THE 3-DIMENSIONAL LEADING-EDGE VORTEX OF A HOVERING MODEL HAWKMOTH

Recent flow visualization experiments with the hawkmoth, Manduca sexta , revealed a small but clear leading-edge vortex and a pronounced thre e-dimensional flow. Details of this flow pattern were studied with a s caled-up, robotic insect ('the flapper') that accurately mimicked the wing movements of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing confirmed the existence of a small, strong a nd stable leading-edge vortex, increasing in size from wingbase to win gtip. Between 25 and 75% of the wing length, its diameter increased ap proximately from 10 to 50% of the wing chord. The leading-edge vortex had a strong axial flow velocity, which stabilized it and reduced its diameter. The vortex separated from the wing at approximately 75% of t he wing length and thus fed vorticity into a large, tangled tip vortex . If the circulation of the leading-edge vortex were fully used for li ft generation, it could support up to two-thirds of the hawkmoth's wei ght during the downstroke. The growth of this circulation with time an d spanwise position clearly identify dynamic stall as the unsteady aer odynamic mechanism responsible for high lift production by hovering ha wkmoths and possibly also by many other insect species.