)] This machine developed by Ingersoll Milling Machine andPathe technologies has four high-speed stitching heads capable of800 stitchesmin

MaterialsThe rst US military aircraft (Fig 6) employed a composite

airframe (wood wire and fabric) The transition from wood andfabric composites to predominately metal construction in the UnitedStates took approximately 10 years This is considerably less timethan the present gradual transition to high modulus composites Thiscontention is supported by a review of the Aeronautical Chamberof Commerce of America Aircraft YearBook from 1921 to 1931

By 1931 the aircraft yearbook identi ed the following fourUS aircraft as employing stressed skin metal constructionBoeing Monomail Thaden T-4 Northrop Alpha and ConsolidatedFleetster(Although the Fleetster employed anall-metal monocoquefuselage initially the wing was of plywood construction Ref 14 in-dicates the aircraft was all metal by 1932)

The rst all-metal airplane an integrally braced monoplane wasdesigned and constructed by Hugo Junkers in 1915 Junkers obtaineda Deutsches Reich patent for an all-metal ying wing in 1910 InRef 15 he indicates that the wing for his initial sheet iron airplaneprototype was a ldquosupporting coverrdquo concept where all tensile com-pressive and shearing forces are taken up by the wing cover Be-cause the welded iron wing weight resulted in a poor airplane rate ofclimb he changed the wing design in subsequent models to ldquoreactbending moments with a more ef cient frame or trestle of tubesrdquo

The rst smooth skin metal monocoque fuselage patent num-ber 1557855 was issued to Flavius E Loudy in 1921 (see Fig 7from Ref 16)

In December 196917 a lecture to the Royal Aeronautical Soci-ety of England titled ldquoThe History of Metal Aircraft Constructionrdquowas given by Marcus Langley In the prolog to this lecture he indi-cated that his lecture was in some way an obituary of metal aircraftconstruction for the following reason ldquoAlthough we shall still have

PAUL ET AL 21

metal aircraft for many years to come quite new materials are be-ginning to appear and they have as many advantages over metal asmetal had over wood I am referring to such materials as carbon bers used in matrices of synthetic resinsrdquo17

Hindsight now shows that this pronouncement and many simi-lar since then were premature The airframe of todayrsquos aircraft 25years later is a hybrid of metal and composites A review of thecurrent status of airframe materials usage (Table 1) shows a fairlyequal balance in the use of aluminum titanium and compositesThis balance is a modern development driven primarily by perfor-mance requirements and affordability A discussion of each materialevolution follows

Aluminum Alloys

For 60 years aluminum alloys have been the primary materialsfor airframes No other single material has played as major a rolein aircraft production as aluminum speci cally the 2000 and 7000series ingot alloys in various heat treatments

In 1944 Alcoa developed 75S (AlndashZnndashMgndashCu) expressly to meetthe aircraft industryrsquos need for higher strength This alloy rst sawservice on the B-29 bomber and according to the 1945 aircraftyearbook ldquoPractically all the new war planes were utilizing highstrength 75Srdquo

The principal aluminum alloy since 1945 however has beenAlcoarsquos 24S (AlndashCundashMgndashMn) that contained the same alloyingelements as the older 17S but in different proportions for greaterstrength Produced in 1917 17S was Alcoarsquos version of Duralumin(from the French word dur meaning hard) patented by Alfred Wilmin Germany in 1908 Wilmrsquos Dural aluminum alloy was used for theZeppelin structure in 1911 Alcoarsquos 17S was used in constructionof the US Navy airship Shenandoah which rst ew in 1923 Thisalloy began the US stressed metal skin semimonocoque structurerevolution

Other countries were also quick to adopt Duralumin monocoquefuselage construction In England the Short Brothers Company builtthe metal Silver Streak biplane in 1919 which was all Duraluminexcept for the wing spars which were steel The wing consisted ofround steel tube spars and Duralumin ribs with right angle angesto which riveted Duralumin covers were joined18

Table 1 Materials use as a percentage of structural weight

Material B-2 FA-18EF F-22 V-22 C-17 AV-8B

Composite 37 22 25 42 08 26Aluminum 27 27 16 mdashmdash 70 47Titanium 23 23 39 mdashmdash 09 mdashmdash

Fig 7 F E Loudy patent 1557855 led 1921

Over the many years of utilizing aluminum materials several im-portant lessons have been learned Reference 19 cites the example ofthe KC-135 aircraft In 1954 the Boeing 367-80 became Americarsquos rst jet transport prototype The KC-135 was a derivative of theBoeing 367-80 (Dash 80) as was the 707 but the lower wing skinof the KC-135 was initially 7178-T6 Al A small critical crack lengthfor this material at the KC-135 wing limit load level led to casesof in-service rapid fracture

The concern about loss of an aircraft from degradation of failsafety from widespread fatigue cracking motivated the US AirForce to make a costly modi cation The lower wing skins werereplaced with 2024-T3 aluminum

In 1982 14 years of service experience with the C-5A resultedin greater emphasis being placed on fracture toughness corro-sion resistance and durability in selecting materials for the C-5B7075-T6 fuselage skin and cargo oor skins were changed to 7475-T761 where corrosion was a problem The wing plank material waschanged from 7075-T6 to 7175-T73 for increased toughness

Predictions as early as 198520 were that ldquolithium containing alu-minum alloys (AlLi) would nd signi cant use in both militaryand civil aircraftrdquo and that the ldquoultimate level of utilization of car-bon ber reinforced polymer matrix composites may be somewhatless than predicted in view of the potential of AlLi alloysrdquo

By the early 1990s however the realization set in that AlLiwas not ready to be a wide-scale direct substitute for conventionalaluminum aerospace alloys although it was being used in someproduction airframe applications in the United States and EuropeAt present it is the high-strength aluminum alloys such as 7150 and7055 with increased compression strength and balanced fracturetoughness and corrosion resistance that are enabling aluminum tomaintain its high level of use in the aerospace industry

Titanium

The rst major structural application of titanium was the DouglasX-3 that utilized 629 lb of Rem-Cru Inc titanium This was a com-mercially pure material used for the aft fuselage boom and stabilatorportions of the aircraft

The X-3 rst ew in 1952 and O A Wheelon a production de-sign engineer with the Douglas Aircraft Company was awarded theWright brothers medal for a published analysis of the use of titaniumin aircraft construction entitled ldquoDesign Methods and Manufactur-ing Techniques With Titaniumrdquo

One of the rst airframe production applications was the F-86Sabre Jet which is noted for a killloss ratio of at least 41 in theKorean War This aircraft which ew as the XP-86 in 1947 em-ployed 600 lb of titanium in the aft fuselage and engine areas (Fig 9)

22 PAUL ET AL

Fig 8 US production of titanium sponge

Fig 9 Titanium aft fuselage inproduction at North American Aviationin 1954

The history and early use of titanium was solidly wedded to theaerospace industry and the annealed alphandashbeta alloy Ti 6Al-4V hasbeen the workhorse of the industry

This alloy has been utilized on aircraft from the B-52 to theF-22 Reference 21 indicates this speci c alloy composition was rst melted and evaluated in 1953 at IIT Research Institute under aUS Air Force contract The Mallory Sharon Titanium Corporationversion of this alloy (MST 6Al-4V) was selected shortly thereafter(1954) for use in Pratt and Whitneyrsquos J-57 turbojet engine (disksblades and other parts) for the B-52

The rst commercial production of titanium named after theTitans of mythology began in 1946 Titanium was pushed into com-mercial production as a structural material in unprecedented timeby the combined efforts of industry and government to meet militaryrequirements

Figure 8 shows how rapidly titanium sponge (re ned metal beforemelting into usable form) production was scaled up In 1956 90 ofthe titanium market was for military aircraft production By 1980less than 20 of aerospace grade was used for military aircraftThe majority of titanium sponge is processed into titanium dioxidepigment for use as ller in paint paper plastics and rubber

The use of titanium has been increasing at the expense ofboth aluminum and composites and will continue to increase inmore unitized assemblies facilitated by combined superplastic-formingdiffusion-bonding welding and casting

The most famous US titanium airplane the YF-12ASR-71 rst ew as the A-12 in 1962 and was fabricated from primarily beta-120VCA alloy (Ti-13V-11Cr-3Al)

Johnson stated in Ref 22 that ldquoof the rst 6000 B-120 piecesfabricated we lost 95rdquo The material was described as being sobrittle that if it fell off your desk it would shatter on the oor

Fig 10a F-15CD conventional aft fuselage

Fig 10b F-15E SPFDB aft fuselage

The most widely used alloy is Ti-6Al-4V This alloy is particularlyamenable to fabrication by superplastic-formingdiffusion-bonding(SPFDB) processes and therefore was the subject of a systematicwell-planned research and development (RampD) thrust by the USAir Force and US Navy over a 15-year period from 1970 to 1985

Reference 23 indicates that the use of SPFDB titanium for theaft fuselage of the F-15E resulted in 726 fewer components and10000 fewer fasteners and achieved 15 weight savings over theCD models (see Figs 10a and 10b)

Actually it has been large forgings and castings that have facil-itated the creation of one-piece substructures Figure 11 shows aTi-6Al-4V F-22 aft-fuselage frame produced by WymanndashGordonThe F-22 wing carry-through bulkhead is fabricated from the largesttitanium forging by surface area to date (96 ft2 6560 lb) Four bulk-heads in the midfuselage are made of titanium The forward and aftboom structure on either side of the aircraftrsquos empennage sectionare formed from integrally stiffened titanium isogrid panels that areelectron beam welded to forged titanium frames The wing attach t-tings and rudder actuator support are hot isostatically pressed (HIP)titanium castings

The A-6E composite wing rear spar is a 27-ft titanium forgingand large one-piece titanium bulkhead forgings are employed by theFA-18 EF aircraft

The BellndashBoeing V-22 Osprey Engineering and ManufacturingDevelopment aircraft ( rst ight 5 February 1997) used a one-piecetitanium casting for the transmission adapter case stow ring that waspreviously 40 separate riveted components This part serves as theprimary support structure for the rotor system proprotor gearboxand engine and nacelle components The Howmet Corporation in-dicates a premium-quality consistently homogenous component is

PAUL ET AL 23

Fig 11 F-22 aft-fuselage frame forging produced on WymanndashGordon50000-ton hydraulic press

achieved by the HIP processing so that allowables do not have to bereduced by conventional casting factors

Recently RMI Titanium of Ohio developed a stronger more dam-age tolerant alloy (Ti-6-22-22S) that is nding application on theF-22 joint strike ghter and X-33 reusable launch vehicle

Timet Corporation alloy 10ndash2ndash3 is being used in the main landinggear and Timetal 21S for the nacelle cowl plug nozzle and Prattand Whitney Aircraft engine components of the Boeing 777 aircraftTitanium usage on Boeing aircraft has increased from 2 emptyweight of the 737 to 8 of the 777 The McDonnell Douglas C-17transport also utilizes alloy 10ndash2ndash3 for landing gear components

The focus of titanium alloy development however has shiftedfrom aerospace to industrial applications New product forms areemerging rapidly Titanium is growing in use for everything fromgolf clubs skis tennis rackets bicycles pots and pans and jewelryto even hip and knee replacements The kitchen sink cannot be faroff

Noting this trend toward increased titanium use a 1991 AdvancedMaterials and Processing magazine article was titled ldquoTitaniumStaves Off Compositesrdquo In reality titanium has grown in use partlybecause of its compatibility with composites both from a thermalexpansion characteristic and resistance to galvanic corrosion whenin contact with carbon epoxy

Advanced Composites

The present generation of composite structures technology ap-plied to the B-2 FA-18EF V-22 and RAH-66 has been in devel-opment for 30 years The present level of maturity is a result of acontinued investment by the RampD structures and materials commu-nities of all three services and NASA No other class of material hasreceived such high level of funding support for such a long time Nu-merous composite structures built from the 1960s to 1980s veri edcomposite structure advantages (primarily weight savings) leadingto more extensive application of composite airframes in the 1990swhen affordability became the major issue

Table 2 is a summary of some of the programs initiated to fos-ter the development and transition of composites Although theseinitiatives were seldom adequately funded to complete all initiallyplanned objectives they nonetheless were able to sustain substantialfunding for some limited period of time

Production in the United States of high modulus ber compositestructures commonly referred to as advanced composites beganwith boron epoxy F-14 and F-15 horizontal stabilizers in the early

Table 2 Advanced composite technology programs

Agency Acronym Program

US Army ACAP Advanced composite airframe programNASA ACEE Aircraft energy ef ciencyUS Air Force DMLCC Design and manufacture of low cost

compositesNASA ACT Advanced composites technologyUS Navy AV-8B Composite structure development programARPAa ACP Affordable composites for propulsionDARPAb APMC Affordable polymer matrix compositesDODcindustry CAI Composites affordability initiative

aAdvanced Research Projects AgencybDefense Advanced Research Projects AgencycDepartment of Defense

1970s The technology was fostered by the US Air Force BoronFilaments and Composites Advanced Development Program ini-tiated in 1964 as a result of a Project Forecast recommendationExperimental carbon epoxy structures were ying in 1970 and car-bon epoxy came to the forefront as the most cost-effective advancedcomposite material in the mid-1970s with production applicationson the F-15 B-1 and F-16 In 1978 carbon epoxy was accepted bythe Navy for F-18 and AV-8B primary wing box structures

Table 3 is a sample of some of the early composite structuredemonstrations Table 3 consists of a list of composite structuresfor which a metal baseline weight was available There have beenso many composite structures built since these and many more arecurrently in development that any list is out of date before it isprinted

There were of course many other composite structures some ofwhich did not have a metal baseline or for which a metal coun-terpart would not even be practical Some of those that come tomind are the Rockwell highly maneuverable advanced technologyremotely piloted research vehicle and the Grumman X-29 forwardswept wing demonstrator Both of these designs exploited the ad-vantageous aeroelastic characteristic of exible composite wings

The bulk of advanced composite material being used in ightin the United States today is fabricated from unidirectional tapeconsisting of AS-4 or IM7 carbon ber in 3501 thermoset epoxymatrix preimpregnated by Hercules Inc

Although developed some time ago the Slingsby T67 Fire ywas selected in 1992 for the US Air Force enhanced ight screener(T3A) The basic airframe started as a wooden construction thatSlingsby redesigned as primarily a wet layup glass ber polymercomposite airframe

Fiberglass-reinforced polymer construction has been employedfor many years in the glider industry It has been arguedpara that theHorton brothers of Germany created the worldrsquos rst compositeaircraft the Ho Va which began as a glider in 1936 The fuselageand wings for the rst Ho V were fabric-covered D-tubes moldedfrom paper- lled phenol resin R Horten noted that such moldedplastic construction promised to reduce production time and cost

The term molded reinforced plastic plane was being used by 1941(see Ref 24) Although nearly every form of material was tried as ller resin-coated wood veneer strips and sheets (usually mahoganyor spruce) were the most popular reinforcing agents Impregnationby liquid phenolic- or urea-type resins was common

Both Japan and Germany built highly successful berglass com-posite soaring planes in the 1950s Several of these are described inRef 1

The 42 by weight composite structure BellBoeing OspreyV-22 rst ew in 1989 The critical link between the blade andthe rotor hub (the yoke) of the V-22 is a composite structure Theyoke carries the blade centrifugal and lifting forces transmits enginetorque and permits lead-lag and apping motion Also each yokearm can accommodate pitch changes

The US Air Forcersquos latest ghter the F-22 Raptor ( rst ight7 September 1997) was the rst aircraft to employ resin transfer

paraData available online at httpwwwnur ugelcom

24 PAUL ET AL

Table 3 Composite structure weight savings

Baseline Composite WeightComponent weight lb weight lb savings

Flight service componentsEmpennage

F-111 horizontal stabilizer 1142 866 24F-14 horizontal stabilizer 1005 825 18F-15 horizontal stabilizer 527 395 25F-5 vertical stabilizer 119 83 30DC-10 vertical stabilizer 1005 834 17L-1011 vertical stabilizer 858 642 25B-737 horizontal stabilizer 262 204 22

WingsAV-8B wing 1143 949 17F-18 wing 1843 1641 11A-6 wing 6392 6400 0a

(selective reinforcement)C-130 wing box 4800 4200 125

FuselageUH-60 rear fuselage 444 399 10AV-8B forward fuselage 229 171 25

(selective reinforcement)CH-53 Aft fuselage 1159 947 18B-1 dorsal longeron 1485 829 44

OtherC-5 slat 241 190 21B-727 elevator 131 98 25DC-10 rudder 91 67 26L0-1011 aileron 140 107 23F-15 speed brake 112 89 21B-1 weapons bay door 147 129 12F-5 ap 34 25 26B-1 ap 87 73 16B-1 slat 74 61 18F-4 rudder 64 42 34F-11 spoiler 20 17 15Space shuttle doors 4150 3196 23DC-9 nose cowl 24 13 46F-16 landing gear door 53 42 20A-7 speed brake 123 74 40

RampD demonstration componentsWing

F-15 wing 2155 1787 17F-100 wing skin 616 475 23

FuselageF-111 fuselage 625 510 18CH-54 tail cone 464 397 14YF-16 fuselage 572 451 21F-5 fuselage 914 677 26

OtherB-1 vertical stabilizer 658 539 18B-1 horizontal stabilizer 3315 2844 14

aLoad capability 18 increase

molding (RTM) for composite part fabrication RTM is used forover 400 parts from inlet lip edges to sine wave wing spars Bothbismaleimide and epoxy parts are RTM processed The number ofparts made from thermoset composites is a 5050 split betweenepoxy resin and bismaleimide (BMI) Exterior skins are all BMIThermoplastics make up about 1 of the airframe by weight Theyare employed for the landing gear and weapons bay doors Auto-mated ber placement technology is utilized to fabricate the F-22composite horizontal stabilizersrsquo pivot shafts Overall 25 of theairframe structural material is composites

Helicopter rotor blade designers established an early position atthe forefront of the development and application of advanced com-posite structures and continue to design to maximize their bene ts

A Vertol CH-47 boronepoxy aft rotor blade was ight tested in1969 Of course glass ber-reinforced blades were in developmentand even production several years before that

Although weight reduction in helicopters is important it has beentheir adaptability to complex aerodynamic shapes that has enabledcomposites to play a major role in helicopter structures The con-cepts of hingeless and bearingless rotors were made practical by the

design exibility of composites Composite blades achieve virtuallyunlimited fatigue life

Most composite applications came about as a result of improvedperformance at an acceptable cost By the mid-1980s the struc-tural ef ciency of composites was fairly well recognized throughoutthe aerospace industry Emphasis then shifted to lowering develop-ment and production cost of composite assemblies the goal beingto achieve lower cost than competing aluminum construction Ex-panded use will require achieving this goal as well as projectedlong-term durability and supportability bene ts

The present emphasis on design to cost will continue facilitatedby integrated product teams (IPTs) and computer-aided design toolsHow well composite producibility and supportability issues are ad-dressed in the future will determine the extent and the rate at whichcomposite airframe applications will expand

Years of experience with composite materials have also resultedin lessons learned For example delaminations were found in thewing skins and substructure of US Marine Corps AV-8B and RAFUK-58 Harriers Investigations revealed that these delaminationswere caused by gaps between the skin and substructure that werenot properly shimmed prior to fastener torque-up during assemblyof the wings To correct this problem and prevent its occurrencein other composite aircraft production the US Navy now requiresthat liquid shimming be used to ll gaps between mating surfacesin composite structures Additionally closer dimensional tolerancesare placed on mating surfaces to ensure a better t

The one projection made by many was that 50or more of todayrsquosairframes would be made from composites This has not happenedfor two reasons 1) the cost delta for todayrsquos composites and 2) thepresent poor out-of-plane load reaction capability of laminated com-posite construction (and our limited ability to design to minimizethis shortcoming)

Design CriteriaUp to the mid-1950s the method for ensuring structural integrity

was essentially the same as that employed by the Wright brothersin 1903 A stress analysis was substantiated by a static test to a loadlevel greater than that expected in ight

The method consists of applying a factor of safety to the ightloads and verifying the structure could sustain this load by testing afull-scale airframe The factor of safety was developed to account forthe following sources of variability25 1)uncertainties in loads 2) in-accuracies in structural analysis 3) variations in material properties4) deterioration during service life and 5) variations in constructionquality

