/,,_-_; (_. _i..i)t,l _. j) AIAA 2000-1327 Flexible Wing Model for Structural Sizing and Multidisciplinary Design Optimization of a Strut-Braced Wing F.H. Gern, A.H. Naghshineh-Pour, E. Sulaeman, R.K. Kapania Vir.ginia.Polytechnic Institute and State Un=vers=ty, Blacksburg, VA and R.T.Haftka Un=vers_ty of Florida, Gainesvile, FL 41st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Meeting & Exhibit 3-6 April 2000 / Atlanta, GA For permission to copy or republish, contactthe American Institute of Aeronautics and Astronautics 1801 Alexander Bell Drive, Suite 500, Reston, Virginia 20191-4344 AIAA-2000-1327 FLEXIBLE WING MODEL FOR STRUCTURAL WING SIZING AND MULTIDISCIPLINARY DESIGN OPTIMIZATION OF A STRUT-BRACED WING Frank H. Gern °, Amir H. Naghshineh-Pour +, Erwin Sulaeman*, and Rakesh K. Kapania I Department of Aerospace and Ocean Engineering Virginia Polytechnic Institute and State University Blacksburg, VA 24061-0203 Raphael T. Haftka i Department of Aerospace Engineering, Mechanical and Engineering Sciences University of Florida Gainesville, FL 32611-6250 Abstract c_ wing-box chord F,v vertical strut force (z-direction) This paper describes a structural and aeroelastic F._h horizontal strut force 0,-direction) model for wing sizing and weight calculation of a Lo# strut vertical offset length strut-braced wing. The wing weight is calculated using M, freestream Mach number a newly developed structural weight analysis module M(y) bending moment considering the special nature of strut-braced wings. A qi(Y) local lift distribution for element i specially developed aeroelastic model enables one to s wing-strut intersection (from wing root) consider wing flexibility and spanload redistribution u unit step function during in-flight maneuvers. The structural model uses Uah, Vab, Wabbackwash, sidewash and downwash a hexagonal wing-box featuring skin panels, stringers, velocity, respectively and spar caps, whereas the aerodynamics part employs V(y) shear force a iinearized transonic vortex lattice method. Thus, the w bending deflection wing weight may be calculated from the rigid or W, engine weight flexible wing spanload. Y_ spanwise engine position (from root) The calculations reveal the significant influence of y spanwise coordinate the strut on the bending material weight of the wing. o;/3 lift coefficients at structural nodes The use of a strut enables one to design a wing with A wing sweep angle thin airfoils without weight penalty. The strut also 0 bending slope influences wing spanload and deformations. Weight /-, vortex strength savings are not only possible by calculation and iterative resizing of the wing structure according to the Introduction actual design loads. Moreover, as an advantage over the cantilever wing, employment of the strut twist Strut-braced wing configurations have been used moment for further load alleviation leads to increased both in the early days of aviation and today's small savings in structural weight. airplanes. Adopting thin airfoil sections required external structural wing support to sustain the Nomenclature aerodynamic loads. However, external structures cause a significant drag penalty. Gradually. it was AR wing aspect ratio understood that the external bracing could be b wing span removed and lower drag could be achieved by c wing chord replacing the wing-bracing structure with a cantilever "Research Associate, MemberAIAA wing with an appropriate wing-box and thickness to *Graduate Student. Student Member AIAA chord ratios. Graduate Student I Professor, Associate Fellow AIAA However, along with the idea of the cantilever Distinguished Professor, Fellow AIAA wing configuration with its aerodynamic advantages, Copyright © 2000 by the authors. Published by the American the concept of the truss-braced wing configuration Institute of Aeronautics and Astronautics, Inc. with permission. 1 American Institute of Aeronautics and Astronautics alsosurvived.Thisis dueto thetirelesseffortsof samepayloadrange.Theyconcludedthatthestrut- WernerPfenningeratNorthropin theearly1950's[1] bracedwingconfigurationreducesthetotalaircraft andhis continuationof theseeffortsuntil thelate weight,eventhoughwingandstrutweightincreased 1980's.Usingastrutoratrussofferstheopportunityto comparedtothecantileverwingcase,whichisdueto increasethewingaspectratio andto decreasethe aerodynamicadvantagesof highaspectratiowings. induceddrag significantlywithout wing weight Furthermore,theresultsshowedafuelweightsavings penaltiesrelativeto a cantileverwing.Also,alower of20%. wingthicknessbecomesfeasiblereducingtransonic The strut-bracedwing conceptoffers the wavedragandhenceresultinginalowerwingsweep. possibilityto reducewing thicknesswithoutthe Reducedwingsweepandhighaspectratiosproduce penaltyof