Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.

Design, Fabrication, and Testing of the DARPA / Wright Lab "Smart Wing"Wind Tunnel Model

Jayanth N. Kudva, Kari Appa, Christopher A. Martin, and A. Peter JardineNorthrop Grumman Corporation

Military Aircraft Systems DivisionEl Segundo, CA, USA - 90245

George Sendeckyj and Terry HarrisWright Laboratory

Wright Patterson AFBDayton, OH

Anna-Marie McGowan and Renee LakeNASA Langley Research Center

Hampton, VA

ABSTRACT

The concept of an adaptive aircraft wing, i.e., whose shapeparameters such as camber, span-wise twist, and thicknesscan be varied to optimize the wing shape for various flightconditions, has been extensively studied by numerousresearchers [1-8]. While the aerodynamic benefits (interms of increased lift/drag ratios, improvedmaneuverability, and delayed flow separation) have beenanalytically and experimentally established, thecomplexity and weight penalty of the designs and actuationmechanisms have limited their practical implementation.Recent developments in sensors and actuators using smartmaterials could potentially alleviate the shortcomings ofprior designs, leading the way to a more practical "smart"adaptive wing which responds to changes in flight andenvironmental conditions by optimally modifying itsshape.

This paper presents the results of recent work conductedunder a Defense Advanced Research Projects Agency(DARPA) contract entitled "Smart Structures and MaterialsDevelopment - Smart Wing". In particular, developmentand testing of the smart wing wind tunnel model arepresented. Limitations and potential benefits of adaptivewing designs are also discussed, along withrecommendations for future work required to develop anoperational smart adaptive wing.

1.0 INTRODUCTION AND BACKGROUND

Since the dawn of manned flight, aircraft engineers havedreamed of adaptive wings to provide optimal flightperformance over a wide range of flight conditions. Theterms active and adaptive are used to broadly convey afamily of concepts wherein the structure senses theenvironment and responds actively to optimizeperformance. For aircraft, concepts include: (1) activefeedback control systems for flutter suppression, loadalleviation, and improvements in

ride quality; and (2) changing the shape of the wing (tovary camber, span-wise twist or airfoil cross-section) foroptimal performance at different flight conditions (take-off, landing, maneuver, and multiple cruise conditions).

Whereas active load alleviation systems are quite welldeveloped and installed on several commercial and militaryaircraft, active flutter suppression systems have yet to beincorporated in operational aircraft. Current experimentalefforts are based on actively deploying conventionalcontrol surfaces.

The theoretical benefits of active control of wing shape arewell known and have also been experimentally validated.Two extensive studies in this area are the mission adaptivewing (MAW) and the active flexible wing (AFW) programs[1-5]. The MAW design used a mechanical actuationsystem to smoothly deploy leading and trailing edgecontrol surfaces which were fully enclosed by flexible wingskins to provide increased efficiency by elimination ofdiscontinuities in the airfoil cross-section. Performancebenefits over a conventional fixed camber wing in thesubsonic regime were demonstrated in flight tests on amodified F-lll. However, the complexities of themechanical actuation system and increase in overall weightrendered the design impractical for fleet operations.

The AFW concept on the other hand involves reducing thewing flexibility (and hence weight). To improve maneuverperformance, the wing was twisted using aerodynamictorque provided by control surface deflections. Aeroelasticperformance degradation was offset using active controls.While the anticipated aerodynamic performance benefitswere somewhat compromised by the increased drag due tothe use of control surfaces to both twist the wing and fornormal flight control operations, the concept hassufficient benefits and a detailed flight test is currentlybeing planned [5].

Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.

The smart wing concept is based on both the AFW andMAW designs and potentially improves the benefits bymaking judicious use of smart materials and structurestechnologies. Under an DARPA/WL contract to NorthropGrumman, the smart wing concept is being investigatedincorporating new ideas in integrated sensing and actuationsystems. Details of the program are discussed below.

