Analysis on Steady State with Deep Stall of T-tail Transport Aircraft and Optimal Recovery Control

Abstract

Although it is well known that the equation of motion of T-tail transport aircraft has stable steady states at high angles of attack due to deep stall, a reliable and prompt recovery procedure has not been established. In this study, bifurcation analysis is conducted to understand post-stall nonlinear behavior of a T-tail airliner model, with deflection of rudder and elevator as parameters. The results show that it is difficult to recover from flat spin, a stable equilibrium point with a high angle of attack, by simply switching the rudder and elevator deflection. In order to propose recovery control, an optimal control problem is solved with the direct collocation method. This problem requires a transition from the flat spin to another stable equilibrium point, which is steady level flight. The optimal control input utilizes the coupling of longitudinal and lateral dynamics to make state variables cross the boundary of attraction in the nonlinear dynamical system, and recovery is achieved in a short time.