The size of a narrow gap traversable by a fixed-wing drone is limited by its wingspan. Inspired by birds, here, we enable the traversal of a gap of sub-wingspan width and height using a morphing-wing drone capable of temporarily sweeping in its wings mid-flight. This maneuver poses control challenges due to sudden lift loss during gap-passage at low flight speeds and the need for precisely timed wing-sweep actuation ahead of the gap. To address these challenges, we first develop an aerodynamic model for general wing-sweep morphing drone flight including low flight speeds and post-stall angles of attack. We integrate longitudinal drone dynamics into an optimal reference trajectory generation and Nonlinear Model Predictive Control framework with runtime adaptive costs and constraints. Validated on a 130 g wing-sweep-morphing drone, our method achieves an average altitude error of 5 cm during narrow-gap passage at forward speeds between 5 and 7 m/s, whilst enforcing fully swept wings near the gap across variable threshold distances. Trajectory analysis shows that the drone can compensate for lift loss during gap-passage by accelerating and pitching upwards ahead of the gap to an extent that differs between reference trajectory optimization objectives. We show that our strategy also allows for accurate gap passage on hardware whilst maintaining a constant forward flight speed reference and near-constant altitude.

翻译：固定翼无人机可穿越的狭窄缝隙尺寸受其翼展限制。受鸟类启发，本研究利用具备飞行中临时收拢机翼能力的变形翼无人机，实现了对亚翼展宽度和高度的缝隙穿越。该机动在低飞行速度下穿越缝隙时因升力突然损失以及需要在缝隙前精确计时执行机翼收拢动作而带来控制挑战。为解决这些挑战，我们首先建立了适用于低飞行速度及失速后迎角条件下、具备通用机翼收拢变形能力的无人机气动模型。我们将无人机纵向动力学整合至具有运行时自适应成本函数与约束的最优参考轨迹生成及非线性模型预测控制框架中。在130克机翼收拢变形无人机上的验证表明，该方法在5至7米/秒前飞速度穿越狭窄缝隙期间，实现了平均5厘米的高度误差，同时在可变阈值距离范围内确保机翼在缝隙附近完全收拢。轨迹分析显示，无人机可通过在缝隙前加速和向上俯仰来补偿穿越期间的升力损失，其补偿程度因参考轨迹优化目标而异。我们证明该策略在硬件上也能实现精确的缝隙穿越，同时保持恒定的前飞速度参考值和近乎恒定的高度。