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were designed to be flexible and the aero-dynamic pressures were exceptionally low.

As aircraft design evolved to providefor constantly increasing engine speedsand load capacities, designing for flexible

wings was discarded due to ever-expand-ing performance demands. The result isthat todays aircraft employ rigid, hingedflaps, ailerons, and other control surfacesthat approximate (albeit crudely) optimalairfoil shapes.

Early morphing effortsIdeally, aircraft wings should alter theirshape discreetly; they should morph asneeded in response to changing flight con-ditions to maximize fuel efficiency andflight performance. However, morphingwings have been an elusive goal for air-craft designers, and many attempts by in-dustry, academia, and national labs havenot yielded a practical design.

In the mid-1980s, the Air Force Re-search Laboratory, partnering with NASA,modified and flight-tested an F-111 in aprogram called Mission Adaptive Wing(MAW). Using conventional rigid-linkmechanisms and fiberglass flex-panels,adaptive wing geometry proved its aero-dynamic superiority over conventionalleading- and trailing-edge flaps throughmost of the F-111 flight profile. Unfortu-

nately, drawbacks in the design, specifi-

cally increases in weight, complexity,packaging, and mechanical performance,significantly offset the aerodynamic bene-fits and impaired further development.

After the MAW program results be-came available, German researchers affili-ated with Airbus began exploring variable-camber wing technology for long- andmedium-range transport aircraft. Theiranalysis suggested that a 3-6% reduction infuel burn might be possible. But the addedstructural mass required to position their

conventional high-lift flaps for camberchange at cruise conditions limited thefuel savings to more like 1%. Withoutsome fundamentally new technology, thislevel of improvement was not sufficient tocontinue work on the project.

Nevertheless, the goals of reducedfuel consumption and lower drag contin-ued to attract interest, especially amonggovernment research agencies. By exploit-ing elasticity of the underlying structureusing compliant mechanisms, we devel-oped variable geometry airfoils in the Mis-sion Adaptive Compliant Wing (MACW)program which was funded by the Air Ve-hicles Directorate of the Air Force Re-search Laboratory, the same organizationthat funded the earlier MAW project.

Distributed elastic shape morphingThe MACW program uses jointless com-pliant mechanisms to morph shapes ondemanda concept pioneered by the firstauthor, who founded FlexSys five yearsago to explore practical applications. Thebasic theories on the design of compliantmechanisms were transformed to precise

and practical shape morphing applicationsby Joel Hetrick at FlexSys.

In contrast to conventional elasticmechanisms that employ flexible hinges,our algorithms generate designs with dis-tributed compliance for enhanced fatigueresistance and ease of manufacture. Dis-tributed compliant mechanisms get theirflexibility from the topology and shape ofthe whole material cross-section ratherthan concentrating the flexion to fixedpoints that can localize fatigue stresses.

This new design paradigm offers ad-

ditional benefits, since the whole adaptivestructure is viewed as a compliant mecha-

It has been known from the very begin-nings of aviation that significant efficiencyand maneuverability benefits can be real-ized by small, controlled deformations offlight surfaces, especially a wings leading

and trailing edges. One challenge facingcurrent research in aircraft developmentis improving the technology that changesthe shape of these control surfaces, espe-cially since the current flaps can generatea substantial fuel-consuming drag whendeployed.

Advances in the design synthesis ofjointless compliant mechanisms for appli-cations to adaptive structures have identi-fied a simple, elegant, yet robust alterna-tive to conventional mechanisms. Thisapproach promises a lightweight, smooth,continuous airfoil with enhanced flightcharacteristics including significant re-duction in drag and improved maneuver-ability. Furthermore, smooth and contin-uous shape morphing allows the airfoil toadapt effectively to different flight condi-tions rather than sacrificing performancewith a fixed geometry airfoil or drag-pro-ducing hinged flaps.

The Wright brothers original proto-type flyer had a saddle that allowedOrville, the pilot, to adjust trim on theends of the wood and fabric wings bychanging his body position. This approach

was possible because the wing structures

Adaptive structures: Movinginto the mainstream

Flexsys mission adaptive compliant trailing edge was designed for a high-altitude, long-endurance air-craft undergoing 10 flap deflection with a 3 twist. The airfoil and the conformal flap combinationwere designed to support an aggressive, 65% laminar boundary layer (chordwise) on the upper surface.

