Disturbance rejection flight control for small fixed-wing unmanned aerial vehicles

A novel disturbance observer-based control scheme is developed for the longitudinal flight dynamics of a small fixed-wing unmanned aerial vehicle in order to improve its flight performance in the presence of wind disturbances. This control scheme uses nonlinear dynamics inversion to address the nonlinearities in the flight dynamics so that it can follow the reference commands in airspeed and height. A novel nonlinear disturbance observer is designed based on the nominal aircraft model to provide estimates of the wind influences and system uncertainties. These estimates are then used to form the compensation control efforts. The developed scheme has been tested in the simulation studies, with scenarios of landing profile tracking and straight flight under wind disturbances and parameter uncertainties. The results have shown a significant improvement in control accuracy and robustness comparing to the baseline controllers and the controller with integral actions. It should be noted that, although the current disturbance observer solution is tailored to the particular problem, this method may be of interest in many similar flight control applications for different aircraft.