Aerodynamics of the hovering hummingbird

Abstract

Despite profound musculoskeletal differences, hummingbirds (Trochilidae) are widely thought to employ aerodynamic mechanisms similar to those used by insects. The kinematic symmetry of the hummingbird upstroke and downstroke has led to the assumption that these halves of the wingbeat cycle contribute equally to weight support during hovering, as exhibited by insects of similar size. This assumption has been applied, either explicitly or implicitly, in widely used aerodynamic models and in a variety of empirical tests. Here we provide measurements of the wake of hovering rufous hummingbirds (Selasphorus rufus) obtained with digital particle image velocimetry that show force asymmetry: hummingbirds produce 75% of their weight support during the downstroke and only 25% during the upstroke. Some of this asymmetry is probably due to inversion of their cambered wings during upstroke. The wake of hummingbird wings also reveals evidence of leading-edge vortices created during the downstroke, indicating that they may operate at Reynolds numbers sufficiently low to exploit a key mechanism typical of insect hovering. Hummingbird hovering approaches that of insects, yet remains distinct because of effects resulting from an inherently dissimilar-avian-body plan.

Leading edge vortex in a slow-flying passerine

“…The difference in LEV strength in the flycatcher when compared with that in hummingbirds could be owing to the difference in upstroke function. The hummingbird generates 25 per cent of its total lift during the upstroke [2], while the flycatcher produces almost no weight support during the upstroke (electronic supplementary material, figure S3b) [4][5][6]. To compensate for a low weight supporting upstroke, the flycatcher needs to generate relatively more lift during the downstroke, which might be achieved by producing a relatively strong LEV.…”

“…Most insects and hummingbirds use horizontal strokeplane hovering, where weight-supporting lift forces are generated during both the downstroke and upstroke [1,2]. Unlike hummingbirds, most conventional birds use inclined stroke-plane hovering where the majority of the flight forces are produced during the downstroke [1,3,4].…”

Evolution of avian flight: muscles and constraints on performance

“…Present understanding is that the pectoralis via downstroke provide the majority of the work and power necessary for flapping flight [12,31] even in hummingbirds, where the upstroke contribution during hovering and slow flight is 25-30% of the total lift production [19,20,62,63]. The fibre composition of the pectoralis is remarkably uniform among most species, including only fast-twitch fibres with relatively limited variation in myosin isoforms [64,65].…”

The effect of body size on the wing movements of pteropodid bats, with insights into thrust and lift production

“…Within the range of body sizes between those extremes, animal flight can differ in subtle but important ways. The mechanics of insect flight differ between fruit flies and hawkmoths, and the way a bird flies also varies from hummingbirds to pigeons to vultures (Combes and Daniel, 2003;Dial and Biewener, 1993;Dickinson and Götz, 1996;Sane, 2003;McGahan, 1973;Warrick et al, 2005). Unlike insects and birds, however, bats have largely been assumed to use similar mechanisms of aerodynamic force production in flight, regardless of size (Bullen and McKenzie, 2002;Hedenström et al, 2007;Norberg and Rayner, 1987), even though bats range in body mass over roughly three orders of magnitude, from the ≤0.002kg bumblebee bat (Craseonycteris thonglongyai) to >1.2kg flying foxes (Pteropus spp.)…”