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nism that can move into complex prede-termined positions with only minimalforce and be locked in place in multipledesired configurations.

In order to morph a load-bearingstructure to different shapes or positions,the structure must have optimum compli-ance for controlled elastic deformation.Development of this concept was fundedby the Air Force Air Vehicles Directorate,in the form of an adaptive trailing-edgeflap for high-altitude, long-endurance air-

craft. The program is slated to culminatein the flight testing of a variable geometryendurance airfoil later this year, to dem-onstrate compliant wing technology forsystem application in both military andcivil aircraft.

While the current program focuses ona trailing-edge test section, parallel effortsare under way in the development of con-formal surfaces for both the leading andtrailing edge, and for rotorcraft as well asfixed-wing airfoils. The research is targetedat minimizing the force required to morphsurfaces during the full flight profile whilemaintaining maximum stiffness to with-stand all external pressure conditions.

Design and testing has accounted foractuator displacement, minimizing shapeerror, overall system weight, bucklingforces, package constraints, overall sys-tem complexity, and material fatigue.

Algorithms have been developed for aseries of kinematic, geometric, and struc-tural optimization steps to create and re-fine the structural design.

The success of the wing design is di-rectly dependent on the morphed trailing

edge maintaining a balanced aerodynamicpressure distribution along the whole air-foil upper surface. It is also focused oneliminating suction peaks around the lead-ing edge, and preventing abrupt changesin surface slope at the entry to the pres-sure recovery region. Typically, several it-erations are necessary to resolve conflictingdesign requirements between the aero-elastic analysis and the compliant struc-ture cross section.

Throughout the design process, skinkinematic requirements are considered to

ensure smooth, wrinkle-free covering ofthe underlying structure. These surfaces

must maintain cohesion and flexibilityunder all environmental conditions, andthrough the full schedule of conformalmovement. A variety of materials are be-ing applied to meet different applications,including aluminum alloys, aluminum-polymer composites, titanium alloys, glass-fiber-reinforced composites, and carbon-

fiber composites.

The fuel/drag challengeStudies at NASA Dryden show that a mere1% reduction in drag would save the U.S.fleet of widebody transport aircraft ap-proximately $140 million/year (at a fuelcost of $0.70/gal). There is wide agree-ment that some sort of smart wingcould provide optimal wing camber withgreatly reduced drag throughout a givencruise envelope. For a medium-rangetransport aircraft with such a wing, the

projected fuel savings should be about 3-5%, depending on mission distance.

Although the aerodynamic studiesfor such a program have projected long-range cruise benefits similar to the Ger-man studies, these programs have lan-guished because the smart trailing-edgehardware never matured.

The structural design of a variable-camber trailing edge targeted for a

medium-range aircraft has weight andcritical flutter speed restrictions competi-tive with conventional flap systems. Forthis case, performance estimates indicatefuel savings in the 5% range on a 4,000-n.mi. mission.

A variable-geometry trailing-edgesurface can minimize drag across the fullaerodynamic lift-coefficient range and fa-vorably shift the buffet boundary, provid-ing enhanced operational flexibility. Asa next step, trailing-edge variable geome-try can be combined with laminar-flow

technology by controlling the chordwisepressure distribution consistent with the

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Fullyattachedflowalongentiresurface

Fullyattachedflowalongentiresurface

Rotation

Rotation

Rotation

Rotation

Direction of

flight

Direction offlight

Advancing bladeAngle of attack = 1 to +4 pitchSpeed = tip speed + max forward speed = Mach

Advancing bladeAOA = 1 to +4 pitchSpeed = tip speed + max forward speed = Mach 0.98

Retreating bladeAOA = +14 to +18 pitchSpeed = Mach 0.36 (or less)Blade stall occurs at outer 1/3 of rotor span

Retreating bladeAOA = +14 to +18 pitch

Speed = Mach 0.36 (or less)No blade stall occurs anywhere along rotor span

Shock inducedflow separation

Carefully controlledactive leading edgemorphing

Separated wake region

Fullyatta

chedflow

,lowdrag

,lifthyst

eresisminim

ized

Flow-separation on the retreating blade can be avoided by simply morphing the leading edgeonce per revolution.