The aircraft yearbook in 1921 states ldquoThe fatigue of metals orprogressive failure from incipient cracking is the greatest bugbearof the designerrdquo The Bureau of Standards was reporting exu-ral fatigue data on duralumin sheets as early as 1922 HoweverRef 1 indicates that the rst mention of fatigue in government air-worthiness regulations did not appear until 1945 The concept ofmetal fatigue goes back a long way to the failures of railway carsand locomotive axles in the 1840s Reference 26 credits Wohler ofGermany in 1860 with the concept of designing for a nite fatiguelife In 1853 French engineers were inspecting axles of horse-drawnmail coaches for cracks and recommending their replacement after60000 km However Ref 27 indicates that it was not until 1914that suf cient test data were available to consider incorporating fa-tigue limits into aerospace structural steel speci cations In 1918the rst full-scale fatigue test of a large aircraft component wascarried out in the United Kingdom at the Royal Aircraft Establish-ment

Many papers were written in the 1940s predicting increases inairframe fatigue failures The authors of Ref 28 forecast increasesin fatigue troubles would stem from the following trends that ex-isted in 1943 1) a drive toward higher cruising speeds 2) the contin-uous increase in the ratio of useful load to gross weight and 3) theincrease in wing loading and gross weight plus an increase in servicelife necessitated by high initial cost

Reference 28 provides stressndashnumber of cycles (SndashN) curves onthe 17S and recommends designing on the basis of life expectancy

PAUL ET AL 25

The authors also recommend determining service loadings fromstatistical ight data recorded on existing aircraft but cautioned thatsuch data are applicable only to airplanes of speed wing loadingsize and mission similar to those in which the data were obtained

The author of Ref 26 indicates that Gassner of Germany hadwarned as early as 1941 that if the higher static strength of thenew aluminum alloy (7000-type) were taken advantage of a shorterfatigue life would result One goal of the researchers of that timewas to obtain suf cient data to be able to reduce factors of safetythat resulted from ignorance of the strength of materials

An International Committee on Aeronautical Fatigue wasfounded in 1951 at the suggestion of Plantema of The NetherlandsIt was two UK Comet crashes in 1954 and a series of US B-47catastrophic accidents in 1958 however that focused US Air Forceattention on structural fatigue and led to The US Air Force AircraftStructural Integrity Program (ASIP) This program adopted a fatiguemethodology approach commonly referred to as safe-life This ap-proach was used in the development of US Air Force aircraft inthe 1960s but was not adequate from the standpoint of addressingmanufacturing and service-induced damage Consequently aircraftdesigned both prior to and subsequent to the safe-life method contin-ued to experience premature failures The US Air Force ASIP wasrevised to include the incorporation of a damage tolerancedurabilityphilosophy into the design process Although initially applied tothe B-1A and modi cations of the F-111 C-5A and F-4 the dam-age tolerancedurability requirements were formally established inMIL-STD-1530A in 1975

The US Navy ASIP is similar It requires fatigue life of thestructure to be designed such that failure of a full-scale test articlewill not occur and that the structure will be free from any defectssuch as cracks deformation loss of modulus disbonding or fastenerhole deformation and delamination when subjected to a minimumof two lifetimes

Spurred on by joint service airframe requirements of the JointAdvanced Strike Technology program the services have developeda joint service guide speci cation for aircraft structures

All US Air Force aircraft since the F-16 have been subject todurabilitydamage tolerance requirements Each of the tests requiredby ASIP has served to identify de ciencies in structural design andanalysis resulting in modi cations to the structure that provided asafer and more supportable airframe

The full-scale airframe static fatigue durabilitydamage toler-ance tests are expensive and therefore there should be a reason-able expectation that they can with minor design modi cationsbe accomplished successfully This generally means conducting anextensive structural element and subcomponent test program in sup-port of the ASIP These tests (Table 4) address all of the critical loadpaths

This type of building block program that is testing large numbersof coupons elements and components prior to full-scale hardwarehas evolved as the most logical risk-reduction approach for newsystem airframe development The consequences of a failure late inthe development cycle of a system are too great to proceed any otherway

Table 4 Airframe design development tests

Aircraft Elements tested

B-2 160000 nonmetals 1450 metallic 6 leading edge74 wing panels 40 engine exhaust panels27 joints

C-17 6000 coupons 400 componentsF-22 Allowables

14500 composites 5300 metalsStructural integrity

4100 metals 8800 compositesReproducibility

80 full and subscale composite 15 titaniumstructures

FA-18 EF 10000 materials characterizationprocessing tests165 elements and components

V-22 9700 coupons 550 elements and components

The opposite is true for advanced technology demonstrators(ATDs) which introduce technology through prototyping thus ac-celerating transition of new technologies Failure in an ATD pro-gram is more accepted as a lesson learned rather than a failure ofthe contractor and procuring agency

Table 4 indicates that composite structures require more extensivedesign and certi cation testing than metals The additional testingrequirements add cost to the design allowables and design certi -cation efforts This was clearly identi ed in Ref 29 published inNovember 1994 as a lesson learned

In the case of composite structural assemblies however whenthe structure satis es all static load conditions fatigue is generallynot a problem The effects of moisture and temperature on designmargins can be established at the coupon and subcomponent levelwith the full-scale component margins adjusted appropriately

The one thing the described programs had in common was a sub-stantial nite element analysis effort (both global and local detailmodels) to support test hardware design and to certify the airframefor adequate design margins These programs demonstrate the im-portance of a thorough analysis effort coupled with a complete criti-cal area design development test program to substantiate the designIt would be unwise and uneconomical to dispense with either

Analysis and Design TechniquesThe introduction of digital computers in the 1950s paved the

way for the now emerging multidisciplinary design technologiesapplicable to the integrated design of a variety of engineering prod-ucts including air and space vehicles This section is a brief outlineof the developments that led to such software systems as ASKASTARDYNE NASTRAN ANSYS ABAQUS and ASTROS

Before discussing the chronology and signi cance of structuralanalysis developments it is appropriate to distinguish between twolevels of analysis 1) analysis of structural elements and 2) analysisof builtup structures

Analysis of Structural Elements

Structural elements or components can be categorized into threetypes line elements surface elements and solids The analysis ofsuch elements has been developed and documented over the last150 years by Euler Lagrange St Venant and others References 30and 31 document the chronology of these developments Over adozen solid mechanics books by Timoshenko constitute a rich com-pendium of analysis methods for structural elements The rangeand applicability of this continuum approach (the solution of dif-ferential equations) is however limited to very restricted cases ofloading boundary conditions and element shapes Such methods asRayleighndashRitz Galerkin and nite differences are approximationsThese methods replace a continuum model by a discretized equiva-lent The result is a set of algebraic equations that can be solved bydigital computers

Analysis of Builtup Structures

Builtup structures are simply constructed out of many structuralelements joined together at their boundaries The differential equa-tion approach for the solution of builtup structures is intractable Asan alternative a number of discretized procedures have been devel-oped since the early 1900s A structure is considered determinate ifits internal forces can be solved by the force and moment equilib-rium equations alone Otherwise it is considered an indeterminatesystem Analyzing complex indeterminate systems was a dauntingtask before the advent of digital computers

The force and displacement methods that emerged in the 1950swere a major breakthrough in structural analysis They refer to thediscretized model of the structure in which the continuum is re-placed by a nite number of grid points Each grid point is assumedto have six degrees of freedom (DOF) in space These DOFs areforces andor displacements In the force method the unknowns arethe set of forces released to make the indeterminate structure deter-minate Enforcement of compatibility conditions results in force-displacement relations connected by a exibility matrix In thedisplacement method the unknowns are the displacements of the

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 3: Evolution of U.S. Military Aircraft Structures Technology

20 PAUL ET AL

surfaces aps and nacelles troop deck oor pylons and fairingsand main landing gear pod

The General Dynamics B-58 which broke 12 world speedrecords was called the bonded bomber because of the extensiveuse of bonded aluminum sandwich construction Developed in thelate 1950s aluminum (2024-T867075-T6) sandwich panels cov-ered 90 of the wings and 80 of the total airframe The entireouter skin of the wing was made up of honeycomb core sandwichand the fuselage was a combination of beaded and honeycomb sand-wich panels Partly because of extremely lightweight sandwich de-sign and partly because of a large bombfuel pod that comprised thelower-half of the fuselage the structure made up only 165 of thetakeoff gross weight (TOGW) and only 14 of the maximum grossweight achieved later in ight refueling

This lightweight airframe design brought with it signi cant struc-tural maintenance problems The Strategic Air Command once es-timated the cost of operating two wings of supersonic B-58 aircraftequaled that of six wings of B-52s

Of course this was not all airframe maintenance The B-58 avion-ics system consisted of 5000 vacuum tubes and transistors The thin-sheet sandwich skin was however susceptible to in-service damagedue to handling walking and punctures and required periodic time-consuming costly inspections

According to Petrushka10 General Dynamics Director of Struc-tures in 1985 if the B-58 had not been retired early ldquoit is fair toconjecture that the extreme maintenance problems with honeycombsandwich could well have become the limiting life factorrdquo10

Some recent record setting aircraft that utilized compositesandwich airframes include the following 1) The graphite com-posite Hexcel honeycomb sandwich Voyager was employed byRichard Rutan and Jeana Yeager to accomplish their nine-daynonstop unrefueled 25000-mile ight around the world in 1987The Voyagerrsquos takeoff weight was more than 10 times the struc-tural weight 2) The high-altitude Boeing-built uncrewed graphiteKevlarreg epoxyNomex honeycomb Condor has a 200-ft wing spanwith a 367 aspect ratio that weighs only 2 lbft2 enabling it to setan altitude record of 66980 ft (for piston powered aircraft) and tostay aloft for 2 1

2days in 1989

Reference 11 indicates that a primary motivation for the develop-ment of the bonded honeycomb structure is the signi cant decreasein the number of parts with the associated decreased fabricationcost over mechanically fastened construction

Minimizing fastener installation time and cost has been a de-sire of manufacturing engineers ever since the rst all-metal air-frames Reference 12 indicates that the CurtisndashWright Corporationexplored stitching of metal aircraft parts during World War II to re-duce aircraft fabrication time associated with riveting and spotweld-ing An automotive industry metal stitcher (Fig 4) was adapted toairframe metal-to-metal and dissimilar material joining Stitchingand weaving is now being employed to create three-dimensional

Fig 4 CurtisndashWright metal stitching machine

Fig 5 Composite material stitching machine

Fig 6 Wright model A delivered 7 September 1908

carbon ber preforms [Such a stitching machine13 is funded bythe NASA Advanced Composites Technology (ACT) program (seeFig 5)] This machine developed by Ingersoll Milling Machine andPathe technologies has four high-speed stitching heads capable of800 stitchesmin

MaterialsThe rst US military aircraft (Fig 6) employed a composite

airframe (wood wire and fabric) The transition from wood andfabric composites to predominately metal construction in the UnitedStates took approximately 10 years This is considerably less timethan the present gradual transition to high modulus composites Thiscontention is supported by a review of the Aeronautical Chamberof Commerce of America Aircraft YearBook from 1921 to 1931

By 1931 the aircraft yearbook identi ed the following fourUS aircraft as employing stressed skin metal constructionBoeing Monomail Thaden T-4 Northrop Alpha and ConsolidatedFleetster(Although the Fleetster employed anall-metal monocoquefuselage initially the wing was of plywood construction Ref 14 in-dicates the aircraft was all metal by 1932)

The rst all-metal airplane an integrally braced monoplane wasdesigned and constructed by Hugo Junkers in 1915 Junkers obtaineda Deutsches Reich patent for an all-metal ying wing in 1910 InRef 15 he indicates that the wing for his initial sheet iron airplaneprototype was a ldquosupporting coverrdquo concept where all tensile com-pressive and shearing forces are taken up by the wing cover Be-cause the welded iron wing weight resulted in a poor airplane rate ofclimb he changed the wing design in subsequent models to ldquoreactbending moments with a more ef cient frame or trestle of tubesrdquo

The rst smooth skin metal monocoque fuselage patent num-ber 1557855 was issued to Flavius E Loudy in 1921 (see Fig 7from Ref 16)

In December 196917 a lecture to the Royal Aeronautical Soci-ety of England titled ldquoThe History of Metal Aircraft Constructionrdquowas given by Marcus Langley In the prolog to this lecture he indi-cated that his lecture was in some way an obituary of metal aircraftconstruction for the following reason ldquoAlthough we shall still have

PAUL ET AL 21

metal aircraft for many years to come quite new materials are be-ginning to appear and they have as many advantages over metal asmetal had over wood I am referring to such materials as carbon bers used in matrices of synthetic resinsrdquo17

Hindsight now shows that this pronouncement and many simi-lar since then were premature The airframe of todayrsquos aircraft 25years later is a hybrid of metal and composites A review of thecurrent status of airframe materials usage (Table 1) shows a fairlyequal balance in the use of aluminum titanium and compositesThis balance is a modern development driven primarily by perfor-mance requirements and affordability A discussion of each materialevolution follows

Aluminum Alloys

For 60 years aluminum alloys have been the primary materialsfor airframes No other single material has played as major a rolein aircraft production as aluminum speci cally the 2000 and 7000series ingot alloys in various heat treatments

In 1944 Alcoa developed 75S (AlndashZnndashMgndashCu) expressly to meetthe aircraft industryrsquos need for higher strength This alloy rst sawservice on the B-29 bomber and according to the 1945 aircraftyearbook ldquoPractically all the new war planes were utilizing highstrength 75Srdquo

The principal aluminum alloy since 1945 however has beenAlcoarsquos 24S (AlndashCundashMgndashMn) that contained the same alloyingelements as the older 17S but in different proportions for greaterstrength Produced in 1917 17S was Alcoarsquos version of Duralumin(from the French word dur meaning hard) patented by Alfred Wilmin Germany in 1908 Wilmrsquos Dural aluminum alloy was used for theZeppelin structure in 1911 Alcoarsquos 17S was used in constructionof the US Navy airship Shenandoah which rst ew in 1923 Thisalloy began the US stressed metal skin semimonocoque structurerevolution

Other countries were also quick to adopt Duralumin monocoquefuselage construction In England the Short Brothers Company builtthe metal Silver Streak biplane in 1919 which was all Duraluminexcept for the wing spars which were steel The wing consisted ofround steel tube spars and Duralumin ribs with right angle angesto which riveted Duralumin covers were joined18

Table 1 Materials use as a percentage of structural weight

Material B-2 FA-18EF F-22 V-22 C-17 AV-8B

Composite 37 22 25 42 08 26Aluminum 27 27 16 mdashmdash 70 47Titanium 23 23 39 mdashmdash 09 mdashmdash

Fig 7 F E Loudy patent 1557855 led 1921

Over the many years of utilizing aluminum materials several im-portant lessons have been learned Reference 19 cites the example ofthe KC-135 aircraft In 1954 the Boeing 367-80 became Americarsquos rst jet transport prototype The KC-135 was a derivative of theBoeing 367-80 (Dash 80) as was the 707 but the lower wing skinof the KC-135 was initially 7178-T6 Al A small critical crack lengthfor this material at the KC-135 wing limit load level led to casesof in-service rapid fracture

The concern about loss of an aircraft from degradation of failsafety from widespread fatigue cracking motivated the US AirForce to make a costly modi cation The lower wing skins werereplaced with 2024-T3 aluminum

In 1982 14 years of service experience with the C-5A resultedin greater emphasis being placed on fracture toughness corro-sion resistance and durability in selecting materials for the C-5B7075-T6 fuselage skin and cargo oor skins were changed to 7475-T761 where corrosion was a problem The wing plank material waschanged from 7075-T6 to 7175-T73 for increased toughness

Predictions as early as 198520 were that ldquolithium containing alu-minum alloys (AlLi) would nd signi cant use in both militaryand civil aircraftrdquo and that the ldquoultimate level of utilization of car-bon ber reinforced polymer matrix composites may be somewhatless than predicted in view of the potential of AlLi alloysrdquo

By the early 1990s however the realization set in that AlLiwas not ready to be a wide-scale direct substitute for conventionalaluminum aerospace alloys although it was being used in someproduction airframe applications in the United States and EuropeAt present it is the high-strength aluminum alloys such as 7150 and7055 with increased compression strength and balanced fracturetoughness and corrosion resistance that are enabling aluminum tomaintain its high level of use in the aerospace industry

Titanium

The rst major structural application of titanium was the DouglasX-3 that utilized 629 lb of Rem-Cru Inc titanium This was a com-mercially pure material used for the aft fuselage boom and stabilatorportions of the aircraft

The X-3 rst ew in 1952 and O A Wheelon a production de-sign engineer with the Douglas Aircraft Company was awarded theWright brothers medal for a published analysis of the use of titaniumin aircraft construction entitled ldquoDesign Methods and Manufactur-ing Techniques With Titaniumrdquo

One of the rst airframe production applications was the F-86Sabre Jet which is noted for a killloss ratio of at least 41 in theKorean War This aircraft which ew as the XP-86 in 1947 em-ployed 600 lb of titanium in the aft fuselage and engine areas (Fig 9)

22 PAUL ET AL

Fig 8 US production of titanium sponge

Fig 9 Titanium aft fuselage inproduction at North American Aviationin 1954

The history and early use of titanium was solidly wedded to theaerospace industry and the annealed alphandashbeta alloy Ti 6Al-4V hasbeen the workhorse of the industry

This alloy has been utilized on aircraft from the B-52 to theF-22 Reference 21 indicates this speci c alloy composition was rst melted and evaluated in 1953 at IIT Research Institute under aUS Air Force contract The Mallory Sharon Titanium Corporationversion of this alloy (MST 6Al-4V) was selected shortly thereafter(1954) for use in Pratt and Whitneyrsquos J-57 turbojet engine (disksblades and other parts) for the B-52

The rst commercial production of titanium named after theTitans of mythology began in 1946 Titanium was pushed into com-mercial production as a structural material in unprecedented timeby the combined efforts of industry and government to meet militaryrequirements

Figure 8 shows how rapidly titanium sponge (re ned metal beforemelting into usable form) production was scaled up In 1956 90 ofthe titanium market was for military aircraft production By 1980less than 20 of aerospace grade was used for military aircraftThe majority of titanium sponge is processed into titanium dioxidepigment for use as ller in paint paper plastics and rubber

The use of titanium has been increasing at the expense ofboth aluminum and composites and will continue to increase inmore unitized assemblies facilitated by combined superplastic-formingdiffusion-bonding welding and casting

The most famous US titanium airplane the YF-12ASR-71 rst ew as the A-12 in 1962 and was fabricated from primarily beta-120VCA alloy (Ti-13V-11Cr-3Al)

Johnson stated in Ref 22 that ldquoof the rst 6000 B-120 piecesfabricated we lost 95rdquo The material was described as being sobrittle that if it fell off your desk it would shatter on the oor

Fig 10a F-15CD conventional aft fuselage

Fig 10b F-15E SPFDB aft fuselage

The most widely used alloy is Ti-6Al-4V This alloy is particularlyamenable to fabrication by superplastic-formingdiffusion-bonding(SPFDB) processes and therefore was the subject of a systematicwell-planned research and development (RampD) thrust by the USAir Force and US Navy over a 15-year period from 1970 to 1985

Reference 23 indicates that the use of SPFDB titanium for theaft fuselage of the F-15E resulted in 726 fewer components and10000 fewer fasteners and achieved 15 weight savings over theCD models (see Figs 10a and 10b)

Actually it has been large forgings and castings that have facil-itated the creation of one-piece substructures Figure 11 shows aTi-6Al-4V F-22 aft-fuselage frame produced by WymanndashGordonThe F-22 wing carry-through bulkhead is fabricated from the largesttitanium forging by surface area to date (96 ft2 6560 lb) Four bulk-heads in the midfuselage are made of titanium The forward and aftboom structure on either side of the aircraftrsquos empennage sectionare formed from integrally stiffened titanium isogrid panels that areelectron beam welded to forged titanium frames The wing attach t-tings and rudder actuator support are hot isostatically pressed (HIP)titanium castings

The A-6E composite wing rear spar is a 27-ft titanium forgingand large one-piece titanium bulkhead forgings are employed by theFA-18 EF aircraft

The BellndashBoeing V-22 Osprey Engineering and ManufacturingDevelopment aircraft ( rst ight 5 February 1997) used a one-piecetitanium casting for the transmission adapter case stow ring that waspreviously 40 separate riveted components This part serves as theprimary support structure for the rotor system proprotor gearboxand engine and nacelle components The Howmet Corporation in-dicates a premium-quality consistently homogenous component is

PAUL ET AL 23

Fig 11 F-22 aft-fuselage frame forging produced on WymanndashGordon50000-ton hydraulic press

achieved by the HIP processing so that allowables do not have to bereduced by conventional casting factors

Recently RMI Titanium of Ohio developed a stronger more dam-age tolerant alloy (Ti-6-22-22S) that is nding application on theF-22 joint strike ghter and X-33 reusable launch vehicle

