Wing beat kinematics of a nectar-feeding bat,Glossophaga soricina,flying at different flight speeds and Strouhal numbers

Abstract

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.