Power reduction and the radial limit of stall delay in revolving wings of different aspect ratio
Power reduction and the radial limit of stall delay in revolving wings of different aspect ratio
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DOI：
10.1098/rsif.2015.0051
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发表时间：
2015-04-06
影响因子：
3.9
通讯作者：
Lentink, David
中科院分区：
文献类型：
--
作者：
Kruyt, Jan W.;van Heijst, GertJan F.;Lentink, David
飞机和直升机使用大展弦比机翼以减少飞行所需的动力，但必须在低迎角下工作以防止气流分离和失速。能够缓慢持续飞行的动物，如蜂鸟，具有低展弦比的翅膀，并且在大迎角下拍打翅膀而不会失速。相反，它们沿着机翼前缘产生一个附着涡流，沿着提升升力。以前的研究已经证明，这种涡流和高升力可以通过以相同的迎角旋转动物翅膀来再现。拍打和旋转动物翅膀是如何延迟失速和降低功率的？据推测，失速延迟是由于肩关节和翼尖之间有一个短的径向距离（用弦长测量）而引起的。这种无量纲的翼长测量值代表了旋转翼和扑翼边界层中惯性力与旋转加速度的相对大小。在这里，我们表明，对于一组展弦比，即动物和飞机的机翼，旋转机翼上前缘涡的附着是由机翼展弦比决定的，而展弦比是相对于旋转中心定义的。在大迎角下，当局部半径小于四个弦长时，涡保持附着，在大展弦比机翼上，涡向外分离。这种径向失速极限解释了为什么在低迎角下（直升机的）旋转大展弦比机翼比（蜂鸟的）低展弦比机翼需要更少的功率，而在高迎角下则相反。
Airplanes and helicopters use high aspect ratio wings to reduce the power required to fly, butmust operate at lowangle of attack to prevent flowseparation and stall. Animals capable of slowsustained flight, such as hummingbirds, have low aspect ratio wings and flap their wings at high angle of attack without stalling. Instead, they generate an attached vortex along the leading edge of the wing that elevates lift. Previous studies have demonstrated that this vortex and high lift can be reproduced by revolving the animal wing at the same angle of attack. How do flapping and revolving animal wings delay stall and reduce power? It has been hypothesized that stall delay derives from having a short radial distance between the shoulder joint and wing tip, measured in chord lengths. This non-dimensionalmeasure ofwing length represents the relative magnitude of inertial forces versus rotational accelerations operating in the boundary layer of revolving and flapping wings. Here we show for a suite of aspect ratios, which represent both animal and aircraft wings, that the attachment of the leading edge vortex on a revolving wing is determined by wing aspect ratio, defined with respect to the centre of revolution. At high angle of attack, the vortex remains attached when the local radius is shorter than four chord lengths and separates outboard on higher aspect ratio wings. This radial stall limit explains why revolving high aspect ratio wings (of helicopters) require less power compared with lowaspect ratio wings (of hummingbirds) at low angle of attack and vice versa at high angle of attack.