Design of a Variable Span Wing for an Unmanned Aerial Vehicle

Marcelo Antonio Borralho da Silveiramarcelo.silveira@ist.utl.pt

Instituto Superior Tecnico, Universidade de Lisboa, Portugal

July 2019

Abstract

Morphing aircraft technologies provide a strategy to change and adapt the aircraft shape to theflight conditions. This approach to aircraft design aspires to find efficiency improvements for futureaircraft. The variable span telescopic wing offers the characteristics of a large span wing with highaspect ratio desirable for efficient low speed flight while accommodating those of a smaller span wingwith low aspect ratio for better flight performance at high subsonic speed. This work presents thedesign of a telescopic wing with span ranging from 3.5m to 5m for a medium-endurance patrol andsurveillance 150kg unmanned aerial vehicle (UAV). Different telescopic wing mechanisms were studiedand described. Methodology for calculating aerodynamic loads, maximum structural stresses and sizingof the telescopic spar elements, based on analytical formulas and aerodynamic calculations using thelifting line theory, is present. The aircraft structure is designed to sustain a load factor of 3.8g withan ultimate load factor of 5.7g. The performance of the variable span wing aircraft was assessed andcompared in terms of drag with two fixed wing aircraft with 4.3m and 5m wingspan respectively. Themorphing wing outperforms the 5m fixed wing for 150kt dive speed with 23% drag reduction and canallow for a 68% increase in mass until no performance benefit is noticed. Compared with the the 4.3mfixed wing the drag reduction achieved is up to 16% at loiter speed of 70kt with weight penalty of 13%.Keywords: Morphing wing, variable span, UAV, structural sizing, performance.

1. Introduction

The aeronautical industry currently focuses ondeveloping an environmentally efficient aviationwith work and research being done on reductionof emissions, engines and alternative fuels, air traf-fic management, safety aspects of air transport, etc.Morphing technologies belong to the new fields andapproaches to aircraft design that strive to find im-provements in aircraft efficiency.

Morphing is referred as ”a set of technologies thatincrease a vehicles performance by manipulatingcertain characteristics to better match the vehiclestate to the environment and task at hand” by Bow-man et al. [1]. Using this definition, technologiessuch as flaps, slats, retractable landing gears andothers associated with conventional aircraft wouldbe considered morphing technologies. This contra-dicts the connotation of radical changes in shape orshape changes that require futuristic technologiesand materials that morphing carries.

Regardless of the type of technology and its ex-tent, materials, actuators and structures it may usethe objectives and aimed capabilities of morphingtechnologies are identified accordingly to Mcgowanet al. [2]: to increase ”the adaptability of the vehi-cle to enable optimized performance at more than

one point in the flight envelope”. From a designstandpoint, this means a morphing aircraft can beview as multiple different vehicles. Whereas con-ventional aircraft commonly are optimized for justone design point, [3, 4].

Conventional aircraft are generally optimized to-wards a single design point therefore this character-istic of morphing aircraft enables them to performthe missions of several conventional aircraft. Addi-tionally, this ability of having a design optimized formore than one flight phase allows a morphing air-craft to perform more missions efficiently or evento perform a mission that would require more thanone conventional aircraft, [2].

Aircraft performance is affected by diverse geo-metric wing parameters. As an example, high sweptwings are more adequate for transonic and super-sonic flight while low swept wings are capable ofgreatly efficient low speed flight. This possibilitieshave been motivating engineers to go further on thedevelopment of ambitious morphing concepts, [5].In the future, morphing aircraft technologies, ac-cordingly to Rodriguez [6], will be used to: ”1. im-prove aircraft performance to expand its flight en-velope; 2. replace conventional control surfaces forflight control to improve performance and stealth;

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3. reduce drag to improve range and; 4. reducevibration or control flutter”.

