Trim and flight – elastic coupling characteristics of a flexible air-breathing hypersonic vehicle

Longitudinal trim and flight-elastic coupling characteristics of a flexible air-breathing hypersonic vehicle (FAHV) are presented and analyzed in this paper. FAHVs with airframe– propulsion integrated configuration and lightweight slender fuselage usually show intricate interactions of air frame, aerodynamics, scramjet engine, elasticity and rigid body flight dynamics, which may result in unusual trim envelope, flight limitation, engine boundaries, and intractable flight elastic coupling problem. Starting with the physics-based models of aerodynamics, scramjet engine and flexible structure, three different flight dynamic models are established for a FAHV, which are rigid body model, aero-elastic coupling model and flight-aero-elastic coupling model. The aero-elastic model describes the interaction of aerodynamics and deformation of the air frame, while the flight-aero-elastic model captures inertial coupling effects between the pitch and normal accelerations of the aircraft and the structural dynamics. The longitudinal trim envelopes are obtained considering limitations of angle of attack, equivalence ratio, elevator deflection and scramjet thermal choking based on both rigid body model and aero-elastic coupling model. Trim differences between rigid body model and aero-elastic coupling model are given. A Modal comparison study based on all these three models is conducted to get an insight of the system-theoretic properties of the flight-elastic coupling. Mode data indicate that there is strong coupling between flight dynamics and fuselage vibration resulting in a highly unstable aeroelastic mode, which may bring challenge for control design.