X-31: Prelude to tailless flight?

Mission:
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 is hundreds of pounds lighter than either the F-18 HARV (external steel vanes plus ballast) or the F-16 MATV (integral nozzle plus ballast)."

Flight controls. The digital flight control system hardware, designed by Honeywell Defense Avionics Systems, uses both conventional and thrust-vectoring control surfaces to maintain precise control of the aircraft throughout its flight envelope. Based on pilot input and feedback signals, the control laws (developed by Deutsche Aerospace) calculate the required thrust deflection in pitch and yaw. The flight-control system translates this deflection command into single-vane deflections. For example, if a yawing moment to the right is required, the left vane moves into the exhaust jet. The upper vane moves into the plume enough to compensate for the pitching moment created by the left vane. The right vane moves out of the plume.

In general, direction of thrust can be deflected at an angle of more than 15 degrees around centerline. In the event of system failure, or if the pilot chooses to disengage thrust vectoring, the flight-control system automatically redistributes commands to the conventional aerodynamic surfaces. "Even if one of the vanes falls off," says Hannes Ross, pre-design department director, Deutsche Aerospace, "it wouldn't put the pilot in jeopardy."

No tailless fighter design has ever been flight tested, and no vertical tailless aircraft has flown supersonically. However, the X-31's all-digital, fly-by-wire flight control system with integrated thrust vectoring readily lends itself to "quasi-tailless" flight experiments. These tests measure in flight the requirements needed to maneuver and control a tailless aircraft.

The quasi-tailless mode uses the plane's aerodynamic surfaces, primarily the rudder, to cancel the stabilizing effect of the vertical tail. This, explains Schellenger, directionally destabilizes the aircraft so it behaves as though all or part of the vertical tail has been removed-without really removing the tail.

Instead, thrust vectoring stabilizes the destabilized aircraft and performs yaw control for maneuver coordination. Moreover, variable destabilization gains permit selection of varying degrees of tail removal. If undesired aircraft motions occur, or if the pilot disengages the mode, the flight-control system quickly reverts to its normal mode of operation. "That's the real attraction of the quasi-tailless feature," says Ross. "Many different tailless designs can be aggressively tested in complete safety."

A special feature of the quasi-tailless control mode provides the option to use the rudder to both destabilize and to emulate the effects of another yaw-control device. Such non-rudder aerodynamic controls, Ross points out, are likely to be part of an aircraft designed to be tailless.

Historic flight. On March 17, 1994, the X-31 climbed to 37,000 feet above the desert floor of Edwards Air Force Base, accelerated to Mach 1.2, and engaged the quasi-tailless mode-a significant first in aviation history. The degree of tail removal was increased incrementally up to full tail off. Performing maneuvers, including 2g turns, the aircraft responded well.

Quasi-tailless experiments at subsonic cruise speeds, and at low approach and landing speeds, will be the next step. These tests will allow investigation of the relationship between degree of destabilization, aggressiveness of maneuver, and aerodynamic yaw control required at selected flight test points. A new control law version that incorporates added logic to emulate a tailless aircraft throughout the entire X-31 flight envelope will follow. The new version will add thrust control of the engine to the thrust-vector control, further enhancing aircraft response in the quasi-tailless mode.

After all quasi-tailless flight data have been evaluated, the X-31 will proceed with the next step in the follow-on program: physically reducing the size of the present vertical tail and rudder. Ability to control the aircraft should thrust vectoring fail will determine the new size of the tail and rudder. These tests will verify the quasi-tailless results with flight data from a truly destabilized airframe.

Finally, after installation of a deployable/retractable stabilization and control device, such as a flip-up, all-moving vertical fin, plans call for removing the vertical tail completely.

Where does this all lead? Post-stall performance has already attracted the attention of air forces around the world. Now, the possibility of replacing aircraft surfaces with vectored thrust capability promises substantial reduction in aircraft weight, aerodynamic drag, fuel consumption, and radar signature. While the latter is important to military personnel, less weight, drag, and fuel consumption are of significant interest to the commercial airline industry.

Militarily or commercially, if the results prove as good as they look, the X-31 program could eventually result in increased employment for aerospace engineers worldwide.

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