The design of morphing mechanisms up totheir implementation faces the difficult task ofnot only providing aerodynamic efficiency improve-ments but also keeping the aircraft structurallystrong, lightweight and controllable. The necessityto adapt shape to the flight regime means the inter-nal structure of morphing aircraft should be able tochange in an efficient and reversible manner. Theskin should be able to accommodate the change ofinternal structure by either sliding or being flexiblewhich renders it a virtually non-load-carrying ele-ment and thus degrades the overall structural stiff-ness. However, the skin still needs to be strongenough to transmit the aerodynamic loads. Ad-ditionally, surface continuity and smoothness andguaranteeing that the gaps and seams are minimalin all aircraft configurations is deeply related to howaerodynamically efficient the aircraft can be. Thesecomplexities lead to a heavier structure and accord-ingly to Jha and Kudva [5] ”demand new effectiveways of changing structural shape with less actu-ation force, along with materials with good struc-tural properties: high strain with no plastic defor-mation, good fatigue resistance, low environmentalimpact, etc”.

The extend to which morphing technologies canimprove the performance of an aircraft is closely re-lated to its role and whether it is frequently changedthroughout the aircraft lifetime and missions. Amulti-role aircraft can benefit greatly from morph-ing technologies that can adapt it to the situationat hand, while an aircraft that remains in the sameflight conditions throughout the majority of its ser-vice time has less to gain by adapting to the sec-ondary conditions.

2. Span Morphing Background

Aircraft morphing concepts are differentiatedinto different categories: wing, fuselage/tail and en-gine morphing. Within the wing concepts are asso-ciated with the change of a particular wing geo-metrical parameter. Three major categories can bedefined: planform change, out-of-plane and airfoil,see figure 1.

The three main parameters that affect the wingplanform are span, chord and sweep. Both chordand span have a directly proportional effect on wingplanform area. As span increases so does the wingarea. But opposite to the chord’s effect, span in-crease also raises the aspect ratio of the wing, a pa-rameter that alters the lift to drag ratio. A higheraspect ratio will result in increased wing efficiencyand both increased range and endurance. Aircraft’sinertia properties also change with a larger span.At low speed induced drag is the dominant drag

Figure 1: Wing morphing concepts types, [3]

component; however, at high flight speeds the liftcoefficient decreases and the friction drag compo-nent surpasses the induced drag component. Thus,aircraft with large wing span and high aspect ratiowill generally have good range and fuel efficiency,but lack maneuverability and have relatively lowcruise speeds. On the other hand low aspect ra-tio aircraft are faster and highly maneuverable, butlose out on aerodynamic efficiency, [7, 8].

A variable-span wing can potentially integrateinto a single aircraft the advantages of both a highspan configuration and a short span one. This mor-phing technology has aroused interest specially formilitary UAVs, which must loiter for extended peri-ods of time during surveillance and be able to switchinto high-speed dash mode to move reconnaissancearea or to attack a target. An increase in span ac-companied by the respective increase in aspect ratioand wing area results in a decrease in the lift dis-tribution for the same flight condition, i.e., for thesame overall lift. Nevertheless, bending moment atthe wing root can considerably increase leading toone major drawback. The need of a heavier wingstructure to support it. ”Thus, both the aerody-namic and the aeroelastic characteristics should beinvestigated in the design of variable-span morph-ing wings”, [3, 4].

3. Telescopic Wing Design Strategy

The aim of this work is to design a telescopicwing for a surveillance UAV and evaluate its feasi-bility and performance. The design process is donein three major steps: First, for a starting point theinitial sizing of an aircraft platform and a typicalmission are defined. Then, concepts for the morph-ing mechanism are develop, selected and described.Finally, a structural analysis, based on analyticalformulas and aerodynamic calculations using thelifting line theory, is approached.

3.1. Design FrameworkAs a starting point, the initial sizing of a medium

endurance and medium altitude UAV platform isdetermined. The UAV model AR5 from TEKEVERis used as reference both in terms of dimensions and

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performance. As the information gathered aboutthis aircraft is limited further market research ofsimilar size and performance aircraft was realizedto find adequate wing loading ratio.

