太阳能无人机翼载很低,在伞降过程中,降落伞的拉力对无人机的姿态影响很大,稳定下降速度与巡航速度可比拟,无人机的气动力不可忽略。而太阳能无人机又需要有较平稳的伞降姿态以减小对机翼上太阳能板的损坏,因此对伞降的姿态控制要求比其他飞机高。为了以最小的代价准确分析这个现象,首先,把无人机和降落伞视为2个刚体,推导了伞降系统的九自由度多体动力学模型,根据机-绳-伞的相对位置关系,采用四元数法推导了伞绳和降落伞的姿态角代数方程,从而减小了动力学方程组的个数和对降落伞参数的依赖。然后,采用空间几何法,根据伞绳的柔性特征,推导了伞绳挂点和结点对无人机的力矩作用关系。接下来,针对算例无人机进行了伞降系统多体动力学模型的飞行仿真,并与飞行试验数据进行了对比,发现位置、速度、姿态等数据一致性都较高、细节保留较好,验证了所提出的伞降着陆仿真分析方法的有效性。最后,采用所建立的多体动力学飞行仿真系统进行了伞绳挂点对俯仰角和飞行轨迹的敏感性分析,得到了前挂点前移对无人机的姿态扰动影响很大,而后挂点的前后移动对姿态扰动的影响相对不明显的结论。
The low wing load of solar-powered UAVs leads to a large impact of the parachute pull on the UAV attitude in the parachute landing process. The steady descent speed is comparable to its cruise speed, while the aerodynamic force of the UAV cannot be ignored. The solar-powered UAV needs a more stable parachute attitude to reduce damage to the solar panels on the wing, therefore requiring higher attitude control of parachute landing than other aircraft. For accurate analysis of this phenomenon at the minimum cost, a nine degrees of freedom multi-body dynamic model of the parachute landing system is firstly derived with the UAV and the parachute regarded as two rigid bodies. According to the relative position relationship of the UAV-rope-parachute, the algebraic equations of the attitude angles of the parachute rope and the parachute are obtained using the quaternion method, thus reducing the number of dynamic equations and the dependence on parachute parameters. Based on the flexible characteristics of the parachute rope, the moment relationship between the attachment point and the node of the parachute rope to the UAV is then derived using the spatial geometry method. Furthermore, the flight simulation of the multi-body dynamic model of the parachute landing system as an example UAV is carried out, and the results are compared with the flight test data. It is found that the data consistency of position, speed and attitude is high, the details are well preserved, and the effectiveness of the proposed parachute landing simulation analysis method is verified. Finally, the sensitivity of the attachment point of the rope to the pitch angle and flight trajectory is analyzed using the established multi-body dynamics flight simulation system. It is concluded that the forward movement of the front attachment has a considerable influence on the pitch angle of the UAV, while the forward and backward movements of the rear attachment has relatively little influence on the pitch angle.
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