Hover flight control of X-shaped flapping wing aircraft considering wing–tail interactions

The dynamic modelling and attitude controller design of a flapping wing aircraft, Beihawk, were investigated in this study. The aircraft features of an X-shaped flapping wing configuration, tail control surfaces installed downstream of the main flapping wings for roll and pitch control, and a tail rotor for yaw control. Unsteady effects including partial leading edge suction, induced flow, and post-stall behaviour, were considered in the aerodynamic modelling. Flapping-wing-induced flow was found to be crucial to the tail control force generation for the utilized aircraft layout. A periodically time-varying wing–tail interaction model was thus established, based on which a double-loop controller was designed to enable stable hovering of the aircraft. The stability of the nonlinear hovering-based manoeuvre was studied by manifold theory, based on the proposed wing–tail interaction model. Simulations and flight experiments on Beihawk were conducted to verify the proposed theory. It was found that the controller considering wing–tail interaction effect enabled more stable hover flight compared with traditional PD controller. In addition, it was found a wider stability region can be obtained by a higher flapping-wing-induced flow velocity. The functions between this velocity and the influencing factors, such as thrust force, wing span and tail installation position, are also presented. Besides, a visual parameter stability boundary between the vertical and pitching velocity is given. Relevant conclusions are vital in active flight envelope protection.