The aircraft main specifications are displayed ontable 1. The key characteristic of the aircraft isthe variable span. The wing is set up to vary al-most continuously its span between 5m and 3.5m,respectively the maximum and minimum span con-figurations. Some considerations were made aboutthe desired performance of the UAV and are de-tailed in table 2.

Table 1: Aircraft specifications

Takeoff mass [kg] 150Length [m] 3.5Span [m] 3.5-5Chord [m] 0.75Wing area [m2] 3.75Aspect ratio 4.7-6.7Tail configuration Inverted V-tailTail span [m] 1.715Tail chord [m] 0.429

Table 2: Aircraft desired performance

Cruise speed [kts] 95Cruise Altitude [m] 3,000Loiter Speed [kts] 70Dive Speed [kts] 150Stall Speed [kts] 50Maximum climb rate [m/s] 3Lift-to-drag ratio 20Take-off roll distance [m] 150

Figure 2: Mission profile

The airfoil profile is one of the most impor-tant aspects of the wing, and has direct impacton the aerodynamic performance. As the aircraftit self, the ideal airfoil performs exceptionally wellin the dominant flight phases, loiter and cruise,and does not limit aircraft performance. The cho-sen Seiling S4233 low Reynolds airfoil and several

low Reynolds (Re) airfoils were considered and thesoftware XFLR5 was used to evaluate their perfor-mance. This software provides several tools to ana-lyze and study both airfoils and wings. The airfoilanalysis tool set is a version of the Xfoil softwareoriginally developed by Mark Drela compiled withC++ and C programming languages. For struc-tural reasons: to ensure enough internal space forstructural elements, airfoils with maximum thick-ness (t/cmax) inferior to 10% of the chord were notconsidered. In the end the Seiling S4233 airfoil (il-lustrated in figure 3) was chosen due to its betteroverall performance.

Figure 3: Seilig S4233 low Reynolds airfoil

3.2. Telescopic WingThe objective of the span morphing concept is to

achieve an efficient wing that can present the de-sired span to better suit the flight condition andto expand the aircraft flight envelope. The tele-scopic wing is intended to provide a maximum wingspan of 5m to the aircraft and a minimum spanof 3.5m. Although the comparison between themaximum and minimum wing span configurationshows the more drastic differences in performanceand flight characteristics there are further benefitsto be explored from the intermediate wing con-figurations. Therefore, the ability to provide anywingspan length between the maximum and mini-mum configurations is desired.

Some wing concepts were considered:Concept A is based on the AdAR design devel-

oped in [9]. The design span morphing capability iscarried out by a telescopic spar. While the structureof the inboard part of the wing is fixed and of con-ventional characteristics, the outboard part of thewing includes sliding ribs and a compliant morphingskin made from a elastomeric matrix composite, seefigure 4. Strained elastic materials experience thePoisson effect; Thus carbon fiber reinforcements areapplied to the skin chord wise to reduce the Poissonratio of the material and the deviations from thedesired aerodynamic shape the wing might suffer.Additionally, a zero Poisson mesh made from poly-mers can be used to provide airfoil shape supportto the skin at different wingspans. Span extensionis actuated by a strap drive system powered withand electrical engine. Due to the compression forcesintroduced by the elastomeric skin the span retrac-tion dispenses an actuation force oriented with themovement of the outboard spar.

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Figure 4: Plant view of span morphing concept A(extended)

The mechanism responsible for varying the spanof concept B is similar to the previous concept;However, instead of using a compliant skin struc-ture, a telescopic rigid skin is adopted, figure 5.The structural loading is carried by a single tele-scoping spar and the shape of the airfoil surface ismaintained with sliding ribs throughout the inboardpart of the wing. The contact between the slidingribs and the skin is intermediated with a low frictioncoefficient material to facilitate smooth movement.The outboard part of the wing that is connected tothe moving beam of the main spar has conventionalfixed structure and its skin slides from inside theinboard telescoping skin.

