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The Wake of a Kestrel (Falco Tinnunculus) in Flapping Flight
1 Department of Zoology, University of Bristol, Woodland Road, Bristol BS8 1UG; Department of Aerospace Engineering, University of Southern California, University Park, Los Angeles, CA 90089-0192, USA.
The structure of the wake behind a kestrel in medium-speed flight down a 36 m length of corridor was analysed qualitatively and quantitatively by stereophotogrammetry of multiple flash photographs of the motion of small soap-covered helium bubbles. The wake consists of a pair of continuous, undulating trailing vortices. The upstroke is therefore aerodynamically active and the circulation appears to remain constant along the wing whose geometry is altered during the course of the wingstroke. It is argued that the flight kinematics, and so the wake structure, of the kestrel may be typical of flapping flight at medium speeds and a flight model based on this wake geometry is presented. Rough estimates of the rate of momentum generated in the wake balance the weight almost exactly and a direct estimate of the induced power requirement from the wake measurements is obtained. The significance of these results for the various alternative aerodynamic descriptions and energetic predictions of models of flapping animal flight is briefly assessed
Key words: flight, wake, kestrel
Accepted on August 19, 1986
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