Flutter tests drive X-56A programme

Washington DC
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The US Air Force Research Laboratory (AFRL) has unveiled a new unmanned test aircraft, the Lockheed Martin X-56A, designed to study active flutter suppression. Two examples of the aircraft will be built, along with four sets of wings - one rigid, to test initial aircraft handling, and three sets with increasing flexibility for flutter tests.

"As we demand more and more from our aircraft in terms of being efficient, that leads us to aircraft configurations that are in inherently very flexible," said Pete Flick, AFRL programme manager.

Flutter occurs when aerodynamic forces reach the wings' resonance point, leading to a self-perpetuating vibration. Flutter has been a factor in a number of aviation catastrophes, but represents a special problem for high altitude long endurance (HALE) aircraft, which typically have long, thin, flexible wings and a very narrow flight envelope at altitude. It is a constant worry for aircraft near the edge of the flight envelope, but can be countered through increasing the wing stiffness. Increased stiffness means increased weight, and thus decreased efficiency.

"The way we do it today is we would typically just add structural weight, but that's a severe penalty," Flick said. "We have a better approach, we feel, and that's to actively control the deflections with surfaces on the wing. That's what led us to this programme."

"There are multiple control surfaces along the aft edge of the vehicle, a total of 10 across the entire span" that will change position as flutter is encountered, he added.

Because of the project's relatively high level of risk, the X-56 is designed to be as low-energy as possible. The aircraft will conduct tests between 90 and 150 knots airspeed (167-278 km/h), and rarely at over 1,000 feet (305m) altitude.

AFRL officials hope the test results can be applied to a nascent unmanned reconnaissance programme called SensorCraft. After approximately 25 hours of flight test with AFRL at Edwards AFB's North Base, scheduled to take up to three months, the aircraft will be sent to NASA's Dryden flight test centre to serve as an aerodynamic test bed.