Figure 5: Plant view of span morphing concept B(extended)

Design concept C is based on a telescopic wingdesign developed in works [4, 10]. The telescopicwing consists of a hollow outer wing (inboard) fromwhich the inner wing (outboard) slides out, figure 6.The hollow inboard part of the wing is made from asandwich made from composite materials that is re-inforced in the spanwise direction with carbon fiberbeams. This structure has to provide both the air-foil surface and the structural stiffness to supportthe aerodynamic forces applied to the wing. Theinner moving wing is structurally conventional andthe actuation options to provide its movement anddetermine its position are varied. The outboardwing moves as a whole. The chord discrepancybetween the fixed wing and the moving wing canbe considerable and this design has the more sig-nificant difficulty mitigating that as the minimumthickness achievable of the skin of the fixed wing islimited by the structural necessities of that element.

Figure 6: Plant view of span morphing concept C(extended)

Table 3 presents an assessment of the design con-cepts considered based on several evaluated param-eters. It is worth noting that the table was con-structed based on knowledge of these concepts andtheir advantages and drawbacks developed on previ-ous studies and implementation efforts (which werereviewed earlier). An evaluation scheme rangingfrom 0 to 5, 5 being the best rating, was used.

Table 3: Concept evaluation matrix

Concept A (AdAR) B CAerodynamics 5 4 2.5Weight 4 4.5 5Manufacture 3 5 3Actuation System 5 3 4Actuation Energy 2.5 5 5Total 19.5 21.5 19.5

Concept B emerges as the best overall option.The telescoping spar design utilizes only two

beams, one fixed that starts from the root of thewing and the second one ends at the tip of the wing.To achieve a large relative span extension the twobeams are designed with the same length. Tak-ing into consideration other telescopic wing mecha-nisms of similar size and similar number of telescop-ing elements, a 10% overlap between the beams atthe most extended configuration was defined. Re-sulting in two 1.375m beams, 1.250 from half ofthe semi span length and 0.125m extra to result inthe 10% overlap. The computed shear and bend-ing moments (which are shown later) revealed thatthe 10% overlap margin was enough margin to keepthe section from being a critical section in terms ofinternal beam stress.

From an aircraft design perspective, structuralweight is something desired to be as low as possi-ble; Thus, the beam cross section shape chosen mustbe apt to sustain torsional and bending loads whilecontaining the minimum amount of material. Whilethe hollow circular cross section is the most ma-terial efficient under torsion, similar shaped closed

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Figure 7: Plant view of the telescopic spar (re-tracted)

loop hollow cross sections perform adequately un-der torsion. The hollow rectangular cross sectionshape was chosen. It allows for a simple overallsetup, fabrication and actuation, while also being aclosely related shape to the wing profile.

Following the design objectives expressed above,an aerospace grade aluminium alloy was chosen forits high weight to strength ratio. The Aluminium2024 T3 alloy is mainly composed by aluminiumand copper; Its properties of interest are: the max-imum yield strength of 345MPa and also the shearstrength of 283MPa and the young modulus of73.1GPa.

Another aspect of the main telescopic spar de-sign is the movement between the two beams. Toprevent jamming and allow for a smooth and man-ageable actuation, it is essential to keep low lev-els of friction between the beams. The bearingsurfaces made from the low friction UHMWPE(Ultra-High-Molecular-Weight-Polyethylene) poly-mer along which the sliding motion of the spar oc-curs are located at the end of each beam, i.e., one atthe outboard end of the fixed beam and one at theinboard end of the moving beam. Sets of discretebearing surfaces made from a low friction polymerplaced on the exterior surface of the inner beam andthe interior surface of the outer beam to facilitatethe movement.

