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«Aeroservoelastic and structural dynamics research on smart structures conducted at NASA Langley Research Center Anna-Maria %vas McGowan, W. Keats ...»

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A vital part of maturing piezoelectric control technologies so that they may be used on full-scale aircraft in the future is developing an understanding of the issues associated with scaling wind-tunnel test results to full-scale and conducting tests on actual full-scale structures. With this in mind, three research activities in the area of active buffeting alleviation using piezoelectric actuators may provide useful results: ( I ) ground tests ofa full-scale F/A-18 airplane using simulated buffet loads, which were completed in February 1998; (2) additional wind-tunnel tests using scaled models to examine scaling issues, which are currently planned to begin in late 1998; and (3) potential future flight test demonstrations, which are tentatively scheduled to begin in 200 I.

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3.2 Ground Testing Using a Full-Scale F/A-18 with Patch Piezoelectric Actuators Ground tests of a full-scale F/A-18 aircraft were conducted under the auspices of The Technical Cooperation Program (TTCP) as well as an international bilateral agreement and involved the USA, Australia, and Canada. The tests were conducted at the Aeronautics and Maritime Research Laboratory (AMRL) in Melbourne, Australia. The test facility used air bags and shakers to simulate buffet loads on the aircraft. Patch piezoelectric actuators were surface-bonded to the vertical tails for active control.

The piezoelectric actuators and control system were designed and built by Active Controls !lperts (ACX) in fulfillment of a Phase I1 SBIR with the Air Force Research Laboratory at Wright Patterson Air Force Base. NASA Langley assisted with control law design and experimental testing.

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ground tests will be used to determine the control Figure 5 Power Spectral Density Functions of Acceleration effectiveness of piezoelectric actuators over a range of Measured Near the Trailing Edge Tip on the simulated flight conditions and for future scaling studies to F/A- 18 Aircraft During Ground Testing be conducted in wind-tunnel tests.

3.3 Future Wind-Tunnel and Flight Tests A scaled version of the buffet load alleviation system used in the ground test will be ground- and wind-tunnel tested on a 16% F/A-18 model at the Transonic Dynamics Tunnel in 1999 as part of the SIDEKIC (Scaling Influences Derived from Experimentally-Known Impact of Controls on buffet-affected tails) program. As the acronym implies, relationships for scaling the performance of piezoelectric actuators will be determined and validated by comparing the full-scale ground test results with scaled ground- and wind-tunnel results. Additionally, plans are underway to measure buffet pressures on other twin-tail configurations, namely the F-22 and the Joint Strike Fighter (JSF).

Plans are also being discussed by the Air Force Research Laboratory, NASA Dryden Flight Center, and NASA Langley for a flight demonstration of active rudder and active piezoelectric actuators in 2001. NASA Langley plans to play an active role in the flight demonstration tests by wind-tunnel testing scaled-models of the proposed buffet load alleviation system concepts prior to the flight tests and by designing control laws for the flight tests.

4. DARPA/ AFRL/ NASA/ NORTHROP GRUMMAN SMART WING PROGRAM

The overall objective of the DARPAI AFRL/ NASA/ Northrop Grumman Smart Wing program is to design, develop and demonstrate the use of smart materials and structures to improve the aerodynamic performance of military aircraft including improvements in lift to drag ratio, maneuver capabilities and aeroelastic effects. The approach includes: 1) designing, fabricating and testing scaled semi-span and full-span wind-tunnel models; 2) addressing power, reliability, packaging and system integration issues; and 3) laying the ground work for technology transition in a potential follow-on program. An overview of the program is presented in reference 23. The Smart Wing program is led by the Northrop Grumman Corporation who was awarded DAEWA contracts for Phase I, which began in January 1995, and Phase 1, which began in September 1997. Phase I and Phase I1 contracts are being monitored by the Air Force Research Laboratory (AFEU) at Wright-Patterson Air Force Base. Wind-tunnel testing during the program is being performed at the NASA Langley TDT via a Memorandum of Agreement between NASA Langley and Northrop Grumman. In addition, the administrative responsibilities of the program are being coordinated through an Interagency Agreement between the AFRL and NASA Langley. Other members of the large team of researchers on the program include: Lockheed Martin Astronautics and Control Systems; Naval Research Labs; Mission Research Corporation; Rockwell Science Center; Fiber & Sensor Technologies, Inc.; Etrema Products, Inc.; SRI International; University of California, Los Angeles; Georgia Institute of Technology; and the University of Texas at Arlington.

