Data from: The wake of hovering flight in bats

Hovering means stationary flight at zero net forward speed, which can be achieved by animals through muscle powered flapping flight. Small bats capable of hovering typically do so with a downstroke in an inclined stroke plane, and with an aerodynamically active outer wing during the upstroke. The magnitude and time history of aerodynamic forces should be reflected by vorticity shed into the wake. We thus expect hovering bats to generate a characteristic wake, but this has until now never been studied. Here we trained nectar-feeding bats, Leptonycteris yerbabuenae, to hover at a feeder and using time-resolved stereoscopic particle image velocimetry in conjunction with high-speed kinematic analysis we show that hovering nectar-feeding bats produce a series of bilateral stacked vortex loops. Vortex visualizations suggest that the downstroke produces the majority of the weight support, but that the upstroke contributes positively to the lift production. However, the relative contributions from downstroke and upstroke could not be determined on the basis of the wake, because wake elements from down- and upstroke mix and interact. We also use a modified actuator disc model to estimate lift force, power and flap efficiency. Based on our quantitative wake-induced velocities, the model accounts for weight support well (108%). Estimates of aerodynamic efficiency suggest hovering flight is less efficient than forward flapping flight, while the overall energy conversion efficiency (mechanical power output/metabolic power) was estimated at 13%.

悬停指净前向速度为零的静止飞行状态，动物可通过肌肉驱动的扑翼飞行实现该状态。具备悬停能力的小型蝙蝠通常在倾斜扑翼平面内完成下扑阶段，并在上挥阶段保持翼外段处于气动活跃状态。尾迹中脱落的涡量应能反映气动作用力的大小与时序变化，因此我们推测悬停蝙蝠会产生具有特征性的尾迹，但截至目前该现象尚未得到研究。

本研究训练食蜜蝙蝠长舌叶鼻蝠（Leptonycteris yerbabuenae）使其在喂食器处实现悬停，并结合时间分辨立体粒子图像测速技术（time-resolved stereoscopic particle image velocimetry）与高速运动学分析（high-speed kinematic analysis），证实悬停的食蜜蝙蝠会产生一系列双边堆叠涡环（bilateral stacked vortex loops）。涡旋可视化结果显示，下扑阶段提供了大部分的体重支撑力，但上挥阶段也对升力产生具有正向贡献。然而，由于下扑与上挥阶段产生的尾迹元素会相互混合并发生相互作用，基于尾迹无法区分两个阶段的相对贡献占比。

本研究同时采用改进型致动盘模型（actuator disc model）估算升力、功率与扑翼效率。基于定量测得的尾迹诱导速度，该模型对体重支撑力的拟合效果极佳，预测值可达实际需求的108%。气动效率估算结果表明，悬停飞行的效率低于前向扑翼飞行；而整体能量转换效率（机械功率输出/代谢功率）经估算约为13%。