The vortex wake of a 'hovering' model hawkmoth

Visualization experiments with Manduca sexta have revealed the presence 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 movements of the wings of a hovering hawkmoth. Smoke released from the leading edge of the flapper wing revealed a small but strong leading-edge vortex on the downstroke. This vortex had a high axial flow velocity and was stable, separating from the wing at approximately 75% of the wing 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 per 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 the body weight of a hawkmoth, confirming that the downstroke is the main provider of lift force.