Nature 384, 626 - 630 (26 December 1996); doi:10.1038/384626a0

Leading-edge vortices in insect flight

Charles P. Ellington, Coen van den Berg^*, Alexander P. Willmott^* & Adrian L. R. Thomas^*

Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB23EJ, UK
^*Present addresses: Faculty of Human Movement Sciences, Vrije Universiteit, Van der Boechorststraat 9, 1081 BT Amsterdam, The Netherlands (C.v.d.B.); Kawachi Millibioflight Project, Japan Science and Technology Corporation (JST), Park Building 3F, 4-7-6 Komaba, Meguro-ku, Tokyo 153, Japan (A.P.W.); Department of Zoology, University of Oxford, South Parks Road, Oxford 0X1 3PS, UK (A.L.R.T.).

INSECTS cannot fly, according to the conventional laws of aerodynamics: during flapping flight, their wings produce more lift than during steady motion at the same velocities and angles of attack¹⁻⁵. Measured instantaneous lift forces also show qualitative and quantitative disagreement with the forces predicted by conventional aerodynamic theories⁶⁻⁹. The importance of high-life aerodynamic mechanisms is now widely recognized but, except for the specialized fling mechanism used by some insect species^1,10−13, the source of extra lift remains unknown. We have now visualized the airflow around the wings of the hawkmoth Manduca sexta and a 'hovering' large mechanical model—the flapper. An intense leading-edge vortex was found on the down-stroke, of sufficient strength to explain the high-lift forces. The vortex is created by dynamic stall, and not by the rotational lift mechanisms that have been postulated for insect flight¹⁴⁻¹⁶. The vortex spirals out towards the wingtip with a spanwise velocity comparable to the flapping velocity. The three-dimensional flow is similar to the conical leading-edge vortex found on delta wings, with the spanwise flow stabilizing the vortex.