X-31: Prelude to tailless flight? : Page 2 of 3

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

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