THE VORTEX WAKE OF A HOVERING MODEL HAWKMOTH

Visualization experiments with Manduca sexta have revealed the presenc e of a leading-edge vortex and a highly three-dimensional flow pattern . To further investigate this important discovery, a scaled-up robotic insect was built (the 'flapper') which could mimic the complex moveme nts of the wings of a hovering hawkmoth. Smoke released from the leadi ng edge of the flapper wing revealed a small but strong leading-edge v ortex on the downstroke. This vortex had a high axial flow velocity an d was stable, separating from the wing at approximately 75% of the win g length. It connected to a large, tangled tip vortex, extending back to a combined stopping and starting vortex from pronation. At the end of the downstroke, the wake could be approximated as one vortex ring p er wing. Based on the size and velocity of the vortex rings, the mean lift force during the downstroke was estimated to be about 1.5 times t he body weight of a hawkmoth, confirming that the downstroke is the ma in provider of lift force.