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Journal of Experimental Biology 165,1-19 (1992)
Published by Company of Biologists 1992

The Profile Drag of a Hawk'S Wing, Measured by Wake Sampling in a Wind Tunnel

C. J. PENNYCUICK ¹, CARLTON E. HEINE ², SEAN J. KIRKPATRICK ¹, and MARK R. FULLER ³

¹ Department of Biology, University of Miami, PO Box 249118, Coral Gables, FL 33124-0421, USA,
² Department of Zoology, Duke University, Durham, NC27706, USA
³ US Fish and Wildlife Service, Patuxent Wildlife Research Center, Laurel, MD 20708, USA

To whom reprint requests should be addressed.

The distribution of dynamic pressure behind a Harris' hawk's wing was sampled using a wake rake consisting of 15 pitot tubes and one static tube. The hawk was holding on to a perch, but at an air speed and gliding angle at which it was capable of gliding. The perch was instrumented, so that the lift developed by the wing was known and the lift coefficient could be calculated. The mean of 92 estimates of profile drag coefficient was 0.0207, with standard deviation 0.0079. Lift coefficients ranged from 0.51 to 1.08. Reynolds numbers were nearly all in the range 143000–194000. The estimates of profile drag coefficient were reconcilable with previous estimates of the wing profile drag of the same bird, obtained by the subtractive method, and also with values predicted by the ‘Airfoil-ii’ program for designing aerofoils, based on a digitized wing profile from the ulnar region of the wing. The thickness of the wake suggested that the boundary layer was mostly or fully turbulent in most observations and separated in some, possibly as an active means of creating drag for control purposes. It appears that the bird could momentarily either increase or decrease the profile drag of specific parts of the wing, by active changes of shape, and it appeared to use the carpo-metacarpal region especially for such control movements. Further investigation in a low turbulence wind tunnel would help to resolve doubts about the possible influence of airstream turbulence on the behaviour of the boundary layer.

Note:
Present address: Department of Zoology, University of Bristol, Woodland Road, Bristol BS8 1UG, England.

Key words: bird, wing, drag, wind tunnel, Parabuteo unicinctus

Accepted on December 20, 1991

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