Flight Path Angle Dynamics of Air-Breathing Hypersonic Vehicles

Abstract : The flight path dynamics of aircraft are often characterized by the presence of a right-half plane zero in the elevator-to-flight path angle transfer function. For most aircraft, the frequency of this zero is high enough that it does not limit the bandwidth of the flight control system. This is not the case, however, with air-breathing hypersonic aircraft. This class of aircraft is characterized by unstable longitudinal dynamics, strong loop interactions, and the presence of non-minimum phase transmission zeros. In the case of flight-path angle and velocity control, the presence of a low frequency transmission zero severely limits the achievable bandwidth. We show that the frequency of the zero is related to the instantaneous center-of-rotation of the aircraft, which is dependent upon the amount of lift produced by the longitudinal control effectors. In order to improve flight-path control, we investigate the feasibility of an aircraft configured with redundant pitch control effectors. The additional effector moves the instantaneous center-of-rotation, and as a result, the location of the zero. The trade-off is that the path-attitude decoupling inherent in hypersonic aircraft becomes more pronounced. Results are given for both a rigid hypersonic aircraft model and a model that includes the effects of the first fuselage bending mode.