How do birds' tails work? Delta-wing theory fails to predict tail shape during flight

Citation

Evans M, Rosen M, Park K & Hedenstrom A (2002) How do birds' tails work? Delta-wing theory fails to predict tail shape during flight. Proceedings of the Royal Society B: Biological Sciences, 269 (1495), pp. 1053-1057. https://doi.org/10.1098/rspb.2001.1901

Abstract
Birds appear to use their tails during flight, but until recently the aerodynamic role that tails fulfil was largely unknown. In recent years delta-wing theory, devised to predict the aerodynamics of high-performance aircraft, has been applied to the tails of birds and has been successful in providing a model for the aerodynamics of a bird's tail. This theory now provides the conventional explanation for how birds' tails work. A delta-wing theory (slender-wing theory) has been used, as part of a variable-geometry model to predict how tail and wing shape should vary during flight at different airspeeds. We tested these predictions using barn swallows flying in a wind tunnel. We show that the predictions are not quantitatively well supported. This suggests that a new theory or a modified version of delta-wing theory is needed to adequately explain the way in which morphology varies during flight.

Keywords
Delta–Wing Theory; Aerodynamic Models; Birds' Tails

Journal
Proceedings of the Royal Society B: Biological Sciences: Volume 269, Issue 1495