Flight Control Design Using Backstepping

Abstract: Aircraft flight control design is traditionally based on linear control theory, due to the existing wealth of tools for linear design and analysis. However, in order to achieve tactical advantages, modern fighter aircraft strive towards performing maneuvers outside the region where the dynamics of flight are linear, and the need for nonlinear tools arises.In this thesis we investigate backstepping as a new framework for nonlinear flight control design. Backstepping is a recently developed design tool for constructing globally stabilizing control laws for a certain class of nonlinear dynamic systems. Flight control laws for two different control objectives are designed. First, general purpose maneuvering is considered, where the angle of attack, the sideslip angle, and the roll rate are the controlled variables. Second, automatic control of the flight path angle control is considered.The key idea of the backstepping designs is to benet from the naturally stabilizing aerodynamic forces acting on the aircraft. The resulting state feedback control laws thereby rely on less knowledge of these forces compared to control laws based on feedback linearization, which today is the prevailing nonlinear design technique within aircraft flight control.The backstepping control laws are shown to be inverse optimal with respect to meaningful cost functionals. This gives the controllers certain gain margins which implies that stability is preserved for a certain amount of control surface saturation.Also, the problem of handling a model error appearing at the input of a nonlinear dynamic system is treated, by considering the model error as an unknown, additive disturbance. Two schemes, based on adaptive backstepping and nonlinear observer design, are proposed for estimating and adapting to such a disturbance.These are used to deal with model errors in the description of the aerodynamic moments acting on the aircraft.The designed control laws are evaluated using realistic aircraft simulation models and the results are highly encouraging.