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Science 10 December 2004:
Vol. 306. no. 5703, pp. 1960 - 1962
DOI: 10.1126/science.1104682
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Reports

Leading-Edge Vortex Lifts Swifts

J. J. Videler,1,2* E. J. Stamhuis,1 G. D. E. Povel2

The current understanding of how birds fly must be revised, because birds use their hand-wings in an unconventional way to generate lift and drag. Physical models of a common swift wing in gliding posture with a 60° sweep of the sharp hand-wing leading edge were tested in a water tunnel. Interactions with the flow were measured quantitatively with digital particle image velocimetry at Reynolds numbers realistic for the gliding flight of a swift between 3750 and 37,500. The results show that gliding swifts can generate stable leading-edge vortices at small (5° to 10°) angles of attack. We suggest that the flow around the arm-wings of most birds can remain conventionally attached, whereas the swept-back hand-wings generate lift with leading-edge vortices.

1 Department of Marine Biology (Experimental Marine Zoology Group), Groningen University, Post Office Box 14, 9750 AA, Haren, Netherlands.
2 Evolutionary Mechanics, Institute of Biology, Leiden University, Post Office Box 9516, 2300 RA Leiden, Netherlands.

* To whom correspondence should be addressed. E-mail: j.j.videler{at}biol.rug.nl

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