anincreasedstructuralweightbyreducing naturallaminarflow dueto low Reynoldsnumbers. thebendingmomentonthewing.However,reduced Consequently,a significantincreasein theoverall wingthicknesstogetherwith shorterwingchords aircraftperformanceisachieved[2],[3]. result in smaller wing-box dimensions,thus A number of strut-bracedwing aircraft significantlyreducingwing-boxtorsionalstiffness configurationshavebeeninvestigatedin thepast.In andrenderingthewingmoresensitiveto aeroelastic continuingPfenninger'swork,KulfanandVachalfrom problemslikeincreasedstaticaeroelasticdeformation theBoeingCompanyperformedpreliminarydesign orreducedflutteranddivergencespeeds.Thepresent studiesandevaluatedtheperformanceof a large approachhighlightsa possibilityto remedythe subsonicmilitary airplane[4]. They compared problemof increasedaeroelasticdeformationsby performanceandeconomicsof acantileverwingwith employmentof the strutmomentinducedon the a strut-bracedwing configuration.Two load wing. conditions,a2.5gmaneuverand1.67taxibumpwere Previously investigatedstrut-bracedwing usedtoperformstructuralanalyses.Theiroptimization conceptsconsideredthestruttoberigidlyattachedto andsensitivityanalysesshowedthathighaspectratio thewing.Therefore,strutbucklingduringnegativeg wingswithlowthicknesstochordratioswouldresult maneuverswasa majordesignissue,renderingthe inasignificantfuelconsumptionreduction. strutveryheavyin orderto overcomethisbuckling For thecantileverconfiguration,a groundstrike constraint[4], [5]. To avoidstrut buckling,the problemaroseduringtaxiing.Thisissuewasresolved presentapproachoffersan innovativeconcept.A byaddingastrutto thewingstructure.Moreover,the telescopingsleevemechanismis employedto have analysisindicatedthat the strut-bracedwing thestrutactiveonlyduringpositivegmaneuvers.For configurationrequireslessfuel(1.6%),andresultsin negativegmaneuvers,thewingactslikeacantilever lowertakeoffgrossweight(1.8%)andlowerempty wing, renderingthe strut bucklingconstraint weight (3%) comparedto the cantileverwing unnecessary.Furthermore,thisarrangementallows configuration.Costcomparisonsshowedthat the one to applya definedstrut forceat the 2.5g operatingcostsof thestrut-bracedwingconfiguration maneuverdesignload insteadof the statically wereslightlylessthanthoseof thecantileverwing indeterminateone obtainedfrom a rigid strut configurationbecauseofalowertakeoffgrossweight. attachment.Thisway,thestrutforceaswellasstrut Parkfromthe BoeingCompanycomparedthe positioncanbe optimizedin orderto achievethe blockfuel consumptionof a struttedwingversusa maximumbenefitsoutofthedesignconcept. cantileverwing[5].Eventhoughheconcludedthatthe To fully exploitthesynergismfrom thestrut- useof a strut savesstructuralwing weight,the bracedwingconcept,anMDOapproachhasbeen significantincreasein thestrutt/c to copewithits chosenfor aircraft design optimization.The bucklingatthe-1.0gloadconditionincreasedthestrut multidisciplinaryteamconsistsof aerodynamics, dragandhencedid not appearpracticalfor this structures,andadetailedinvestigationofinterference transportaircraftdueto a higherfuel consumption drag.Theaerodynamicanalysisusessimplemodels comparedtothecantilevercase. forinduceddrag,parasitedrag,andinterferencedrag. Anotherstudyonstrut-bracedwingconfigurations All analysesarelinkedtogether,andtheperformance wasconductedbyTurrizianietal.[6].Theyaddressed of thestrut-bracedwingaircraftisthenoptimizedfor fuel efficiencyadvantagesof a strut-bracedwing minimumtake-off-grossweight[3],[7],[8]. businessjet employinganaspectratioof 25overan TheMDOapproachhasbeenimplementedin equivalentconventionalwingbusinessjet with the severalaircraft designs.Grossmanet al. [9] 2 AmericanInstituteofAeronauticsandAstronautics investigatedthe interactionof aerodynamicand strut design, a vertical strut offset was considered as structuraldesignof a compositesailplanesubjectto to achieve a significant reduction in wing/strut aeroelastic,structural,andaerodynamicconstraintsto interference drag. increasetheoverallperformance.Theyshowedthat themultidisciplinarydesigncanyieldresultssuperior Load Cases to the onesobtainedfromthe sequentialmethod. To determine the bending material weight of the Anotherexampleistheapplicationof MDOtoaHigh strut-braced wing, two maneuver load conditions SpeedCivilTransport(HSCT).A significantefforthas (2.5g maneuver, -1.0g pushover) and a taxi bump (- beenmadeat the MultidisciplinaryAnalysisand 1.0g) are considered to be design critical. For the - Design(MAD)centerof VirginiaTechto perform 1.0g pushover and for the -2.0g taxi bump, the strut is MDOof anHSCT.Severalmethodsweredeveloped not active and the wing acts like a cantilever beam. for thebetteruseof theMDOapproachfor aircraft Since the strut is not supporting the wing in these conceptualandpreliminarydesign.Moreinformation