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tial application to the universal accep-tance that it enjoys today.

Rotorcraft applicationsRotorcraft is another category of aircraftwhere adaptive compliant structures tech-nology offers significant promise. Whilethe research is not as far along as that forfixed-wing aircraft, applying variable-geometry technology along both the lead-ing and trailing edge of rotor blades couldhave an even greater impact.

An Army-funded research project re-cently resulted in a variable-geometryhelicopter rotor blade section with an em-bedded compliant mechanism. The vari-able-geometry leading edge, with0-10deflection capability, modifies the aero-dynamics so that dynamic stall on the re-treating blade is delayed, giving the crafthigher speed and better maneuveringperformance.

In this program, we designed andfabricated a 3-ft-span, full-scale chord ro-

tor blade to demonstrate 0-10 leading-edge camber change at 6 Hz (once perrevolution). The blade was designed withhigh-strength materials to withstand pres-sure loads and centrifugal loads for 4,500hr of service life (220 million cycles fa-tigue life).

In a helicopter blade, the blade crosssection that is best for the advancingblade is far from optimal for the sameblade in its retreating phase. The deficiencymanifests itself in reduced lift and a dy-namic shock event as the blade azimuth

changes, along with vibration stress, audi-ble sound, and structural wear. These

substantially limit the performance in allrotorcraft, such as maximum speed andaltitude, as well as impacting the struc-

tural life of rotor blades and the cost ofoperation and maintenance.

The morphed leading-edge structurebegins with the airfoil shape optimal forgood high-speed performance in the ad-vancing blade phase and then changes toa cambered design that optimizes airfoilperformance as the blade retreats. It thencycles back to the advancing blade config-uration, once per revolution or at a rateup to seven times per second. This behav-ior allows the morphing aircraft structureto maintain an optimal profile throughout

its entire azimuthal circuit, thereby offer-ing substantial gains in speed and maneu-verability (12-25%) and about 10% in-crease in payload.

The reductions in drag, vibration,and wear will significantly improve per-formance of next-generation rotorcraft farbeyond the current achievable maximum,promising higher speeds, lower drag andfuel requirements, better maneuverabil-ity, and quieter operation, not to mentionlonger usable blade life and reducedmaintenance.

Another project is aimed at develop-ing a variable-geometry trailing edge forrotorcraft. This research addresses twoadditional challenges: producing lift at allrotor azimuthal positions and minimumdrag (low-frequency control), and vibra-tion damping using small trailing-edgedeflections at five cycles per revolution(high-frequency control).

The successful development of morphingtechnology will impact not only fixed-wing and rotorcraft vehicles, but other

aerodynamic devices such as wind turbineblades. Adaptive airfoils can also improvethe performance of air and fluid channels,engine inlets, and underwater vehiclesand systems.

In short, better performing airfoilsoffer a whole new generation of more effi-cient, quieter, faster, and highly maneu-verable and reliable vehicles and systems,yielding a whole new opportunity for in-dustries which are so dependent on fuelefficiency. Sridhar Kota

Joel A. Hetrick

Russell F. Osborn Jr.FlexSys

18 AEROSPACE AMERICA/SEPTEMBER 2006

laminar-flow requirements of variablecamber airfoils. This could significantlyboost fuel savings potential in the nextgeneration of transport aircraft, perhapsinto the 12% range.

In large part, the analysis work hasalready been done and the adaptive struc-tures design algorithms have been devel-oped and applied with success in endur-ance aircraft wing geometry. The next stepis to bring this advanced military-leveltechnology into the commercial aircraftarena to realize significant fuel savings. Itmay be worth remembering that the con-cept of winglets took a long time from ini-

Without leading-edge morphing

With leading-edge morphing

Morphing the leading edge of a rotor blade as it approaches the retreating side delays dynamic stall,thereby enabling the helicopter to fly faster, maneuver better, carry heavier payload, and reducestall-induced vibration.

A laminar-flow fl ight test model fitted withvariable geometry trailing edge was tested ina high-speed subsonic wind tunnel at WrightPatterson AFB in June. The model was sched-uled for performance flight test inAugust onScaled Composites White Knight aircraft.