Timet Corporation alloy 10ndash2ndash3 is being used in the main landinggear and Timetal 21S for the nacelle cowl plug nozzle and Prattand Whitney Aircraft engine components of the Boeing 777 aircraftTitanium usage on Boeing aircraft has increased from 2 emptyweight of the 737 to 8 of the 777 The McDonnell Douglas C-17transport also utilizes alloy 10ndash2ndash3 for landing gear components

The focus of titanium alloy development however has shiftedfrom aerospace to industrial applications New product forms areemerging rapidly Titanium is growing in use for everything fromgolf clubs skis tennis rackets bicycles pots and pans and jewelryto even hip and knee replacements The kitchen sink cannot be faroff

Noting this trend toward increased titanium use a 1991 AdvancedMaterials and Processing magazine article was titled ldquoTitaniumStaves Off Compositesrdquo In reality titanium has grown in use partlybecause of its compatibility with composites both from a thermalexpansion characteristic and resistance to galvanic corrosion whenin contact with carbon epoxy

Advanced Composites

The present generation of composite structures technology ap-plied to the B-2 FA-18EF V-22 and RAH-66 has been in devel-opment for 30 years The present level of maturity is a result of acontinued investment by the RampD structures and materials commu-nities of all three services and NASA No other class of material hasreceived such high level of funding support for such a long time Nu-merous composite structures built from the 1960s to 1980s veri edcomposite structure advantages (primarily weight savings) leadingto more extensive application of composite airframes in the 1990swhen affordability became the major issue

Table 2 is a summary of some of the programs initiated to fos-ter the development and transition of composites Although theseinitiatives were seldom adequately funded to complete all initiallyplanned objectives they nonetheless were able to sustain substantialfunding for some limited period of time

Production in the United States of high modulus ber compositestructures commonly referred to as advanced composites beganwith boron epoxy F-14 and F-15 horizontal stabilizers in the early

Table 2 Advanced composite technology programs

Agency Acronym Program

US Army ACAP Advanced composite airframe programNASA ACEE Aircraft energy ef ciencyUS Air Force DMLCC Design and manufacture of low cost

compositesNASA ACT Advanced composites technologyUS Navy AV-8B Composite structure development programARPAa ACP Affordable composites for propulsionDARPAb APMC Affordable polymer matrix compositesDODcindustry CAI Composites affordability initiative

aAdvanced Research Projects AgencybDefense Advanced Research Projects AgencycDepartment of Defense

1970s The technology was fostered by the US Air Force BoronFilaments and Composites Advanced Development Program ini-tiated in 1964 as a result of a Project Forecast recommendationExperimental carbon epoxy structures were ying in 1970 and car-bon epoxy came to the forefront as the most cost-effective advancedcomposite material in the mid-1970s with production applicationson the F-15 B-1 and F-16 In 1978 carbon epoxy was accepted bythe Navy for F-18 and AV-8B primary wing box structures

Table 3 is a sample of some of the early composite structuredemonstrations Table 3 consists of a list of composite structuresfor which a metal baseline weight was available There have beenso many composite structures built since these and many more arecurrently in development that any list is out of date before it isprinted

There were of course many other composite structures some ofwhich did not have a metal baseline or for which a metal coun-terpart would not even be practical Some of those that come tomind are the Rockwell highly maneuverable advanced technologyremotely piloted research vehicle and the Grumman X-29 forwardswept wing demonstrator Both of these designs exploited the ad-vantageous aeroelastic characteristic of exible composite wings

The bulk of advanced composite material being used in ightin the United States today is fabricated from unidirectional tapeconsisting of AS-4 or IM7 carbon ber in 3501 thermoset epoxymatrix preimpregnated by Hercules Inc

Although developed some time ago the Slingsby T67 Fire ywas selected in 1992 for the US Air Force enhanced ight screener(T3A) The basic airframe started as a wooden construction thatSlingsby redesigned as primarily a wet layup glass ber polymercomposite airframe

Fiberglass-reinforced polymer construction has been employedfor many years in the glider industry It has been arguedpara that theHorton brothers of Germany created the worldrsquos rst compositeaircraft the Ho Va which began as a glider in 1936 The fuselageand wings for the rst Ho V were fabric-covered D-tubes moldedfrom paper- lled phenol resin R Horten noted that such moldedplastic construction promised to reduce production time and cost

The term molded reinforced plastic plane was being used by 1941(see Ref 24) Although nearly every form of material was tried as ller resin-coated wood veneer strips and sheets (usually mahoganyor spruce) were the most popular reinforcing agents Impregnationby liquid phenolic- or urea-type resins was common

Both Japan and Germany built highly successful berglass com-posite soaring planes in the 1950s Several of these are described inRef 1

The 42 by weight composite structure BellBoeing OspreyV-22 rst ew in 1989 The critical link between the blade andthe rotor hub (the yoke) of the V-22 is a composite structure Theyoke carries the blade centrifugal and lifting forces transmits enginetorque and permits lead-lag and apping motion Also each yokearm can accommodate pitch changes

The US Air Forcersquos latest ghter the F-22 Raptor ( rst ight7 September 1997) was the rst aircraft to employ resin transfer

paraData available online at httpwwwnur ugelcom

24 PAUL ET AL

Table 3 Composite structure weight savings

Baseline Composite WeightComponent weight lb weight lb savings

Flight service componentsEmpennage

F-111 horizontal stabilizer 1142 866 24F-14 horizontal stabilizer 1005 825 18F-15 horizontal stabilizer 527 395 25F-5 vertical stabilizer 119 83 30DC-10 vertical stabilizer 1005 834 17L-1011 vertical stabilizer 858 642 25B-737 horizontal stabilizer 262 204 22

WingsAV-8B wing 1143 949 17F-18 wing 1843 1641 11A-6 wing 6392 6400 0a

(selective reinforcement)C-130 wing box 4800 4200 125

FuselageUH-60 rear fuselage 444 399 10AV-8B forward fuselage 229 171 25

(selective reinforcement)CH-53 Aft fuselage 1159 947 18B-1 dorsal longeron 1485 829 44

OtherC-5 slat 241 190 21B-727 elevator 131 98 25DC-10 rudder 91 67 26L0-1011 aileron 140 107 23F-15 speed brake 112 89 21B-1 weapons bay door 147 129 12F-5 ap 34 25 26B-1 ap 87 73 16B-1 slat 74 61 18F-4 rudder 64 42 34F-11 spoiler 20 17 15Space shuttle doors 4150 3196 23DC-9 nose cowl 24 13 46F-16 landing gear door 53 42 20A-7 speed brake 123 74 40

RampD demonstration componentsWing

F-15 wing 2155 1787 17F-100 wing skin 616 475 23

FuselageF-111 fuselage 625 510 18CH-54 tail cone 464 397 14YF-16 fuselage 572 451 21F-5 fuselage 914 677 26

OtherB-1 vertical stabilizer 658 539 18B-1 horizontal stabilizer 3315 2844 14

aLoad capability 18 increase

molding (RTM) for composite part fabrication RTM is used forover 400 parts from inlet lip edges to sine wave wing spars Bothbismaleimide and epoxy parts are RTM processed The number ofparts made from thermoset composites is a 5050 split betweenepoxy resin and bismaleimide (BMI) Exterior skins are all BMIThermoplastics make up about 1 of the airframe by weight Theyare employed for the landing gear and weapons bay doors Auto-mated ber placement technology is utilized to fabricate the F-22composite horizontal stabilizersrsquo pivot shafts Overall 25 of theairframe structural material is composites

Helicopter rotor blade designers established an early position atthe forefront of the development and application of advanced com-posite structures and continue to design to maximize their bene ts

A Vertol CH-47 boronepoxy aft rotor blade was ight tested in1969 Of course glass ber-reinforced blades were in developmentand even production several years before that

Although weight reduction in helicopters is important it has beentheir adaptability to complex aerodynamic shapes that has enabledcomposites to play a major role in helicopter structures The con-cepts of hingeless and bearingless rotors were made practical by the

design exibility of composites Composite blades achieve virtuallyunlimited fatigue life

Most composite applications came about as a result of improvedperformance at an acceptable cost By the mid-1980s the struc-tural ef ciency of composites was fairly well recognized throughoutthe aerospace industry Emphasis then shifted to lowering develop-ment and production cost of composite assemblies the goal beingto achieve lower cost than competing aluminum construction Ex-panded use will require achieving this goal as well as projectedlong-term durability and supportability bene ts

The present emphasis on design to cost will continue facilitatedby integrated product teams (IPTs) and computer-aided design toolsHow well composite producibility and supportability issues are ad-dressed in the future will determine the extent and the rate at whichcomposite airframe applications will expand

Years of experience with composite materials have also resultedin lessons learned For example delaminations were found in thewing skins and substructure of US Marine Corps AV-8B and RAFUK-58 Harriers Investigations revealed that these delaminationswere caused by gaps between the skin and substructure that werenot properly shimmed prior to fastener torque-up during assemblyof the wings To correct this problem and prevent its occurrencein other composite aircraft production the US Navy now requiresthat liquid shimming be used to ll gaps between mating surfacesin composite structures Additionally closer dimensional tolerancesare placed on mating surfaces to ensure a better t

The one projection made by many was that 50or more of todayrsquosairframes would be made from composites This has not happenedfor two reasons 1) the cost delta for todayrsquos composites and 2) thepresent poor out-of-plane load reaction capability of laminated com-posite construction (and our limited ability to design to minimizethis shortcoming)

Design CriteriaUp to the mid-1950s the method for ensuring structural integrity

was essentially the same as that employed by the Wright brothersin 1903 A stress analysis was substantiated by a static test to a loadlevel greater than that expected in ight

The method consists of applying a factor of safety to the ightloads and verifying the structure could sustain this load by testing afull-scale airframe The factor of safety was developed to account forthe following sources of variability25 1)uncertainties in loads 2) in-accuracies in structural analysis 3) variations in material properties4) deterioration during service life and 5) variations in constructionquality

The aircraft yearbook in 1921 states ldquoThe fatigue of metals orprogressive failure from incipient cracking is the greatest bugbearof the designerrdquo The Bureau of Standards was reporting exu-ral fatigue data on duralumin sheets as early as 1922 HoweverRef 1 indicates that the rst mention of fatigue in government air-worthiness regulations did not appear until 1945 The concept ofmetal fatigue goes back a long way to the failures of railway carsand locomotive axles in the 1840s Reference 26 credits Wohler ofGermany in 1860 with the concept of designing for a nite fatiguelife In 1853 French engineers were inspecting axles of horse-drawnmail coaches for cracks and recommending their replacement after60000 km However Ref 27 indicates that it was not until 1914that suf cient test data were available to consider incorporating fa-tigue limits into aerospace structural steel speci cations In 1918the rst full-scale fatigue test of a large aircraft component wascarried out in the United Kingdom at the Royal Aircraft Establish-ment

Many papers were written in the 1940s predicting increases inairframe fatigue failures The authors of Ref 28 forecast increasesin fatigue troubles would stem from the following trends that ex-isted in 1943 1) a drive toward higher cruising speeds 2) the contin-uous increase in the ratio of useful load to gross weight and 3) theincrease in wing loading and gross weight plus an increase in servicelife necessitated by high initial cost

Reference 28 provides stressndashnumber of cycles (SndashN) curves onthe 17S and recommends designing on the basis of life expectancy

PAUL ET AL 25

The authors also recommend determining service loadings fromstatistical ight data recorded on existing aircraft but cautioned thatsuch data are applicable only to airplanes of speed wing loadingsize and mission similar to those in which the data were obtained

The author of Ref 26 indicates that Gassner of Germany hadwarned as early as 1941 that if the higher static strength of thenew aluminum alloy (7000-type) were taken advantage of a shorterfatigue life would result One goal of the researchers of that timewas to obtain suf cient data to be able to reduce factors of safetythat resulted from ignorance of the strength of materials

An International Committee on Aeronautical Fatigue wasfounded in 1951 at the suggestion of Plantema of The NetherlandsIt was two UK Comet crashes in 1954 and a series of US B-47catastrophic accidents in 1958 however that focused US Air Forceattention on structural fatigue and led to The US Air Force AircraftStructural Integrity Program (ASIP) This program adopted a fatiguemethodology approach commonly referred to as safe-life This ap-proach was used in the development of US Air Force aircraft inthe 1960s but was not adequate from the standpoint of addressingmanufacturing and service-induced damage Consequently aircraftdesigned both prior to and subsequent to the safe-life method contin-ued to experience premature failures The US Air Force ASIP wasrevised to include the incorporation of a damage tolerancedurabilityphilosophy into the design process Although initially applied tothe B-1A and modi cations of the F-111 C-5A and F-4 the dam-age tolerancedurability requirements were formally established inMIL-STD-1530A in 1975

The US Navy ASIP is similar It requires fatigue life of thestructure to be designed such that failure of a full-scale test articlewill not occur and that the structure will be free from any defectssuch as cracks deformation loss of modulus disbonding or fastenerhole deformation and delamination when subjected to a minimumof two lifetimes

Spurred on by joint service airframe requirements of the JointAdvanced Strike Technology program the services have developeda joint service guide speci cation for aircraft structures

All US Air Force aircraft since the F-16 have been subject todurabilitydamage tolerance requirements Each of the tests requiredby ASIP has served to identify de ciencies in structural design andanalysis resulting in modi cations to the structure that provided asafer and more supportable airframe

The full-scale airframe static fatigue durabilitydamage toler-ance tests are expensive and therefore there should be a reason-able expectation that they can with minor design modi cationsbe accomplished successfully This generally means conducting anextensive structural element and subcomponent test program in sup-port of the ASIP These tests (Table 4) address all of the critical loadpaths

This type of building block program that is testing large numbersof coupons elements and components prior to full-scale hardwarehas evolved as the most logical risk-reduction approach for newsystem airframe development The consequences of a failure late inthe development cycle of a system are too great to proceed any otherway

Table 4 Airframe design development tests

Aircraft Elements tested

B-2 160000 nonmetals 1450 metallic 6 leading edge74 wing panels 40 engine exhaust panels27 joints

C-17 6000 coupons 400 componentsF-22 Allowables

14500 composites 5300 metalsStructural integrity

4100 metals 8800 compositesReproducibility

80 full and subscale composite 15 titaniumstructures

FA-18 EF 10000 materials characterizationprocessing tests165 elements and components

V-22 9700 coupons 550 elements and components

The opposite is true for advanced technology demonstrators(ATDs) which introduce technology through prototyping thus ac-celerating transition of new technologies Failure in an ATD pro-gram is more accepted as a lesson learned rather than a failure ofthe contractor and procuring agency

Table 4 indicates that composite structures require more extensivedesign and certi cation testing than metals The additional testingrequirements add cost to the design allowables and design certi -cation efforts This was clearly identi ed in Ref 29 published inNovember 1994 as a lesson learned

In the case of composite structural assemblies however whenthe structure satis es all static load conditions fatigue is generallynot a problem The effects of moisture and temperature on designmargins can be established at the coupon and subcomponent levelwith the full-scale component margins adjusted appropriately

The one thing the described programs had in common was a sub-stantial nite element analysis effort (both global and local detailmodels) to support test hardware design and to certify the airframefor adequate design margins These programs demonstrate the im-portance of a thorough analysis effort coupled with a complete criti-cal area design development test program to substantiate the designIt would be unwise and uneconomical to dispense with either

Analysis and Design TechniquesThe introduction of digital computers in the 1950s paved the

way for the now emerging multidisciplinary design technologiesapplicable to the integrated design of a variety of engineering prod-ucts including air and space vehicles This section is a brief outlineof the developments that led to such software systems as ASKASTARDYNE NASTRAN ANSYS ABAQUS and ASTROS

Before discussing the chronology and signi cance of structuralanalysis developments it is appropriate to distinguish between twolevels of analysis 1) analysis of structural elements and 2) analysisof builtup structures

Analysis of Structural Elements

Structural elements or components can be categorized into threetypes line elements surface elements and solids The analysis ofsuch elements has been developed and documented over the last150 years by Euler Lagrange St Venant and others References 30and 31 document the chronology of these developments Over adozen solid mechanics books by Timoshenko constitute a rich com-pendium of analysis methods for structural elements The rangeand applicability of this continuum approach (the solution of dif-ferential equations) is however limited to very restricted cases ofloading boundary conditions and element shapes Such methods asRayleighndashRitz Galerkin and nite differences are approximationsThese methods replace a continuum model by a discretized equiva-lent The result is a set of algebraic equations that can be solved bydigital computers

Analysis of Builtup Structures

Builtup structures are simply constructed out of many structuralelements joined together at their boundaries The differential equa-tion approach for the solution of builtup structures is intractable Asan alternative a number of discretized procedures have been devel-oped since the early 1900s A structure is considered determinate ifits internal forces can be solved by the force and moment equilib-rium equations alone Otherwise it is considered an indeterminatesystem Analyzing complex indeterminate systems was a dauntingtask before the advent of digital computers

The force and displacement methods that emerged in the 1950swere a major breakthrough in structural analysis They refer to thediscretized model of the structure in which the continuum is re-placed by a nite number of grid points Each grid point is assumedto have six degrees of freedom (DOF) in space These DOFs areforces andor displacements In the force method the unknowns arethe set of forces released to make the indeterminate structure deter-minate Enforcement of compatibility conditions results in force-displacement relations connected by a exibility matrix In thedisplacement method the unknowns are the displacements of the

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 4: Evolution of U.S. Military Aircraft Structures Technology

PAUL ET AL 21

metal aircraft for many years to come quite new materials are be-ginning to appear and they have as many advantages over metal asmetal had over wood I am referring to such materials as carbon bers used in matrices of synthetic resinsrdquo17

Hindsight now shows that this pronouncement and many simi-lar since then were premature The airframe of todayrsquos aircraft 25years later is a hybrid of metal and composites A review of thecurrent status of airframe materials usage (Table 1) shows a fairlyequal balance in the use of aluminum titanium and compositesThis balance is a modern development driven primarily by perfor-mance requirements and affordability A discussion of each materialevolution follows

Aluminum Alloys

For 60 years aluminum alloys have been the primary materialsfor airframes No other single material has played as major a rolein aircraft production as aluminum speci cally the 2000 and 7000series ingot alloys in various heat treatments

In 1944 Alcoa developed 75S (AlndashZnndashMgndashCu) expressly to meetthe aircraft industryrsquos need for higher strength This alloy rst sawservice on the B-29 bomber and according to the 1945 aircraftyearbook ldquoPractically all the new war planes were utilizing highstrength 75Srdquo

The principal aluminum alloy since 1945 however has beenAlcoarsquos 24S (AlndashCundashMgndashMn) that contained the same alloyingelements as the older 17S but in different proportions for greaterstrength Produced in 1917 17S was Alcoarsquos version of Duralumin(from the French word dur meaning hard) patented by Alfred Wilmin Germany in 1908 Wilmrsquos Dural aluminum alloy was used for theZeppelin structure in 1911 Alcoarsquos 17S was used in constructionof the US Navy airship Shenandoah which rst ew in 1923 Thisalloy began the US stressed metal skin semimonocoque structurerevolution

Other countries were also quick to adopt Duralumin monocoquefuselage construction In England the Short Brothers Company builtthe metal Silver Streak biplane in 1919 which was all Duraluminexcept for the wing spars which were steel The wing consisted ofround steel tube spars and Duralumin ribs with right angle angesto which riveted Duralumin covers were joined18

Table 1 Materials use as a percentage of structural weight

Material B-2 FA-18EF F-22 V-22 C-17 AV-8B

Composite 37 22 25 42 08 26Aluminum 27 27 16 mdashmdash 70 47Titanium 23 23 39 mdashmdash 09 mdashmdash

Fig 7 F E Loudy patent 1557855 led 1921

Over the many years of utilizing aluminum materials several im-portant lessons have been learned Reference 19 cites the example ofthe KC-135 aircraft In 1954 the Boeing 367-80 became Americarsquos rst jet transport prototype The KC-135 was a derivative of theBoeing 367-80 (Dash 80) as was the 707 but the lower wing skinof the KC-135 was initially 7178-T6 Al A small critical crack lengthfor this material at the KC-135 wing limit load level led to casesof in-service rapid fracture

The concern about loss of an aircraft from degradation of failsafety from widespread fatigue cracking motivated the US AirForce to make a costly modi cation The lower wing skins werereplaced with 2024-T3 aluminum

In 1982 14 years of service experience with the C-5A resultedin greater emphasis being placed on fracture toughness corro-sion resistance and durability in selecting materials for the C-5B7075-T6 fuselage skin and cargo oor skins were changed to 7475-T761 where corrosion was a problem The wing plank material waschanged from 7075-T6 to 7175-T73 for increased toughness

