digital mock-up: a useful tool in aircraft design

ICAS 2000 CONGRESS

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DIGITAL MOCK-UP: A USEFUL TOOL IN AIRCRAFT DESIGN

D. Camatti, S. Corpino, M. PasquinoDepartment of Aerospace Engineering (DIASP), Politecnico di Torino

ABSTRACT

This paper shows the activity we are nowinvolved in, focused on the improvement ofconceptual design methodologies by theintegration with modern parametric 3D CADsoftware tools. This is implemented both atsystem level and at subsystem level and theintegration between them is optimised. Theresult is a Digital Mock-Up at ConceptualLevel, whose usefulness will be shown.

1 INTRODUCTION

Our research activity at the Department ofAeronautic and Space Engineering at thePolitecnico di Torino is focused on aircraftconceptual design. We are currently working atit as Aerospace Engineering PhD studentsthough two of us had already gotten involvedinto it as undergraduate students carrying outtheir graduation theses. Our activity fits into thebroader research which has been carried outthroughout [1] [2] [3] [4] by the SystemsEngineering Research Group at DIASP whoseprofessor co-ordinator is Prof. S. Chiesa. Theresearch group draws inspiration from thefundamental studies of Prof. G. Gabrielli [5] [6].

Fig.1: Topics involved in group activity

Figure 1 is an attempt to explain andunderline how all different aspects of our

research are connected with each other, as atypical feature of Systems Engineering.

D. Camatti, S. Corpino, M. Pasquino

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On one hand figure 1 shows theeducational purpose of our activity which iswidely discussed in [1] (paper presented at thisvery congress), on the other it underlines howour research fits into the broader aircraftconceptual design research. Figure 1 alsoillustrates that the main feature of our work hasbeen the introduction of the new parametric 3DCAD techniques which have gained importanceand have become essential if not pre-eminent.The main role of new parametric 3D CADtechniques, how these techniques have beenused within the aircraft conceptual designactivity and the study turned towards theoptimisation of 3D CAD tool utilisation havealready been discussed in [7] and will be talkedover in the next paragraph.

Figure 1 also reminds the use of 3D CADsoftware tool within the Digital Mock-Upanalysis, that is the study of the subsystemsinstallation inside the airframe. We have alreadyapplied this technique in a previous work [8]which shows the usefulness of the DigitalMock-Up at Conceptual Level (DMUCL).Paragraph 3 will deal with the effective carryingout of the DMUCL. Eventually paragraph 4 willconsider the advantages that a DMUCL analysiscan bring. These advantages partly have beenalready discussed in [7], partly stem from thepossibility of the DMUCL integration in amethodology now elaborated [9].

The entire research activity presented inthis paper has been tailored on an advancedtrainer vehicle (with also possible operationalemployments) named SCALT (SupersonicCombat Affordable Light Trainer or SafeCompetitive Advanced Light Trainer) andelaborated at DIASP. The SCALT study andcarrying out are widely debated in [10] and [11].

2 PARAMETRIC 3D CAD WITHIN THECONCEPTUAL DESIGN ACTIVITY :ADVANTAGES AND PROBLEMS TO BESOLVED

At the beginning of a new design different kindsof choices have to be made:

• qualitative choices, mainly architecturalchoices such as the shape of the bodies,

and others like the kind of material touse, etc. ;

• quantitative choices, such as the sizes ofdifferent parts, the values of functionalparameters (for example the enginerating required, rate of flow in onboardsystems, forces acting on aerodynamicsurfaces, etc.)

• The main activities to perform in orderto make these choices are:

• conceiving and carrying out the drawingof the object you are studying. Noticethat in some cases the 2D drawings areintegrated by solid models (wooden-made or in polypro foam, for example);

• building up a mathematical or a physicalmodel of the object, that leads you to fixnumerical value for parameters.

The sequence of these two steps, oftenrepeated several times, characterises the typicaldesign activity (see fig.2).

Fig.2: Design activity


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When dealing with simple items, theconceiving activity may be fulfilling.Conversely when complexity increases adetailed and quantitative definition becomesnecessary.

Conceiving and carrying out the drawingactivity has been scarcely used until now in thedevelopment of aircraft conceptual designmethodologies. The conceptual design wascarried out only by the adoption of analyticalor/and numerical algorithms. Notwithstandingthe effort required, it did not lead to asatisfactory definition of the item. A typicalexample is the estimation of fuel tanks capacityby calculating the internal volume of the wing.

The introduction of parametric three-dimensional CAD software integrated into theconceptual design methodology represents, inour opinion, a great improvement.

The possibility to visualise the object in avery realistic way, and to observe it fromdifferent directions thanks to rotation tool,represents a substantial contribution to thedesigner.