2.0 SMART WING REQUIREMENTS, DESIGN,AND TESTINGUnder the smart wing program, three key features are beingstudied: 1) hingeless, smoothly contoured trailing edge(TE) control surfaces, 2) variable wing twist, and 3) real-time pressure distribution data for feedback control. Toevaluate the concepts and quantify performanceimprovements, two 16% scaled models (of a presentgeneration fighter aircraft), one conventional and the otherincorporating the above features (Figures 1 and 2), havebeen fabricated and tested in a wind tunnel to quantifyperformance benefits of the smart wing concept. Prior toundertaking the design, actuation requirements for thesmart wing were established.

Flap (Electrically Actuated)

Aileron

Fiber Opticsensors

Figure 1. Conventional Wing Model2.1 RequirementsDetails of the requirements analysis performed in theprogram are provided in References 8 and 9. Figure 3shows the actuation rates needed for various flightoperations. Figure 4 shows calculated (nominal) values oftorque at wing mid-span and tip to achieve 2 and 5 degreesof twist for a full-scale aircraft and scaled models. (Thetorque requirements increase essentially as the fourth powerof the geometric scaling factor - the values shown in thefigure are slightly different because of differences in thematerials used.) While it is feasible to achieve the torquerequirements for the models, it is obvious that meetingscaling requirements will be a significant challenge totransition this technology to a full-scale aircraft. This isdiscussed further in Section 3.

Controureduontroi surtaces

SMA Torque Tubes

Surface ContouredUsing SMAs

Figure 2. Smart Wing Model

Flight ConditionCruiseTake-off/LandingManeuverFlutter Suppression

Actuation Rate0.3 to 1 Hz0.3 to 1 Hz1 to 10 Hz

50 to 100 Hz

Figure 3. Actuation Rate Requirements

WingTwist@ 50%span@ wingtip

Full-scale

2.0x1 06

0.16x106

16%Model

1.5x103

3.0x1 02

10%Model220

165

Twist(deg)

2°

5°

Figure 4. Nominal Torque Requirements forWing Twisting (in.lbs.)

2.2 DesignWing Twist: Several design concepts (Figure 5) wereconsidered for twisting the wing for the wind tunnelmodels. Initial trade studies indicated that the integratedtorque box concept was structurally most efficient.However, on further examination, the design presentedsevere manufacturing difficulties and appears to besomewhat impractical. Hence the shape memory alloy(SMA) torque tube actuation was chosen and a design withtwo concentric tubes as shown in Figure 5A wasimplemented. This technique functioned well in the tunnel,but because the final wind tunnel model was significantlystiffer than the scaled model (primarily due to escalationof wing skin and spar web thickness from the originalscaled values to prevent local panel buckling), maximumwing tip twist of only about 1.25 degrees was realized. Ifthe stiffness were scaled exactly, 3 to 5 degrees of twist atthe wing tip could easily have been achieved. Furtherdetails of the torque tube design are presented in Reference10 and 11.

Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.

(a) NESTED TORQUE TUBES (b) TWIN NESTED SMATORQUE TUBES

__ SMAEmMdtdSMA Tendons TerrWnttton

(2 Anugonutlc Sets) PW> (Typ)

Figure 5. Wing Twist Activation Concepts

Adaptive Control Surfaces: The aerodynamic benefits ofcontoured hingeless surfaces are well known [1-3]. Toimplement these types of control surfaces, SMA basedactuation systems are ideal because of their high force andhigh strain capabilities [12, 13]. Figure 6 shows aschematic of the adaptive control surfaces with embeddedSMA wires in top and bottom face sheets which providetwo-way "antagonistic" actuation. Figure 7 shows the finalsystem used for the wind tunnel model. Approximatelyforty 20 mil diameter wires were used to obtain theequivalent of ten degrees of rotation. Because of thecomplex thermo-mechanical behavior of the SMA wires, i twas essential to incorporate sensors to determine the trueposition of the control surfaces. The most suitable sensorswere fiber-optic sensors, and a suite of extrinsic Fabry-Perot interferometric (EFPI) strain sensors were embeddedin the control surfaces and calibrated to provide an accuratemeasure of control surface actuation. This information wasused for feedback to command, achieve and maintain adesired deflection.

