Very small insects use novel wing flapping and drag principle to generate the weight-supporting vertical force
Very small insects use novel wing flapping and drag principle to generate the weight-supporting vertical force
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DOI：
10.1017/jfm.2018.668
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发表时间：
2018-09-19
影响因子：
3.7
通讯作者：
Sun, Mao
中科院分区：
文献类型：
--
作者：
Cheng, Xin;Sun, Mao
空气粘度对昆虫翅膀周围气流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毫米），粘性效应是如此之大，以至于它们的大同伴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。为了阐明其空气动力机制，我们测量了微小黄蜂Encarsia formosa （0.6 mm R）在悬停和极慢上升飞行时的机翼运动学，并计算和分析了空气动力。我们发现这些昆虫有两种不同寻常的翅膀运动。一种是“划桨”：翅膀快速向下和向后移动，就像划桨一样。另一种是先前发现的韦斯-福格“飞动”。划桨产生所需垂直力的70%，而韦斯-福格“投掷”产生另外30%。桨翼主要产生近似向上指向的阻力，从而产生垂直力。由于每次划桨都会产生起动流，因此阻力本质上是不稳定的，而且在相同的速度和迎角下，阻力要比稳定运动时大得多。此外，我们的计算表明，如果小黄蜂采用大昆虫通常的翅膀运动（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的1/3；也就是说，他们必须使用特殊的机翼运动来克服通常使用的扑翼运动学所遇到的大粘性效应的问题。我们首次观察到非常小的昆虫使用阻力来支撑它们的重量，并解释了当阻力原理被应用时如何产生净垂直力。
The effect of air viscosity on the flow around an insect wing increases as insect size decreases. For the smallest insects (wing length R below 1 mm), the viscous effect is so large that lift-generation mechanisms used by their larger counterparts become ineffective. How the weight-supporting vertical force is generated is unknown. To elucidate the aerodynamic mechanisms responsible, we measure the wing kinematics of the tiny wasp Encarsia formosa (0.6 mm R) in hovering or very slow ascending flight and compute and analyse the aerodynamic forces. We find that the insects perform two unusual wing motions. One is 'rowing': the wings move fast downward and backward, like stroking oars. The other is the previously discovered Weis-Fogh 'fling'. The rowing produces 70% of the required vertical force and the Weis-Fogh 'fling' the other 30 %. The oaring wing mainly produces an approximately up-pointing drag, resulting in the vertical force. Because each oaring produces a starting flow, the drag is unsteady in nature and much greater than that in steady motion at the same velocities and angles of attack. Furthermore, our computation shows that if the tiny wasps employed the usual wing kinematics of the larger insects (flapping back and forth in a horizontal plane), the vertical force produced would be only 1/3 of that by the real wing kinematics; i.e. they must use the special wing movements to overcome the problem of large viscous effects encountered by the commonly used flapping kinematics. We have observed for the first time very small insects using drag to support their weight and we explain how a net vertical force is generated when the drag principle is applied.