Lindhe Norberg Ulla M, Winter York
Department of Zoology, Göteborg University, Box 463, SE-405 30 Göteborg, Sweden.
J Exp Biol. 2006 Oct;209(Pt 19):3887-97. doi: 10.1242/jeb.02446.
High-speed film analysis showed that the wing beat kinematics in Glossophaga soricina change gradually with increasing flight speed, indicating that there is no sudden gait change at any particular, critical, flight speed. The flight of two adult specimens was studied over a range of flight speeds (1.23-7.52 ms(-1)) in a 30 m long flight tunnel. During the upstroke in hovering and slow flight there is a tip-reversal or supination of the handwings, which thus produce a backward flick. This backward motion successively disappears at speeds V approximately 3.2 ms(-1), above which the wingtip path becomes more vertical or directed upwards-forwards relative to the still air (the stroke plane angle increasing with flight speed as alpha=44.8V(0.29)). We found no correlations between either span ratio SR (the ratio of the wing span on the upstroke to that on the downstroke) and V, or downstroke ratio (the duration of the downstroke divided by the total stroke period) and V. On the other hand, SR decreases significantly with increasing wing beat frequency f, SR proportional to f(-0.40). The Strouhal number (St=f x amplitude/V), a dimensionless parameter describing oscillating flow mechanisms and being a predictor of the unsteadiness of the flow, decreases with the speed as St proportional to V(-1.37). Close to the theoretical minimum power speed (4-6 m s(-1)) G. soricina operates with a Strouhal number in the region 0.17<St<0.22, which is associated with efficient lift and thrust production. At slower speeds, 3.4-4 m s(-1), St is 0.25-0.4, which is still within the favourable region. But at speeds below 3 m s(-1) St becomes higher (0.5<St<0.68), indicating that unsteady effects become important, with unfavourable lift and thrust production as a result. Only at these speeds do the bats perform the backward flick during the upstroke, which may produce thrust. This may serve as a compensation in some bats and birds to increase aerodynamic performance.
高速影片分析显示，索氏长舌蝠的翅膀拍动运动学特征会随着飞行速度的增加而逐渐变化，这表明在任何特定的临界飞行速度下都不会出现突然的步态变化。在一条30米长的飞行隧道中，研究了两个成年标本在一系列飞行速度（1.23 - 7.52米/秒）下的飞行情况。在悬停和慢速飞行的上冲程过程中，前翅会出现尖端反转或内旋，从而产生向后的轻弹动作。这种向后的运动在速度约为3.2米/秒时相继消失，高于此速度时，相对于静止空气，翅尖路径变得更加垂直或向上向前（冲程平面角度随着飞行速度增加，α = 44.8V(0.29)）。我们发现展弦比SR（上冲程时的翼展与下冲程时的翼展之比）与V之间、下冲程比（下冲程持续时间除以总冲程周期）与V之间均无相关性。另一方面，SR随着翅膀拍动频率f的增加而显著降低，SR与f(-0.40)成正比。斯特劳哈尔数（St = f×振幅/V）是一个描述振荡流动机理且可预测流动不稳定性的无量纲参数，它随着速度降低，St与V(-1.37)成正比。接近理论最小功率速度（4 - 6米/秒）时，索氏长舌蝠的斯特劳哈尔数在0.17 < St < 0.22范围内，这与高效的升力和推力产生相关。在较慢速度3.4 - 4米/秒时，St为0.25 - 0.4，仍处于有利区域。但在速度低于3米/秒时，St会变得更高（0.5 < St < 0.68），这表明不稳定效应变得重要，导致升力和推力产生不利。只有在这些速度下，蝙蝠在上冲程时才会执行向后轻弹动作，这可能会产生推力。这在一些蝙蝠和鸟类中可能起到补偿作用，以提高空气动力学性能。
