Volume 10, Number 5 • September/October 2002 • Aerospace Technology Development

Ground Tests Complete on Active Aeroelastic Wing

The upper wing surfaces of the Active Aeroelastic Wing F/A-18 test aircraft are covered with accelerometers and other sensors during ground vibration tests at NASA Dryden Flight Research Center.

Engineers and technicians at NASA Dryden Flight Research Center have wrapped up the last major ground tests before beginning the first research flights in a project to demonstrate that twisting or warping flexible wings can enhance aircraft performance.

The ground vibration and structural mode interaction tests on the Active Aeroelastic Wing (AAW) F/A-18A test aircraft were completed in mid-
September, according to Dryden’s AAW project manager Denis Bessette.

A joint program of the US Air Force Research Laboratory (AFRL), Boeing’s Phantom Works and NASA Dryden, Active Aeroelastic Wing is researching the use of lighter-weight flexible wings for improved maneuverability of future high-performance military aircraft. The program intends to demonstrate improved aircraft roll control through aerodynamically induced wing twist on a full-scale manned supersonic aircraft.

“The project reflects a return to aviation’s beginnings, when the Wright brothers devised a primitive wing-warping method to control the Wright Flyer, and a gateway to the future, a future where aircraft will sense their environment, morph and adapt their shape to the existing flight conditions,” said Bessette. “These future aircraft will take advantage of years of evolutionary lessons exhibited in bird-like flight.”

AAW research could also enable thinner, higher aspect-ratio wings on future aircraft, which could result in reduced aerodynamic drag, allowing greater range or payload, and improved fuel efficiency.
“Active Aeroelastic Wing technology is important to the Air Force because it represents a new approach to designing wings and is applicable to a wide variety of future air vehicle concepts that are under study,” said Pete Flick, AAW program manager for the AFRL Air Vehicles Directorate. “The AAW design approach removes some constraints that limit conventional wing design, opening up the envelope for future designers.”

During the most recent tests, the F-18 rested on three large airbags, while electro-mechanical shakers induced vibrations into the wings at varying amplitudes and frequencies. Test instrumentation measured how the structure reacted as these vibrations propagated through the aircraft to determine potentially adverse effects.

In the ground vibration tests, the F-18’s hydraulics were powered up, but the control surfaces were inactive. The structural mode interaction tests take the process one step further, with the flight controls operating and the interaction of the flight control surfaces with the aircraft structure observed. This test assures that vibrations caused by the actions of the flight controls are damped or suppressed, rather than reinforcing each other to cause large, uncontrolled vibrations or “flutters” that could lead to catastrophic failure of the aircraft structure.

The testbed F/A-18A, provided by the US Navy, was modified with additional actuators, a split
leading-edge flap actuation system and thinner wing skins that allowed the outer wing panels to twist up to five degrees. The traditional wing control surfaces—trailing edge ailerons and the leading and trailing edge flaps—were used to provide the aerodynamic force needed to twist or “warp” the wing. Project engineers hoped to obtain almost equivalent roll performance of production F/A-18s at transonic and supersonic speeds without using the horizontal stabilators and with smaller control surface deflections.

A six-month long structural loads testing program on the F/A-18’s modified wings, one of the most extensive tests ever performed in Dryden’s Flight Loads Laboratory, was conducted in 2001. As part of those tests, the wings were subjected to loads up to 70 percent of the design limit load of the
airplane, with load distribution over the wings a particularly critical item. Q

For more information, contact Denis Bessette at NASA Dryden Flight Research Center, phone: 661/276-3110. Please mention you read about it in Innovation.

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