Predictions as early as 198520 were that ldquolithium containing alu-minum alloys (AlLi) would nd signi cant use in both militaryand civil aircraftrdquo and that the ldquoultimate level of utilization of car-bon ber reinforced polymer matrix composites may be somewhatless than predicted in view of the potential of AlLi alloysrdquo

By the early 1990s however the realization set in that AlLiwas not ready to be a wide-scale direct substitute for conventionalaluminum aerospace alloys although it was being used in someproduction airframe applications in the United States and EuropeAt present it is the high-strength aluminum alloys such as 7150 and7055 with increased compression strength and balanced fracturetoughness and corrosion resistance that are enabling aluminum tomaintain its high level of use in the aerospace industry

Titanium

The rst major structural application of titanium was the DouglasX-3 that utilized 629 lb of Rem-Cru Inc titanium This was a com-mercially pure material used for the aft fuselage boom and stabilatorportions of the aircraft

The X-3 rst ew in 1952 and O A Wheelon a production de-sign engineer with the Douglas Aircraft Company was awarded theWright brothers medal for a published analysis of the use of titaniumin aircraft construction entitled ldquoDesign Methods and Manufactur-ing Techniques With Titaniumrdquo

One of the rst airframe production applications was the F-86Sabre Jet which is noted for a killloss ratio of at least 41 in theKorean War This aircraft which ew as the XP-86 in 1947 em-ployed 600 lb of titanium in the aft fuselage and engine areas (Fig 9)

22 PAUL ET AL

Fig 8 US production of titanium sponge

Fig 9 Titanium aft fuselage inproduction at North American Aviationin 1954

The history and early use of titanium was solidly wedded to theaerospace industry and the annealed alphandashbeta alloy Ti 6Al-4V hasbeen the workhorse of the industry

This alloy has been utilized on aircraft from the B-52 to theF-22 Reference 21 indicates this speci c alloy composition was rst melted and evaluated in 1953 at IIT Research Institute under aUS Air Force contract The Mallory Sharon Titanium Corporationversion of this alloy (MST 6Al-4V) was selected shortly thereafter(1954) for use in Pratt and Whitneyrsquos J-57 turbojet engine (disksblades and other parts) for the B-52

The rst commercial production of titanium named after theTitans of mythology began in 1946 Titanium was pushed into com-mercial production as a structural material in unprecedented timeby the combined efforts of industry and government to meet militaryrequirements

Figure 8 shows how rapidly titanium sponge (re ned metal beforemelting into usable form) production was scaled up In 1956 90 ofthe titanium market was for military aircraft production By 1980less than 20 of aerospace grade was used for military aircraftThe majority of titanium sponge is processed into titanium dioxidepigment for use as ller in paint paper plastics and rubber

The use of titanium has been increasing at the expense ofboth aluminum and composites and will continue to increase inmore unitized assemblies facilitated by combined superplastic-formingdiffusion-bonding welding and casting

The most famous US titanium airplane the YF-12ASR-71 rst ew as the A-12 in 1962 and was fabricated from primarily beta-120VCA alloy (Ti-13V-11Cr-3Al)

Johnson stated in Ref 22 that ldquoof the rst 6000 B-120 piecesfabricated we lost 95rdquo The material was described as being sobrittle that if it fell off your desk it would shatter on the oor

Fig 10a F-15CD conventional aft fuselage

Fig 10b F-15E SPFDB aft fuselage

The most widely used alloy is Ti-6Al-4V This alloy is particularlyamenable to fabrication by superplastic-formingdiffusion-bonding(SPFDB) processes and therefore was the subject of a systematicwell-planned research and development (RampD) thrust by the USAir Force and US Navy over a 15-year period from 1970 to 1985

Reference 23 indicates that the use of SPFDB titanium for theaft fuselage of the F-15E resulted in 726 fewer components and10000 fewer fasteners and achieved 15 weight savings over theCD models (see Figs 10a and 10b)

Actually it has been large forgings and castings that have facil-itated the creation of one-piece substructures Figure 11 shows aTi-6Al-4V F-22 aft-fuselage frame produced by WymanndashGordonThe F-22 wing carry-through bulkhead is fabricated from the largesttitanium forging by surface area to date (96 ft2 6560 lb) Four bulk-heads in the midfuselage are made of titanium The forward and aftboom structure on either side of the aircraftrsquos empennage sectionare formed from integrally stiffened titanium isogrid panels that areelectron beam welded to forged titanium frames The wing attach t-tings and rudder actuator support are hot isostatically pressed (HIP)titanium castings

The A-6E composite wing rear spar is a 27-ft titanium forgingand large one-piece titanium bulkhead forgings are employed by theFA-18 EF aircraft

The BellndashBoeing V-22 Osprey Engineering and ManufacturingDevelopment aircraft ( rst ight 5 February 1997) used a one-piecetitanium casting for the transmission adapter case stow ring that waspreviously 40 separate riveted components This part serves as theprimary support structure for the rotor system proprotor gearboxand engine and nacelle components The Howmet Corporation in-dicates a premium-quality consistently homogenous component is

PAUL ET AL 23

Fig 11 F-22 aft-fuselage frame forging produced on WymanndashGordon50000-ton hydraulic press

achieved by the HIP processing so that allowables do not have to bereduced by conventional casting factors

Recently RMI Titanium of Ohio developed a stronger more dam-age tolerant alloy (Ti-6-22-22S) that is nding application on theF-22 joint strike ghter and X-33 reusable launch vehicle

Timet Corporation alloy 10ndash2ndash3 is being used in the main landinggear and Timetal 21S for the nacelle cowl plug nozzle and Prattand Whitney Aircraft engine components of the Boeing 777 aircraftTitanium usage on Boeing aircraft has increased from 2 emptyweight of the 737 to 8 of the 777 The McDonnell Douglas C-17transport also utilizes alloy 10ndash2ndash3 for landing gear components

The focus of titanium alloy development however has shiftedfrom aerospace to industrial applications New product forms areemerging rapidly Titanium is growing in use for everything fromgolf clubs skis tennis rackets bicycles pots and pans and jewelryto even hip and knee replacements The kitchen sink cannot be faroff

Noting this trend toward increased titanium use a 1991 AdvancedMaterials and Processing magazine article was titled ldquoTitaniumStaves Off Compositesrdquo In reality titanium has grown in use partlybecause of its compatibility with composites both from a thermalexpansion characteristic and resistance to galvanic corrosion whenin contact with carbon epoxy

Advanced Composites

The present generation of composite structures technology ap-plied to the B-2 FA-18EF V-22 and RAH-66 has been in devel-opment for 30 years The present level of maturity is a result of acontinued investment by the RampD structures and materials commu-nities of all three services and NASA No other class of material hasreceived such high level of funding support for such a long time Nu-merous composite structures built from the 1960s to 1980s veri edcomposite structure advantages (primarily weight savings) leadingto more extensive application of composite airframes in the 1990swhen affordability became the major issue

Table 2 is a summary of some of the programs initiated to fos-ter the development and transition of composites Although theseinitiatives were seldom adequately funded to complete all initiallyplanned objectives they nonetheless were able to sustain substantialfunding for some limited period of time

Production in the United States of high modulus ber compositestructures commonly referred to as advanced composites beganwith boron epoxy F-14 and F-15 horizontal stabilizers in the early

Table 2 Advanced composite technology programs

Agency Acronym Program

US Army ACAP Advanced composite airframe programNASA ACEE Aircraft energy ef ciencyUS Air Force DMLCC Design and manufacture of low cost

compositesNASA ACT Advanced composites technologyUS Navy AV-8B Composite structure development programARPAa ACP Affordable composites for propulsionDARPAb APMC Affordable polymer matrix compositesDODcindustry CAI Composites affordability initiative

aAdvanced Research Projects AgencybDefense Advanced Research Projects AgencycDepartment of Defense

1970s The technology was fostered by the US Air Force BoronFilaments and Composites Advanced Development Program ini-tiated in 1964 as a result of a Project Forecast recommendationExperimental carbon epoxy structures were ying in 1970 and car-bon epoxy came to the forefront as the most cost-effective advancedcomposite material in the mid-1970s with production applicationson the F-15 B-1 and F-16 In 1978 carbon epoxy was accepted bythe Navy for F-18 and AV-8B primary wing box structures

Table 3 is a sample of some of the early composite structuredemonstrations Table 3 consists of a list of composite structuresfor which a metal baseline weight was available There have beenso many composite structures built since these and many more arecurrently in development that any list is out of date before it isprinted

There were of course many other composite structures some ofwhich did not have a metal baseline or for which a metal coun-terpart would not even be practical Some of those that come tomind are the Rockwell highly maneuverable advanced technologyremotely piloted research vehicle and the Grumman X-29 forwardswept wing demonstrator Both of these designs exploited the ad-vantageous aeroelastic characteristic of exible composite wings

The bulk of advanced composite material being used in ightin the United States today is fabricated from unidirectional tapeconsisting of AS-4 or IM7 carbon ber in 3501 thermoset epoxymatrix preimpregnated by Hercules Inc

Although developed some time ago the Slingsby T67 Fire ywas selected in 1992 for the US Air Force enhanced ight screener(T3A) The basic airframe started as a wooden construction thatSlingsby redesigned as primarily a wet layup glass ber polymercomposite airframe

Fiberglass-reinforced polymer construction has been employedfor many years in the glider industry It has been arguedpara that theHorton brothers of Germany created the worldrsquos rst compositeaircraft the Ho Va which began as a glider in 1936 The fuselageand wings for the rst Ho V were fabric-covered D-tubes moldedfrom paper- lled phenol resin R Horten noted that such moldedplastic construction promised to reduce production time and cost

The term molded reinforced plastic plane was being used by 1941(see Ref 24) Although nearly every form of material was tried as ller resin-coated wood veneer strips and sheets (usually mahoganyor spruce) were the most popular reinforcing agents Impregnationby liquid phenolic- or urea-type resins was common

Both Japan and Germany built highly successful berglass com-posite soaring planes in the 1950s Several of these are described inRef 1

The 42 by weight composite structure BellBoeing OspreyV-22 rst ew in 1989 The critical link between the blade andthe rotor hub (the yoke) of the V-22 is a composite structure Theyoke carries the blade centrifugal and lifting forces transmits enginetorque and permits lead-lag and apping motion Also each yokearm can accommodate pitch changes

The US Air Forcersquos latest ghter the F-22 Raptor ( rst ight7 September 1997) was the rst aircraft to employ resin transfer

paraData available online at httpwwwnur ugelcom

24 PAUL ET AL

Table 3 Composite structure weight savings

Baseline Composite WeightComponent weight lb weight lb savings

Flight service componentsEmpennage

F-111 horizontal stabilizer 1142 866 24F-14 horizontal stabilizer 1005 825 18F-15 horizontal stabilizer 527 395 25F-5 vertical stabilizer 119 83 30DC-10 vertical stabilizer 1005 834 17L-1011 vertical stabilizer 858 642 25B-737 horizontal stabilizer 262 204 22

WingsAV-8B wing 1143 949 17F-18 wing 1843 1641 11A-6 wing 6392 6400 0a

(selective reinforcement)C-130 wing box 4800 4200 125

FuselageUH-60 rear fuselage 444 399 10AV-8B forward fuselage 229 171 25

(selective reinforcement)CH-53 Aft fuselage 1159 947 18B-1 dorsal longeron 1485 829 44

OtherC-5 slat 241 190 21B-727 elevator 131 98 25DC-10 rudder 91 67 26L0-1011 aileron 140 107 23F-15 speed brake 112 89 21B-1 weapons bay door 147 129 12F-5 ap 34 25 26B-1 ap 87 73 16B-1 slat 74 61 18F-4 rudder 64 42 34F-11 spoiler 20 17 15Space shuttle doors 4150 3196 23DC-9 nose cowl 24 13 46F-16 landing gear door 53 42 20A-7 speed brake 123 74 40

RampD demonstration componentsWing

F-15 wing 2155 1787 17F-100 wing skin 616 475 23

FuselageF-111 fuselage 625 510 18CH-54 tail cone 464 397 14YF-16 fuselage 572 451 21F-5 fuselage 914 677 26

OtherB-1 vertical stabilizer 658 539 18B-1 horizontal stabilizer 3315 2844 14

aLoad capability 18 increase

molding (RTM) for composite part fabrication RTM is used forover 400 parts from inlet lip edges to sine wave wing spars Bothbismaleimide and epoxy parts are RTM processed The number ofparts made from thermoset composites is a 5050 split betweenepoxy resin and bismaleimide (BMI) Exterior skins are all BMIThermoplastics make up about 1 of the airframe by weight Theyare employed for the landing gear and weapons bay doors Auto-mated ber placement technology is utilized to fabricate the F-22composite horizontal stabilizersrsquo pivot shafts Overall 25 of theairframe structural material is composites

Helicopter rotor blade designers established an early position atthe forefront of the development and application of advanced com-posite structures and continue to design to maximize their bene ts

A Vertol CH-47 boronepoxy aft rotor blade was ight tested in1969 Of course glass ber-reinforced blades were in developmentand even production several years before that

Although weight reduction in helicopters is important it has beentheir adaptability to complex aerodynamic shapes that has enabledcomposites to play a major role in helicopter structures The con-cepts of hingeless and bearingless rotors were made practical by the

design exibility of composites Composite blades achieve virtuallyunlimited fatigue life

Most composite applications came about as a result of improvedperformance at an acceptable cost By the mid-1980s the struc-tural ef ciency of composites was fairly well recognized throughoutthe aerospace industry Emphasis then shifted to lowering develop-ment and production cost of composite assemblies the goal beingto achieve lower cost than competing aluminum construction Ex-panded use will require achieving this goal as well as projectedlong-term durability and supportability bene ts

The present emphasis on design to cost will continue facilitatedby integrated product teams (IPTs) and computer-aided design toolsHow well composite producibility and supportability issues are ad-dressed in the future will determine the extent and the rate at whichcomposite airframe applications will expand

Years of experience with composite materials have also resultedin lessons learned For example delaminations were found in thewing skins and substructure of US Marine Corps AV-8B and RAFUK-58 Harriers Investigations revealed that these delaminationswere caused by gaps between the skin and substructure that werenot properly shimmed prior to fastener torque-up during assemblyof the wings To correct this problem and prevent its occurrencein other composite aircraft production the US Navy now requiresthat liquid shimming be used to ll gaps between mating surfacesin composite structures Additionally closer dimensional tolerancesare placed on mating surfaces to ensure a better t

The one projection made by many was that 50or more of todayrsquosairframes would be made from composites This has not happenedfor two reasons 1) the cost delta for todayrsquos composites and 2) thepresent poor out-of-plane load reaction capability of laminated com-posite construction (and our limited ability to design to minimizethis shortcoming)

Design CriteriaUp to the mid-1950s the method for ensuring structural integrity

was essentially the same as that employed by the Wright brothersin 1903 A stress analysis was substantiated by a static test to a loadlevel greater than that expected in ight

The method consists of applying a factor of safety to the ightloads and verifying the structure could sustain this load by testing afull-scale airframe The factor of safety was developed to account forthe following sources of variability25 1)uncertainties in loads 2) in-accuracies in structural analysis 3) variations in material properties4) deterioration during service life and 5) variations in constructionquality

The aircraft yearbook in 1921 states ldquoThe fatigue of metals orprogressive failure from incipient cracking is the greatest bugbearof the designerrdquo The Bureau of Standards was reporting exu-ral fatigue data on duralumin sheets as early as 1922 HoweverRef 1 indicates that the rst mention of fatigue in government air-worthiness regulations did not appear until 1945 The concept ofmetal fatigue goes back a long way to the failures of railway carsand locomotive axles in the 1840s Reference 26 credits Wohler ofGermany in 1860 with the concept of designing for a nite fatiguelife In 1853 French engineers were inspecting axles of horse-drawnmail coaches for cracks and recommending their replacement after60000 km However Ref 27 indicates that it was not until 1914that suf cient test data were available to consider incorporating fa-tigue limits into aerospace structural steel speci cations In 1918the rst full-scale fatigue test of a large aircraft component wascarried out in the United Kingdom at the Royal Aircraft Establish-ment

Many papers were written in the 1940s predicting increases inairframe fatigue failures The authors of Ref 28 forecast increasesin fatigue troubles would stem from the following trends that ex-isted in 1943 1) a drive toward higher cruising speeds 2) the contin-uous increase in the ratio of useful load to gross weight and 3) theincrease in wing loading and gross weight plus an increase in servicelife necessitated by high initial cost

Reference 28 provides stressndashnumber of cycles (SndashN) curves onthe 17S and recommends designing on the basis of life expectancy

PAUL ET AL 25

The authors also recommend determining service loadings fromstatistical ight data recorded on existing aircraft but cautioned thatsuch data are applicable only to airplanes of speed wing loadingsize and mission similar to those in which the data were obtained

The author of Ref 26 indicates that Gassner of Germany hadwarned as early as 1941 that if the higher static strength of thenew aluminum alloy (7000-type) were taken advantage of a shorterfatigue life would result One goal of the researchers of that timewas to obtain suf cient data to be able to reduce factors of safetythat resulted from ignorance of the strength of materials

An International Committee on Aeronautical Fatigue wasfounded in 1951 at the suggestion of Plantema of The NetherlandsIt was two UK Comet crashes in 1954 and a series of US B-47catastrophic accidents in 1958 however that focused US Air Forceattention on structural fatigue and led to The US Air Force AircraftStructural Integrity Program (ASIP) This program adopted a fatiguemethodology approach commonly referred to as safe-life This ap-proach was used in the development of US Air Force aircraft inthe 1960s but was not adequate from the standpoint of addressingmanufacturing and service-induced damage Consequently aircraftdesigned both prior to and subsequent to the safe-life method contin-ued to experience premature failures The US Air Force ASIP wasrevised to include the incorporation of a damage tolerancedurabilityphilosophy into the design process Although initially applied tothe B-1A and modi cations of the F-111 C-5A and F-4 the dam-age tolerancedurability requirements were formally established inMIL-STD-1530A in 1975

The US Navy ASIP is similar It requires fatigue life of thestructure to be designed such that failure of a full-scale test articlewill not occur and that the structure will be free from any defectssuch as cracks deformation loss of modulus disbonding or fastenerhole deformation and delamination when subjected to a minimumof two lifetimes

Spurred on by joint service airframe requirements of the JointAdvanced Strike Technology program the services have developeda joint service guide speci cation for aircraft structures

All US Air Force aircraft since the F-16 have been subject todurabilitydamage tolerance requirements Each of the tests requiredby ASIP has served to identify de ciencies in structural design andanalysis resulting in modi cations to the structure that provided asafer and more supportable airframe

The full-scale airframe static fatigue durabilitydamage toler-ance tests are expensive and therefore there should be a reason-able expectation that they can with minor design modi cationsbe accomplished successfully This generally means conducting anextensive structural element and subcomponent test program in sup-port of the ASIP These tests (Table 4) address all of the critical loadpaths

This type of building block program that is testing large numbersof coupons elements and components prior to full-scale hardwarehas evolved as the most logical risk-reduction approach for newsystem airframe development The consequences of a failure late inthe development cycle of a system are too great to proceed any otherway

Table 4 Airframe design development tests

Aircraft Elements tested

B-2 160000 nonmetals 1450 metallic 6 leading edge74 wing panels 40 engine exhaust panels27 joints

C-17 6000 coupons 400 componentsF-22 Allowables

14500 composites 5300 metalsStructural integrity

4100 metals 8800 compositesReproducibility

80 full and subscale composite 15 titaniumstructures

FA-18 EF 10000 materials characterizationprocessing tests165 elements and components

V-22 9700 coupons 550 elements and components

The opposite is true for advanced technology demonstrators(ATDs) which introduce technology through prototyping thus ac-celerating transition of new technologies Failure in an ATD pro-gram is more accepted as a lesson learned rather than a failure ofthe contractor and procuring agency

Table 4 indicates that composite structures require more extensivedesign and certi cation testing than metals The additional testingrequirements add cost to the design allowables and design certi -cation efforts This was clearly identi ed in Ref 29 published inNovember 1994 as a lesson learned

In the case of composite structural assemblies however whenthe structure satis es all static load conditions fatigue is generallynot a problem The effects of moisture and temperature on designmargins can be established at the coupon and subcomponent levelwith the full-scale component margins adjusted appropriately