The wing rib is a plate like structure situatedin the plane of the two-dimensional airfoil profilewhich provides the airfoil shape to the surface ofthe wing and its main function is to transmit theaerodynamic loads from the skin to the internal loadbearing structures. With 6 sliding rib in the inboardsection of the wing and 8 fixed ribs on the outboardsection in the maximum wingspan configuration thesliding ribs are at 281.3mm from each other, themaximum distance; In the minimum wingspan con-figuration the sliding rib spacing is 93.8mm. Whilethe distance between the fixed ribs is constant at458.3mm.

The sliding ribs are simply in contact with boththe the main spar and the airfoil skin. While thethe other ribs are fixed to both and can transfer ef-fectively moments and forces, the sliding ribs need

Figure 8: Isometric view of the sliding rib

an additional structure to provide the leverage todeal with out of plane forces that can be amplifiedwith the slightest deviation between the rib planeand the plane perpendicular to the spanwise direc-tion. In figure 8 can be seen a box like structurethat involves the spar through a length consider-ably larger than the thickness of the rib is used toprovide the alignment stability necessary.

3.3. Structural Strength and SizingThe telescopic wing is designed according to the

norm: Certification Specifications for Normal, Util-ity, Aerobatic, and Commuter Category AeroplanesCS-23 that states: ”Strength requirements are spec-ified in terms of limit loads (the maximum loads tobe expected in service) and ultimate loads (limitloads multiplied by prescribed factors of safety)”,[11]. The norm specifies that a normal categoryaeroplane should be design for a limit load factor of3.8g. The general aviation factor of security of 1.5,also specified in the CS-23 norm, should be appliedto the limit load factor; Thus, the ultimate load fac-tor is set to 5.7g. This is the critical load that thestructure has to sustain without failing.

(a) (b)

Figure 9: Velocity-load diagrams: (a) 5m wingspanconfiguration (maximum) (b) 3.5m wingspan con-figuration (minimum)

In figure 9 it is presented the aircraft Velocity-load (V-n) diagrams of two wing configurationswith 5m and 3.5m wingspan and 150kg of mass.The air properties of the international standard at-mosphere at sea level were considered. The criticalflight situations in terms of structure integrity for

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both wing configurations are identified by points Aand B. Point A being the wing stall condition ata 5.7 load factor and point B is a 5.7 loading atdive speed. The major load bearing element of thewing is the telescopic spar and it is assumed to sus-tain the full aerodynamic loading. Wing structureweight is not considered.

For the sake of identifying the critical flightphase, the airflow characteristics around the wingare assessed with the non-linear lifting line theorymethod analysis provided by the XFLR5 software toestimate the distribution of the aerodynamic forcesover the wing span.

A script was developed using MATLAB thatcomputes the shear and moment diagrams of thetelescopic spar for a given load distribution. Ittakes as inputs the beam dimensions and its po-sitions, i.e., how extended is the telescoping mech-anism and a semi-span load distribution data filethat is gathered from XFLR5. These diagrams areused to find the maximum cross sectional loadingthroughout each beam of the telescopic spar.

Some considerations and simplifications weretaken:

• Beams are considered rigid bodies;

• The interaction between the two elements ofthe telescopic spar was simplified: points Aand C are considered the only contact pointsbetween the beams and the reaction forces arediscrete and simply vertical.

The free-body schematic of each beam-elementand its loading can be seen in figure 10.

Figure 10: Free body diagram of each telescopicspar element

dVydx

= −py (1)

where Vy(x) is the shear distribution and py(x)is the load distribution.

dMz

dx= −Vy (2)

where Mz(x) is the moment distribution.The integration of these formulas, equations 1

and 2, will provide the same result as using thefree body diagram method: considering the beamvirtually split at section x; Then, Vy(x) and Mz(x)will respectively be equal to the summation of forces

and moments applied from section x to either end ofthe beam, so as to maintain conservation of linearand angular momentum.

