The aerodynamics of hovering insect flight. IV. Aerodynamic mechanisms

TLDR

The aerodynamic mechanisms of lift generation in hovering insect flight involve a gradual stall of thin profiles at Re < 10⁵, as discussed through theoretical considerations and experimental studies. Leading‑edge separation bubbles form when flow detaches at the sharp leading edge and re‑attaches downstream, creating a recirculating region that increases effective camber and thickness and thus lift at low Reynolds numbers. Measured lift coefficients show that leading‑edge separation bubbles are a prominent feature in insect flight, and similar effects may occur in fast forward flight of birds.

Abstract

Theoretical considerations and available experimental studies are combined for a discussion on the aerodynamic mechanisms of lift generation in hovering animal flight. A comparison of steady-state thin-aerofoil theory with measured lift coefficients reveals that leading edge separation bubbles are likely to be a prominent feature in insect flight. Insect wings show a gradual stall that is characteristic for thin profiles at Reynolds numbers (Re) less than about 105. In this type of stall, flow separates at the sharp leading edge and then re-attaches downstream to the upper wing surface, producing a region of limited separation enclosing a recirculating flow. The resulting leading edge bubble enhances the camber and thickness of the thin profile, improving lift at low Re. Some of the results for bird wing profiles indicate that the complications of leading edge bubbles might even be found in the fast forward flight of birds.