The one thing the described programs had in common was a sub-stantial nite element analysis effort (both global and local detailmodels) to support test hardware design and to certify the airframefor adequate design margins These programs demonstrate the im-portance of a thorough analysis effort coupled with a complete criti-cal area design development test program to substantiate the designIt would be unwise and uneconomical to dispense with either

Analysis and Design TechniquesThe introduction of digital computers in the 1950s paved the

way for the now emerging multidisciplinary design technologiesapplicable to the integrated design of a variety of engineering prod-ucts including air and space vehicles This section is a brief outlineof the developments that led to such software systems as ASKASTARDYNE NASTRAN ANSYS ABAQUS and ASTROS

Before discussing the chronology and signi cance of structuralanalysis developments it is appropriate to distinguish between twolevels of analysis 1) analysis of structural elements and 2) analysisof builtup structures

Analysis of Structural Elements

Structural elements or components can be categorized into threetypes line elements surface elements and solids The analysis ofsuch elements has been developed and documented over the last150 years by Euler Lagrange St Venant and others References 30and 31 document the chronology of these developments Over adozen solid mechanics books by Timoshenko constitute a rich com-pendium of analysis methods for structural elements The rangeand applicability of this continuum approach (the solution of dif-ferential equations) is however limited to very restricted cases ofloading boundary conditions and element shapes Such methods asRayleighndashRitz Galerkin and nite differences are approximationsThese methods replace a continuum model by a discretized equiva-lent The result is a set of algebraic equations that can be solved bydigital computers

Analysis of Builtup Structures

Builtup structures are simply constructed out of many structuralelements joined together at their boundaries The differential equa-tion approach for the solution of builtup structures is intractable Asan alternative a number of discretized procedures have been devel-oped since the early 1900s A structure is considered determinate ifits internal forces can be solved by the force and moment equilib-rium equations alone Otherwise it is considered an indeterminatesystem Analyzing complex indeterminate systems was a dauntingtask before the advent of digital computers

The force and displacement methods that emerged in the 1950swere a major breakthrough in structural analysis They refer to thediscretized model of the structure in which the continuum is re-placed by a nite number of grid points Each grid point is assumedto have six degrees of freedom (DOF) in space These DOFs areforces andor displacements In the force method the unknowns arethe set of forces released to make the indeterminate structure deter-minate Enforcement of compatibility conditions results in force-displacement relations connected by a exibility matrix In thedisplacement method the unknowns are the displacements of the

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 5: Evolution of U.S. Military Aircraft Structures Technology

22 PAUL ET AL

Fig 8 US production of titanium sponge

Fig 9 Titanium aft fuselage inproduction at North American Aviationin 1954

The history and early use of titanium was solidly wedded to theaerospace industry and the annealed alphandashbeta alloy Ti 6Al-4V hasbeen the workhorse of the industry

This alloy has been utilized on aircraft from the B-52 to theF-22 Reference 21 indicates this speci c alloy composition was rst melted and evaluated in 1953 at IIT Research Institute under aUS Air Force contract The Mallory Sharon Titanium Corporationversion of this alloy (MST 6Al-4V) was selected shortly thereafter(1954) for use in Pratt and Whitneyrsquos J-57 turbojet engine (disksblades and other parts) for the B-52

The rst commercial production of titanium named after theTitans of mythology began in 1946 Titanium was pushed into com-mercial production as a structural material in unprecedented timeby the combined efforts of industry and government to meet militaryrequirements

Figure 8 shows how rapidly titanium sponge (re ned metal beforemelting into usable form) production was scaled up In 1956 90 ofthe titanium market was for military aircraft production By 1980less than 20 of aerospace grade was used for military aircraftThe majority of titanium sponge is processed into titanium dioxidepigment for use as ller in paint paper plastics and rubber

The use of titanium has been increasing at the expense ofboth aluminum and composites and will continue to increase inmore unitized assemblies facilitated by combined superplastic-formingdiffusion-bonding welding and casting

The most famous US titanium airplane the YF-12ASR-71 rst ew as the A-12 in 1962 and was fabricated from primarily beta-120VCA alloy (Ti-13V-11Cr-3Al)

Johnson stated in Ref 22 that ldquoof the rst 6000 B-120 piecesfabricated we lost 95rdquo The material was described as being sobrittle that if it fell off your desk it would shatter on the oor

Fig 10a F-15CD conventional aft fuselage

Fig 10b F-15E SPFDB aft fuselage

The most widely used alloy is Ti-6Al-4V This alloy is particularlyamenable to fabrication by superplastic-formingdiffusion-bonding(SPFDB) processes and therefore was the subject of a systematicwell-planned research and development (RampD) thrust by the USAir Force and US Navy over a 15-year period from 1970 to 1985

Reference 23 indicates that the use of SPFDB titanium for theaft fuselage of the F-15E resulted in 726 fewer components and10000 fewer fasteners and achieved 15 weight savings over theCD models (see Figs 10a and 10b)

Actually it has been large forgings and castings that have facil-itated the creation of one-piece substructures Figure 11 shows aTi-6Al-4V F-22 aft-fuselage frame produced by WymanndashGordonThe F-22 wing carry-through bulkhead is fabricated from the largesttitanium forging by surface area to date (96 ft2 6560 lb) Four bulk-heads in the midfuselage are made of titanium The forward and aftboom structure on either side of the aircraftrsquos empennage sectionare formed from integrally stiffened titanium isogrid panels that areelectron beam welded to forged titanium frames The wing attach t-tings and rudder actuator support are hot isostatically pressed (HIP)titanium castings

The A-6E composite wing rear spar is a 27-ft titanium forgingand large one-piece titanium bulkhead forgings are employed by theFA-18 EF aircraft

The BellndashBoeing V-22 Osprey Engineering and ManufacturingDevelopment aircraft ( rst ight 5 February 1997) used a one-piecetitanium casting for the transmission adapter case stow ring that waspreviously 40 separate riveted components This part serves as theprimary support structure for the rotor system proprotor gearboxand engine and nacelle components The Howmet Corporation in-dicates a premium-quality consistently homogenous component is

PAUL ET AL 23

Fig 11 F-22 aft-fuselage frame forging produced on WymanndashGordon50000-ton hydraulic press

achieved by the HIP processing so that allowables do not have to bereduced by conventional casting factors

Recently RMI Titanium of Ohio developed a stronger more dam-age tolerant alloy (Ti-6-22-22S) that is nding application on theF-22 joint strike ghter and X-33 reusable launch vehicle

Timet Corporation alloy 10ndash2ndash3 is being used in the main landinggear and Timetal 21S for the nacelle cowl plug nozzle and Prattand Whitney Aircraft engine components of the Boeing 777 aircraftTitanium usage on Boeing aircraft has increased from 2 emptyweight of the 737 to 8 of the 777 The McDonnell Douglas C-17transport also utilizes alloy 10ndash2ndash3 for landing gear components

The focus of titanium alloy development however has shiftedfrom aerospace to industrial applications New product forms areemerging rapidly Titanium is growing in use for everything fromgolf clubs skis tennis rackets bicycles pots and pans and jewelryto even hip and knee replacements The kitchen sink cannot be faroff

Noting this trend toward increased titanium use a 1991 AdvancedMaterials and Processing magazine article was titled ldquoTitaniumStaves Off Compositesrdquo In reality titanium has grown in use partlybecause of its compatibility with composites both from a thermalexpansion characteristic and resistance to galvanic corrosion whenin contact with carbon epoxy

Advanced Composites

The present generation of composite structures technology ap-plied to the B-2 FA-18EF V-22 and RAH-66 has been in devel-opment for 30 years The present level of maturity is a result of acontinued investment by the RampD structures and materials commu-nities of all three services and NASA No other class of material hasreceived such high level of funding support for such a long time Nu-merous composite structures built from the 1960s to 1980s veri edcomposite structure advantages (primarily weight savings) leadingto more extensive application of composite airframes in the 1990swhen affordability became the major issue

Table 2 is a summary of some of the programs initiated to fos-ter the development and transition of composites Although theseinitiatives were seldom adequately funded to complete all initiallyplanned objectives they nonetheless were able to sustain substantialfunding for some limited period of time

Production in the United States of high modulus ber compositestructures commonly referred to as advanced composites beganwith boron epoxy F-14 and F-15 horizontal stabilizers in the early

Table 2 Advanced composite technology programs

Agency Acronym Program

US Army ACAP Advanced composite airframe programNASA ACEE Aircraft energy ef ciencyUS Air Force DMLCC Design and manufacture of low cost

compositesNASA ACT Advanced composites technologyUS Navy AV-8B Composite structure development programARPAa ACP Affordable composites for propulsionDARPAb APMC Affordable polymer matrix compositesDODcindustry CAI Composites affordability initiative

aAdvanced Research Projects AgencybDefense Advanced Research Projects AgencycDepartment of Defense

1970s The technology was fostered by the US Air Force BoronFilaments and Composites Advanced Development Program ini-tiated in 1964 as a result of a Project Forecast recommendationExperimental carbon epoxy structures were ying in 1970 and car-bon epoxy came to the forefront as the most cost-effective advancedcomposite material in the mid-1970s with production applicationson the F-15 B-1 and F-16 In 1978 carbon epoxy was accepted bythe Navy for F-18 and AV-8B primary wing box structures

Table 3 is a sample of some of the early composite structuredemonstrations Table 3 consists of a list of composite structuresfor which a metal baseline weight was available There have beenso many composite structures built since these and many more arecurrently in development that any list is out of date before it isprinted

There were of course many other composite structures some ofwhich did not have a metal baseline or for which a metal coun-terpart would not even be practical Some of those that come tomind are the Rockwell highly maneuverable advanced technologyremotely piloted research vehicle and the Grumman X-29 forwardswept wing demonstrator Both of these designs exploited the ad-vantageous aeroelastic characteristic of exible composite wings

The bulk of advanced composite material being used in ightin the United States today is fabricated from unidirectional tapeconsisting of AS-4 or IM7 carbon ber in 3501 thermoset epoxymatrix preimpregnated by Hercules Inc

Although developed some time ago the Slingsby T67 Fire ywas selected in 1992 for the US Air Force enhanced ight screener(T3A) The basic airframe started as a wooden construction thatSlingsby redesigned as primarily a wet layup glass ber polymercomposite airframe

Fiberglass-reinforced polymer construction has been employedfor many years in the glider industry It has been arguedpara that theHorton brothers of Germany created the worldrsquos rst compositeaircraft the Ho Va which began as a glider in 1936 The fuselageand wings for the rst Ho V were fabric-covered D-tubes moldedfrom paper- lled phenol resin R Horten noted that such moldedplastic construction promised to reduce production time and cost

The term molded reinforced plastic plane was being used by 1941(see Ref 24) Although nearly every form of material was tried as ller resin-coated wood veneer strips and sheets (usually mahoganyor spruce) were the most popular reinforcing agents Impregnationby liquid phenolic- or urea-type resins was common

Both Japan and Germany built highly successful berglass com-posite soaring planes in the 1950s Several of these are described inRef 1

The 42 by weight composite structure BellBoeing OspreyV-22 rst ew in 1989 The critical link between the blade andthe rotor hub (the yoke) of the V-22 is a composite structure Theyoke carries the blade centrifugal and lifting forces transmits enginetorque and permits lead-lag and apping motion Also each yokearm can accommodate pitch changes

The US Air Forcersquos latest ghter the F-22 Raptor ( rst ight7 September 1997) was the rst aircraft to employ resin transfer

paraData available online at httpwwwnur ugelcom

24 PAUL ET AL

Table 3 Composite structure weight savings

Baseline Composite WeightComponent weight lb weight lb savings

Flight service componentsEmpennage

F-111 horizontal stabilizer 1142 866 24F-14 horizontal stabilizer 1005 825 18F-15 horizontal stabilizer 527 395 25F-5 vertical stabilizer 119 83 30DC-10 vertical stabilizer 1005 834 17L-1011 vertical stabilizer 858 642 25B-737 horizontal stabilizer 262 204 22

WingsAV-8B wing 1143 949 17F-18 wing 1843 1641 11A-6 wing 6392 6400 0a

(selective reinforcement)C-130 wing box 4800 4200 125

FuselageUH-60 rear fuselage 444 399 10AV-8B forward fuselage 229 171 25

(selective reinforcement)CH-53 Aft fuselage 1159 947 18B-1 dorsal longeron 1485 829 44

OtherC-5 slat 241 190 21B-727 elevator 131 98 25DC-10 rudder 91 67 26L0-1011 aileron 140 107 23F-15 speed brake 112 89 21B-1 weapons bay door 147 129 12F-5 ap 34 25 26B-1 ap 87 73 16B-1 slat 74 61 18F-4 rudder 64 42 34F-11 spoiler 20 17 15Space shuttle doors 4150 3196 23DC-9 nose cowl 24 13 46F-16 landing gear door 53 42 20A-7 speed brake 123 74 40

RampD demonstration componentsWing

F-15 wing 2155 1787 17F-100 wing skin 616 475 23

FuselageF-111 fuselage 625 510 18CH-54 tail cone 464 397 14YF-16 fuselage 572 451 21F-5 fuselage 914 677 26

OtherB-1 vertical stabilizer 658 539 18B-1 horizontal stabilizer 3315 2844 14

aLoad capability 18 increase

molding (RTM) for composite part fabrication RTM is used forover 400 parts from inlet lip edges to sine wave wing spars Bothbismaleimide and epoxy parts are RTM processed The number ofparts made from thermoset composites is a 5050 split betweenepoxy resin and bismaleimide (BMI) Exterior skins are all BMIThermoplastics make up about 1 of the airframe by weight Theyare employed for the landing gear and weapons bay doors Auto-mated ber placement technology is utilized to fabricate the F-22composite horizontal stabilizersrsquo pivot shafts Overall 25 of theairframe structural material is composites

Helicopter rotor blade designers established an early position atthe forefront of the development and application of advanced com-posite structures and continue to design to maximize their bene ts

A Vertol CH-47 boronepoxy aft rotor blade was ight tested in1969 Of course glass ber-reinforced blades were in developmentand even production several years before that

Although weight reduction in helicopters is important it has beentheir adaptability to complex aerodynamic shapes that has enabledcomposites to play a major role in helicopter structures The con-cepts of hingeless and bearingless rotors were made practical by the

design exibility of composites Composite blades achieve virtuallyunlimited fatigue life

Most composite applications came about as a result of improvedperformance at an acceptable cost By the mid-1980s the struc-tural ef ciency of composites was fairly well recognized throughoutthe aerospace industry Emphasis then shifted to lowering develop-ment and production cost of composite assemblies the goal beingto achieve lower cost than competing aluminum construction Ex-panded use will require achieving this goal as well as projectedlong-term durability and supportability bene ts

The present emphasis on design to cost will continue facilitatedby integrated product teams (IPTs) and computer-aided design toolsHow well composite producibility and supportability issues are ad-dressed in the future will determine the extent and the rate at whichcomposite airframe applications will expand

Years of experience with composite materials have also resultedin lessons learned For example delaminations were found in thewing skins and substructure of US Marine Corps AV-8B and RAFUK-58 Harriers Investigations revealed that these delaminationswere caused by gaps between the skin and substructure that werenot properly shimmed prior to fastener torque-up during assemblyof the wings To correct this problem and prevent its occurrencein other composite aircraft production the US Navy now requiresthat liquid shimming be used to ll gaps between mating surfacesin composite structures Additionally closer dimensional tolerancesare placed on mating surfaces to ensure a better t

The one projection made by many was that 50or more of todayrsquosairframes would be made from composites This has not happenedfor two reasons 1) the cost delta for todayrsquos composites and 2) thepresent poor out-of-plane load reaction capability of laminated com-posite construction (and our limited ability to design to minimizethis shortcoming)

Design CriteriaUp to the mid-1950s the method for ensuring structural integrity

was essentially the same as that employed by the Wright brothersin 1903 A stress analysis was substantiated by a static test to a loadlevel greater than that expected in ight

The method consists of applying a factor of safety to the ightloads and verifying the structure could sustain this load by testing afull-scale airframe The factor of safety was developed to account forthe following sources of variability25 1)uncertainties in loads 2) in-accuracies in structural analysis 3) variations in material properties4) deterioration during service life and 5) variations in constructionquality

The aircraft yearbook in 1921 states ldquoThe fatigue of metals orprogressive failure from incipient cracking is the greatest bugbearof the designerrdquo The Bureau of Standards was reporting exu-ral fatigue data on duralumin sheets as early as 1922 HoweverRef 1 indicates that the rst mention of fatigue in government air-worthiness regulations did not appear until 1945 The concept ofmetal fatigue goes back a long way to the failures of railway carsand locomotive axles in the 1840s Reference 26 credits Wohler ofGermany in 1860 with the concept of designing for a nite fatiguelife In 1853 French engineers were inspecting axles of horse-drawnmail coaches for cracks and recommending their replacement after60000 km However Ref 27 indicates that it was not until 1914that suf cient test data were available to consider incorporating fa-tigue limits into aerospace structural steel speci cations In 1918the rst full-scale fatigue test of a large aircraft component wascarried out in the United Kingdom at the Royal Aircraft Establish-ment

Many papers were written in the 1940s predicting increases inairframe fatigue failures The authors of Ref 28 forecast increasesin fatigue troubles would stem from the following trends that ex-isted in 1943 1) a drive toward higher cruising speeds 2) the contin-uous increase in the ratio of useful load to gross weight and 3) theincrease in wing loading and gross weight plus an increase in servicelife necessitated by high initial cost

Reference 28 provides stressndashnumber of cycles (SndashN) curves onthe 17S and recommends designing on the basis of life expectancy

PAUL ET AL 25

The authors also recommend determining service loadings fromstatistical ight data recorded on existing aircraft but cautioned thatsuch data are applicable only to airplanes of speed wing loadingsize and mission similar to those in which the data were obtained

The author of Ref 26 indicates that Gassner of Germany hadwarned as early as 1941 that if the higher static strength of thenew aluminum alloy (7000-type) were taken advantage of a shorterfatigue life would result One goal of the researchers of that timewas to obtain suf cient data to be able to reduce factors of safetythat resulted from ignorance of the strength of materials

An International Committee on Aeronautical Fatigue wasfounded in 1951 at the suggestion of Plantema of The NetherlandsIt was two UK Comet crashes in 1954 and a series of US B-47catastrophic accidents in 1958 however that focused US Air Forceattention on structural fatigue and led to The US Air Force AircraftStructural Integrity Program (ASIP) This program adopted a fatiguemethodology approach commonly referred to as safe-life This ap-proach was used in the development of US Air Force aircraft inthe 1960s but was not adequate from the standpoint of addressingmanufacturing and service-induced damage Consequently aircraftdesigned both prior to and subsequent to the safe-life method contin-ued to experience premature failures The US Air Force ASIP wasrevised to include the incorporation of a damage tolerancedurabilityphilosophy into the design process Although initially applied tothe B-1A and modi cations of the F-111 C-5A and F-4 the dam-age tolerancedurability requirements were formally established inMIL-STD-1530A in 1975

The US Navy ASIP is similar It requires fatigue life of thestructure to be designed such that failure of a full-scale test articlewill not occur and that the structure will be free from any defectssuch as cracks deformation loss of modulus disbonding or fastenerhole deformation and delamination when subjected to a minimumof two lifetimes

Spurred on by joint service airframe requirements of the JointAdvanced Strike Technology program the services have developeda joint service guide speci cation for aircraft structures

All US Air Force aircraft since the F-16 have been subject todurabilitydamage tolerance requirements Each of the tests requiredby ASIP has served to identify de ciencies in structural design andanalysis resulting in modi cations to the structure that provided asafer and more supportable airframe

The full-scale airframe static fatigue durabilitydamage toler-ance tests are expensive and therefore there should be a reason-able expectation that they can with minor design modi cationsbe accomplished successfully This generally means conducting anextensive structural element and subcomponent test program in sup-port of the ASIP These tests (Table 4) address all of the critical loadpaths

This type of building block program that is testing large numbersof coupons elements and components prior to full-scale hardwarehas evolved as the most logical risk-reduction approach for newsystem airframe development The consequences of a failure late inthe development cycle of a system are too great to proceed any otherway

Table 4 Airframe design development tests

Aircraft Elements tested

B-2 160000 nonmetals 1450 metallic 6 leading edge74 wing panels 40 engine exhaust panels27 joints

C-17 6000 coupons 400 componentsF-22 Allowables

14500 composites 5300 metalsStructural integrity

4100 metals 8800 compositesReproducibility

80 full and subscale composite 15 titaniumstructures

FA-18 EF 10000 materials characterizationprocessing tests165 elements and components

