Remotely Piloted Aircraft Demonstrations Flutter Suppression Using A Classic Flight Controller

The remotely piloted X-56A subscale aircraft has demonstrated with a classic flight controller in flight that flutter can be suppressed at 110 knots, or about 127 mph in the a lower weight configuration. The flight Sept. 15 at NASA’s Armstrong Flight Research Center in California was intended to validate and improve aerodynamic computer models as well as determine aircraft characteristics at the higher airspeed, said Cheng Moua, X-56A project manager.

The focus of test flights shifts this month from building good aerodynamic models to predict the speed at which flutter will happen to demonstrating suppression at higher speeds with a modern, robust flight controller. “Suppressing flutter with this type of aircraft is groundbreaking,” Moua said. “This is awesome.”

The X-56A is intended to validate enabling technology for designing aircraft with highly flexible, lightweight wings. The use of less structurally-rigid wings could be critical to future long-range, fuel efficient airliners. The experimental aircraft is investigating the destructive vibration known as flutter, which such wings can be susceptible.

A combination of ground vibration tests and loads testing helped improve models, but it is the flight data that is significantly refining the models as the classical controller was modified and tuned. “We want to understand the airplane and how it behaves,” Moua explained. “When we had flown the modern controller earlier in the program, it didn’t perform as we expected because the models were not as accurate as we thought. The modern controller relies on a highly accurate model of aircraft.”

In the new phase of flights, the tests will begin at 70 knots, or about 81 mph, and progress in increments of 10 knots, or about 11.5 mph, to build confidence in the models and advance toward the flutter suppression goal. He likened the controllers to high-performance sports cars where one is good, but the other is better. “We are close to suppressing flutter with the new controller, we are almost there,” he added. “There is more uncertainty as you approach flutter with the classical controller, while the new controller is more robust.”

Many factors lead to flutter, but changing fuel weight during flight is a key. When the aircraft is heavier, it doesn’t encounter flutter until it reaches a higher speed. However, as it becomes lighter, it can experience flutter at a much slower speed.

Lockheed Martin developed X-56A aircraft for the U.S. Air Force Research Laboratory and transferred the aircraft to Armstrong for flight research. The program is funded through NASA’s Advanced Air Transport Technology project, NASA’s Flight Demonstration Capabilities project and the U.S. Air Force Research Laboratory.

(Images provided with NASA news release)

FMI: www.nasa.gov