Modern CAD software allow to obtain:• a “solid model” of an object. The so

called solid model is the geometricdigital representation of the real object.It is the visual expression of thecomputer internal mathematicalrepresentation of the object.

• a parametric model of an object. Thisfeature allows us to integrate CADsoftware in a computerisedmathematical procedure. Theparametric feature means that it is notnecessary to specify the object’s sizeonce and for all, but it is possible todefine it everytime. Furthermore, thereis the possibility to assign explicitrelations between different dimensionsso that changing one of them, everydepending data changes as aconsequence.

Figure 3 shows the new conceptual designmethodology defined and set up with regard tothe SCALT project, as discussed in [7] [8] [10][11].

P E R F O R M A N C EM IS S IO N P R O F IL E S ,

O P E R A T IO N A LC A P A B IL IT Y

R E Q U IR E M E N T San d P R E L IM IN A R Y

H Y P O T H E S IS

F IR S T H Y P O T H E S ISA B O U T W E IG H T ,P R O P U L S IO N a n dA R C H IT E C T U R E

L A Y O U T

CFD

FEM

H Y P O T H E S IS A B O U TS T R U C T U R A L L O A D S

S U B S Y S T E M SD E F IN IT IO N a n d

W E IG H T E S T IM A T IO N

A C C E S S A B IL IT Y ,R E P A IR A B IL IT Y

V E R IF IC A T IO N a n dF IR S T H Y P O T H E S IS

a b o u t A S S E M B L YP R O C E D U R E S

Fig.3: new conceptual design methodology

Unfortunately there are also someproblems related with the utilisation of CADsoftware. They are shown in figure 4, which


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compares advantages and disadvantages of thistool. In particular, the generality of CADrepresents a problem in some way. In fact, thismeans that the same object can be obtained indifferent ways (one of which is probably betterthan the others) and that there are not standardsequences to follow. This suggested us to begina research activity which aims at definingstandard optimised procedures. This target hasbeen

pursued during the development of theSCALT,

which gave us the possibility to conceive,develop and test the new methodologyintegrated with the drawing procedure. SCALTproject represents the first case study; theresearch turned forward the optimisation ofdrawing procedures is carried on through othercase studies [7] [8].

PARAMETRIC CAD-3DADVANTAGES PROBLEMS

• Realisticvisualisation.

• Parametric design.• Characteristics

calculation duringthe phase ofdesign.

• Generalcharacteristics ofthe software, notdirectly addressedto aeronauticalapplications.

• Several differentways to obtain thesame item.

Fig.4: CAD characteristics

3 DIGITAL MOCK UP AT CONCEPTUALLEVEL

The 3D parametric CAD tool reveals its powerand its problems also in the subsystemdefinition phase. On one hand it is possible toobtain a complete and precise concept of theaircraft, making the results of the design muchmore reliable than those with the classicmethodologies. On the other hand the problemsrelated to the procedures still remain. All layoutsubsystems, included the structural scheme,have been foreseen. Each one, excluding thestructural scheme, has been defined on the basisof a block scheme layout. Later on they have

been sized by mathematical algorithms or bysimulation models. The study of the installationof sized systems on the airframe has beencarried on by the definition of the Digital Mock-up at Conceptual Level (D.M.U.C.L.). Figure 5sums up the procedure used to build theDMUCL of the SCALT aircraft.

The usefulness of the DMUCL consists inthe possibility of:

• obtaining a detailed definition of theitem you are studying (for example anaircraft);

• verifying the actual possibility of theairframe to contain all systems foreseenand check the right positioning of eachequipment;

• allowing a very accurate and easyestimation of the centre of gravity andgeometrical quantities;

• allowing preliminary studies ofmaintenance characteristics.

• studying the interface betweenequipments and structural elements.

Figures from 6 to 11 show the results of theDMUCL study applied to the SCALTsubsystems.

S U B S Y S T E M S lev el

d e sig n o fsub syste m s

in te g r a tio n o fsub syste m s

L ib rary

in s ta l la tio nin teg ra tio n w ithsim u la t ion m o d e ls

(M A T L A B -S IM U L IN K )

D M U C L

Fig.5: DMUCL procedure


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Fig.6: Structural layout

Fig.7: Flight controls and landing-gears

Fig.8: Hydraulic and electrical systems

Fig.9: Fuel system and secondary power

Fig.10: Avionic system and computing

Fig.11: Armament and furnishing


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The study of Digital Mock-Up lead us tothe following observations:

• it is possible to note the solution chosenfor the secondary power system: it isfully integrated with ECS and APU,based on AMAG (Aircraft MountedAccessory Gear Box) and discussed in[9] and [11];