V-22 9700 coupons 550 elements and components

The opposite is true for advanced technology demonstrators(ATDs) which introduce technology through prototyping thus ac-celerating transition of new technologies Failure in an ATD pro-gram is more accepted as a lesson learned rather than a failure ofthe contractor and procuring agency

Table 4 indicates that composite structures require more extensivedesign and certi cation testing than metals The additional testingrequirements add cost to the design allowables and design certi -cation efforts This was clearly identi ed in Ref 29 published inNovember 1994 as a lesson learned

In the case of composite structural assemblies however whenthe structure satis es all static load conditions fatigue is generallynot a problem The effects of moisture and temperature on designmargins can be established at the coupon and subcomponent levelwith the full-scale component margins adjusted appropriately

The one thing the described programs had in common was a sub-stantial nite element analysis effort (both global and local detailmodels) to support test hardware design and to certify the airframefor adequate design margins These programs demonstrate the im-portance of a thorough analysis effort coupled with a complete criti-cal area design development test program to substantiate the designIt would be unwise and uneconomical to dispense with either

Analysis and Design TechniquesThe introduction of digital computers in the 1950s paved the

way for the now emerging multidisciplinary design technologiesapplicable to the integrated design of a variety of engineering prod-ucts including air and space vehicles This section is a brief outlineof the developments that led to such software systems as ASKASTARDYNE NASTRAN ANSYS ABAQUS and ASTROS

Before discussing the chronology and signi cance of structuralanalysis developments it is appropriate to distinguish between twolevels of analysis 1) analysis of structural elements and 2) analysisof builtup structures

Analysis of Structural Elements

Structural elements or components can be categorized into threetypes line elements surface elements and solids The analysis ofsuch elements has been developed and documented over the last150 years by Euler Lagrange St Venant and others References 30and 31 document the chronology of these developments Over adozen solid mechanics books by Timoshenko constitute a rich com-pendium of analysis methods for structural elements The rangeand applicability of this continuum approach (the solution of dif-ferential equations) is however limited to very restricted cases ofloading boundary conditions and element shapes Such methods asRayleighndashRitz Galerkin and nite differences are approximationsThese methods replace a continuum model by a discretized equiva-lent The result is a set of algebraic equations that can be solved bydigital computers

Analysis of Builtup Structures

Builtup structures are simply constructed out of many structuralelements joined together at their boundaries The differential equa-tion approach for the solution of builtup structures is intractable Asan alternative a number of discretized procedures have been devel-oped since the early 1900s A structure is considered determinate ifits internal forces can be solved by the force and moment equilib-rium equations alone Otherwise it is considered an indeterminatesystem Analyzing complex indeterminate systems was a dauntingtask before the advent of digital computers

The force and displacement methods that emerged in the 1950swere a major breakthrough in structural analysis They refer to thediscretized model of the structure in which the continuum is re-placed by a nite number of grid points Each grid point is assumedto have six degrees of freedom (DOF) in space These DOFs areforces andor displacements In the force method the unknowns arethe set of forces released to make the indeterminate structure deter-minate Enforcement of compatibility conditions results in force-displacement relations connected by a exibility matrix In thedisplacement method the unknowns are the displacements of the

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 6: Evolution of U.S. Military Aircraft Structures Technology

PAUL ET AL 23

Fig 11 F-22 aft-fuselage frame forging produced on WymanndashGordon50000-ton hydraulic press

achieved by the HIP processing so that allowables do not have to bereduced by conventional casting factors

Recently RMI Titanium of Ohio developed a stronger more dam-age tolerant alloy (Ti-6-22-22S) that is nding application on theF-22 joint strike ghter and X-33 reusable launch vehicle

Timet Corporation alloy 10ndash2ndash3 is being used in the main landinggear and Timetal 21S for the nacelle cowl plug nozzle and Prattand Whitney Aircraft engine components of the Boeing 777 aircraftTitanium usage on Boeing aircraft has increased from 2 emptyweight of the 737 to 8 of the 777 The McDonnell Douglas C-17transport also utilizes alloy 10ndash2ndash3 for landing gear components

The focus of titanium alloy development however has shiftedfrom aerospace to industrial applications New product forms areemerging rapidly Titanium is growing in use for everything fromgolf clubs skis tennis rackets bicycles pots and pans and jewelryto even hip and knee replacements The kitchen sink cannot be faroff

Noting this trend toward increased titanium use a 1991 AdvancedMaterials and Processing magazine article was titled ldquoTitaniumStaves Off Compositesrdquo In reality titanium has grown in use partlybecause of its compatibility with composites both from a thermalexpansion characteristic and resistance to galvanic corrosion whenin contact with carbon epoxy

Advanced Composites

The present generation of composite structures technology ap-plied to the B-2 FA-18EF V-22 and RAH-66 has been in devel-opment for 30 years The present level of maturity is a result of acontinued investment by the RampD structures and materials commu-nities of all three services and NASA No other class of material hasreceived such high level of funding support for such a long time Nu-merous composite structures built from the 1960s to 1980s veri edcomposite structure advantages (primarily weight savings) leadingto more extensive application of composite airframes in the 1990swhen affordability became the major issue

Table 2 is a summary of some of the programs initiated to fos-ter the development and transition of composites Although theseinitiatives were seldom adequately funded to complete all initiallyplanned objectives they nonetheless were able to sustain substantialfunding for some limited period of time

Production in the United States of high modulus ber compositestructures commonly referred to as advanced composites beganwith boron epoxy F-14 and F-15 horizontal stabilizers in the early

Table 2 Advanced composite technology programs

Agency Acronym Program

US Army ACAP Advanced composite airframe programNASA ACEE Aircraft energy ef ciencyUS Air Force DMLCC Design and manufacture of low cost

compositesNASA ACT Advanced composites technologyUS Navy AV-8B Composite structure development programARPAa ACP Affordable composites for propulsionDARPAb APMC Affordable polymer matrix compositesDODcindustry CAI Composites affordability initiative

aAdvanced Research Projects AgencybDefense Advanced Research Projects AgencycDepartment of Defense

1970s The technology was fostered by the US Air Force BoronFilaments and Composites Advanced Development Program ini-tiated in 1964 as a result of a Project Forecast recommendationExperimental carbon epoxy structures were ying in 1970 and car-bon epoxy came to the forefront as the most cost-effective advancedcomposite material in the mid-1970s with production applicationson the F-15 B-1 and F-16 In 1978 carbon epoxy was accepted bythe Navy for F-18 and AV-8B primary wing box structures

Table 3 is a sample of some of the early composite structuredemonstrations Table 3 consists of a list of composite structuresfor which a metal baseline weight was available There have beenso many composite structures built since these and many more arecurrently in development that any list is out of date before it isprinted

There were of course many other composite structures some ofwhich did not have a metal baseline or for which a metal coun-terpart would not even be practical Some of those that come tomind are the Rockwell highly maneuverable advanced technologyremotely piloted research vehicle and the Grumman X-29 forwardswept wing demonstrator Both of these designs exploited the ad-vantageous aeroelastic characteristic of exible composite wings

The bulk of advanced composite material being used in ightin the United States today is fabricated from unidirectional tapeconsisting of AS-4 or IM7 carbon ber in 3501 thermoset epoxymatrix preimpregnated by Hercules Inc

Although developed some time ago the Slingsby T67 Fire ywas selected in 1992 for the US Air Force enhanced ight screener(T3A) The basic airframe started as a wooden construction thatSlingsby redesigned as primarily a wet layup glass ber polymercomposite airframe

Fiberglass-reinforced polymer construction has been employedfor many years in the glider industry It has been arguedpara that theHorton brothers of Germany created the worldrsquos rst compositeaircraft the Ho Va which began as a glider in 1936 The fuselageand wings for the rst Ho V were fabric-covered D-tubes moldedfrom paper- lled phenol resin R Horten noted that such moldedplastic construction promised to reduce production time and cost

The term molded reinforced plastic plane was being used by 1941(see Ref 24) Although nearly every form of material was tried as ller resin-coated wood veneer strips and sheets (usually mahoganyor spruce) were the most popular reinforcing agents Impregnationby liquid phenolic- or urea-type resins was common

Both Japan and Germany built highly successful berglass com-posite soaring planes in the 1950s Several of these are described inRef 1

The 42 by weight composite structure BellBoeing OspreyV-22 rst ew in 1989 The critical link between the blade andthe rotor hub (the yoke) of the V-22 is a composite structure Theyoke carries the blade centrifugal and lifting forces transmits enginetorque and permits lead-lag and apping motion Also each yokearm can accommodate pitch changes

The US Air Forcersquos latest ghter the F-22 Raptor ( rst ight7 September 1997) was the rst aircraft to employ resin transfer

paraData available online at httpwwwnur ugelcom

24 PAUL ET AL

Table 3 Composite structure weight savings

Baseline Composite WeightComponent weight lb weight lb savings

Flight service componentsEmpennage

F-111 horizontal stabilizer 1142 866 24F-14 horizontal stabilizer 1005 825 18F-15 horizontal stabilizer 527 395 25F-5 vertical stabilizer 119 83 30DC-10 vertical stabilizer 1005 834 17L-1011 vertical stabilizer 858 642 25B-737 horizontal stabilizer 262 204 22

WingsAV-8B wing 1143 949 17F-18 wing 1843 1641 11A-6 wing 6392 6400 0a

(selective reinforcement)C-130 wing box 4800 4200 125

FuselageUH-60 rear fuselage 444 399 10AV-8B forward fuselage 229 171 25

(selective reinforcement)CH-53 Aft fuselage 1159 947 18B-1 dorsal longeron 1485 829 44

OtherC-5 slat 241 190 21B-727 elevator 131 98 25DC-10 rudder 91 67 26L0-1011 aileron 140 107 23F-15 speed brake 112 89 21B-1 weapons bay door 147 129 12F-5 ap 34 25 26B-1 ap 87 73 16B-1 slat 74 61 18F-4 rudder 64 42 34F-11 spoiler 20 17 15Space shuttle doors 4150 3196 23DC-9 nose cowl 24 13 46F-16 landing gear door 53 42 20A-7 speed brake 123 74 40

RampD demonstration componentsWing

F-15 wing 2155 1787 17F-100 wing skin 616 475 23

FuselageF-111 fuselage 625 510 18CH-54 tail cone 464 397 14YF-16 fuselage 572 451 21F-5 fuselage 914 677 26

OtherB-1 vertical stabilizer 658 539 18B-1 horizontal stabilizer 3315 2844 14

aLoad capability 18 increase

molding (RTM) for composite part fabrication RTM is used forover 400 parts from inlet lip edges to sine wave wing spars Bothbismaleimide and epoxy parts are RTM processed The number ofparts made from thermoset composites is a 5050 split betweenepoxy resin and bismaleimide (BMI) Exterior skins are all BMIThermoplastics make up about 1 of the airframe by weight Theyare employed for the landing gear and weapons bay doors Auto-mated ber placement technology is utilized to fabricate the F-22composite horizontal stabilizersrsquo pivot shafts Overall 25 of theairframe structural material is composites

Helicopter rotor blade designers established an early position atthe forefront of the development and application of advanced com-posite structures and continue to design to maximize their bene ts

A Vertol CH-47 boronepoxy aft rotor blade was ight tested in1969 Of course glass ber-reinforced blades were in developmentand even production several years before that

Although weight reduction in helicopters is important it has beentheir adaptability to complex aerodynamic shapes that has enabledcomposites to play a major role in helicopter structures The con-cepts of hingeless and bearingless rotors were made practical by the

design exibility of composites Composite blades achieve virtuallyunlimited fatigue life

Most composite applications came about as a result of improvedperformance at an acceptable cost By the mid-1980s the struc-tural ef ciency of composites was fairly well recognized throughoutthe aerospace industry Emphasis then shifted to lowering develop-ment and production cost of composite assemblies the goal beingto achieve lower cost than competing aluminum construction Ex-panded use will require achieving this goal as well as projectedlong-term durability and supportability bene ts

The present emphasis on design to cost will continue facilitatedby integrated product teams (IPTs) and computer-aided design toolsHow well composite producibility and supportability issues are ad-dressed in the future will determine the extent and the rate at whichcomposite airframe applications will expand

Years of experience with composite materials have also resultedin lessons learned For example delaminations were found in thewing skins and substructure of US Marine Corps AV-8B and RAFUK-58 Harriers Investigations revealed that these delaminationswere caused by gaps between the skin and substructure that werenot properly shimmed prior to fastener torque-up during assemblyof the wings To correct this problem and prevent its occurrencein other composite aircraft production the US Navy now requiresthat liquid shimming be used to ll gaps between mating surfacesin composite structures Additionally closer dimensional tolerancesare placed on mating surfaces to ensure a better t

The one projection made by many was that 50or more of todayrsquosairframes would be made from composites This has not happenedfor two reasons 1) the cost delta for todayrsquos composites and 2) thepresent poor out-of-plane load reaction capability of laminated com-posite construction (and our limited ability to design to minimizethis shortcoming)

Design CriteriaUp to the mid-1950s the method for ensuring structural integrity

was essentially the same as that employed by the Wright brothersin 1903 A stress analysis was substantiated by a static test to a loadlevel greater than that expected in ight

The method consists of applying a factor of safety to the ightloads and verifying the structure could sustain this load by testing afull-scale airframe The factor of safety was developed to account forthe following sources of variability25 1)uncertainties in loads 2) in-accuracies in structural analysis 3) variations in material properties4) deterioration during service life and 5) variations in constructionquality

The aircraft yearbook in 1921 states ldquoThe fatigue of metals orprogressive failure from incipient cracking is the greatest bugbearof the designerrdquo The Bureau of Standards was reporting exu-ral fatigue data on duralumin sheets as early as 1922 HoweverRef 1 indicates that the rst mention of fatigue in government air-worthiness regulations did not appear until 1945 The concept ofmetal fatigue goes back a long way to the failures of railway carsand locomotive axles in the 1840s Reference 26 credits Wohler ofGermany in 1860 with the concept of designing for a nite fatiguelife In 1853 French engineers were inspecting axles of horse-drawnmail coaches for cracks and recommending their replacement after60000 km However Ref 27 indicates that it was not until 1914that suf cient test data were available to consider incorporating fa-tigue limits into aerospace structural steel speci cations In 1918the rst full-scale fatigue test of a large aircraft component wascarried out in the United Kingdom at the Royal Aircraft Establish-ment

Many papers were written in the 1940s predicting increases inairframe fatigue failures The authors of Ref 28 forecast increasesin fatigue troubles would stem from the following trends that ex-isted in 1943 1) a drive toward higher cruising speeds 2) the contin-uous increase in the ratio of useful load to gross weight and 3) theincrease in wing loading and gross weight plus an increase in servicelife necessitated by high initial cost

Reference 28 provides stressndashnumber of cycles (SndashN) curves onthe 17S and recommends designing on the basis of life expectancy

PAUL ET AL 25

The authors also recommend determining service loadings fromstatistical ight data recorded on existing aircraft but cautioned thatsuch data are applicable only to airplanes of speed wing loadingsize and mission similar to those in which the data were obtained

The author of Ref 26 indicates that Gassner of Germany hadwarned as early as 1941 that if the higher static strength of thenew aluminum alloy (7000-type) were taken advantage of a shorterfatigue life would result One goal of the researchers of that timewas to obtain suf cient data to be able to reduce factors of safetythat resulted from ignorance of the strength of materials

An International Committee on Aeronautical Fatigue wasfounded in 1951 at the suggestion of Plantema of The NetherlandsIt was two UK Comet crashes in 1954 and a series of US B-47catastrophic accidents in 1958 however that focused US Air Forceattention on structural fatigue and led to The US Air Force AircraftStructural Integrity Program (ASIP) This program adopted a fatiguemethodology approach commonly referred to as safe-life This ap-proach was used in the development of US Air Force aircraft inthe 1960s but was not adequate from the standpoint of addressingmanufacturing and service-induced damage Consequently aircraftdesigned both prior to and subsequent to the safe-life method contin-ued to experience premature failures The US Air Force ASIP wasrevised to include the incorporation of a damage tolerancedurabilityphilosophy into the design process Although initially applied tothe B-1A and modi cations of the F-111 C-5A and F-4 the dam-age tolerancedurability requirements were formally established inMIL-STD-1530A in 1975

The US Navy ASIP is similar It requires fatigue life of thestructure to be designed such that failure of a full-scale test articlewill not occur and that the structure will be free from any defectssuch as cracks deformation loss of modulus disbonding or fastenerhole deformation and delamination when subjected to a minimumof two lifetimes

Spurred on by joint service airframe requirements of the JointAdvanced Strike Technology program the services have developeda joint service guide speci cation for aircraft structures

All US Air Force aircraft since the F-16 have been subject todurabilitydamage tolerance requirements Each of the tests requiredby ASIP has served to identify de ciencies in structural design andanalysis resulting in modi cations to the structure that provided asafer and more supportable airframe

The full-scale airframe static fatigue durabilitydamage toler-ance tests are expensive and therefore there should be a reason-able expectation that they can with minor design modi cationsbe accomplished successfully This generally means conducting anextensive structural element and subcomponent test program in sup-port of the ASIP These tests (Table 4) address all of the critical loadpaths

This type of building block program that is testing large numbersof coupons elements and components prior to full-scale hardwarehas evolved as the most logical risk-reduction approach for newsystem airframe development The consequences of a failure late inthe development cycle of a system are too great to proceed any otherway

Table 4 Airframe design development tests

Aircraft Elements tested

B-2 160000 nonmetals 1450 metallic 6 leading edge74 wing panels 40 engine exhaust panels27 joints

C-17 6000 coupons 400 componentsF-22 Allowables

14500 composites 5300 metalsStructural integrity

4100 metals 8800 compositesReproducibility

80 full and subscale composite 15 titaniumstructures

FA-18 EF 10000 materials characterizationprocessing tests165 elements and components

V-22 9700 coupons 550 elements and components

The opposite is true for advanced technology demonstrators(ATDs) which introduce technology through prototyping thus ac-celerating transition of new technologies Failure in an ATD pro-gram is more accepted as a lesson learned rather than a failure ofthe contractor and procuring agency

Table 4 indicates that composite structures require more extensivedesign and certi cation testing than metals The additional testingrequirements add cost to the design allowables and design certi -cation efforts This was clearly identi ed in Ref 29 published inNovember 1994 as a lesson learned

In the case of composite structural assemblies however whenthe structure satis es all static load conditions fatigue is generallynot a problem The effects of moisture and temperature on designmargins can be established at the coupon and subcomponent levelwith the full-scale component margins adjusted appropriately

The one thing the described programs had in common was a sub-stantial nite element analysis effort (both global and local detailmodels) to support test hardware design and to certify the airframefor adequate design margins These programs demonstrate the im-portance of a thorough analysis effort coupled with a complete criti-cal area design development test program to substantiate the designIt would be unwise and uneconomical to dispense with either

Analysis and Design TechniquesThe introduction of digital computers in the 1950s paved the

way for the now emerging multidisciplinary design technologiesapplicable to the integrated design of a variety of engineering prod-ucts including air and space vehicles This section is a brief outlineof the developments that led to such software systems as ASKASTARDYNE NASTRAN ANSYS ABAQUS and ASTROS

Before discussing the chronology and signi cance of structuralanalysis developments it is appropriate to distinguish between twolevels of analysis 1) analysis of structural elements and 2) analysisof builtup structures

Analysis of Structural Elements

Structural elements or components can be categorized into threetypes line elements surface elements and solids The analysis ofsuch elements has been developed and documented over the last150 years by Euler Lagrange St Venant and others References 30and 31 document the chronology of these developments Over adozen solid mechanics books by Timoshenko constitute a rich com-pendium of analysis methods for structural elements The rangeand applicability of this continuum approach (the solution of dif-ferential equations) is however limited to very restricted cases ofloading boundary conditions and element shapes Such methods asRayleighndashRitz Galerkin and nite differences are approximationsThese methods replace a continuum model by a discretized equiva-lent The result is a set of algebraic equations that can be solved bydigital computers

Analysis of Builtup Structures

Builtup structures are simply constructed out of many structuralelements joined together at their boundaries The differential equa-tion approach for the solution of builtup structures is intractable Asan alternative a number of discretized procedures have been devel-oped since the early 1900s A structure is considered determinate ifits internal forces can be solved by the force and moment equilib-rium equations alone Otherwise it is considered an indeterminatesystem Analyzing complex indeterminate systems was a dauntingtask before the advent of digital computers

The force and displacement methods that emerged in the 1950swere a major breakthrough in structural analysis They refer to thediscretized model of the structure in which the continuum is re-placed by a nite number of grid points Each grid point is assumedto have six degrees of freedom (DOF) in space These DOFs areforces andor displacements In the force method the unknowns arethe set of forces released to make the indeterminate structure deter-minate Enforcement of compatibility conditions results in force-displacement relations connected by a exibility matrix In thedisplacement method the unknowns are the displacements of the

