Gust Response and Control of Very Flexible Aircraft.

Abstract

This dissertation studies the response of very flexible aircraft models to gust disturbances and introduces methods for controlling the aircraft within this environment. A new control technique for minimizing the bending moment induced on the aircraft by a gust disturbance is developed. The curvature of the wing, which is proportional to the bending moment, is assumed to be a state available for feedback. A controller is then designed to use the control surfaces to minimize the deviations in wing curvature caused by a gust encounter or by maneuvering flight. The controller is validated on a linearized aircraft model and then in a fully nonlinear simulation. A two-loop control architecture for tracking longitudinal trajectory commands is presented. A new outer loop is introduced which uses a combination of proportional, integral, derivative as well as sliding mode control. The body frame velocities are tracked using a dynamic inversion inner loop. Control of the lateral states is demonstrated using an output feedback controller designed using a reduced order linear aircraft model. This control scheme is first demonstrated in still air and then multiple discrete and stochastic gust disturbances are added. The wing curvature controller is then added to the system, allowing for reduced bending moments and virtually no degradation in tracking performance. Finally, the control architecture is augmented with reference and extended command governors to enforce constraints on the wing curvature during maneuvering flight with gust disturbances. The closed loop sensitivity of this class of aircraft to changes in stiffness parameters is also studied. Aircraft models previously used for the open loop study are simulated in the closed loop system described above. The VFA models are given longitudinal trajectory commands in both still air and with a gust disturbance. Conclusions are then drawn about methods of VFA aircraft design to maximize open and closed loop performance during gust encounter.