X-31 in banked flight.
Two X-31 Enhanced Fighter Maneuverability (EFM) demonstrators flew at the NASA Dryden Flight Research Center, Edwards, CA, and at Palmdale, CA, to obtain data that may apply to the design of highly maneuverable next-generation fighters. The program ended in June 1995.
The X-31 program demonstrated the value of thrust vectoring (directing engine exhaust flow) coupled with advanced flight control systems, to provide controlled flight during close-in air combat at very high angles of attack. The result of this increased maneuverability was a significant advantage over most conventional fighters.
"Angle of Attack" (AoA or alpha) is an engineering term used to describe the angle of an aircraft's wings relative to the incoming wind direction. During maneuvers, pilots often fly at extreme angles of attack - with the nose pitched up while the aircraft continues in its original direction. This can lead to loss of control and result in the loss of the aircraft and the pilot.
Three thrust vectoring paddles mounted on the X-31's airframe adjacent to the engine nozzle directed the exhaust flow to provide control in pitch (moving the nose up or down) and yaw (moving it right or left) to improve control. Made of graphite epoxy, the paddles could sustain temperatures of up to 1,500 degrees centigrade for extended periods of time. In addition the X-31s were configured with movable forward canards and eventually with fixed aft strakes. The canards were small wing-like structures set on a line between the nose and the leading edge of the wing. The strakes were set on the same line between the trailing edge of the wing and the engine exhaust. Both supplied additional pitch control in tight maneuvering situations.
The X-31 operated with a digital fly-by-wire flight control system. It included four digital flight control computers with no analog or mechanical back-up. Three synchronous main computers drove the flight control surfaces. The fourth computer served as a tie-breaker in case the three main computers produced conflicting commands.
The X-31 research program produced technical data at high angles of attack. This information gave engineers and aircraft designers a better understanding of aerodynamics, effectiveness of flight controls and thrust vectoring, and airflow phenomena at high angles of attack. This may lead to design methods providing better maneuverability in future high-performance aircraft and make them safer to fly.