A further MATLAB script was created capable ofcalculating the maximum normal and shear stressesin a section of a hollow rectangular beam. Thescript takes into consideration the geometry of thesection, i.e., height, width and thickness of the hol-low rectangular section. Shear and bending mo-ment are additional inputs. The calculations aremade following the equations 3 for normal stressand 4 for shear stress.

σxx = −Mzy

Izz(3)

where y is the distance from the neutral surface,in the case of a symmetrical cross section the neutralsurface coincides with the x axis, and where Izz isthe centroidal moment of inertia of the section, [12].

τxy =VyQ

Izzb(4)

where Q is the first area moment.

Izz =wh3 − (w − 2t)(h− 2t)3

12(5)

Q =wt(h− t)

2+t(2h− 4t)(h− 2t)

4(6)

b = 2t (7)

where w, h and t are respectively the width,height and thickness of the hollow rectangular crosssection.

Since there are benefits in producing the light-est possible aircraft, another set of scripts was de-veloped to find the dimensions of the cross sectionwith the least area still capable of supporting theload applied to the beam and present a maximumnormal stress below the maximum yield strength ofthe material of the beam.

4. Structural Strength and Sizing Results

After identifying the maximum bending momentand shear force applied to the section of each beam,a final beam geometry capable of sustaining theloading and with the smallest area was found. Thecritical structural case, i.e., the flight condition thatgenerated the largest cross sectional loading corre-sponds to the point A of V-n diagram of the 5 meterspan wing, figure 9 (a). This corresponds to a stallcondition of the maximum wingspan with a 5.7gload factor. The minimum area geometry found ispresented below. In figure 11 the cross sections ofboth the inboard beam and the outboard beam arerepresented inside the airfoil profile (thickness is not

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Figure 11: Final telescopic spar geometry schematic

Table 4: Final telescopic spar dimensions

Beam w [mm] h [mm] t [mm] Area [mm2]AB 75.6 158.0 1.03 476.9CD 79.7 164.0 0.15 73.0

represented). The full dimensions are presented intable 4.

The figure 12 shows the lift distribution over thesemi span of the wing for the maximum and mini-mum wingspan achieved by the morphing wing forthe stall condition (case A).

Figure 12: Semi span lift distribution for maximumand minimum wingspan configurations (case A: n=5.7; stall condition)

In figures 13 and 14, the bending moment andshear force through the wing semi span are illus-trated for maximum 5m span wing and for the stallcondition at the ultimate load factor of n= 5.7.

From these shear and bending moment diagramsthe maximum sectional loading of each beam isgiven in table 5. The maximum normal stress andshear stress present in the cross section of the tele-scopic spar beams is exhibited in tables 6.

Table 5: Maximum section loading: Case A StallSpeed

Beam Max M [kN.m] Max S [kN]AB 4.748 5.001CD 0.782 3.367

Figure 13: Bending moment diagram of the maxi-mum wingspan (5m) wing at stall condition and n=5.7

Figure 14: Shear force diagram of the maximumwingspan (5m) wing at stall condition and n= 5.7

Table 6: Maximum normal and shear stress: CaseA Stall Speed (5m span)

Beam Max σxx [MPa] Max τxy [MPa]AB 344.85 34.80CD 344.79 150.84

Table 7 presents the maximum deflection resultsof each beam of the telescopic spar, as well as aconservative estimate of the maximum wing deflec-tion. Additionally, it is presented the angle definedby the three beam/wing points: tip, deflected tipand root, the latter being the vertex.

Table 7: Maximum deflection: Case A Stall Speed(5m span)

Beam Tip deflection [mm] angle [◦]AB 13.5 0.6◦

CD 2.6 0.1◦

AD (both) 53.4 1.2◦

By analyzing these results, it is possible to inferthe following observations:

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• The space between the beam elements of thetelescopic spar where they overlap is differentfor the vertical sides to the horizontal sides ofthe beams. Between the horizontal wall it mea-sures 3.88mm and between the vertical walls itmeasures 5.70mm.

