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«a paper presented at SPIE’s 1996 Symposium on Smart Structures and Integrated Systems Jennifer L. Pinkerton Anna-Maria R. McGowan Robert W. Moses ...»

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During the open-loop entry in July 1995, transfer functions were acquired for the response of the vertical tail to rudder (and buffet) inputs, to piezoelectric actuator (and buffet) inputs, to tip vane (and buffet) inputs, and to embedded cylinder (and buffet) inputs. Surface pressure data was also acquired. Based on the open-loop data, the rudder and the piezoelectric actuators appeared the most effective candidates. In addition to these open-loop measurements, several preliminary control laws were tested using the rudder, where in each case, the control law was designed using classical control methods. Data for several of these preliminary rudder-based control laws reduced the RMS response in the first bending mode by as much as 35%. This preliminary closed-loop result gave valuable insight into active buffeting alleviation for the November 1995 closed-loop entry. During the November entry, sixteen control laws were tested for the rudder while six control laws were tested for the piezoelectric actuators. Each control law reduced the power spectral density of the first bending mode by as much as 60% using gains well below the actuators’ physical limits. The results of one control law for the piezoelectric actuators are shown in Figure 14. Pressure measurements were acquired on both surfaces of the flexible and rigid vertical tails during numeraus open-loop and closed-loop conditions at various angles of attack.

The results of these wind-tunnel tests illustrate that buffeting alleviation of vertical tails can be accomplished using active control of the rudder or of piezoelectric actuators. A better understanding of control law design for active buffeting alleviation has been gained from this investigation. The pressure measurements offer insight into the flowfield around the tail during buffet. +

3.3.2 Future DlanS

Tests in the TDT to investigate potential adaptive control concepts for buffeting alleviation are planned for later this decade.

In addition to wind-tunnel investigations, the United States Air Force (USAF) and NASA, through the auspices of The Technical Cooperative Program (TTCP), have initiated a collaborative buffeting alleviation study involving research organizations and industry from the United States, Australia, and Canada. The objective of this program is to ground test a full-scale F- 18 vertical tail that incorporates a buffeting alleviation system employing piezoelectric actuators. The ground tests are expected to be performed in Australia at the Aeronautical and Maritime Research Laboratory in Melbourne. Plans are to pursue a flight test demonstration of the concept at NASA Dryden Flight Research Center in 1999 if the ground test program proves successful.

3.4 Airfoil THUNDER Testing to Ascertain Characteristics (ATTACH) project

The Airfoil THUNDER Testing to Ascertain Characteristics (ATTACH) project was a feasibility study that focused on identifiing (1) the material characteristics, such as creep, hysteresis, and fatigue, and (2) the airfoil shaping effectiveness of the new THUNDER piezoelectric technology under aerodynamic loading. Characterization of the material behavior of THUNDER was the objective for Phase I, while Phase I1 examined its ability to reduce drag over an airfoil. The following sections present both the approach and preliminary findings of both phases of testing.

The testbed used for both phases of testing in the ATTACH project was the 0.25-inch thick, 1.5-inch wide, 5-inch long, generic, roughly-symmetric airfoil shown in Figure 15. This airfoil was supported by two 0.25-inch thick, IO-inch long sidewalls that extended through 85% of the length of the test section, creating a nearly two-dimensional flow condition. A single 1.5-inch wide, 2.5-inch long rectangular wafer of THUNDER was placed near the leading edge of the airfoil to act as the first half ofthe upper surface. To smooth the airfoiuwafer interface, a sheet of thin fiberglass material was wrapped over the upper surface of the airfoil/wafer combination and held in place by a flexible latex membrane. A photograph of the installed model is shown in Figure 16.

3.4.1 Phase I testing

The initial series of tests conducted during the ATTACH project were performed to identify the creep, hysteresis, and fatigue characteristics of a THUNDER wafer under aerodynamic loading. For this experiment, a total of 60 conditions were tested, consisting of combinations of the following parameters: five angles of attack (-2O, Oo, +2O, +4",+6O), four steady-state input voltages (-102 V, +lo2 V, -170 V, +170 V), and three tunnel velocities (wind-off, 20 m/s, 35 m/s).

