Variable-Response Research Aircraft
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The Princeton Variable-Response Research Aircraft (VRA) is a highly modified single-engine North American Aviation Navion A airplane designed for research on flight dynamics, flying qualities, and control. The most distinguishing feature of the VRA is the pair of vertical side-force-generating surfaces mounted midway between wing roots and tips, but equally important elements of its design are the digital fly-by-wire (DFBW) control system, first installed in 1978, that parallels the standard Navion's mechanical control system and the fast-acting wing flaps that produce negative as well as positive lift. In operation, a safety pilot/test conductor has direct control of the aircraft through the mechanical system, while the test subject controls the aircraft through the experimental electronic system.
Although limited to an airspeed of about 200 km/hr (125 mph), the VRA can simulate the motions of other aircraft types through independent, closed-loop control of all the forces and moments acting on the airplane. Feedback control of motion variables to the control surfaces allows the natural frequencies, damping ratios, and time constants of the Navion airframe to be shifted to values representative of other airplanes, while the direct-force surfaces (side force and lift) provide realistic accelerations in the cockpit.
Consider, for example, pure pitching and yawing motions of the Boeing 747 airliner. The aircraft rotates about its center of mass, which is considerably aft of the cockpit; consequently, substantial lateral and vertical motions and accelerations are felt in the cockpit when simple angular rotations occur. The VRA can simulate these lateral and vertical motions through the actions of its direct-force control surfaces. Conversely, these surfaces make it possible to decouple or modify the VRA's natural modes of motion for experimentation. Thus, the VRA can execute a turn without banking or can bank without turning. Its control surfaces also can be used to simulate turbulence for test purposes.
Having completed over 20 years of research at Princeton University's Flight Research Laboratory (see External Link), the VRA and its sister ship, the Avionics Research Aircraft (which is virtually identical to the VRA but does not have side-force panels) currently are owned and operated by the University of Tennessee Space Research Institute.
