92The VAAC Harrier

The DERA vectored thrust aircraft advanced flight control (VAAC) Harrier is a research aircraft sponsored by MOD which is being used to research control concepts in handling and control. The goal is to demonstrate that through the application of new approaches and design methods, a major reduction in pilot workload can be assured. More background information can be found in Reference [5], The aircraft is a modified two-seater training aircraft; a safety pilot flies from the front cockpit and the test pilot flies from the rear cockpit. The test pilot flies the experimental flight control system (FCS) implemented on a hardware system which is monitored independently by a separate system. The monitor provides an envelope of flight parameters within which the aircraft is permitted to fly with the experimental FCS selected. The monitor follows the control law demands and returns control to the safety pilot if any predetermined parameter which makes up the flight envelope is violated or potentially dangerous situations arise. In this manner the experimental control law is not required to be safety-critical, and hence new ideas can be tried out without going through rigorous flight-clearance procedures.

Figure 9.1 shows a schematic diagram of the aircraft. On a typical approach to landing the pilot will attempt to maintain a steady flight path (GAMMAD) while decelerating to the hover alongside the ship. To do this he has to alternately move the nozzle and throttle levers with his left hand while stabilising the aircraft in pitch with his right hand. Movements to nozzle and throttle couple into pitch requiring compensation from the stick. The objective of the experimental control law is to take care of this coupling and give the pilot two primary commands, namely speed demand and flight-path demand. A control law with this strategy is called a two-inceptor law.

From Figure 9.1 it can be seen that the control law has three primary surfaces with which to control the longitudinal motion, namely throttle, nozzle and tailplane. The tailplane back drives the reation control system when the nozzles are down. The three degrees of freedom are used to control pitch attitude, forward speed and vertical speed.

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