Multivariable control of gas turbines for ASTOVL applications

The Harrier Aircraft is capable of `short take off and Vertical Landing´ (STOVL) but is incapable of supersonic flight. Designs for a supersonic replacement generally involve a power plant with more degrees of freedom, requiring more complex control. The Harrier power plant is a single Pegasus gas turbine. The fuel flow to the engine determines the thrusts developed at the two front nozzles and two rear nozzles, in a predetermined way. Control is relatively straightforward since a single input, fuel flow, determines the thrust output, and only small changes of pitch moment are produced. High pressure air from the engine is ducted to the wing tips and to the aircraft tail for the reaction control system which enables aircraft pitch, roll, and yaw to be controlled and trimmed during hover and transition. The article describes work on power plant control for an aircraft concept known as Advanced vectored Thrust. Studies have been confined mostly to the hover case, using thermodynamic simulations of the engine, together with a simple airframe model. The first step was the design of a suitable engine control system, which was subsequently modelled and integrated with the engine simulation. This was then included within the airframe model with a simplified controller in order to investigate the response of the airframe to engine thrust changes