Hovering Passerine

According to their kinematic characteristics, the modes of bird-hovering flight are typically classified as symmetric, notably employed by hummingbirds, and asymmet­ric, which is often observed in other birds, such as the Japanese White-Eye (Zosterops japonicus) and Gouldian Finch (Erythrura gouldiae), both of which belong to the order Passeriforme. For a hovering passerine, only the downstroke produces the lift force necessary to support the bird’s weight; the upstroke is aerodynamically inac­tive and produces no lift [4] [389] [390]. Hence, even without producing lift forces continuously throughout the entire cycle of wing beating, a passerine is able to hover.

Chang et al. [391] presented experimental measurements to support the notion that a hovering passerine (Japanese White-Eye, Zosterops japonicus) can employ an unconventional mechanism of “ventral clap” to produce lift for weight support. They claimed that this ventral clap can first abate and then augment lift production during the downstroke. The net effect of the ventral clap on lift production is positive because the extent of lift augmentation is greater than the extent of lift abatement, as shown in Figure 3.62, where the PIV data illustrate the flow structures associated with the various phase of the flapping motion. As discussed in this section and by Trizila et al. [301], LEV and TEV flows as well as the downward jet flow are observ­able, contributing to the complicated balancing and net generation of aerodynamic forces.

In Chang et al.’s study [391], two methods based on wake topology were applied to evaluate the locomotive forces. The first is primarily associated with the Kutta – Joukowski lift theorem [392]-[394]. For a bird executing quasi-steady level flight, the total lift acting on its wings must be equal to its weight. The second method to evaluate the locomotive force is based on a vortex-ring model that obtains a time – averaged force from a calculation of the momentum change (i. e. impulse), divided by the generation period of a vortex ring shed by a flapping or beating appendage of a locomoting animal [342] [395] [396]. Although these methods are simple and convenient, as discussed in Section 3.6, such approaches encounter difficulties in offering an accurate account and they are not based on first principles.