Flatness-based path planning for fixed-wing UAVs in tight formation under synergistic aerodynamic constraints

Tight formation flight, as a significant way for fixed-wing unmanned aerial vehicle (UAV) to execute missions, generates synergistic aerodynamic effects that significantly influence the motion decision-making and control of UAVs. In aerial refueling missions, this is manifested as complex aerodynamic effects such as vortices affecting the path planning of the refueling UAV. This paper proposes a path-planning method for fixed-wing UAVs to conduct aerial refueling under the constraints of synergistic aerodynamics. Firstly, an environment constraint model for vortex distribution is obtained from aerodynamic experimental data of the refueling formation. Subsequently, by utilizing the differential flatness property of fixed-wing UAVs, the nonlinear system states and control variables are mapped to linear functions of flat outputs. This allows the establishment of segment constraints for the path, enabling the use of a key-point heuristic algorithm in the flat output space to generate the aerial refueling flight path. Furthermore, a flat output minimum snap algorithm is applied for multi-constraint optimization of the flight path, resulting in a smooth and feasible optimal path. Simulation experiments demonstrate the effectiveness and advancement of the proposed path-planning method under the influence of vortices.