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AAW - Phase 2 flights proving concept
Feb. 2005
Dryden's F/A-18 active aeroelastic wing flights resumed in December, continuing validation of the concept that flexible wings can be twisted, or warped, in flight to control aircraft roll.

The F/A-18 Active Aeroelastic Wing executes a Phase 2 flight over Mojave terrain. NASA Photo by Jim Ross Image Right: The F/A-18 Active Aeroelastic Wing executes a Phase 2 flight over Mojave terrain. NASA Photo by Jim Ross

AAW Project Manager Larry Myers said the project's second phase will establish whether in-flight research results match predictions and expectations Dryden researchers calculated based on phase one flight data obtained in 2002.

"We've demonstrated four subsonic and three supersonic flights where we've taken advantage of the aeroelasticity of the wing," Myers said. "Initial results indicate the aeroelastic effects - the AAW concept, if you will - is proven out."

Another 20 to 25 flights are anticipated before the program wraps up by the end of March or early April.

Phase one, in 2003, included about 50 flights made over more than five months. Those missions evaluated the control surfaces' effectiveness in twisting the wing at different speeds and altitudes. That process, called parameter identification, allowed for development of control laws that would direct the aircraft's software to use wing elasticity effectively.

F/A-18 takes off on an AAW test
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Quicktime 5.9 MB F/18A Active Aeroelastic Wing Test
In the second phase, the new control laws in the aircraft's software are being used to twist the wings - unlike in a traditional F/A-18, which attains roll control through deflection of its horizontal stabilizers. While the first phase showed the wings functioned well, the second phase is designed to verify that with the new software, the AAW can attain similar and effective roll.

Production F/A-18 aircraft currently have rigidity built into the wings to reduce vibration (called flutter) on them. Flutter is reduced during an aircraft's design and testing phases to keep vibrations from weakening its structure in flight. The AAW wings make use of that flexibility as part of the control system.

"On the top and bottom surfaces of the wing we've changed the materials and thickness back to the pre-production flexibility of the original F/A-18," Meyer explained. "To take advantage of the aeroelasticity of this wing, we've designed control laws that (maximize use of) the inboard and outboard leading-edge flaps to control them separately, to exploit the wing's aeroelasticity."

The AAW concept of using wing twist for roll control is an idea as old as the first successful sustained, powered and controlled aircraft, flown more than 101 years ago. The Wright brothers warped the wings of their Wright Flyer to obtain roll control.

Image Right: NASA's F/A-18 Active Aeroelastic Wing rolls into a hard left-hand turn during a December research flight. NASA Photo by Carla Thomas The concept dates to the past, but also has implications for future aircraft. Successful AAW flights could lead to reduced weight, which would enable more carrying capacity or better fuel economy. It also could impact aircraft design through the use of wing twist to control an aircraft, thus eliminating the need for a horizontal tail, Myers said.

Image Right: NASA's F/A-18 Active Aeroelastic Wing rolls into a hard left-hand turn during a December research flight. NASA Photo by Carla Thomas

In addition, AAW technologies could have applications for Uninhabited Air Vehicles. UAVs able to loiter at high altitudes could one day impact telecommunications and disaster relief, and potentially lead to future Dryden research projects as new designs, software and integrated systems are developed, he added.

Work is not imminent for the AAW after completion of phase-two testing, but researchers are advocating additional flights to examine buffering on the F/A-18's vertical tail at high angles of attack. Those maneuvers cause vibrations on the tail that shorten the service life of the flight control surface.

"We want to try to control that vibration with an active rudder and, eventually, with piezoelectric actuators (actuators composed of ceramic materials that change shape when voltage is applied)," Myers said. "Piezoelectric work at Langley (Research Center) could be used in this research, which would require quick, active actuators that work at high frequency."

Those very fast actuators were developed at Langley Research Center, Hampton, Va., and could be one of the first steps necessary for development of a future, morphing aircraft, he added. The quick, high-frequency actuators also could have applications for other research, including wingtip torsion.

Even before phases one and two of the current project, Dryden was involved in researching the AAW wing technology.

The AAW wing was tested in the Dryden loads lab for six months. According to Myers, about 104 loads pads were bonded to the lower wing skin and control surfaces, covering 60 percent of the testbed's surface area. Thirty-two hydraulic jacks simultaneously exerted up to 15 PSI of force on all 104 pads, either in tension or compression. In addition, 202 strain gage channels and 72 load test points were run.

The forces applied to the wing were up to 70 percent of the design load limit, 20 percent greater than average loads standards and roughly enough vertical load to lift four F/A-18s off the ground, Myer said.

+ View Volume 47, Issue 1, February 2005 Dryden X-Press

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Jay Levine
X-Press Editor