Final approach and flare control of a flexible aircraft in crosswind landings

In this paper, the flight simulation and control of a flexible aircraft in landing is presented. The discrete form of hybrid-state equations of motion in terms of quasi coordinates takes into account the coupling between elastic and rigid-body degrees of freedom and provides the framework for the controller design and simulation. The wings of the aircraft are modeled as cantilever beams undergoing bending and torsion about their elastic axis. Distributed variables are discretized in space using the Galerkin method, and unsteady aerodynamic forces are computed using strip theory and finite state induced-flow theory. A landing condition in which the elastic aircraft encounters a crosswind all the way before touchdown is considered. To design the control system, nonlinear coupled equations of motion are linearized about the trim condition and separated into two sets of decoupled equations, in which the elastic variables affect lateral-directional equations. The landing phase is divided into the final approach and flare phases and, for each phase, separate controllers are designed for longitudinal- and lateral-directional channels. An optimal-integral feedforward control scheme is implemented to accomplish the autolanding, while suppressing the wind effects on the flight path. The performance of the control system is examined through nonlinear simulation