Modelling and Attitude Control of an Agile Fixed Wing UAV based on Nonlinear Dynamic Inversion

An estimation model is key in the design and development of an unmanned aerial vehicle's control system. This work presents a complete methodology for modelling the dynamics of a fixed-wing UAV for aerobatic maneuvers. The UAV dynamic non-linear model considered uses total variables instead of a nominal values and perturbation values about certain trimmed conditions for conventional flight envelopes. Such modelling allows the expansion of the flight envelope of an unmanned aerial vehicle to cover the full range of the angle of attack. The quaternion formulation is used since it eliminates the nonlinearity of the aerodynamics due to Euler angles for high angle of attacks. The objective is to have a complete and accurate representation of a highly dynamic fixed wing UAV capable of making aerobatic maneuvers without encountering singularities. A set of controllers are then designed for the inner and outer loops for attitude control using nonlinear dynamic inversion in cascade with a PI controller. Simulations carried out show robust tracking of reference attitude angles.