Air-breathing Hypersonic Flight vehicle (AHFV) features airframe-integrated scramjet engines, which may result in the coupling of the aerodynamic force and the thrust even in nominal flight condition. The coupling of the aerodynamic force and the thrust affects dramatically the static and dynamic behaviors of the AHFV. Another major issue characterizing the AHFV flight dynamics is wide range of flight conditions. Due to the extremely wide range of flight conditions, the dynamic characters of the AHFV vary much more over the flight envelope than conventional jet aircraft. The goal of this paper is to investigate the longitudinal dynamic characters of the AHFV with the coupling of the aerodynamic force and the thrust over large envelope. In order to understand deeply the effect of vehicle parameters or stability derivatives on vehicle instability and non-minimum phase at different flight conditions over large envelope, emphasis is placed on approximate formulae for describing the modes of motion and unstable zero associated with flight path angle. The analytical approximate formulae show that the effect of the coupling on short period and speed ranges on phugoid. The numerical results apply the approximate solution provided in this paper is accurate enough to describe the impacts of vehicle stability derivatives on dynamic properties and to yield an improved insight of longitudinal flight dynamics. Applying a 3 DOF nonlinear model for the longitudinal dynamics, AHFV steady horizontal flight static characteristics (e.g. trimmable region, trimmed controls and trimmed angle of attack) and dynamic characteristics (e.g. instability and unstable zero associated with flight path angle) how to change over large flight envelope are examined. The results indicate that the vehicle is characterized by instability and non-minimum phase with significant coupling.
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