Flight Dynamics and Control of a Transitioning Quadrotor Biplane Tailsitter

This article presents an in-depth flight dynamics analysis of a quadrotor biplane tailsitter and proposes novel dynamic inversion (DI) flight control laws for autonomous hover-to-cruise transition. As the basis for the synthesis and demonstration of such control laws, a flight dynamics model is developed that also accounts for rotor dynamics and rotor-on-wing interactions. The flight dynamics model is trimmed and linearized at discrete increments in flight speed, from hover to cruise flight. The order of the linearized models is reduced by means of residualization, a subset of singular perturbation theory, to enable stability analysis and control design. The stability and response properties are analyzed both at hover and in cruise flight in terms of eigenvalues, motion modes, and frequency responses. A multiloop DI control law is developed, where an outer velocity loop tracks commanded longitudinal, lateral, and vertical ground velocities in the heading frame and computes the desired pitch and roll attitudes for the inner loop to follow. The inner attitude loop ensures stability, disturbance rejection, and appropriate dynamic response about the roll, pitch, and yaw axes. To demonstrate the proposed control strategy and investigate performance limits, two distinct trim and closed-loop transition trajectories are considered: one in which the vehicle performs the transition with a vertical climb component, and another in which it performs a level, forward-only translation. Closed-loop simulations based on the full nonlinear dynamics are used to demonstrate autonomous hover-to-cruise transitions and to assess the minimum feasible transition time before the onset of rotor stall.