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 7: Evolution of U.S. Military Aircraft Structures Technology

24 PAUL ET AL

Table 3 Composite structure weight savings

Baseline Composite WeightComponent weight lb weight lb savings

Flight service componentsEmpennage

F-111 horizontal stabilizer 1142 866 24F-14 horizontal stabilizer 1005 825 18F-15 horizontal stabilizer 527 395 25F-5 vertical stabilizer 119 83 30DC-10 vertical stabilizer 1005 834 17L-1011 vertical stabilizer 858 642 25B-737 horizontal stabilizer 262 204 22

WingsAV-8B wing 1143 949 17F-18 wing 1843 1641 11A-6 wing 6392 6400 0a

(selective reinforcement)C-130 wing box 4800 4200 125

FuselageUH-60 rear fuselage 444 399 10AV-8B forward fuselage 229 171 25

(selective reinforcement)CH-53 Aft fuselage 1159 947 18B-1 dorsal longeron 1485 829 44

OtherC-5 slat 241 190 21B-727 elevator 131 98 25DC-10 rudder 91 67 26L0-1011 aileron 140 107 23F-15 speed brake 112 89 21B-1 weapons bay door 147 129 12F-5 ap 34 25 26B-1 ap 87 73 16B-1 slat 74 61 18F-4 rudder 64 42 34F-11 spoiler 20 17 15Space shuttle doors 4150 3196 23DC-9 nose cowl 24 13 46F-16 landing gear door 53 42 20A-7 speed brake 123 74 40

RampD demonstration componentsWing

F-15 wing 2155 1787 17F-100 wing skin 616 475 23

FuselageF-111 fuselage 625 510 18CH-54 tail cone 464 397 14YF-16 fuselage 572 451 21F-5 fuselage 914 677 26

OtherB-1 vertical stabilizer 658 539 18B-1 horizontal stabilizer 3315 2844 14

aLoad capability 18 increase

molding (RTM) for composite part fabrication RTM is used forover 400 parts from inlet lip edges to sine wave wing spars Bothbismaleimide and epoxy parts are RTM processed The number ofparts made from thermoset composites is a 5050 split betweenepoxy resin and bismaleimide (BMI) Exterior skins are all BMIThermoplastics make up about 1 of the airframe by weight Theyare employed for the landing gear and weapons bay doors Auto-mated ber placement technology is utilized to fabricate the F-22composite horizontal stabilizersrsquo pivot shafts Overall 25 of theairframe structural material is composites

Helicopter rotor blade designers established an early position atthe forefront of the development and application of advanced com-posite structures and continue to design to maximize their bene ts

A Vertol CH-47 boronepoxy aft rotor blade was ight tested in1969 Of course glass ber-reinforced blades were in developmentand even production several years before that

Although weight reduction in helicopters is important it has beentheir adaptability to complex aerodynamic shapes that has enabledcomposites to play a major role in helicopter structures The con-cepts of hingeless and bearingless rotors were made practical by the

design exibility of composites Composite blades achieve virtuallyunlimited fatigue life

Most composite applications came about as a result of improvedperformance at an acceptable cost By the mid-1980s the struc-tural ef ciency of composites was fairly well recognized throughoutthe aerospace industry Emphasis then shifted to lowering develop-ment and production cost of composite assemblies the goal beingto achieve lower cost than competing aluminum construction Ex-panded use will require achieving this goal as well as projectedlong-term durability and supportability bene ts

The present emphasis on design to cost will continue facilitatedby integrated product teams (IPTs) and computer-aided design toolsHow well composite producibility and supportability issues are ad-dressed in the future will determine the extent and the rate at whichcomposite airframe applications will expand

Years of experience with composite materials have also resultedin lessons learned For example delaminations were found in thewing skins and substructure of US Marine Corps AV-8B and RAFUK-58 Harriers Investigations revealed that these delaminationswere caused by gaps between the skin and substructure that werenot properly shimmed prior to fastener torque-up during assemblyof the wings To correct this problem and prevent its occurrencein other composite aircraft production the US Navy now requiresthat liquid shimming be used to ll gaps between mating surfacesin composite structures Additionally closer dimensional tolerancesare placed on mating surfaces to ensure a better t

The one projection made by many was that 50or more of todayrsquosairframes would be made from composites This has not happenedfor two reasons 1) the cost delta for todayrsquos composites and 2) thepresent poor out-of-plane load reaction capability of laminated com-posite construction (and our limited ability to design to minimizethis shortcoming)

Design CriteriaUp to the mid-1950s the method for ensuring structural integrity

was essentially the same as that employed by the Wright brothersin 1903 A stress analysis was substantiated by a static test to a loadlevel greater than that expected in ight

The method consists of applying a factor of safety to the ightloads and verifying the structure could sustain this load by testing afull-scale airframe The factor of safety was developed to account forthe following sources of variability25 1)uncertainties in loads 2) in-accuracies in structural analysis 3) variations in material properties4) deterioration during service life and 5) variations in constructionquality

The aircraft yearbook in 1921 states ldquoThe fatigue of metals orprogressive failure from incipient cracking is the greatest bugbearof the designerrdquo The Bureau of Standards was reporting exu-ral fatigue data on duralumin sheets as early as 1922 HoweverRef 1 indicates that the rst mention of fatigue in government air-worthiness regulations did not appear until 1945 The concept ofmetal fatigue goes back a long way to the failures of railway carsand locomotive axles in the 1840s Reference 26 credits Wohler ofGermany in 1860 with the concept of designing for a nite fatiguelife In 1853 French engineers were inspecting axles of horse-drawnmail coaches for cracks and recommending their replacement after60000 km However Ref 27 indicates that it was not until 1914that suf cient test data were available to consider incorporating fa-tigue limits into aerospace structural steel speci cations In 1918the rst full-scale fatigue test of a large aircraft component wascarried out in the United Kingdom at the Royal Aircraft Establish-ment

Many papers were written in the 1940s predicting increases inairframe fatigue failures The authors of Ref 28 forecast increasesin fatigue troubles would stem from the following trends that ex-isted in 1943 1) a drive toward higher cruising speeds 2) the contin-uous increase in the ratio of useful load to gross weight and 3) theincrease in wing loading and gross weight plus an increase in servicelife necessitated by high initial cost

Reference 28 provides stressndashnumber of cycles (SndashN) curves onthe 17S and recommends designing on the basis of life expectancy

PAUL ET AL 25

The authors also recommend determining service loadings fromstatistical ight data recorded on existing aircraft but cautioned thatsuch data are applicable only to airplanes of speed wing loadingsize and mission similar to those in which the data were obtained

The author of Ref 26 indicates that Gassner of Germany hadwarned as early as 1941 that if the higher static strength of thenew aluminum alloy (7000-type) were taken advantage of a shorterfatigue life would result One goal of the researchers of that timewas to obtain suf cient data to be able to reduce factors of safetythat resulted from ignorance of the strength of materials

An International Committee on Aeronautical Fatigue wasfounded in 1951 at the suggestion of Plantema of The NetherlandsIt was two UK Comet crashes in 1954 and a series of US B-47catastrophic accidents in 1958 however that focused US Air Forceattention on structural fatigue and led to The US Air Force AircraftStructural Integrity Program (ASIP) This program adopted a fatiguemethodology approach commonly referred to as safe-life This ap-proach was used in the development of US Air Force aircraft inthe 1960s but was not adequate from the standpoint of addressingmanufacturing and service-induced damage Consequently aircraftdesigned both prior to and subsequent to the safe-life method contin-ued to experience premature failures The US Air Force ASIP wasrevised to include the incorporation of a damage tolerancedurabilityphilosophy into the design process Although initially applied tothe B-1A and modi cations of the F-111 C-5A and F-4 the dam-age tolerancedurability requirements were formally established inMIL-STD-1530A in 1975

The US Navy ASIP is similar It requires fatigue life of thestructure to be designed such that failure of a full-scale test articlewill not occur and that the structure will be free from any defectssuch as cracks deformation loss of modulus disbonding or fastenerhole deformation and delamination when subjected to a minimumof two lifetimes

Spurred on by joint service airframe requirements of the JointAdvanced Strike Technology program the services have developeda joint service guide speci cation for aircraft structures

All US Air Force aircraft since the F-16 have been subject todurabilitydamage tolerance requirements Each of the tests requiredby ASIP has served to identify de ciencies in structural design andanalysis resulting in modi cations to the structure that provided asafer and more supportable airframe

The full-scale airframe static fatigue durabilitydamage toler-ance tests are expensive and therefore there should be a reason-able expectation that they can with minor design modi cationsbe accomplished successfully This generally means conducting anextensive structural element and subcomponent test program in sup-port of the ASIP These tests (Table 4) address all of the critical loadpaths

This type of building block program that is testing large numbersof coupons elements and components prior to full-scale hardwarehas evolved as the most logical risk-reduction approach for newsystem airframe development The consequences of a failure late inthe development cycle of a system are too great to proceed any otherway

Table 4 Airframe design development tests

Aircraft Elements tested

B-2 160000 nonmetals 1450 metallic 6 leading edge74 wing panels 40 engine exhaust panels27 joints

C-17 6000 coupons 400 componentsF-22 Allowables

14500 composites 5300 metalsStructural integrity

4100 metals 8800 compositesReproducibility

80 full and subscale composite 15 titaniumstructures

FA-18 EF 10000 materials characterizationprocessing tests165 elements and components

V-22 9700 coupons 550 elements and components

The opposite is true for advanced technology demonstrators(ATDs) which introduce technology through prototyping thus ac-celerating transition of new technologies Failure in an ATD pro-gram is more accepted as a lesson learned rather than a failure ofthe contractor and procuring agency

Table 4 indicates that composite structures require more extensivedesign and certi cation testing than metals The additional testingrequirements add cost to the design allowables and design certi -cation efforts This was clearly identi ed in Ref 29 published inNovember 1994 as a lesson learned

In the case of composite structural assemblies however whenthe structure satis es all static load conditions fatigue is generallynot a problem The effects of moisture and temperature on designmargins can be established at the coupon and subcomponent levelwith the full-scale component margins adjusted appropriately

The one thing the described programs had in common was a sub-stantial nite element analysis effort (both global and local detailmodels) to support test hardware design and to certify the airframefor adequate design margins These programs demonstrate the im-portance of a thorough analysis effort coupled with a complete criti-cal area design development test program to substantiate the designIt would be unwise and uneconomical to dispense with either

Analysis and Design TechniquesThe introduction of digital computers in the 1950s paved the

way for the now emerging multidisciplinary design technologiesapplicable to the integrated design of a variety of engineering prod-ucts including air and space vehicles This section is a brief outlineof the developments that led to such software systems as ASKASTARDYNE NASTRAN ANSYS ABAQUS and ASTROS

Before discussing the chronology and signi cance of structuralanalysis developments it is appropriate to distinguish between twolevels of analysis 1) analysis of structural elements and 2) analysisof builtup structures

Analysis of Structural Elements

Structural elements or components can be categorized into threetypes line elements surface elements and solids The analysis ofsuch elements has been developed and documented over the last150 years by Euler Lagrange St Venant and others References 30and 31 document the chronology of these developments Over adozen solid mechanics books by Timoshenko constitute a rich com-pendium of analysis methods for structural elements The rangeand applicability of this continuum approach (the solution of dif-ferential equations) is however limited to very restricted cases ofloading boundary conditions and element shapes Such methods asRayleighndashRitz Galerkin and nite differences are approximationsThese methods replace a continuum model by a discretized equiva-lent The result is a set of algebraic equations that can be solved bydigital computers

Analysis of Builtup Structures

Builtup structures are simply constructed out of many structuralelements joined together at their boundaries The differential equa-tion approach for the solution of builtup structures is intractable Asan alternative a number of discretized procedures have been devel-oped since the early 1900s A structure is considered determinate ifits internal forces can be solved by the force and moment equilib-rium equations alone Otherwise it is considered an indeterminatesystem Analyzing complex indeterminate systems was a dauntingtask before the advent of digital computers

The force and displacement methods that emerged in the 1950swere a major breakthrough in structural analysis They refer to thediscretized model of the structure in which the continuum is re-placed by a nite number of grid points Each grid point is assumedto have six degrees of freedom (DOF) in space These DOFs areforces andor displacements In the force method the unknowns arethe set of forces released to make the indeterminate structure deter-minate Enforcement of compatibility conditions results in force-displacement relations connected by a exibility matrix In thedisplacement method the unknowns are the displacements of the

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 8: Evolution of U.S. Military Aircraft Structures Technology

PAUL ET AL 25

The authors also recommend determining service loadings fromstatistical ight data recorded on existing aircraft but cautioned thatsuch data are applicable only to airplanes of speed wing loadingsize and mission similar to those in which the data were obtained

The author of Ref 26 indicates that Gassner of Germany hadwarned as early as 1941 that if the higher static strength of thenew aluminum alloy (7000-type) were taken advantage of a shorterfatigue life would result One goal of the researchers of that timewas to obtain suf cient data to be able to reduce factors of safetythat resulted from ignorance of the strength of materials

An International Committee on Aeronautical Fatigue wasfounded in 1951 at the suggestion of Plantema of The NetherlandsIt was two UK Comet crashes in 1954 and a series of US B-47catastrophic accidents in 1958 however that focused US Air Forceattention on structural fatigue and led to The US Air Force AircraftStructural Integrity Program (ASIP) This program adopted a fatiguemethodology approach commonly referred to as safe-life This ap-proach was used in the development of US Air Force aircraft inthe 1960s but was not adequate from the standpoint of addressingmanufacturing and service-induced damage Consequently aircraftdesigned both prior to and subsequent to the safe-life method contin-ued to experience premature failures The US Air Force ASIP wasrevised to include the incorporation of a damage tolerancedurabilityphilosophy into the design process Although initially applied tothe B-1A and modi cations of the F-111 C-5A and F-4 the dam-age tolerancedurability requirements were formally established inMIL-STD-1530A in 1975

The US Navy ASIP is similar It requires fatigue life of thestructure to be designed such that failure of a full-scale test articlewill not occur and that the structure will be free from any defectssuch as cracks deformation loss of modulus disbonding or fastenerhole deformation and delamination when subjected to a minimumof two lifetimes

Spurred on by joint service airframe requirements of the JointAdvanced Strike Technology program the services have developeda joint service guide speci cation for aircraft structures

All US Air Force aircraft since the F-16 have been subject todurabilitydamage tolerance requirements Each of the tests requiredby ASIP has served to identify de ciencies in structural design andanalysis resulting in modi cations to the structure that provided asafer and more supportable airframe

The full-scale airframe static fatigue durabilitydamage toler-ance tests are expensive and therefore there should be a reason-able expectation that they can with minor design modi cationsbe accomplished successfully This generally means conducting anextensive structural element and subcomponent test program in sup-port of the ASIP These tests (Table 4) address all of the critical loadpaths

This type of building block program that is testing large numbersof coupons elements and components prior to full-scale hardwarehas evolved as the most logical risk-reduction approach for newsystem airframe development The consequences of a failure late inthe development cycle of a system are too great to proceed any otherway

Table 4 Airframe design development tests

Aircraft Elements tested

B-2 160000 nonmetals 1450 metallic 6 leading edge74 wing panels 40 engine exhaust panels27 joints

C-17 6000 coupons 400 componentsF-22 Allowables

14500 composites 5300 metalsStructural integrity

4100 metals 8800 compositesReproducibility

80 full and subscale composite 15 titaniumstructures

FA-18 EF 10000 materials characterizationprocessing tests165 elements and components

V-22 9700 coupons 550 elements and components

The opposite is true for advanced technology demonstrators(ATDs) which introduce technology through prototyping thus ac-celerating transition of new technologies Failure in an ATD pro-gram is more accepted as a lesson learned rather than a failure ofthe contractor and procuring agency

Table 4 indicates that composite structures require more extensivedesign and certi cation testing than metals The additional testingrequirements add cost to the design allowables and design certi -cation efforts This was clearly identi ed in Ref 29 published inNovember 1994 as a lesson learned

In the case of composite structural assemblies however whenthe structure satis es all static load conditions fatigue is generallynot a problem The effects of moisture and temperature on designmargins can be established at the coupon and subcomponent levelwith the full-scale component margins adjusted appropriately

The one thing the described programs had in common was a sub-stantial nite element analysis effort (both global and local detailmodels) to support test hardware design and to certify the airframefor adequate design margins These programs demonstrate the im-portance of a thorough analysis effort coupled with a complete criti-cal area design development test program to substantiate the designIt would be unwise and uneconomical to dispense with either

Analysis and Design TechniquesThe introduction of digital computers in the 1950s paved the

way for the now emerging multidisciplinary design technologiesapplicable to the integrated design of a variety of engineering prod-ucts including air and space vehicles This section is a brief outlineof the developments that led to such software systems as ASKASTARDYNE NASTRAN ANSYS ABAQUS and ASTROS

Before discussing the chronology and signi cance of structuralanalysis developments it is appropriate to distinguish between twolevels of analysis 1) analysis of structural elements and 2) analysisof builtup structures

Analysis of Structural Elements

Structural elements or components can be categorized into threetypes line elements surface elements and solids The analysis ofsuch elements has been developed and documented over the last150 years by Euler Lagrange St Venant and others References 30and 31 document the chronology of these developments Over adozen solid mechanics books by Timoshenko constitute a rich com-pendium of analysis methods for structural elements The rangeand applicability of this continuum approach (the solution of dif-ferential equations) is however limited to very restricted cases ofloading boundary conditions and element shapes Such methods asRayleighndashRitz Galerkin and nite differences are approximationsThese methods replace a continuum model by a discretized equiva-lent The result is a set of algebraic equations that can be solved bydigital computers

Analysis of Builtup Structures

Builtup structures are simply constructed out of many structuralelements joined together at their boundaries The differential equa-tion approach for the solution of builtup structures is intractable Asan alternative a number of discretized procedures have been devel-oped since the early 1900s A structure is considered determinate ifits internal forces can be solved by the force and moment equilib-rium equations alone Otherwise it is considered an indeterminatesystem Analyzing complex indeterminate systems was a dauntingtask before the advent of digital computers

The force and displacement methods that emerged in the 1950swere a major breakthrough in structural analysis They refer to thediscretized model of the structure in which the continuum is re-placed by a nite number of grid points Each grid point is assumedto have six degrees of freedom (DOF) in space These DOFs areforces andor displacements In the force method the unknowns arethe set of forces released to make the indeterminate structure deter-minate Enforcement of compatibility conditions results in force-displacement relations connected by a exibility matrix In thedisplacement method the unknowns are the displacements of the

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 9: Evolution of U.S. Military Aircraft Structures Technology

26 PAUL ET AL

grid points and the applied forces are the known quantities The twoare connected by the stiffness matrix of the structure Collectivelythe stiffness and exibility (displacement and force) methods arelabeled as matrix structural analysis The six papers in Ref 32 byArgyris and Kelsey provided a comprehensive outline of these meth-ods in 1954 Matrix structural analysis developed along two distinctpaths force method and displacement method Each has advantagesand drawbacks in their application to complex structures

The publication of the nite element concept by Turner et al33 in1956 is a signi cant event in the development of automated struc-tural analysis The basic premise of the nite element formulation isto divide the continuum into small domains and view the total systemas a summation of the small domains called the nite elements Thestiffness of these elements can be generated independently with thetotal system stiffness matrix assembled by a summation after the ap-propriate coordinate transformations This procedure provided thenecessary breakthrough for automated analysis by computers

The pace of nite element analysis development was acceleratedby the three matrix methods conferences sponsored by the USAir Force Flight Dynamics Laboratory (AFFDL) and the Air ForceInstitute of Technology in 1965 1968 and 197134iexcl36

In the 1960s several government laboratories and aerospace com-panies initiated the development of large-scale structural analysissoftware AFFDL sponsored the development of the FORMAT soft-ware system at Douglas Aircraft Company and the MAGIC softwaresystem at Bell Aerospace Company FORMAT and MAGIC pro-vided the motivation and imputus for the development of general-purpose structural analysis software pursued by NASA in 1964

The combination of the NASA Goddard Research Center lead-ership the Computer Sciences Corporationrsquos system software ex-pertise and MacNeal and Swendler Corporationrsquos solution expe-rience of practical problems produced the software system calledNASTRAN NASTRAN was rst released in 1968 The commer-cial version of NASTRAN was started in 1972 under the name ofMSCNASTRAN It has emerged as truly general-purpose softwareby making the model input independent of the details of the mathe-matical operations internal to the software It now contains extensivenonlinear analysis heat transfer aeroelasticity and thermal analysismodules A more complete discussion of this development may befound in Ref 37 It in turn provided the imputus for the developmentof such programs as the structural analysis programs (SAP) seriesANSYS and ABAQUS

