Inner loop control of supersonic 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 significant aeroelastic interactions. Two LQG-type controllers are designed using the frequency-domain additive uncertainty formulation to ensure robustness to unmodeled flexible modes. The first controller is based on a fourth-order model containing only the rigid-body modes, while the second controller is based on an eighth-order model that additionally includes the two most prominent flexible modes. The performance obtainable from the fourth-order controller is not adequate, while the eighth-order controller is found to provide better performance. Frequency domain and time-domain (Lyapunov) methods are subsequently used to assess the robustness of the eighth-order controller to parametric uncertainties in the design model