• Using the aluminium alloy 2420 T3 the beamAB has a mass of 1822.9g and the beam CDhas a mass equal to 279.2g.

• The distinction between the lift distributionsof the maximum and minimum wingspan con-figurations is quite clear. As of the result ofboth generating the same overall amount of lift(the area below each line is equal to half thelift produced by the wing), the 5m wing hasa lower intensity lift distribution over a largerspan. Oppositely, the 3.5m wing’s lift distribu-tion has higher values over a smaller span.

• From the bending moment diagrams we can seethat its maximum value is located at the rootof the wing, which is the expected behavior ofa cantilever beam.

5. Aerodynamic Performance Studies

First, the lift and drag polars of the airfoil profilewere computed for a large range of Reynolds num-ber so as to provide the ability to assess the wingover a likewise large range of flight speeds. TheReynolds numbers analyzed ranged from 60,000 upto 6,000,000.

XFLR5 non-linear lifting line theory method(LLT) was used. It estimates the wing aerody-namic characteristics, including aerodynamic coef-ficient over the whole wing: CL, CD, CM, etc., aswell as the spanwise distribution of the local aero-dynamic coefficients and other flow parameters.

A level flight analysis, i.e., at constant total liftwhile varying speed and angle of attack, was per-formed for the wing for five different span config-urations: 3.5m, 4m, 4.5m and 5m to characterisethe morphing wing and 4.3m to represent the fixedwing. Wing polars are displayed in figures 15 and16. The wing lift and drag coefficients are calcu-lated using the area of the maximum wingspan con-figuration.

The cruise and loiter performance of the morph-ing wing and the fixed wings were studied. In cruiseflight the objective is to maximize the distance pos-sible to be travelled, by producing the necessarylift to level flight with the least drag possible. It isthe condition that for a constant altitude the cruisespeed is the one that provides the maximum valueof the ratio CL/CD. The loiter flight phase is usedto maximize the endurance of the aircraft in that

Figure 15: Wing lift coefficient with angle of attackfor different span wings

Figure 16: Polar curves for different span configu-rations

flight condition. Loiter speed is the flight speedthat will have the maximum value of CL

3/CD2, [7].

Depending on the flight speed some configura-tions are more efficient than others, i.e., have ahigher lift to drag ratio. The lift coefficient nec-essary for level flight decreases as the airspeedincreases and the wing configurations with morearea will generate more drag; Thus from a certainpoint the most efficient wing configuration changes.Therefore, it is possible to approximate of the op-timum CL/CD curve with speed for the morphingwing using several wingspan configurations. Thatgraph is displayed in figure 17.

Figure 17: Optimal CL/CD curve with level flightairspeed for the telescopic wing

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Similarly to the cruise conditions the optimalcurve to maximize available flight endurance canbe approximated as plotted in figure 18.

Figure 18: Optimal CL3/2/CD curve with level

flight airspeed for the telescopic wing

The drag produced by the wings was calculatedfor loiter, cruise and dive speeds. Table 8 presentsthese results for all wing configurations and bothfixed wings. Furthermore, the comparison betweenthe drag of each configuration and the drag of theoptimal morphing wing is provided in percentage ofthe optimal drag for the corresponding speed.

Table 8: Wing drag for different speeds with a com-parison with the 5m span wing

Wingspan Drag (Vl=70 kt) Drag (Vc=90 kt) Drag (Vd=150 kt)[N] ∆ [%] [N] ∆ [%] [N] ∆ [%]

3.5m 84.8 +62.2 61.5 +29.6 72.8 0optimal 52.3 0 47.5 0 72.8 05m 52.3 0 47.5 0 94.4 +29.74.3m 62.6 +19.8 51.1 +7.6 83.2 +14.3

In table 9 are the estimated weight penalties ofthe morphing wing relative to the 4.3m and 5mspan fixed wing. This measure corresponds to thenecessary extra weight the optimal telescopic wingaircraft would have to carry to nullify the aerody-namic benefits of the morphing wing regarding thefixed wing. This parameter is valuable in assessinga morphing application as it provides the additionalweight limit the morphing device must not cross toprovide aerodynamic benefits.