The data obtained during this experiment conclusively identified the presence of both creep and hysteresis of the wafer under wind-on (loaded) and wind-off (unloaded) conditions. An example of wafer displacement in response to five cycles of applied voltage is shown in Figure 17. The test conditions were wind-off with the model at zero degrees angle of attack. In the figure, creep is characterized by the increasing positive (up) and negative (down) displacements exhibited by the wafer while under the constant applied voltages of +lo2 V, respectively. Hysteresis appears as the repeatable symmetric "loops," which in this case are: offset slightly due to creep. Figure 18 provides a comparison of the wafer displacement responses for the wind-off and 20 m/s conditions, again at zero degrees angle of attack. Creep and hysteresis are still apparent for the wind-on condition, but the presence of the flow contributes to smaller positive displacements (compared to wind-off) for the same applied voltage. Wafer displacements are smaller at lower tunnel velocities (not shown) and higher at higher angles of attack (also not shown).

During this phase of testing, the health of the wafer was frequently checked to gain a preliminary understanding of material fatigue. After two weeks of testing, the performance of the wafer began to noticeably degrade. During subsequent examination, no visible flaws were found, but a 33% drop in capacitance was discovered, and repoling returned the wafer to its full capabilities. Thus, similar to other piezoelectric adaptive materials, the performance of THUNDER appears to be a function of capacitance.

3.4.2 Phase I1 testing

With the objective of determining the ability of the THUNDER wafer to reduce drag over the airfoil, tests were conducted at the 40 wind-on conditions described above. For purposes of this feasibility study, it was assumed that variations in drag were directly proportional to velocity changes in the wake of the model. Comparisons of wake velocity for different test conditions, therefore, provided qualitative indications of the drag reducing potential of this piezoelectric actuator for this subscale model. Velocity measurements were taken by traversing a hot film anemometer velocity probe in 0.125-inch increments through the center of the test section sufficiently aft of the airfoil trailing edge to allow the wake to return to tunnel static pressure.

Results from this phase of testing were consistent with the results from Phase I. Velocity and angle of attack had the anticipated effect on drag. Positive applied voltages, which expanded the upper surface of the airfoil up to meet the flow, had the effect of reducing drag. Negative applied voltages produced the opposite effect. These results were obtained using only 32% of the maximum unloaded capability of the wafer. Thus, greater drag reductions would be expected if that percentage is increased.

The results of this research effort indicate that the new THUNDER actuator technology is a promising candidate for future airfoil shaping investigations. Despite decreases in the displacements of the wafer due to aerodynamic loading, noticeable drag reductions were obtained.


This paper has presented brief descriptions of four innovative in-house research programs from the Aeroelasticity Branch at the NASA Langley Research Center. In these programs, adaptive materials and integrated systems are used for either active aeroelastic control or passive aerodynamic shape control. Active wing flutter suppression and reduction in wing response at speeds below flutter were demonstrated in the PARTI program using piezoelectric actuators. Demonstration of an integrated flutter suppression system using adaptive neural network control is being studied through the ANCAR program. The ACROBAT program demonstrated reduced buffeting of the vertical tails of an F- 18 scale model using piezoelectric actuators and an active rudder. And the airfoil shaping potential of a new piezoelectric actuator technology called THUNDER was demonstrated in the ATTACH project.


1. Renken, J. H., “Mission - Adaptive Wing Camber Control Systems for Transport Aircraft,” AIAA-85-5006, AIAA 3rd Applied Aerodynamics Conference, Colorado Springs, Colorado, October 14-16, 1985.

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25, pp. 297-328, 1988.

3. Redeker, G.; Wichmann, G.; and Oelker, H. C., “Aerodynamic Investigations Toward an Adaptive Airfoil for a Transonic Transport Aircraft,” Journal of Aircraft, Vol. 23, No. 5, pp. 398-405, May 1986.

4. Sandford, M. C.; Abel, I.; and Gray, D. L., Development and Demonstration of a Flutter Suppression System Using Active Controls, NASA TR R-450, December 1975.

5. Newsom, J. R. and Abel, I., Active Control of Aeroelastic Response, NASA TM-83179, July 1981.

6. Waszak, M. R. and Srinathkumar, S., “Active Flutter Suppression: Control System Design and Experimental Validation.” AIAA Pauer No. 91-2629. August 1991.


7. Hwang, C.; Winther, B. A.; and Mills, G. R., Demonstration of Active Wing/Store Flutter Suppression Systems, AFFDL TR-78-65, June 1980.

8. Ashley, Holt; Rock, Stephen M.; Digumarthi, Ramarao; Chaney, Kenneth; and Eggers, Alfred J., Jr., Active Control for Fin Buffet Alleviation, WL-TR-93-3099, January 1994.

