A FEEDBACK LINEARISATION BASED NONLINEAR CONTROLLER SYNTHESIS TO RECOVER AN UNSTABLE AIRCRAFT FROM POST- STALL REGIME

Dynamics of the aircraft configuration considered in this paper shows a unique characteristic that mere are no stable attractors in the entire high angle of attack flight envelope. As a result, once the aircraft has departed from die normal flight regime, no standard technique can be applied to recover me aircraft. In mis paper, using feedback linearisation technique, a nonlinear controller is designed at high angles of attack which is engaged after the aircraft departs from normal flight regime. This controller stabilises the aircraft into a stable spin. Then a set of synthetic pilot inputs are applied to cause an automatic transition from the spin equilibrium to low angles of attack where the second controller is connected. This controller is a normal gain scheduled controller designed to have a large domain of attraction at low angles of attack. It traps me aircraft into a low angle of attack level flight This entire concept of recovery has been verified using six degree of freedom nonlinear simulation. Feedback linearisation technique used to design a controller ensures internal stability only if the nonlinear plant has stable zero dynamics. Since zero dynamics depend upon the selection of outputs, a new method of choosing outputs is described to obtain a plant which has stable zero dynamics.