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First published online September 19, 2006
Journal of Experimental Biology 209, 3887-3897 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02446
Wing beat kinematics of a nectar-feeding bat, Glossophaga soricina, flying at different flight speeds and Strouhal numbers
1 Department of Zoology, Göteborg University, Box 463, SE-405 30
Göteborg, Sweden
2 Department of Biology, Bielefield University, Germany
* Author for correspondence (e-mail: ulla.lindhe{at}zool.gu.se)
Accepted 17 July 2006
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
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 
=44.8V0.29). 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
f-0.40. The
Strouhal number (St=fxamplitude/V), a dimensionless
parameter describing oscillating flow mechanisms and being a predictor of the
unsteadiness of the flow, decreases with the speed as
StV-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.
Key words: flight, wing beat kinematics, Strouhal number, bat, Glossophaga soricina
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