9. Weisshaar, T. A., “Aeroservoelastic Control with Active Materials - Progress and Promise,” Proceedings of the CEAS International Forum on Aeroelasticity and Structural Dynamics, Manchester UK, June 1995.

10. Lichtenwalner, P. F. and Little, G. R., “Adaptive neural control of aeroelastic response,’’ SPIE Vol. 27I7: Smart Structures and Integrated Systems, Publication pending.

1 1. Heeg, J.; Miller, J. M.; and Doggett, R. V., Attenuation of Empennage Buflet Response Through Active Control of Damping Using Piezoelectric Material, NASA TM- 107736, February 1993.

12. Crawley, E. F.; Warkentin, D. J.; et al., Feasibility Analysis of Piezoelectric Devices, MIT-SSL 5-88, Space Systems Laboratory, Massachusetts Institute of Technology, Cambridge, Mass., January 1988.

13. Leeks, Tamara J. and Weisshaar, Terrence A., “Optimization of unsymmetric actuators for maximum panel deflection control,” SPIE Vol. 2443: Smart Structures and Integrated Systems, pp. 62-74, San Diego, CA, February 27March 3, 1995.

14. Weisshaar, Terrence A,, “Active Aeroelastic Tailoring with Advanced Materials,” 3 Ist Aircraft Symposium, pp. 34-45, Gifu, Japan, Nov. 10-1 1, 1993.

15. Layton, Jeffrey B., “An Analysis of Flutter Suppression Using Adaptive Materials Including Power Consumption,” AIAA-95-1191-CP, pp. 299-305, 1995.

16. Weisshaar, T. A., “Aeroservoe~astic Control Concepts with Active Materials,” ASME International Mechanical Engineering Congress Exposition, Special Symposium on Aeroelasticity and FluidStructure Interaction Problems, Proceedings of the 1994 ASME Winter Meeting, November 1994.

17. The NASA Langley Transonic Dynamics Tunnel, LWP-799, September 1969.

18. Heeg. J., Analytical and Experimental Investigation of Flutter Suppression by Piezoelectric Actuation, NASA TPMarch 1993.

19. Crawley, Edward F. and Anderson, Eric H., “Detailed Models of Piezoceramic Actuation of Beams,” Journal of Intelligent Material Systems and Structures, Vol. 1., pp. 4-25, January 1990.

20. Takahashi, Sadayuki., “Longitudinal Mode Multilayer Piezoceramic Actuators,” Ceramic Bulletin 65, The American Ceramic Society, pp. 1 156-1 157, 1986.

2 1. Haertling, Gene H., “Rainbow Ceramics -- A New Type of Ultra-High-Displacement Actuator, American Ceramic ” Society Bulletin, Vol. 73, No. 1, pp. 93-96, January 1994.

22. Hellbaum, Richard F., et al., Patent Application Number LAR15348- 1.

23. Heeg, J.; McGowan, A-M. R.; Crawley, E. F.; and Lin, C. Y., “The Piezoelectric Aeroelastic Response Tailoring Investigation: Analysis and Open-Loop Testing,” Proceedings of the CEAS International Forum on Aeroelasticity and Structural Dynamics, Manchester UK, June 1995.

24. Lin, C. Y. ; Crawley, E. F.; and Heeg, J., “Open-Loop and Preliminary Closed-Loop Results of a Strain Actuated Active Aeroelastic Wing,” Proceedings of the 36th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics, and Materials Conference, New Orleans, LA, April 1995.

25. McGowan, Anna-Maria Rivas; Heeg, Jennifer; and Lake, Renee C., “Results From Wind-Tunnel Testing from the Piezoelectric Aeroelastic Response Tailoring Investigation,” Proceedings of the 1996 SDM Conference, Publication pending.

26. Reich, G. W. and Crawley, E. F., Design and Modeling of an Active Aeroelastic Wing, SERC #4-94, Massachusetts Institute of Technology, February 1993.

27. Durham, Michael 14.; Keller, Donald F.; Bennett, Robert M.; Wieseman, Carol D., A Status Report on a Modeifor Benchmark Active Controls Testing, NASA TM- 107582, April 1991.

28. Rivera, Jose A., Jr.; Dansbeny, Bryan E.; et al., “Experimental Flutter Results with Steady and Unsteady Pressure Measurements of a Rigid Wing on a Flexible Mount System,” AIM 9 / - f O / O, April 1991.

29. Soloway, D. and Haley, P., Neural Generalized Predictive Control, NASA TM pending publication, 1996.

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