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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

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Research Article, Biomechanics of Flight

The implications of low-speed fixed-wing aerofoil measurements on the analysis and performance of flapping bird wings

G. R. Spedding^1,*, A. H. Hedenström², J. McArthur¹ and M. Rosén²

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; 10⁴Re10⁵, 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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