Abstract
Active Aeroelastic Wing (AAW) Technologyrepresents a new design approach for aircraft wingstructures. The technology uses static aeroelasticdeformations as a net benefit during maneuvering.AAW is currently being matured through a flightresearch program1; however, transition of thetechnology to future systems will require educatingdesigners in multiple disciplines of this new designapproach. In order to realize the full benefits of AAW,aeroelastic effects will need to be accounted for fromthe beginning of the design process. Conceptual designdecisions regarding parameters such as wing aspectratio, wing thickness-to-chord ratio (t/c), and wingtorque box geometry may be influenced, if designerschoose to utilize AAW.This paper presents recent efforts in developingconceptual aircraft design guidance for AAWtechnology and identifies improvements to the designprocess that could facilitate future AAW designapplications. This process involves using results fromaeroelastic design methods, typically used inpreliminary design, with conventional conceptualdesign methods. This approach will allow aeroelasticeffects to be considered in making conceptual designdecisions.
Introduction
Conventional aircraft design philosophy views theaeroelastic deformation of an aircraft wing as having anegative impact on aerodynamic and controlperformance. The twisting of a wing due to ailerondeflection during a roll maneuver can produce thephenomena of aileron reversal. Aileron reversal is thepoint where the deflection of the aileron produces norolling moment. That is, the rolling moment producedby the change in camber due to aileron deflection isoffset by the effective reduction in wing angle of attackdue to the aeroelastic wing twist. Aircraft designershave generally tried to limit the effects of aeroelasticdeformation by designing geometrically stiff planforms(low aspect ratio, high t/c), increasing structural weightto provide additional stiffness, and/or using horizontaltails to provide supplemental roll moment. Aconventional wing design presents a severe compromisebetween aerodynamic, control, and structuralperformance.Active Aeroelastic Wing (AAW) technology is a newwing structural design approach that integrates flightcontrol design to enhance aerodynamic, control, andstructural performance.2 AAW exploits inherentstructural flexibility as a control advantage, utilizingboth leading and trailing edge control surfaces toaeroelastically shape the wing. The entire wing acts asa control surface, with the leading and trailing edgesurfaces acting as tabs. The power of the air stream isused to twist the wing into a favorable shape. Thedegree of deformation is not necessarily any more thanfor a conventional wing; however, the deformation isadvantageous instead of adverse to the maneuver (SeeFigure 1). AAW can be used to generate large rollcontrol authority at higher dynamic pressures, andenables maneuver load control for both symmetric andasymmetric maneuvers. AAW does not require “smartstructures”, advanced actuation concepts, or adaptivecontrol law techniques; however, AAW maycomplement these other advanced technologies. Thekey difference between AAW and the conventionalapproach is the exploitation of aeroelastic methodsthroughout the design process.LE and TE used,Twist advantageousTELE Aeroelastic twistTE only used,Adverse twistTEAAW ConventionalV∞LEFigure 1. AAW vs. Conventional Roll ManeuverThe AAW approach removes static aeroelasticconstraints in the wing design. Previous studies have10-2shown that an AAW can generate sufficient rollmoment without the need for a horizontal tail to providesupplemental roll moment.2,3,4 AAW expands thedesign space for a design team by enabling thinner,higher aspect ratio wings to be weight competitive withgeometrically stiffer planforms. AAW technology iscurrently being matured through a full-scale flightresearch program.1 While this full-scale demonstrationand characterization of AAW is absolutely necessary tovalidate the technology, transition to future air vehicleswill ultimately depend on educating aircraft designerson the AAW design approach. The objective of thispaper is to present findings of a lightweight fighterdesign study to aid future conceptual design teams inthe application of AAW technology.
Impact of AAW on Conceptual Design Decisions
Conceptual aircraft design results in the specification ofthe vehicle geometry that will best meet the missionand design requirements. Conceptual designersquantify a number of conceptual design parameterssuch as wing area, aspect ratio, thickness-to-chord ratio(t/c), taper ratio, sweep angle, etc. The AAW designapproach enables designers to consider configurationsoutside the conventional design space. Because theAAW approach enables designers to use staticaeroelastic deformation as a net advantage, thinnerand/or higher aspect ratio wings can be effectivelyemployed. Previous AAW design feasibility studieshave demonstrated the benefits of AAW by expandingthis design space.2,3,4,12 In addition, these studiesindicate that AAW may enable configurations withdramatically reduced horizontal tail area. Based oncurrent design methods, conceptual designers wouldfind it difficult to choose the best configuration for anAAW design, because AAW represents a dramaticchange in the design paradigm. Designers trying toemploy AAW would likely have many questions andfew answers. How high of an aspect ratio is feasible?How low of a wing t/c is feasible? Where should theleading and trailing edge spars be located? How shouldthe control surfaces be sized and located? In order toeffectively exploit AAW technology, designers willneed benchmark design studies to reference and adesign process that enables the quantification offlexibility effects on aerodynamics, controlperformance, loads, and structural weight
.Limitations in the Conventional Design Process
Conceptual designers typically use a combination ofempirical and relatively low fidelity analytical methods,and simplify the design problem by makingassumptions such as a rigid structure for the purposes ofestimating aerodynamic and control performance.Designers will, in large part, quantify design parametersbased on experience and a historical database ofexisting aircraft. The methods are generally aneffective approach early in design, but theireffectiveness can be limited when designing for manynew technologies, such as AAW. These empiricalmethods were developed from a database that does notinclude AAW designs, and AAW represents arevolutionary shift in the design paradigm. Likewise,the analytical methods typically employed duringconceptual design are not likely to be multidisciplinaryand, therefore, do not account for interactions such asflexibility effects on aerodynamics, controlperformance, loads, and structural weight. The currentapproach to a conceptual aircraft design would be toconstrain the design space early in the design to avoid“problems”, like static aeroelastic effects, as the designprogresses. These constraints would be based on thedesigners’ experience.In designing with the AAW philosophy, quantifying theeffects of airframe flexibility is an absolute necessity.In order to account for flexibility, it is necessary toemploy methods such as TSO 5 or higher fidelity finiteelement based methods such as ASTROS13 orNASTRAN14. The problem with using such methods toinfluence conceptual design decisions is the timerequired to build the models and perform the analysesand/or design optimizations. Typically a conceptualdesign will undergo many changes very rapidly, and itis difficult to build the models and perform the higherfidelity analyses quickly enough to influence theconceptual design decisions. A design environmentthat includes parameterization of design and analysismodels and associativity between the models andconceptual design parameters would enable higherfidelity models to be updated as the conceptual designparameters are changed. With this capability, higherfidelity methods could be employed to make betterdecisions during conceptual design.


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