Aeroelastic Control-Oriented Modeling of an Airbreathing Hypersonic Vehicle

Abstract

Modeling aeroelastic effects for an airbreathing hypersonic vehicle is challenging due to its tightly integrated airframe and propulsion system that leads to significant deflections in the thrust vector caused by flexing of the airframe. These changes in the orientation of the thrust vector in turn introduce low-frequency oscillations in the flight-path angle, which make control system design a challenging task. The airbreathing hypersonic vehicle considered here is assumed to be a thin-walled structure, where deformations due to axial, bending, shear, and torsional loads are modeled using the six independent displacements of a rigid cross section. Complex interactions between the airframe and the associated hypersonic flowfield may introduce lightly damped high-frequency modes. Such high-frequency free vibration modes are computed accurately using a previously developed novel scheme. The nonlinear equations of motion for the flexible aircraft are derived; these equations are then linearized about a computed cruise condition, and a selective modal analysis is carried out on the linearized system. Furthermore, a linear quadratic regulator is designed to compensate for perturbations in initial conditions, where a novel and practical approach is used to populate the weighting matrices of the linear quadratic regulator objective function.