• most equipments of the hydraulicsystem are located in the rear part ofthe fuselage beneath the engine. As aresult the maintainability improvesbecause of two reasons: first because ofthe high concentration of elementswithin the same area and secondbecause of the reduced number of pipesand connections which may bedisconnected in case the rear part of thefuselage has to be removed. It is clearthat this area has to be protected byfire-extinguishing devices;

• the fuel system is fully contained in thewing (external, internal and centralsections fuel tanks) with two motor-driven booster pumps for each tank;

• the air and oil coolers, using fuel asrefrigerant, are grouped in three areasonly. Two of them are located nearAMAG and thus have to be protectedby fire hazards;

• the 27mm Mauser Gun is located at thebottom of the fuselage and thus makingthe reloading easier;

• avionics equipments and subsystemscontrol computers are easily accessible.Most of them are located on the upperwing centre section sides avoiding theinterference between Maintenancetechnicians;

• flight control systems consist of: linearactuators for ailerons, rudders andspoilers placed near the respectiveaerodynamic surfaces and thus easilyaccessible; three drive units forforeplane, leading edge flaps and flap-elevons placed in accessible areas ofthe fuselage. Each drive unit,constituted by two hydraulic motors, is

connected by shafting to the actuators.In particular there are four linearjackscrews for foreplane and flap-elevons. These last ones will be morewidely discussed in the nextparagraph.

Figure 12 shows the complete DMUCL.The present figure is a very important result as itdemonstrates the physical feasibility for allsubsystems foreseen to be installed on theairframe. It has to be remembered that thetraditional preliminary design methods allowedthe installation test to be accomplished onlysuccessively. Figure 13 and 14 then show howsimple CAD controls make it very easy toevaluate quantities. In particular, figure 13verifies the fuel capacity of the wing structure,centre section included, by calculating thevolume of the integrated tanks.

Fig.12: Complete digital mock-up


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Fig.13: Fuel capacity evaluation

Fig.14: Subsystems centre of gravity evaluation

Figure 14 illustrates the definition of thecentre of gravity co-ordinates for thesubsystems, since the weights estimation hasbeen previously accomplished within thesubsystems design analysis phase (atsubsystems level and, in many cases, atcomponents level).

4 DMUCL AND INTEGRATION WITHRAMS METHODOLOGY

As already been stated, DMUCL represents avery good means to integrate the ConceptualDesign with a methodology [9] turned towardsdealing with Reliability, Availability,Maintainability and Safety characteristics from

which the acronym RAMS derives. Figure 15schematically shows the RAMS methodologyunderlining how technical and logisticalactivities follow one another. The technicalactivities are mainly the conceptual design andthe development and the definition of thesupport. Logistical tools used are partlytraditional, partly originally made up within themethodology development itself. They are:

• PFMEA (Preliminary Failure Modesand Effects Analysis), useful forperforming the Safety and Reliabilityproject design;

• FMECA (Failure Modes and Effectsand Criticality Analysis);

• RCM (Reliability CentredMaintenance), useful for optimising theMaintenance program;

• MPFMEA (Maintenance ProcessFailure Modes and Effects Analysis):i.e. FMEA performed on the successionof operations of a maintenance processin order to prevent critical situations orprobable mistake sources.

Figure 15 illustrates how RAMSmethodology might be integrated with DMUCL.

P F M E Ain s ta lla tio n i s

c r itica l?

R C M

yes

n o

o k

m a in te n a n c e ta sk

M P F M E A

?

Fig. 15: RAMS methodology

SCALT flap-elevon actuation (fig.16) hasbeen considered as subsystem example.

As it can be noted the system (at “blackboxes” level) is constituted by:

a) one drive unit encompassing twohydraulic motors (fed by two differenthydraulic circuits) which may add uptheir own torque when both twoclutches are engaged to give one torquevalue as drive unit output. The clutchesmake also one single hydraulic motor


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to be disengaged possible when thatmotor happens to brake down;

b) two shafts to transfer torque at highangular speed towards both the right andleft flap-elevon jackscrew linearactuators;

Fig.16: Flap-elevon actuation system

c) these actuators get mechanic power (lowtorque value and high angular speed)from the shafting coming from the DriveUnit and transfer high force at low linearspeed to the flaps-elevator themselves.

