Air-Breathing Hypersonic Vehicles (ABHV) represent a new class of Hypersonic Aircraft that

offer distinct advantages compared to other aircraft types. These vehicles can be cost-
effective for accessing space and may function as single-stage-to-orbit aircraft. With the

development of scramjet technology and successful flight demonstrations, the era of ABHV
is rapidly approaching. Given their exceptionally high speeds, it is crucial to possess a
comprehensive understanding of the flight dynamics inherent to these vehicles. Such
understanding is essential for designing robust control systems capable of maneuvering the
aircraft effectively at these velocities. At high speeds, the flight dynamics become
significantly intertwined with the propulsion system, and flexibility effects also become
noteworthy, introducing further nonlinearity to the system. Therefore, analysing the impact
of these factors on the vehicle's flight quality is imperative. A coupled nonlinear model is
utilized to describe the longitudinal dynamics of Air-Breathing Hypersonic Vehicles (ABHV),
incorporating flexible dynamics and a thrust model dependent on the angle of attack and
the fuel-to-air ratio. Modal analysis is conducted at various operating points to examine the
aircraft's modes. The results indicate a notable aeroelastic mode, while the short-period
mode demonstrates a strong coupling with the fuselage's bending mode.

Moreover, the linearized aircraft dynamics are found to be unstable, evidenced by open-
loop simulation results, and exhibit non-minimum phase behaviour within the chosen

vehicle range. Consequently, assessing these modes' effects is vital to design a stable
control system. Bifurcation analysis has proven to be a powerful nonlinear analysis
technique, offering significant insights into system dynamics. It enables the exploration of
global system dynamics, identification of stable and unstable regions, and examination of
parameter variations, such as control surfaces or controller gain effects on the flight system.
Based on the insights gained from this analysis, suitable linear or nonlinear control
strategies will be implemented to evaluate the system's performance under various
maneuvering conditions.