The Active Aeroelastic Wing (AAW) project at Dryden recently completed a major milestone on its pathway to flight.
In September, the aircraft completed a six-month, two-phase loads test series in Dryden's Flight Loads Laboratory, the most extensive structural loads testing ever performed in the facility.
Active Aeroelastic Wing will showcase a 21st century twist on an old-fashioned aircraft control technology - a high-tech derivative of wing warping pioneered by the Wright brothers almost a century ago. AAW will investigate use of lighter-weight flexible wings for improved maneuverability of high-performance aircraft through aerodynamically-induced wing twist on a full-scale aircraft. The program is jointly funded and managed by the U.S. Air Force Research Laboratory (AFRL) and Dryden, with Boeing's Phantom Works as the prime contractor. Dubbed "back to the future" technology by AFRL Program Manager Edmund Pendleton, AAW is considered a first step toward development of "morphing" aircraft which can change their shape in flight to meet varying aerodynamic requirements.
The testbed F/A-18A - NASA No. 853 - was provided by the U.S. Navy's Naval Aviation Systems Team at Patuxent River, Md. Its wings have been modified with additional actuators, a split leading edge flap and thinner skins on the rear portion of the wings that will allow the outer wing panels to twist up to five degrees. Moving the traditional wing control surfaces - trailing edge ailerons and flaps and the leading edge flaps - up or down will provide the aerodynamic force needed to twist or "warp" the wing.
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The Active Aeroelastic Wing (AAW) Program aims to develop data and structural modeling techniques and tools to design lighter, more flexible high-aspect-ratio wings for future high-performance aircraft, which could mean more economical operations or greater payload capability.
NASA Photo / Tony Landis
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In addition, extensive strain gauge instrumentation was installed, particularly on the left wing. A new "hump" fairing atop the fuselage behind the cockpit canopy houses a flight deflection measurement system. The system, first used on the Highly Maneuverable Aircraft Technology (HiMAT) Remotely Piloted Research Vehicle and later on the X-29, incorporates an optical sensor package in the dorsal window pod on the fuselage and 16 infrared light emitting diode markers positioned on the upper surface of the left wing to measure the twisting and bending of the wing during flight.
The structural loads tests on the F/A-18's modified wings began with wing torsional (twist) testing in early April, followed by more extensive structural loads calibration testing.
Dryden aerostructures engineer Bill Lokos, who headed the team that planned and conducted the structural loads analyses, said the tests were "the most extensive ground loads test that we've done" in the loads lab, and represented "what will actually happen in flight."
Lokos noted that 104 loads pads were bonded to the lower wing skin and control surfaces, covering 60 percent of the surface area. Thirty-two hydraulic jacks exerted up to 15 PSI of force simultaneously on all 104 pads, either in compression or tension. On top of that, 202 strain gauge channels and 72 load test points were run.
"These were higher pressures and tension then what we've done in past 30 years," he added. "Other tests have had a higher number of test points or load pads, but not in combination."
Lokos also noted that exerting forces up to 70 percent of the F/A-18's design limit load was far above the usual 20 per cent standard for most loads tests at Dryden.
"That's roughly enough vertical load to lift four F-18s off the ground," he added.
Lokos noted that the complicated tests required far more set-up and teardown time than was consumed in performing the actual tests.
"It took 10 times as much time to do the set up as it did to do the actual torsional stiffness (wing twist) tests," he said. "In the loads calibration tests, the ratio was two or three to one in set up to actual test time."
Following completion of the structural loads tests, the modified twin-engine former fighter plane entered the Air Force Flight Test Center's paint hangar, emerging three weeks later in its new red, white and blue.
AAW project manager Denis Bessette said considerable work still needs to be done before first-phase parameter-identification flight tests can begin next spring.
"What we have left are final instrumentation installation and checks, installation and checkout of the flight control computers, a ground vibration test, a structural mode interaction test, control room to aircraft telemetry checkout, as well as a final flight control computer verification and validation test to complete," he said. "Then we can fly."
With the more flexible wing and an enhanced flight control system, project engineers hope to demonstrate maneuver performance benefits at both subsonic and supersonic speeds due to wing aeroelastic effects alone, without using differential stabilator movements to assist in roll control.
AAW research could enable thinner, higher aspect ratio wings on future aircraft that could result in reduced aerodynamic drag, allowing greater range or payload and improved fuel efficiency. In addition, such wings should be lighter and could be cheaper to manufacture. Data obtained from flight tests at Dryden will provide benchmark criteria as guidance for future aircraft designs.