Fig.17: Flap-elevon actuation system installation choice

Fig.18: Flap-elevon actuation system installation choice – lateral view


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ITEM Q.TY FAILUREMODE EFFECT PROBABILITY CRITICALITY

jack-screw 2 brokensystem blocked (bycomputer control toavoid asymmetry)

low high

shafting 2 brokensystem blocked (bycomputer control toavoid asymmetry)

very low high

one motor notrunning

marginal less ofperformances high low

drive unit 1total inefficiency System blocked very low high

Tab.1: preliminary FMEA of flaps-elevons actuation system

PFMEA, as tab. 1 shows, may point out,even though on the basis of a preliminarydefinition, the need for very basic configurationchoices to be made like the idea of adopting aDrive Unit with two motors fed by differentcircuits. In this way if one engine breaks down(and that is likely to happen taking into accountthe complexity of a motor itself and even moreits servo valve, as well as the possibility of afailure in the feeding circuit) that event may stillbe considered as a low criticality effect event.The high criticality effect event will just occurin case of total inefficiency due to the breakingdown of both motors during the same flight. It isclear that is a very low probable event.

Considering now the subsystem installationon-board aircraft, the RCM application hasrevealed the necessity of undertaking scheduledinspections of the Drive Unit. In fact, as alreadywell known, the advantages of redundancy arealmost inessential if at the beginning of eachsingle flight both parallel branches of therespective Reliability Black Diagram areguaranteed to be working when set in action.The first installation study aimed at lining upthe power shafting coming from the Drive Unitand the jackscrews installed into the wing rear“back strakes”. Thanks to the installation of theDrive Unit beneath the wing centre section,immediately before the AMAG, the shafting runparallel to the y axis. The MPFMEA revealedthat the Drive Unit disassembly would becritical since the Drive Unit was almost

completely enclosed between AMAG, wingcentre section and A.C. generators (fig.17). Theproblems encountered led to the choice of theinstallation of the Drive Unit abaft the enginespar frame. The Drive Unit is now locatedbeneath the actuators as figure 17 and 18 show.The shafts are now inclined and thus a wee bitlonger than it was before. The accessibility hasbeen enhanced allowing the accessibilityoperations within an area where most of thehydraulic apparatus are gathered to beuniformed. Moreover this solution avoids thenecessity of disconnecting hydraulic systems incase the rear part of the fuselage downstreamthe engine spar frame has to be removed.

5 CONCLUSIONS

Taking into account the enhanced definition andprecision level attained for the ConceptualDesign activity as well as the possibility ofnarrowing the study down to fundamentaldetailed choices (remembering in particular theexample illustrated in the previous paragraph)we do believe in the usefulness of DMUCL.

ACKNOWLEGEMENTS:

The authors are very grateful to Ing. N. Violafor her precious contribution.

REFERENCES

[1] S. Chiesa, L. Borello, P. Maggiore, An academicalexperience on aircraft design: affordable advanced


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jet trainer”, 22nd ICAS Congress, Harrogate, UK,August 2000.

[2] S. Chiesa, Un metodo di progetto preliminare pervelivoli da trasporto, 4th AIDAA National Congress,Milan, Italy, September 1977.

[3] L. Borello, S. Chiesa, P. Maggiore, Avamprogettomirato all’efficienza di sistema, 11th AIDAA NationalCongress, Forlì, Italy, October 1991.

[4] S. Chiesa, P. Maggiore, Hypersonic aircraft designmethodology proceedings, 19th ICAS Congress,Anahaim, CA, USA, September 1994.

[5] G. Gabrielli, Un metodo per la determinazione dellasuperficie alare e del suo allungamento nel progettodei velivoli, Vol. 86, Accademia della Scienze, Turin,Italy, 1951/52.

[6] G. Gabrielli, Lezioni sulla scienza del progetto diaeromobili, Ed. Levrotto e Bella, Turin, Italy, 1975.

[7] S. Corpino Utilizzo del CAD 3D parametrico perstudi di configurazione in avamprogetto:applicazione ad un velivolo da addestramentoavanzato, Graduation Thesis in AerospaceEngineering, Polytechnic of Turin, October 1999.

[8] S. Chiesa, M. Di Sciuva, D. Camatti, S. Corpino, M.Pasquino, Utilizzo del CAD 3D parametrico per studidi configurazione e Digital Mock-Up nel progettoconcettuale, 15th AIDAA Congress, Turin, Italy,Novembre 1999.

[9] S. Chiesa, D. Camatti, G. Luda di Cortemiglia,Design to System Effectiveness Methodology, SOLE’99 International Logistics Symposium, Las Vegas,NV, USA, Aug 30-Sep 3, 1999

[10] S. Chiesa, L. Borello, M. Di Sciuva, P. Maggiore,Proposta di velivolo da addestramento avanzatomirato alla sicurezza: valutazione di efficacia eefficienza e analisi del rischio, 15th AIDAACongress, Turin, Italy, Novembre 1999.

[11] M. Pasquino, Proposta di velivolo da addestramentoavanzato mirato alla sicurezza: studio diconfigurazione e valutazione di efficacia edefficienza, Graduation Theses, Polytechnic of Turin,October 1999.

digital mock-up: a useful tool in aircraft design

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