In 1960 Lucien Schmit proposed coupling nite element analysisand mathematical programming methods to accomplish structuraldesign optimization The optimization problem is stated as a min-imization or maximization of an objective function subjected to aset of behavioral andor manufacturing constraints Three basic ele-ments of an optimization problem are 1) de nition of a performanceor objective function 2) de nition of a set of constraints and 3) iden-ti cation of the variables that de ne the functions The variablesde ne an n-dimensional space in which the search for the optimumcan be carried out by various linear and nonlinear programmingmethods There are a variety of gradient and nongradient methodsavailable to carry out this search for the optimumReference 38 givesa comprehensive review of these methods and software programs

The scope of structural optimization expanded into multidisci-plinary design optimization (MDO) in the 1980s and 1990s MDOis de ned here as an optimization problem where the constraintsandor objective functions are derived from more than one disci-pline In the context of airframe design optimization the constraintsand the objective functions are coming from static and dynamicaeroelasticity such as lift and control surface effectiveness utterdivergence stresses displacements frequencies and possibly fromelectromagnetics as well There are now a number of commercialprograms available for MDO ASTROS ELFINI LAGRANGENASTRAN and GENESIS are some examples These softwaredevelopments also spawned extensive development in pre- and post-processing software Some examples are PATRAN IDEAS andHYPERMESH

The nite element based computer-aided design tools availabletoday not only improve the design teamrsquos productivity but facilitate

Table 5 Concurrent engineering teams

Aircraft Number IPTs

Lockheed F-22 84 (divided into several tiers)BoeingBell V-22 72McDonnellNorthrop FA-18EF 407General Electric F414 46 secondary 8 primary integrated

concurrent engineering designdevelopment teams

Pratt and Whitney F-119 100

exploration of several design concept options and assessment ofthe design conceptrsquos sensitivity to design modi cations and moreimportant their impact on system cost

We have evolved from aerospace companies having their ownproprietary nite element programs to commercial nite elementsoftware systems for integration of the nite element method intomultidisciplinary tools capable of automated optimum design in aconcurrent engineering environment

The one area that can be considered as having undergone a revolu-tionary change in a relatively short period of time is the applicationof CADCAM and computer-aided engineering tools Automateddesign tools implemented by integrated product development teamshave truly revolutionized the design process

Concurrent engineering facilitated by computers is providingthese designbuild teams with the disciplines necessary to designtool and manufacture major structural assemblies Examples of suchteams are given in Table 5

For those of us who have watched the slide rule replaced by thecomputer the drafting table replaced by workstations blueprintsreplaced by digital data representations and physical mockups re-placed by electronic models it has truly been revolutionary Con-current engineering implemented by integrated product teams sup-ported by computer-aided design tools has brought to the forefronta more balanced design process Design teams are able to integratethe sometimes con icting requirements for increased performanceaffordability maintainability and survivability

FutureHaving said that aircraft structural design is evolutionary not rev-

olutionary does not mean that there has not been speci c technolo-gies that have had signi cant impact in the past and others that arepresently in an early stage of development that will have profoundimpact in the future Of the technologies currently under develop-ment there are three areas that offer the potential for revolutionarychange to aerospace structures

These areas are multifunctional structures simulation-based pro-totyping and affordable composite structures These technologieshave the potential to produce step increases in airframe ef ciencyand functionality

Multifunctional Structures

Multifunctional structures include concepts that extend airframefunctionality to perform tasks beyond load reaction to increase sur-vivability lethality and aero- and thermal ef ciency and to reducemanufacturing cost while maintaining or improving reliability sup-portability and repairability Candidate technologies include con-formal load bearing antennas integral heat exchangers uidic jetsfor ight control and thrust vectoring embedded electro-optical sen-sors and defensive shields for high-power lasers and microwaves

Multifunctional structures may contain actuators and sensors thatwill allow them to alter their mechanical state (position or velocity)and or mechanical characteristics (stiffness or damping)

Bene ts of such structures include aeroelastic control load al-leviation and elimination of detrimental dynamic oscillations atreduced structural weight while simultaneously achieving a struc-tural integrity equivalent to present safety requirements

Multifunctional structures are in an early stage of developmentbut already bene ts can be foreseen in reduced life-cycle costand reduced direct operating cost through improvements in both

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 10: Evolution of U.S. Military Aircraft Structures Technology

PAUL ET AL 27

Fig 12 Step functions in airframe ef ciency

performance and maintainability Activeadaptive structures struc-ture health monitoring and structureavionics integration are threeareas presently being pursued

There is a potential to reduce inspections on both new and repairedairframes thereby reducing maintenance costs Eventually multi-functional structures are expected to develop to the point where theycan facilitate on-demand and in situ monitoring of damage Oncethey become reliable enough costly airframe teardown inspectionsneed only be performed when there is a fault indication

All three elements sensing processing and actuation must besupported and matured simultaneously before the stepwise improve-ment in airframe ef ciency shown in Fig 12 is realized A multi-functional airframe that integrates antenna functions and electroniccountermeasures is a future step increase in technology with signif-icant bene ts for both air and space structures

The concept of a structure with the capability of sensing andautomatically responding to its environment is one which offersthe potential of extremely attractive advantages in the operation ofstructures in any environment

Simulation-Based Prototyping

Solid modeling coupled with feature-based design software andadvanced visualization technology is already enabling the designerto change design variables and to evaluate the effect of these changeson the response characteristics of the structure in real time It hasbecome commonplace to display stress contours deformations andvibration mode shapes in computerized color graphical depictionsfor highlighting critical areas

ASTROS is an example of a tool that has been developed for facil-itating integrated product design ASTROS provides a mechanismfor effective communication across different disciplines includingaerodynamics ight controls and electrodynamics In the futurethere will be a system of design tools that will facilitate virtualprototyping and enable simulation of advanced technologies andcon gurations before physical ight

Near-term projected applications of simulation-based prototyp-ing include cockpit design maintainability assessments and morecost-effective training More relevant to the airframe developer isthe potential to explore human interaction with the design before itis manufactured

Alternative con gurations can be explored relative to their ease ofmanufacture and ease of assembly With the capability to immersethe designer the pilot and or maintainer in the design customerfamiliarity with the product can begin before it is produced

In the concurrent engineering arena CADCAM software will domore than facilitate communication it will be applied to both designof subsections and assemblies Design-to-build team members willbe able to electronically interact to modify the same digital modelVirtual collaboration engineering already exists for true integratedproduct and process development and real-time visualization andevaluation of design concepts and manufacturing processes in aseamless simulation environment Many more alternative structurallayouts will be able to be explored in the conceptual design phase

Shared databases will enable the design data to pass directly tonumerical controlled programming

New powerful microprocessors will enable logistics personnelwith laptops to conduct structural health monitoring and supportedby knowledge-based expert systemsand neuralnetworks to evaluatethe signi cance of damage on the life of the system

Affordable Composite Structures

Advanced composite structures can facilitate a high degree ofsubsystem integration because their mechanical thermal and elec-tromagnetic properties are tailorable and amenable to embeddedsensors actuators and subsystems

Composites are becoming cost effective in subelement areaswhere they have not been in the past Rapid progress in textilesubelements fabricated by resin lm infusion and RTM of braidedand woven preforms is being made Fuselage frames and cutoutreinforcements are near-term applications Because textile compos-ites provide integral reinforcement in the Z direction signi cantimprovements in intralaminar and interlaminar strength can be ob-tained to react and to transfer out-of-plane loads These advantagesshould offset the disadvantages of lower stiffness and compressionstrength for braided and woven composites vs laminates

The potential payoff of applying textile technology to sandwichstructures will allow elimination of the skin-to-core adhesive bondproblems Because there is an integral link between upper and lowerface sheets there is no debonding Impact damage is minimizedbecause through the thickness bers serve to block delaminationgrowth and thereby localize damage

Another approach to improving the out-of-plane properties oflaminated composite structure is Z-pinning technology Z pinninghas been developed by the Aztex Corporation in cooperation withthe US Air Force Research Laboratory the US Naval Air WarfareCenter and airframers

The technology introduces reinforcing pins through the thicknessThe pins provide increases in pull-off loads and offer a mechanicalinterlocking capability to inhibit crack propagation and provide afailndashsafe linkage if a crack initiates

Z-pinning technology uses an ultrasonic insertion device to pressdiscrete pins into composite laminates with a handheld or auto-mated insertion capability The US Air Force and Navy are sup-porting programs to characterize the structural and cost bene ts ofthis technology Speci c focus has been directed at understandinghow the technology can be used to modify failure criteria in com-posites enhance ballistic survivability and reduce cost through theelimination of fasteners

Electron-beam (E-beam) curing offers great potential for lower-ing composite structure processing costs Cationic epoxy resin com-posite parts can be nonautoclave cured in minutes In the electronbeam process the electronrsquos kinetic energy is deposited directly inthe material volume rather than by surface heating and thermal dif-fusion Both aircraft and space vehicle structures are being producedby this method of processing Examples are a 14-in-diam 3-ft-longintegral fuel tank for the US Armyrsquos Longfog tactical missile by

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 11: Evolution of U.S. Military Aircraft Structures Technology

28 PAUL ET AL

Oak Ridge National Laboratory and an Aerospatiale lament woundrocket motor case The latter program did not employ resins or pro-cessing techniques required to produce structures with the propertiesand quality required of airframes It did demonstrate the feasibilityto produce structures limited in size only by the facilities requiredfor shielding E-beams At its present stage in development E-beamprocessing technology is far from mature and has to be characterizedas high risk but with high potential to reduce processing costs

Compression strain allowables after impact still design most com-posite structures However design concepts based on hybrids ofglass and graphite and the use of textile preforms are showing thepotential to eliminate durabilitydamage tolerance as a design driverfor many fuselage structural members The importance of this is sig-ni cant in light of the damage tolerance durability issues of presentmetallic structure

Composite airframe applications will continue to grow at thesteady pace of the past A major increase in the use of compositeswill be in the automobile industry and civil infrastructure Becauseof the magnitude of these applications lower cost composite ma-terials will become available for aerospace use Also the emphasison lower cost airframes will encourage use of innovative designthat exploits low-cost manufacturing techniques and up-front sys-tem arrangements that allow optimum load paths The combinationof these two developments will fundamentally shift the airframecost vs weight curve

The design optimization of composites has already facilitatedaeroelastic tailoring whereby coupled bending and torsional defor-mations are employed constructively to improve aerodynamic ef -ciency The integration of computational structural mechanics withother disciplines such as uid dynamics and controls can enable theimplementation of adaptive structures with multifunctional capa-bilities This will require the maturing of multidisciplinary designtools with the ability to analyze controlndashstructure interactions

Such design tools will enable us to do a better job of solving theproblems that nature has already solved In 1845 Foucault observedthe following39

Man made machines have a great number of parts entirely dis-tinct one from the other which only touch each other at certainload transfer points In animals all the parts adhere together thereis a connection of tissue between any two given parts of the bodyThis is rendered necessary by the function of nutrition

Simulation-based prototyping will provide the mechanism bywhich these new design concepts can be developed and tried outin a low risk but yet realistic environment The following vision ofthe authors of Ref 40 will then be reality ldquoA Virtual Demonstra-tionDesign process which enables technology validation in virtualspace with user con dence and acceptance In other words all as-pects of modeling and simulation can be developed to the point thata technology can be developed and matured by analysisrdquo

ConclusionsIf airframes for future systems are established at the conceptual

level on the basis of historical trends as has been done in the pastfor aircraft then basic approaches to react external loads load pathde nition and adjacent structural member joining will be accom-plished in the same manner as the last 60 years that is the rivetedskinstringerframe concept will be perpetuated

Todayrsquos 165-ft wing span 585000-lb TOGW C-17 transport uti-lizes the same basic aluminum stressed skin 3-spar multirib structuredeveloped for the 1935 DC-3 95-ft wing span 24000-lb TOGWtransport

The signi cant advancement in structural ef ciency required foraffordable spacesystems will require breaking from the skinstringerframe concept mold Employing new-engineered materials such asadvanced composites and advanced alloys in existing structural con-cepts does not exploit their full potential

Multifunctional structure design is leading the way to new inno-vative concepts as revolutionary as Northroprsquos idea of utilizing thewing covers to carry bending loads

Adaptive structures capable of active load control compensatingfor damage or improving aerodynamic ef ciency are facilitated bymultifunctional structures and in turn are an enabling technologyfor adaptive aircraft The development of multifunctioning adap-tive structures exploiting engineered material capabilities should bethe forcing function to break from the traditional skinstringerframemold Although still in an early development stage design synthesisand optimization tools are being developed that can graphically dis-play stress contours deformations and mode shapes for many struc-tural arrangements This results in more ef cient structural memberplacement

Engineered materials and tailored adaptive structures will not inthe future be buzzwords but rather enabling technology for morestructurally ef cientaffordable airframes

Wilbur Wright 13 May 1900 noted ldquoIt is possible to y withoutmotors but not without knowledge and skill This I conceive tobe fortunate for man by reason of his greater intellect can morereasonably hope to equal birds in knowledge than to equal naturein the perfection of her machineryrdquo

At the 15th National Space Symposium 5ndash8 April 1999 GoldinNASA Administrator indicated that NASA was looking for biolog-ically inspired solutions in just about everything NASA does Hedescribed the lowly cockroach as follows

Its brain is much simpler than the human brain yet it is ableto operate a six-legged transporter mechanism operate light andthermal sensors operate gradient detectors such as a rapid changein light levels deploy escape mechanisms control a survival sys-tem (lays eggs before dying when poisoned) and has food searchcapabilities We have almost no clue to programming such a com-plex device (and have it regenerate itself)

References1Hoff N ldquoInnovation in Aircraft Structures Fifty Years Ago and Todayrdquo

AIAA Paper 84-0840 19842Gibbs-Smith C Aviation An Historical Survey from Its Origins to the

End of World War II Her Majestyrsquos Stationery Of ce London 1970 p 1623Coleman T Jack Northrop and The Flying Wing The Story Behind the

Stealth Bomber Paragon House New York 1988 p 164Howe P ldquoQueen of the Transportsrdquo Flying Magazine Vol 38 No 4

1946 p 275Miller R and Sawers D The Technical Development of Modern Avi-

ation Praeger Publishers New York 1970 pp 65 666Thetford O Aircraft of the Royal Air Force 1918ndash58 Putnam London

1958 p 4367Rouse M and Ambur D ldquoDamage Tolerance and Failure Analysis of

Composite Geodesically Stiffened Compression Panelrdquo Journal of AircraftVol 33 No 3 1996 pp 582ndash588

8Hoff N and Mautner S ldquoSandwich Constructionrdquo Aerospace Engi-neering Review Vol 3 No 8 1944 pp 29ndash63

9Rhodes T ldquoHistory of the Airframe Part 3rdquo Aerospace EngineeringVol 9 No 9 1989 p 32

10Petrushka E ldquoStructural Evolution B-58 to F-16rdquo AIAAAir ForceMuseum Dayton OH April 1985 p 4-2

11Logan T and Soudak U ldquoBonded Aluminum Honeycomb AircraftFlight Surface Primary Structure Applicationrdquo Journal of Aircraft Vol 20No 9 1983 p 805

12Barbe M ldquoMetal Stitching of Aircraft Partsrdquo Aero Digest Vol 41No 5 1942 p 118

13Miller J and Thrash P ldquoFabrication Assembly and Checkout of anAdvanced Stitching Machinerdquo 11th Dept of DefenseNASAFederal Avi-ation Administration Conf on Advanced Composites in Structural DesignWL-TR-97-3009 Vol 2 Oct 1996 pp 13-55ndash13-73

14Janersquos All The Worlds Aircraft New York 1932 p 291c15Junkers H ldquoMetal Airplane Constructionrdquo Journal of the Royal Aero-

nautical Society Vol 27 Sept 1923 p 43216Loudy F Metal Airplane Structure New York 1938 p 38317Langley M ldquoThe History of Metal Aircraft Constructionrdquo Aeronauti-

cal Journal of the Royal Aeronautical Society Vol 75 Jan 1971 p 2818Barnes C Shorts Aircraft Since 1900 Fallbrook California 1967

p 16319Lincoln J ldquoLife Management Approach for USAF Aircraftrdquo AGARD

CP 506-Structures and Materials Panel Fatigue Management Conf BathUK Dec 1991 p 17-2

20Pope G ldquoStructural Materials in Aeronautics Prospects and Perspec-tivesrdquo 10th European Pioneers Day Lecture Paris France 25 April 1985

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6

Page 12: Evolution of U.S. Military Aircraft Structures Technology

PAUL ET AL 29

also Zeitschrift fuer Flugwissenschaften und Weltraumforschung JournalVol 9 SeptndashOct 1985 p 282

21Crossly F ldquoAircraft Applications of Titanium A Review of the Pastand Potential for the Futurerdquo AIAA Paper 81-0893 May 1981

22Johnson C ldquoDevelopment of The Lockheed SR-71 Blackbirdrdquo Lock-heed Horizons Burbank California Winter 19811982 p 15

23Tuegel E Pruitt M and Hefti L ldquoSPFDB Takes Offrdquo AdvancedMaterials and Processes Vol 136 July 1989 p 37

24Simonds J E ldquoPlastics in Aircraftrdquo Aviation Magazine Vol 140No 5 1941 pp 75 124

25Negaard G ldquoThe History of the Aircraft Structural Integrity ProgramrdquoASIAC Rept 6801B WrightndashPatterson AFB June 1980 p 4

26Schutz W ldquoA History of Fatiguerdquo Engineering Fracture MechanicsVol 54 No 2 1996 p 263

27Jenkin C ldquoFatigue in Metalsrdquo Journal of the Royal Aeronautical So-ciety Vol 27 March 1923 p 101

28Bland R and Sandorff P ldquoThe Control of Life Expectancy inAirplaneStructuresrdquo Aeronautical Engineering Review Vol 2 Aug 1943 p 8

29Vosteen L and Hadcock R ldquoComposite Chronicles A Study of theLessons Learned in Development Production and Service of CompositeStructuresrdquo NASA CR 4620 Nov 1994 p 14

30Todhunter I and Peterson K History of the Theory of ElasticityCambridge Univ Press England Vol 1 1886 and Vol 2 1893

31Timoshenko S History of Strength of Materials McGrawndashHill NewYork 1953 pp 1ndash439

32Argyris H and Kelsey S Energy Theorems and Structural Analysis(collection of six papers published in Aircraft Engineering 1954 and 1955)Vol 26 No 308 Oct 1954 p 347 Vol 26 No 309 Nov 1954 p 383

Vol 27 No 312 Feb 1955 p 42 Vol 27 No 313 March 1955 p 80Vol 27 No 314 April 1955 p 125 Vol 27 No 315 May 1955 p 145Butterworths London 1960

33Turner M Clough R Martin C and Topp L ldquoStiffness and De ec-tion Analysis of Complex Structuresrdquo Journal of the Aeronautical SciencesVol 23 No 9 1956 pp 805ndash823

34Proceedings of the Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-66-80 WrightndashPatterson AFB OH Nov 1966

35Proceedings of the 2nd Conference on Matrix Methods in StructuralMecahnics US Air Force Flight Dynamics Lab AFFDL-TR-68-150WrightndashPatterson AFB OH Dec 1969

36Proceedings of the 3rd Conference on Matrix Methods in Structural Me-chanics US Air Force Flight Dynamics Lab AFFDL-TR-71-160 WrightndashPatterson AFB OH Dec 1973

37Schmidt L ldquoStructural Design by Systematic Synthesisrdquo Proceed-ings of the 2nd National Conference on Electronic Computation AmericanSociety of Civil Engineers New York 1960 pp 105ndash122

38Venkayya V ldquoStructural Optimization A Review and Some Recom-mendationsrdquo International Journal for Numerical Methods in EngineeringVol 13 No 2 1978 pp 205ndash228

39Marey E ldquoAnimal Mechanisms A Treatise on Terrestrial andAerial Locomotionrdquo Chapter VII Animal Mechanisms New York 1874p 67

40Moorhouse D and Paul D ldquoThe Use of Technology Demonstratorsto Reduce Systems Acquisition Costrdquo Future Aerospace Technology in theService of the Alliance Ecole Polytechnique Palaiseau France AGARDCP 600 Affordable Combat Aircraft Vol 1 Dec 1997 p A2-6