The aerodynamics of hovering insect flight. IV. Aerodynamic mechanisms

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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.