On Longitudinal Control of High Speed Aircraft in the Presence of Aeroelastic Modes

Longitudinal control system design is considered for a linearized dynamic model of a supersonic transport aircraft concept characterized by relaxed static stability and signi cant aeroelastic interactions. Two LQG-type controllers are designed using the frequencydomain additive uncertainty formulation to ensure robustness to unmodeled exible modes. The rst controller is based on a 4th-order model containing only the rigid-body modes, while the second controller is based on an 8th-order model that additionally includes the two most prominent exible modes. The performance obtainable from the 4th-order controller is not adequate, while the 8th-order controller is found to provide better performance. Frequency-domain and time-domain (Lyapunov) methods are subsequently used to assess the robustness of the 8th-order controller to parametric uncertainties in the design model. Summary Longitudinal control system design is considered for a linearized dynamic model of a supersonic transport aircraft concept. The model consists of rigid-body modes and 18 aeroelastic modes. Two LQG-type controllers are designed using the frequency-domain additive uncertainty formulation to represent the unmodeled exible modes. The rst controller is based on a 4th-order model containing only the rigid-body modes, while the second controller is based on an 8th-order model that additionally includes the two most prominent exible modes. Both controllers are designed to provide stability robustness in the presence of unmodeled exible modes. The performance obtainable from the 4th-order design is not adequate. The 8th-order controller can provide better performance, although it cannot substantially improve the exible mode damping ratios. Frequency-domain and time-domain (Lyapunov) methods are subsequently used to assess the robustness of the 8th-order controller to parametric uncertainties in the design model. The frequency-domain methods are found to give less conservative bounds on permissible parametric uncertainties than the time-domain methods, but are still overly conservative compared to the bounds obtained by numerical simulation. The results also indicate that a single actuator may not be su cient to obtain higher controller performance, and underscore the need for further research on reducing the conservatism of Lyapunov-based methods.