Falling with style: bats perform complex aerial rotations by adjusting wing inertia

The remarkable maneuverability of flying animals results from precise movements of their highly specialized wings. Bats have evolved an impressive capacity to control their flight, in large part due to their ability to modulate wing shape, area, and angle of attack through many independently controlled joints. Bat wings, however, also contain many bones and relatively large muscles, and thus the ratio of bats’ wing mass to their body mass is larger than it is for all other extant flyers. Although the inertia in bat wings would typically be associated with decreased aerial maneuverability, we show that bat maneuvers challenge this notion. We use a model-based tracking algorithm to measure the wing and body kinematics of bats performing complex aerial rotations. Using a minimal model of a bat with only six degrees of kinematic freedom, we show that bats can perform body rolls by selectively retracting one wing during the flapping cycle. We also show that this maneuver does not rely on aerodynamic forces, and furthermore that a fruit fly, with nearly massless wings, would not exhibit this effect. Similar results are shown for a pitching maneuver. Finally, we combine high-resolution kinematics of wing and body movements during landing and falling maneuvers with a 52-degree-of-freedom dynamical model of a bat to show that modulation of wing inertia plays the dominant role in reorienting the bat during landing and falling maneuvers, with minimal contribution from aerodynamic forces. Bats can, therefore, use their wings as multifunctional organs, capable of sophisticated aerodynamic and inertial dynamics not previously observed in other flying animals. This may also have implications for the control of aerial robotic vehicles.

飞行类动物卓越的机动性源于其高度特化翼部的精准运动。蝙蝠演化出了出众的飞行控制能力，这在很大程度上得益于它们能通过众多独立受控的关节调节翼形、翼面积与攻角。不过，蝙蝠的翼部同时包含大量骨骼与相对发达的肌肉，因此其翼质量与体质量的比值高于所有现存其他飞行类动物。
尽管蝙蝠翼部的惯量通常会被认为会降低空中机动性，但我们的研究推翻了这一认知。我们采用基于模型的跟踪算法，对执行复杂空中旋转动作的蝙蝠进行翼部与躯体运动学参数的测量。通过仅包含6个运动学自由度的简化蝙蝠模型，我们证实蝙蝠可在拍打周期内选择性收拢单侧翼来完成躯体滚转动作。我们还发现，该机动动作并不依赖气动力；此外，翼部近乎无质量的果蝇无法实现这类效果。类似的结论也适用于俯仰机动动作。
最终，我们将着陆与跌落机动过程中翼部与躯体运动的高分辨率运动学数据，与包含52个运动学自由度的蝙蝠动力学模型相结合，证实了在着陆与跌落机动过程中，翼部惯量调节在蝙蝠重新定向的过程中占据主导地位，气动力仅起到极小的辅助作用。因此，蝙蝠可将其翼部作为多功能器官，实现此前其他飞行类动物未曾观测到的复杂气动力与惯量动力学控制。该研究结论或许也能为空中机器人车辆的控制提供参考。