During Phase I of the program, a 16% semi-span model (“Smart Wing”) of a high-performance fighter aircraft was designed and fabricated incorporating three key features: 1) variable spanwise wing twist, 2 ) hingeless, smoothly contoured trailing edge control surfaces and 3) fiber optic pressure and strain transducers. Another identically-scaled model of conventional construction (hinged control surfaces and no wing twist) was fabricated and used as a baseline for comparison. On the Smart Wing model, wing twist is accomplished through the use of two shape memory alloy-actuated torque tubes. The in-board and out-board SMA torque tubes are approximately 5 and 4 inches long, respectively. The torque is transferred from the SMA torque tubes to the wing box via steel shafts. The schematic in Figure 6 depicts the wing twist concept. The hingeless aileron and flap are actuated using shape memory alloy (SMA) tendons. As shown in Figure 7, the hingeless control surface concept reduces the separated flow region on the wing thereby increasing lift to drag ratio, and, as a secondary effect, increasing the aeroelastic reversal speed.





Figure 6 : Schematic of Smart Concepts on the Smart Wing Model The first wind-tunnel test in Phase I took place at the Langley TDT in May 1996. A photograph of the Smart Wing model in the TDT is shown in Figure 8. During the test, 1.25 degrees of twist was achieved using both SMA torque tubes resulting in approximately an 8% improvement in rolling moment relative to the untwisted conventional wing. The hingeless control surfaces deployed up to 10 degrees, providing between an 8 and 18% increase in rolling moment and approximately an 8% increase in lift, as compared to the results obtained using the conventional control surfaces deployed through the same angles. A complete summary of wind-tunnel test results is presented in reference 24.

The second wind-tunnel test of Phase I is planned for Summer 1998. Phase I1 of the Smart Wing program includes plans to further mature the technologies developed in Phase I, investigate new actuation concepts, and study near term technology transition issues needed to apply the smart concepts to unmanned aircraft. Two wind-tunnel tests in the Langley TDT are currently planned for Phase 11.

Conventional Control Surface

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6. FUTURE RESEARCH IN FIXED WING AEROELASTICITY

Future activities in controlling aeroelastic phenomena may include novel uses of conventional control devices or a combination of smart and conventional devices. Using new technologies to address aeroelastic issues in air vehicle design can potentially impact many aspects of the behavior of the vehicle such as structural fatigue, passenger comfort, structural weight, aerodynamic and flight control performance, fuel consumption, and the flight envelope. To foster a clearer understanding of how applying certain technologies in specific aeroelastic applications can affect various aspects of aircraft performance, an in-house study is underway at NASA Langley to develop a systems analysis tool that will incorporate aeroservoelastic applications of new technologies. In addition, a parallel effort is underway to survey the aerospace industry to determine what areas within aeroelasticity are viewed as needing improvement to significantly enhance vehicle design.

Emphasis will be placed on two topics: (1) how aeroelastic phenomena affect specific aspects of vehicle designs and (2) what are considered substantial quantitative improvements / changes in those designs. This effort is an important part of ensuring that the research activities at NASA Langley address relevant issues in the near and far term. Additionally, these efforts will aid in identifying relevant, high-payoff aeroelastic applications of new technologies and establishing realistic metrics for each application.

7. ACTIVE TWIST ROTOR (ATR) Recent analytical and experimental investigationsz8”* have indicated that helicopter rotor blades embedded with interdigitated-electrode poled, piezoelectric fiber composite layers (active fiber composites) should be capable of meeting the performance requirements necessary for a usehl individual blade control (IBC) ~ystem’~. this reason, twist-actuated For helicopter rotor systems using active fiber composites (AFC) have become the focus of several advanced IBC research activities at NASA Langley. These research activities and plans for hrther active twist rotor research are described below.

A conceptual drawing of the Active Twist Rotor (ATR) concept is shown in Figure 11. The ATR blade employs embedded piezoelectric AFC plies (layers) to generate large amplitudes of dynamic blade twisting. Mathematical models indicate that from 1 to 2 degrees of twist amplitude over a relatively wide frequency bandwidth are possible using the high strain actuation capabilities of AFC plies. Such levels of twist actuation authority are also possible with only modest increases in blade weight and low levels of power consumption. Figure 12 display:: conceptual drawing of the AFC actuator layer. The AFC actuator employs an interdigitated electrode poling method (IDE) to generate large directional actuation strains in the



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