X-31: Prelude to tailless flight?

Demonstrate the use of thrust vectoring-coupled with advanced flight controls-to provide enhanced flight maneuverability at very high angles of attack. Mission accomplished: Flight tests yield an almost 10:1 kill ratio, far surpassing the optimistic 3:1 ratio predicted by simulation.

The slower the speed, the smaller an aircraft's radius of turn. As any fighter pilot knows, tighter turns mean earlier weapons launch. Unfortunately, conventional aircraft offer limited control at slow speed, and they can fall out of control at stall speed-just when achieving the smallest turn radius.

The X-31 program at Edwards Air Force Base, CA, demonstrates how the ability to maneuver beyond stall limits-allowing very high attack angles-improves a fighter's chances of winning the close-in-combat dogfight. Two design components contributed to the program's success: aerodynamics optimized for post-stall maneuvering and multi-axis, thrust-vectoring capability.

Based on the European Fighter Aircraft, with refinements developed by Rockwell International and Deutsche Aerospace AG (formerly Messerschmitt-Bolkow-Blohm GmbH), the X-31 features a delta/canard configuration. Its center of gravity sits aft of the subsonic center of lift, making the layout unstable in pitch at subsonic speeds. In combination with the delta wing's large surface area and high leading edge sweep, however, the design offers superior supersonic performance.

The "long-coupled" canards, located further from the wing than "short-coupled" configurations, also function differently than conventional canards. Designed for pitch control and trim rather than lift, they move into the wind at increasing angle of attack, maintaining control effectiveness throughout post-stall maneuvers. Should the thrust-vectoring system fail, the canards assist in aerodynamic recovery.

Fixed aft and nose strakes complete the aerodynamic package. The aft strakes supply extra nose-down pitch-control authority from very high angles of attack, while the small nose strakes help control side slip.

Thrust-vectoring control. General Electric's 404 engine-powerplant of the F-18, F-117, X-29, and F-20-provides the thrust-to-weight ratio needed for supermaneuverability. It also resists flow distortion resulting from high angles of attack, large yaw rates, and big sideslip angles. Combined with a belly intake, the engine allows full-power operation, even at extreme angles of attack.

At the program's start, thrust vectoring presented a problem, since no multi-axis nozzle was available. The X-31's solution: three composite vanes arranged 120 degrees apart. Mounted on the aft fuselage with nimonic alloy fittings, the vanes deflect into the exhaust to generate as much as 17% engine thrust in any lateral direction. Constructed of lightweight, heat-resistant carbon-carbon material, the vanes can sustain temperatures as high as 1,500 degrees C for extended periods of time. When not being used for maneuvers, the vanes trail outside the exhaust plume, automatically tracking the jet plume boundary during power changes and change-of-flight condition to minimize effectiveness deadband.

Because the vanes, actuators, and support structure were designed into the aircraft from the beginning, overall effect on weight remains minimal. Moreover, no added aircraft ballast is needed. In fact, says Harvey Schellenger, X-31 chief engineer at Rockwell, the net weight of the vane system totals about the same as the added weight of an integral nozzle. "Without the need for ballast," he points out, "the X-31 thrust-vectoring system

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