Nonlinear Velocity Control Strategy and Trajectory Planning for Parafoils Using a Restricted 6-DOF Model

The flexibility of parafoils introduces hysteresis and issues of insufficient control force in their management. Traditional approaches that employ geometric curves for segmented trajectory planning fail to capture certain flight characteristics of parafoils, complicating the achievement of autonomous control. Meanwhile, complex consideration of the flexibility-induced aerodynamic changes will result in a redundant computational burden, diminishing the real-time performance of the parafoil system, rendering it unsuitable in actual airdrop missions. This study optimizes the six-degree-of-freedom (6-DOF) motion model for parafoils by considering the hysteresis and nonlinear characteristics caused by flexibility and introduces a method for trajectory planning that smoothly transitions based on parafoil flight speed. Analysis of this model revealed hysteresis and nonlinear changes in the three-axis velocity within the parafoil body coordinate system owing to control. The proposed velocity planning method is efficient, optimizing the control energy while providing specific control instructions. Using this method, the parafoil system achieves a landing position accuracy within ±0.3 m in simulations and within ±2 m in flight tests.