An online path planning and a guidance law method are proposed for the unpowered forced landing problem of the hybrid Vertical Take-Off and Landing (VTOL) Unmanned Aerial Vehicles(UAVs). According to the casual initial location and heading when the UAV losses power, a three-dimensional path planning method that satisfies both dynamic and terminal constraints is developed. Using the geometric planning method for designing the extended Dubins plane path, three-dimensional curves are generated based on the gliding stability and performance. Since the unpowered path following is susceptible to wind disturbances in low speed and the curvature of planed curves is discontinuous, a three-dimensional guidance algorithm based on nonlinear model predictive control is proposed. Combined with the lateral-longitudinal decoupled guidance dynamic, nonlinear factors such as following error, external wind disturbance and path curvature discontinuity are modeled as the system output constraints, and guide commands are solved through rolling in real time. Finally, the validation of the online path planning method and the guidance law are simulated and verified. The simulation results show that the proposed online planning method is suitable for the emergency landing from casual locations and headings, and the proposed guidance algorithm has the capability to resist wind disturbance and follow the three-dimensional path accurately.
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