Insects versus birds: the great divide

*Insects generally fly in the world of laminar flow with substantial , persistent vortices swirling around the wings and trailing off into the wake . Understanding of their high -lift mechanisms has advanced greatly in re cent years . Most of the lift is attributable to a large leading -edge vortex (LEV) created by the sharp leading edge of the wing. We have further studied the vortex dynamics and wing f orces using model wings rotating like propellers, which mimics the flapping phase of the wingbeat and avoids the confounding effects when the wing flips over at either end of the cycle. The leading -edge flow separation necessary to create the LEV causes a loss of leading -edge suction, and th e resultant aerodynamic force is approximately normal to the chord. At the high angles of attack needed to produce the LEV, a large drag force is therefore inevitable. The low L/D matters little to the insects: they just arrange the flapping motion to point the resultant force in the desired direction for weight support and thrust, taking advantage of the high drag as well as the high lift. However, the high drag of the insect flight mechanism is obviously unsuitable for fixed and rotary wing devices. The wings of large insects approach a chord -based Reynolds number of 10000 , where the separated shear layer from the sharp leading edge starts to become turbulent. By Re = 20 000 the turbulence destroys the coherence of the LE V, causing periodic shedding of vorticity and a three -fold reduction in the li ft coefficients. All but the smallest b irds live permanently in the turbulent regime , and thus are unlikely to exploit the insect flight mechanism. It may be that turbulence is t he great dividing line between them and the insects. However, there are always exceptions. Hummingbirds range in size from 2 to 20 grams , and fly across the spectrum from laminar to turbulent flow. Pilot tests of hummingbird wings at Re = 5000 reveal that maximum lift coefficients are very similar for insects and hummingbirds, but drag coefficients are much smaller , suggesting that the flow remains attached over the upper wing surface. The hummingbird wing far outperforms the insect LEV mechanism and is muc h less restrictive: it would be suitable for fixed wing and rotary wing designs in addition to flapping ones, and would be suitable for turbulent a s well as laminar applications. Amongst the large insects, dragonflies also have high L/D ratios that indicat e attached flow, and they are one of the few successful insect gliders. Our knowledge of transitional Re aerodynamics would benefit greatly from further study of such exceptions.