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First published online December 28, 2007
Journal of Experimental Biology 211, 215-223 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.007823
Research Article, Biomechanics of Flight |
The implications of low-speed fixed-wing aerofoil measurements on the analysis and performance of flapping bird wings
1 Aerospace and Mechanical Engineering, University of Southern California, Los
Angeles, CA 90089-1191, USA
2 Department of Theoretical Ecology, Lund University, SE 223-62, Lund,
Sweden
* Author for correspondence (e-mail: geoff{at}usc.edu)
Accepted 23 May 2007
Summary
Bird flight occurs over a range of Reynolds numbers (Re;
104
Re105, where Re is a
measure of the relative importance of inertia and viscosity) that includes
regimes where standard aerofoil performance is difficult to predict, compute
or measure, with large performance jumps in response to small changes in
geometry or environmental conditions. A comparison of measurements of fixed
wing performance as a function of Re, combined with quantitative flow
visualisation techniques, shows that, surprisingly, wakes of flapping bird
wings at moderate flight speeds admit to certain simplifications where their
basic properties can be understood through quasi-steady analysis. Indeed, a
commonly cited measure of the relative flapping frequency, or wake
unsteadiness, the Strouhal number, is seen to be approximately constant in
accordance with a simple requirement for maintaining a moderate local angle of
attack on the wing. Together, the measurements imply a fine control of
boundary layer separation on the wings, with implications for control
strategies and wing shape selection by natural and artificial fliers.
Key words: animal flight, aerofoil, lift-drag polar, wake analysis, Reynolds number
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  P. Henningsson, G. R. Spedding, and A. Hedenstrom Vortex wake and flight kinematics of a swift in cruising flight in a wind tunnel J. Exp. Biol., March 1, 2008; 211(5): 717 - 730. [Abstract] [Full Text] [PDF]
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