A modified version of the EFPI strain sensor was alsodeveloped for pressure sensing; the design, testing andperformance of this sensor is discussed in detail inReference 14.

CONVENTIONALCOMPOSITE

LAMINATE

FULLY ACTUATED POSITIONFULL STRESS IN «1 SMA WIRES >

NO STRESS IN »2 SMA WIRES

SANDWICH CORE '

FULLY ACTUATED POSITIONFULL STRESS IN *2 SMA WIRESNO STRESS IN «1 SMA WIRES

SMA COMPOSITEFACESHEET*!

FABRICATEDSHAPE AND

— — POWER-OFFPOSITION

SMA COMPOSITEFACESHEET*2

Figure 6. Antagonistic Actuation Concept

SMA WIRECENTER LAMINATIOi(GLASS EPOXY)CORE (PHENOLIC)

TAILCAPWIRE TERMINATION(MECHANICAL/ELECTRICAL)TERMINATION STRIP (TORLON)

FACE SHEETS (SILICONE)TERMINATION STRIP (TORLON)

Figure 7. Details of Smart Trailing EdgeDesign2.3 Fabrication and Assembly

The basic planform of the wind tunnel models consists offour spars and three ribs and is shown in Figure 8.

Figure 8. Layout of the Wind Tunnel Models

Details of the SMA torque tube fabrication, training andintegration into the structure are discussed in Reference 11;a few pertinent issues are summarized here.

The SMA torque tubes were manufactured from a hot rolledUnimet metal rod obtained from Special Metals company.Due to the inherent difficulty in machining TiNi, the rodswere cut into 4 inch lengths using EIcetrospark DischargeMachining (EDM), followed by gun drilling to produce arough tube, and then further EDM processed to obtain thefinal tube.

Because of a lack of accurate knowledge of the complexthermo-mechanical behavior of SMA torque tubes, theirtraining was a trial and error process, requiring manyiterations.

Connection of the torque tubes to the wing structure toensure maximum torque transfer as well as provide easyassembly required an innovative design. Figure 9 shows a

Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.

schematic of the connections. Root, mid- and tip ribs wereequipped with structurally integral tangs withcorresponding tight fitting slots in the torque tubeconnector to transmit the twist to the various sections.Stainless steel shafts provided the link between the wingstructure and the torque tubes. The key-way slot permittedlength adjustments and the offset holes accommodatedangular adjustments. The attachments were secured usinghardened stainless pins.

photograph of the smart wing model is shown in Figure 11.

In addition to pressure data at approximately 130 points,six component balance data were recorded. Data reduction iscurrently under progress but preliminary results areencouraging. Approximately 8% increase in rolling wasobtained due to a wing twist of only 1.25 degrees (Figure12). The hingeless control surface typically providedbetween 8 and 18% increase in rolling moment compared toa conventional design (Figure 13) .

SSHAFT

TAPEREDSLOTFORRIB MSB

Figure 9: Schematic of Connection Design ofSMA Torque Tube to Wing Structure

2.4 Testing

Figure 10 shows the range of test parameters used at theNASA Langley tests conducted during May 1996. A

Pressure(psf)

2200,1100

Aileron(deg)

0, 5, 10

Flap(d»g)

±(0,5,10)

Wing TipTwist(dag)0,125

Q(psf)

40-120

Figure 10. Wind Tunnel Test Parameters

Figure 11. Smart Wing Wind Tunnel Modelwith SMA Flap (inset)

-0.25 1

Figure 12. Rolling Moment Comparison ofTwisted and Untwisted Wing

1.20

50 150 Q (psf)

Figure 13. Rolling Moment ComparisonConventional and Smart Wing Models

of

Significant improvements in the pressure distribution dueto delayed flow separation at the trailing edge was alsoobserved in the smart wing model (Figure 14). While themaximum improvement was seen at the trailing edge, theoverall pressure distribution also improved, resulting in anet lift increase. Further details of the model design,testing and test results are provided in References 15 and16.

Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.

S o

flow control etc. A system level optimization approach,wherein the load distribution is varied to optimizeperformance at different flight conditions using aeroelastictailoring and a judicious combination of conventional andsmart control systems, is recommended. A phase two smartwing effort along these lines is currently being planned.

4.0 CONCLUDING REMARKSSensors, actuators and innovative designs incorporatingsmart materials and structures technologies could lead tothe design of a truly adaptive "smart" aircraft wing whichwould provide optimal performance at all points in theflight regime by changing its shape parameters andactively responding to external loads and operatingconditions. On-going work sponsored by DARPA and theUnited Air Force are laying the foundation for the eventualrealization of this goal. While preliminary results areencouraging, much work remains to be done, particularly inthe areas of design, optimization, system levelintegration, and cost-benefits analysis.

Figure 14. Comparison of the PressureDistribution in the Conventional and SmartWing Models.

As can be seen from the results, the performanceimprovements of the smart wing design, quantified in thewind tunnel tests, are significant; these improvementsscale-up directly to a full-scale aircraft, i.e., similarpercentage improvements can be expected in operationalaircraft if the concepts are implemented. However, asdiscussed earlier, scale-up of the actuation systems is thecritical issue and will be addressed in Phase 2 of theprogram.

3.0 FUTURE PLANS AND TECHNOLOGYTRANSITIONAfter data reduction and assessment of the results from thefirst wind tunnel test, efforts will be directed towards a)addressing manufacturing and system reliability issues, b)unsteady maneuver conditions and flutter suppression. Inthis regard, extendible leading edge control surfaces whichcan be activated at 30 to 50 Hz appear to have significantpotential. The final goal of the smart wing developmenteffort is to facilitate incorporation of smart material andstructures technologies to improve the performance ofmilitary aircraft. This includes take-off and landing, cruise,maneuver, and aeroelastic stability boundary conditions.

As mentioned earlier, the raw requirements for wingtwisting, control surface contouring etc., scale-up as thefourth power of the linear dimension and a directimplementation of the concepts discussed here may not befeasible and in fact, may not be the most efficient way toproceed. Smart wing technologies will need to beconsidered as one part of the total solution to obtainadaptive wing designs along with other related conceptssuch as aeroelastic tailoring, active flutter suppression,

6.0 ACKNOWLEDGMENTS

Much of the work reported here was performed under aUnited States Defense Advanced Research Projects Agency(DARPA) contract to a team led by Northrop Grumman,entitled "Smart Materials and Structures Development -Smart Wing". The contract is monitored by the U. S. AirForce Wright Laboratory, Wright Patterson Air Force Base,OH. The DARPA program manager is Dr. Robert Crowe.The support and technical guidance provided by Dr. Croweand Dr. Janet Sater (IDA) is greatly appreciated. The windtunnel testing was conducted at the NASA LangleyTransonic Dynamic Tunnel (TDT) in Hampton, VA; besidesthe NASA co-authors. Dr. Tom Noll, Mr. Bob Moses, Ms.Sherry Hoadley, and Dr. Tony Rivera provided excellentsupport above and beyond the call of duty during sixstrenuous weeks of wind tunnel testing.

Besides the authors, key members of the smart wingdevelopment team at Northrop Grumman include Mr. LarryJasmin (hardware design), Mr. Lewis Scherer (wind tunneltesting), Mr. John Flanagan (model fabrication) and Mr.Rich Votava (instrumentation).

Other members of the team include Dr. Bernie Carpenter ofLockheed Martin (SMA control surface development), Mr.Mark West, Mission Research Corporation (systemsoftware), and Mr. Paul Duncan, Fiber and SensorTechnologies (fiber optic sensors). The authors would alsolike to thank Mr. Mark Roberts and Ms. Carlene Lau for thediligent and dedicated help they provided in preparing thispaper.

