Flight on the two-dimensional air-water interface, with body weight supported by surface tension, is a unique locomotion strategy well adapted for the environmental niche on the surface of water. Although previously described in phylogenetically basal aquatic insects like stone flies, the biomechanics of interfacial flight has never been analyzed. Here, we report interfacial flight as an adapted behaviour in water-lily beetles (Galerucella nymphaeae, Linnaeus 1758) which are also dexterous airborne fliers. We present the first quantitative biomechanical model of interfacial flight in insects, uncovering an intricate interplay of capillary, aerodynamic and neuromuscular forces. We show that water-lily beetles use their tarsal claws to attach themselves to the interface, via a fluid contact line pinned at the claw. We investigate the kinematics of interfacial flight trajectories using high-speed imaging and construct a mathematical model describing the flight dynamics. Our results show that nonlinear surface tension forces make interfacial flight energetically expensive compared to airborne flight at the relatively high speeds characteristic of water-lily beetles, and cause chaotic dynamics to arise naturally in these regimes. We identify the crucial roles of capillary-gravity wave drag and oscillatory surface tension forces which dominate interfacial flight, showing that the air-water interface presents a radically modified force landscape for flapping wing flight compared to air.
Very small insects use novel wing flapping and drag principle to generate the weight-supporting vertical force
空气粘度对昆虫翅膀周围流动的影响随着昆虫尺寸的减小而增大。对于最小的昆虫（翼长R小于1 mm），粘性效应是如此之大，以至于它们较大的对应物所使用的升力产生机制变得无效。支撑重量的垂直力是如何产生的尚不清楚。为了阐明空气动力学机制负责，我们测量了微小的黄蜂Encarsia formosa（0.6 mm R）在悬停或非常缓慢的上升飞行中的机翼运动学，并计算和分析气动力。我们发现，昆虫执行两个不寻常的翅膀运动。一种是划船：翅膀快速向下和向后移动，就像划桨一样;另一种是以前发现的Weis-Fogh投掷。划船产生70%的垂直力，Weis-Fogh投掷产生另外30%。桨翼主要产生一个近似向上指向的阻力，从而产生垂直力。因为每一次划桨都产生一个起动流，所以阻力本质上是不稳定的，并且在相同的速度和攻角下比稳定运动中的阻力大得多。此外，我们的计算表明，如果小黄蜂采用较大昆虫的通常的翅膀运动学（在水平面上来回拍打），产生的垂直力将只有真正的翅膀运动学的三分之一;也就是说，它们必须使用特殊的翅膀运动来克服通常使用的拍打运动学遇到的大粘性效应的问题。我们第一次观察到非常小的昆虫使用阻力来支撑它们的重量，并解释了当应用阻力原理时如何产生净垂直力。
Advancing flight physics through natural adaptation and animal learning
Energy and Sustainability Research Institute Groningen, Faculty of Science and Engineering, University of Groningen（格罗宁根能源与可持续发展研究所，科学工程学院，格罗宁根大学）；Groningen Institute for Evolutionary Life Sciences, University of Groningen（格罗宁根进化生命科学研究所，格罗宁根大学）
“Cicada：a Heavy but Agile Flyer”是提交给2011年APS-DFD流体运动画廊的流体动力学视频。与其他昆虫相比，蝉可以产生更高的升力来克服它们庞大的体重。这种隐藏的机制可能有助于设计携带大型有效载荷的微型飞行器（MAV）。然而，关于蝉如何利用翅膀完成各种飞行的研究文献还很缺乏。本文利用高速摄影测量系统和三维曲面重建技术，揭示了机翼在自由前飞过程中的运动和变形。气动性能的研究使用内部沉浸边界法基于计算流体动力学（CFD）求解器。
