Experiments on the Weis-Fogh mechanism of lift generation by insects in hovering flight. Part 1. Dynamics of the ‘fling’

TLDR

The study proposes minor revisions to the Weis‑Fogh–Lighthill clap‑and‑fling model for insect hovering lift. Experiments show that a leading‑edge separation vortex generates most of the circulation, which is convected into a tip vortex forming a downward‑moving ring fed by quasi‑steady separation bubbles as the wings open at high angle of attack. The resulting circulation exceeds inviscid predictions and explains the large lift forces observed in insects.

Abstract

From a series of experiments using simplified mechanical models we suggest certain minor modifications to the Weis-Fogh (1973)–Lighthill (1973) explanation of the so-called ‘clap and fling’ mechanism for the generation of large lift coefficients by insects in hovering flight. Of particular importance is the production and motion of a leading edge, separation vortex that accounts for virtually all of the circulation generated during the initial phase of the ‘fling’ process. The magnitude of this circulation is substantially larger than that calculated using inviscid theory. During the motion that subsequently separates the wings, the vorticity over each of them is convected and combined to become a tip vortex of uniform circulation spanning the space between them. This combined vortex moves downwards as a part of a ring, of large impulse, that is then continuously fed from quasi-steady separation bubbles that move with the wings as they continue to open at a large angle of attack. Such effects are able to account for the large lift forces generated by the insect.