Meeting Abstract
Textbook descriptions of bird flight discuss the relatively elliptical planform of avian wings as evidence that birds achieve minimum induced drag by enabling constant downwash from tip to tip. To test this description, we measured the wakes of gliding birds by tracking up to 22,000 neutrally buoyant helium-filled soap bubbles at each time point. Our hypothesis was that the action of the tail would compensate for lift lost over the body and enable constant downwash from wingtip to wingtip, minimizing induced drag. Instead, we found that the spread and pitch of the tail produces a strong jet of air that far exceeds the expected downwash; that is, the birds are not elliptically loaded at these slow, self-selected glide speeds. A frequently overlooked action of the tail is to minimize viscid drag, which, in aircraft, is typically a negligible contribution due to higher Reynolds numbers. We found that, at the intermediate Reynolds numbers of avian gliding, the contribution of viscous drag is of the same magnitude as the induced (inviscid) drag. Viscous drag minimization predicts constant area loading, with downwash proportional to chord length at each spanwise position. The measured downwash distribution behind our birds, with a strong downward jet behind the body/tail, is consistent with drag minimization, but reveals a compromise between elliptical loading and constant area loading.
