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High aerodynamic lift from the tail reduces drag in gliding raptors

Usherwood, J R; Cheney, J A; Song, J; Windsor, S P; Stevenson, J P J; Dierksheide, U; Nila, A; Bomphrey, R J


J R Usherwood

J A Cheney

J Song

S P Windsor

J P J Stevenson

U Dierksheide

A Nila

R J Bomphrey


Many functions have been postulated for the aerodynamic role of the avian tail during steady-state flight. By analogy with conventional aircraft, the tail might provide passive pitch stability if it produced very low or negative lift. Alternatively, aeronautical principles might suggest strategies that allow the tail to reduce inviscid, induced drag: if the wings and tail act in different horizontal planes, they might benefit from biplane-like aerodynamics; if they act in the same plane, lift from the tail might compensate for lift lost over the fuselage (body), reducing induced drag with a more even downwash profile. However, textbook aeronautical principles should be applied with caution because birds have highly capable sensing and active control, presumably reducing the demand for passive aerodynamic stability, and, because of their small size and low flight speeds, operate at Reynolds numbers two orders of magnitude below those of light aircraft. Here, by tracking up to 20,000, 0.3 mm neutrally buoyant soap bubbles behind a gliding barn owl, tawny owl and goshawk, we found that downwash velocity due to the body/tail consistently exceeds that due to the wings. The downwash measured behind the centreline is quantitatively consistent with an alternative hypothesis: that of constant lift production per planform area, a requirement for minimizing viscous, profile drag. Gliding raptors use lift distributions that compromise both inviscid induced drag minimization and static pitch stability, instead adopting a strategy that reduces the viscous drag, which is of proportionately greater importance to lower Reynolds number fliers.


Usherwood, J. R., Cheney, J. A., Song, J., Windsor, S. P., Stevenson, J. P. J., Dierksheide, U., …Bomphrey, R. J. (2020). High aerodynamic lift from the tail reduces drag in gliding raptors. Journal of Experimental Biology, 223,

Journal Article Type Article
Acceptance Date Jan 6, 2020
Publication Date Feb 10, 2020
Deposit Date Feb 22, 2020
Publicly Available Date Feb 24, 2020
Print ISSN 0022-0949
Publisher The Company of Biologists
Peer Reviewed Peer Reviewed
Volume 223
Public URL


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