Copyright© 1997, American Institute of Aeronautics and Astronautics, Inc.

7.0 REFERENCES

1. Hall, J.M., "Executive Summary AFIT/F-111 MissionAdaptive Wing," WRDC-TR-89-3083, September1989.

2. Wong, K.J., "AFIT/F111 Mission Adaptive Wing Liftand Drag Flight Test Results," AFFTC-TR-86-42, FinalReport, March 1987.

3. Cogburn, L.T., "AFTI/F111 Mission Adaptive WingFlutter and Aeroservoelastic Test Program," AFFTC-TR-86-42, Final Report, April 1987.

4. Miller, G.D., "Active Flexible Wing (AFW)Technology," AFWAL-TR-87-3096, February 1988.

5. Pendelton, E., et. al., "A Flight Research Program forActive Aeroelastic Wing Technology," AIAA Paper No.96-1574, presented at the 37th AIAA SDM Conference,Salt Lake City, UT, April 15-17, 1996.

6. Fielding, J.P., and Vaziry-Zanjany, M.A.F.,"Reliability, Maintainability, and Development CostImplications of Variable Camber Wings," AeronauticalJournal, pp. 183-195, May 1996.

7. Redeker, G., Wichmann, G., and Oelker, H-C.,"Aerodynamic Investigations Toward an AdaptiveAirfoil for a Transonic Transport Aircraft," Journal ofAircraft, Vol. 23, No. 5, pp. 398-405, May 1986.

8. Kudva, J.N., et al., "Overview of the ARPA/WL 'SmartStructures and Material Development-Smart Wing'Contract," Paper No. 2721-02, SPJE North AmericanConference on "Smart Structures and Materials," SanDiego, CA, February 26-29. 1996.

9. Kudva, J. N., et al., "Adaptive smart wing design formilitary aircraft: requirements, concepts and payoffs."Paper No. 2447-04, SPIE North American Conferenceon "Smart Structures and Materials," San Diego,California, February 26 - March 3, 1995.

10. Jardine, A. P., et al., "Shape Memory Alloy TiNiActuators for Twist Control of Smart Wing Designs,"Paper No. 2716-10, SPIE North American Conferenceon "Smart Structures and Materials," San Diego,California, February 26 - February 29, 1996.

11. Jardine, A. P., Flanagan, J., Jasmin, L.. and Carpenter,B. F., "Smart Wing Shape Memory Alloy ActuatorDesign and Performance," Paper No. 3044-04, SPIE

North American Conference on "Smart Structures andMaterials," San Die, CA, March 3-6, 1997.

12. Maclean, B.J., Carpenter, B.F., Draper, J. S., andMisra, M.S., "A Shape Memory Actuated CompliantControl Surface, " SPIE, Vol. 1917, "Smart Structuresand Intelligent Systems," 1993, pp. 809-818.

13. Misra, M.S., et. al., "Adaptive Structure DesignEmploying Shape Memory Actuators," Paper No. 15,AGARD-CP-531, presented at the 75th meeting of theAGARD Structures and Materials Panel, Lindau,Germany, 5-7 October 1992.

14. Duncan, P. G., Jones, M. E., Shinpaugh, K. A.,Poland, S. H., Murphy, K. A., and Claus, R.O.,"Optical Fiber Pressure Sensors for AdaptiveWings", Paper No. 3042-41, SPIE North AmericanConference on "Smart Structures and Materials," SanDiego, CA, March 3-6, 1997.

15. Martin, C. A., Jasmin, L., Appa, K.. and Kudva, J. N.,"Smart Wing Wind Tunnel Model Design", Paper No.3044-03, SPIE North American Conference on "SmartStructures and Materials," San Diego, CA, March 3-6,1997.

16. Scherer, L. B., Martin, C. A., Appa, K., Kudva, J. N.,and West, M., "Smart Wing Wind Tunnel TestResults", Paper No. 3044-05, SPIE North AmericanConference on "Smart Structures and Materials," SanDiego, CA, March 3-6, 1997.