An experiment consisting of flight tests on the X-31A airplane (see figure) has demonstrated the ability to use thrust vectoring to replace the functions of stabilization and turn coordination usually required of a rudder and vertical tail. Comments by the pilot indicated no difference in handling qualities for the majority of the tests flown. The experiment showed that the greatest demand was placed on the thrust-vectoring system at low thrust settings and high roll accelerations. It was demonstrated that a higher level of interaction between the engine and flight-control system will be needed for future reduced-tail or tailless aircraft with thrust-vector control. This experiment helped to introduce thrust vectoring as a new design dimension for future aircraft.

The quasi-tailless concept involves the use of in-flight simulation to assess the effect of partial to total removal of the vertical tail; in implementing this concept in the experiment, the rudder control surface was used to cancel the stabilizing effects of the vertical tail. Yaw-thrust-vector deflections were used to restabilize and control the aircraft. The quasi-tailless mode was flown supersonically with gentle maneuvering. Precise approaches and ground attack profiles were flown subsonically with more aggressive maneuvering.

The X-31A Airplane was operated with thrust vectoring in a quasi-tailless mode to test and demonstrate advanced control concepts.

The supersonic quasi-tailless test showed that maneuvers typically required of transport aircraft could be controlled by thrust vectoring for fairly high levels of instability. A tail-reduction setting of 70 percent was used during part of the experiment and was found to be equivalent to an instability-amplitude-doubling time of approximately 170 ms. The fidelity of the sideslip feedback measurement was found to be a critical factor in determining the amount of destabilization achieved by the quasi-tailless system. The sideslip feedback path included an equivalent delay of approximately 67 ms from complementary filtering of flight-test-boom and inertial parameters, and a deadband caused by misaligned dual redundant sideslip vanes. These factors both influenced the level of destabilization achieved in flight.

To accomplish the objectives of the subsonic tests, it was necessary to maneuver the airplane more aggressively. The precise-approach tests provided a first look at the use of thrust vectoring as a primary means of control at low power settings. The flying qualities were found to be independent of tail-reduction setting up to a setting of 50 percent (an instability-amplitude-doubling time of about 0.92 s). High throttle activity coupled with a lag on the thrust-estimation algorithm resulted in errors in the thrust-control loop gain as high as 4 dB. Because of the high loop-gain margin of the X-31A, these errors did not produce any noticeable stability problems. The tests showed that an accurate and redundant on-board thrust-estimation algorithm is necessary in an integrated propulsion/flight-control system. Either the design of such a system should provide for better estimation of thrust changes resulting from rapid throttle movements, or else a high stability margin for the thrust-vector control loop should be required.

All precise approaches were flown in clear air. A limited nonlinear simulation study showed that even with the deadband and lag in the sideslip feedback path, no significant handling-qualities problems were introduced with simulated turbulence. The issues of ride qualities and rejection of disturbances in the presence of atmospheric turbulence would be better addressed with a real tailless or reduced-tail vehicle. Because of limitations on frequency response and fidelity, the in-flight quasi-tailless system is not accurate enough to reproduce the true directionally unstable behavior characteristic of the response of an aircraft to turbulence.

A reasonable approach to the engine-out failure condition must be developed. If this approach is to include emergency devices that would be deployed to regain directional stability, then the cost, weight, and complexity of such devices must be considered. The cost and weight of adding a thrust-vector system are being reduced by production of axisymmetric thrust-vectoring engines. The X-31A quasi-tailless flight test experiment showed that tailless and reduced-tail fighter-type airplanes are feasible. When the thrust-vectoring capability and reduced-tail configuration are incorporated into the design from the beginning, the benefits of lower drag, reduced structural complexity, and reduced radar cross section could, potentially, outweigh the concomitant added complexity.

This work was done by John Bosworth and Patrick Stoliker of Dryden Flight Research Center. DRC-96-12