Two X-31A aircraft were built by Rockwell International, Downey, California (now part of Boeing) and Daimler-Benz Aerospace (DASA), Germany, in an international research program funded by the US Advanced Research Projects Agency (ARPA) and the German Federal Ministry of Defense (FMOD). First flight occurred on October 11, 1990. Under the auspices of the International Test Organization (IT), the program logged an X-plane record total of 555 research flights in 55 months using 14 pilots from NASA, US Navy, US Marine Corps, US Air Force, German Air Force, Rockwell International and Daimler-Benz.

Unlike the F-18 HARV (High Alpha Research Vehicle), and the F-15 ACTIVE (Advanced Control Technology for Integrated Vehicles), vectored thrust was integral to the original design of the aircraft. It had a delta wing, relatively small canards, and no horizontal tail. The agility of the X-31 airplane was obtained by augmenting the aerodynamic control surfaces with the use of a vectored thrust system. A digital flight control system blended the aerodynamic and thrust vector controls in response to the immediate flight conditions and thrust available.

The thrust vector system provided pitch and yaw forces by deflecting carbon-carbon composite paddles into the engine exhaust plume. The amount of deflection was proportional to the amount of thrust. In much the same way that aerodynamic control surfaces require larger deflections at low airspeeds, the paddles required larger deflections at low power settings. Consequently, the digital flight control system on the X-31 aircraft would determine the most suitable deflection for a given amount of thrust.

The X-31 program differed from most of the NACA or NASA predecessor programs in its emphasis on military applications. During flights with US and German service pilots, the X-31 aircraft was evaluated in a variety of simulated air combat roles, including air-to-air close combat, ground attack, military maneuvers and aircraft carrier operations.

This airplane was capable of controlled flight at both angles of attack greater than 70 degrees and during post-stall conditions beyond the aerodynamic limits of any conventional aircraft. During dog fights with comparable but non-thrust-vectored aircraft, the X-31 maneuverability was clearly superior. Controllability at high alpha gave it almost a helicopter-like ability to put on the brakes, do an about-face and fire from within the opponent's turning radius.

In 1994 researchers began the Quasi-tailless Flight Test Experiment. The digital flight control system on the X-31 aircraft was re-programmed to simulate partial to total vertical tail removal. This was done by using the rudder to cancel the stabilizing effects of the vertical tail, and yaw thrust-vector commands to restabilize and control the airplane. The "size" of the tail was reduced by varying the amount of destabilization.

The potential advantage of a reduced tail size include lower drag, less weight, reduced structural complexity and, for military applications, reduced radar cross-section.

The quasi-tailless flights were done in two phases, each needing different software loads. The first phase evaluated flight at an altitude of 38,000 feet and at a speed of Mach 1.2, which represented a supersonic cruise condition where a reduction in tail size would result in large drag savings. Maneuvers flown were mostly limited to the gentle turns and 30-degree banks consistent with those required of a supersonic transport-type airplane.

After flights simulating a 70-percent tail reduction, the pilot commented that "the aircraft response was satisfactory for all values of tail-off."

The second phase extended to a larger subsonic flight envelope, with simulated carrier approaches and ground attack runs.

The carrier task required maintaining a precise flightpath, as if on approach to a carrier landing. A nominal flight path of 2.5 degrees was chosen, and the aircraft was flown with gear down and speed brakes out. Approaches were waved-off at 100 feet above the ground however, since the landing gear on the plane were not designed for the slamming, carrier-type landing.

The ground attack profiles were more radical. For example, the 15-degree glideslope bomb run was initiated from an altitude of 4000 feet AGL (above ground level) and 350 KIAS (knots indicated airspeed). The X-31 airplane would then execute a 4 g turn (i.e., one producing forces on the pilot and aircraft equal to four times the force of gravity at sea level) to acquire the target, make the run and then pull out with evasive maneuvering. Another profile consisted of approaches from 14,000 feet AGL to a 45-degree glideslope leading to a pre-selected ground target.

Precision approaches and bomb runs in the quasi-tailless modes brought attention to flight deficiencies in the system, with unacceptable roll performance, sideslips and target overshoots because the capabilities of the thrust vectoring and flight control systems were saturated.

The X-31 definitely proved the feasibility of a tailless aircraft, but showed that future tailless designs will require higher levels of interaction between engine power settings and the flight control system.

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