LEADING-EDGE VORTICES IN INSECT FLIGHT

INSECTS cannot fly, according to the conventional laws of aero dynamic s: during flapping flight, their wings produce more lift than during s teady motion at the same velocities and angles of attack(1-5). Measure d instantaneous lift forces also show qualitative and quantitative dis agreement with the forces predicted by conventional aerodynamic theori es(6-9). The importance of high-life aerodynamic mechanisms is now wid ely recognized but, except for the specialized fling mechanism used by some insect species(1,10-13), the source of extra lift remains unknow n. 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 downstroke, of sufficien t strength to explain the high-lift forces. The vortex is created by d ynamic stall, and not by the rotational lift mechanisms that have been postulated for insect flight(14-16). The vortex spirals out towards t he wingtip with a spanwise velocity comparable to the flapping velocit y, The three-dimensional flow is similar to the conical leading-edge v ortex found on delta wings, with the spanwise flow stabilizing the vor tex.