Bifurcation analysis of a high incidence aircraft with nonlinear dynamic inversion control

Abstract

This work investigates the use of bifurcation techniques as a method for control law analysis and design. A hypothetical high incidence research model (HHIRM) is analysed in its open loop state using bifurcation methods with respect to a varying longitudinal control surface with all lateral controls set to zero. The results are used to identify problems in the flight envelope, such as the onset of spins due to lateral coupling at high angles of attack, and are used as control law specifications. A fully nonlinear control law is designed using the method of nonlinear dynamic inversion (NDI) to remove these unwanted effects by using the lateral control surfaces. This control law is implemented and the closed loop model is again analysed using bifurcation methods to investigate the effectiveness of the control law and to see whether any additional undesirable effects have been introduced. The results show how the stable, decoupled, straight and level flight region is extended to higher incidence angles, so extending the safe flight envelope. Potential implementation problems of the control law are also highlighted by the analysis.