Table 9: Optimal morphing wing weight penaltiesto match fixed 4.3m and 5m span wing drag

Weight Penalty [%] Vl=70 kt Vc=90 kt Vd=150 kt4.3m (fixed) 13.2% 9.5% 39.1%5m (fixed) 0% 0% 68.1%

Comparing the optimal morphing wing aircraftto the fixed wings aircraft, results clearly show that

the optimal telescopic wing outperforms the 5mspan fixed wing at air speeds raging from about100kt to the maximum air speed in terms of effi-ciency.

Comparing the optimal morphing wing aircraft tothe 4.3m span fixed wing we cannot see such a sig-nificant benefit at the higher speed range. Nonethe-less, the morphing wing also outperforms the 4.3mwing at the lower air speed range, which is the moreimportant region for surveillance missions.

6. Conclusions

The main purpose of this work was to design atelescopic wing for a medium endurance patrol andsurveillance UAV. The morphing telescopic wingconcept capable of changing wingspan in fight wasdesigned and its structure and aerodynamic perfor-mance were studied.

As a starting point, the aircraft platform as wellas the desired wing aerodynamic shape were definedbased on the TEKEVER AR5. A market researchof UAVs with similar size and mission role allowedto complement the information gathered and helpedto characterize mission profile and performance ob-jectives. As the assessment of a morphing technol-ogy is entirely case dependent, this characterizationof the aircraft platform was used to compare themorphing wing performance to a conventional fixedwing solution.

Several concepts and mechanisms to attain thedesired in flight span variation and wing shape wereinvestigated. The chosen concept was defined andits critical elements were sized. The aircraft has adesign maximum load factor of 3.8g and an ultimateload factor of 5.7g as prescribed in the EASA CS-23norm. The velocity-load diagrams of the morphingwing aircraft were analysed and a methodology tofind the critical point structure wise and thereuponprovide a structure capable of supporting the ulti-mate load factor was developed and implemented.

The aerodynamic assessment of the telescopicwing performance was performed using the liftingline theory method through the XFLR5 software.Comparison between the morphing wing perfor-mance and both a fixed wing optimized for loiterand a fixed wing that represented a compromise so-lution was carried out. It can be concluded themorphing wing will outperform significantly the op-timized fixed wing in the extremes of the operatingrange. If the comparison is against the compromisesolution for the fixed wing then the morphing wingwill marginally or moderately outperform the fixedwing in the vast majority of the flight envelope.

The morphing wing displays a drag reductioncompared to the 5m wing optimized for the sameloiter speed of about 23% for an air speed equal to150kt; Compared with the 4.3m wing the morphing

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wing reaches a drag reduction of about 16% and12% for air speeds of 70kt and 150kt respectively.

6.1. Future Work

• Aerodynamic analysis with higher fidelity CFDmethods that can account for the morphingwing discontinuities will provide better insightsinto the performance of the telescopic wing andwhat benefits it can give.

• Finite element structural analysis that stud-ies all structural elements can help to trimdown structure weight and assure structural in-tegrity. With structure and its weight defined,a stability analysis can be done.

• The design decision between the morphingwing or an alternative fixed wing should be as-sessed over the entire extension of the aircraft’stypical missions. The actuation system, powerand propulsion should be determined and stud-ied so the energy expended by the morphingaircraft over the mission profile can be com-pared to that expended by the fixed wing air-craft.

• Other morphing devices and its conjoined in-tegration may be investigated to provide addi-tional morphing capabilities such as sweep orcamber morphing to further expand the flightenvelope.

• Perform experimental tests to assure the feasi-bility of the telescopic mechanism and corrob-orate the computational calculations.

References

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