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A recovery system for an unmanned aerial vehicle (UAV) using a steerable parafoil is an attractive concept. However, due to the complex interaction between the parafoil and the UAV, this parafoil system has not seen widespread use in the UAV recovery. Under the influence of the UAV, the suspension lines of the system are not always tight, so most of the existing models do not work. To analyze the parafoil and UAV interaction when the suspension lines are tight or slack, this study presents a method for improving a multibody dynamic model for a parafoil-UAV system. The parafoil and UAV are modeled as usual, while the suspension lines are modeled as a combination of several linear viscoelastic elements. All models are coupled, and the nonlinear equations of motion are then derived. To analyze the influence of the invalid suspension lines, this improved model has been compared with an 8-degrees of freedom model. The comparisons demonstrate that the simulation results of the improved model and the 8-DoF model are similar under a small control input. However, under a large control input, the results become significantly different. In an actual flight test, the accuracy of this improved model is found to be better than the 8-DoF model. Finally, an attitude optimal control system is designed for this improved model. The performance of this autonomous control is presented at the end of this paper.

An Improved Nonlinear Multibody Dynamic Model for a Parafoil-UAV System