The wake of hovering flight in bats

Abstract

Hovering means stationary flight at zero net forward speed, which can be achieved by animals through muscle powered flapping flight. Small bats capable of hovering typically do so with a downstroke in an inclined stroke plane, and with an aerodynamically active outer wing during the upstroke. The magnitude and time history of aerodynamic forces should be reflected by vorticity shed into the wake. We thus expect hovering bats to generate a characteristic wake, but this has until now never been studied. Here we trained nectar-feeding bats, Leptonycteris yerbabuenae, to hover at a feeder and using time-resolved stereoscopic particle image velocimetry in conjunction with high-speed kinematic analysis we show that hovering nectar-feeding bats produce a series of bilateral stacked vortex loops. Vortex visualizations suggest that the downstroke produces the majority of the weight support, but that the upstroke contributes positively to the lift production. However, the relative contributions from downstroke and upstroke could not be determined on the basis of the wake, because wake elements from down-and upstroke mix and interact. We also use a modified actuator disc model to estimate lift force, power and flap efficiency. Based on our quantitative wake-induced velocities, the model accounts for weight support well (108%). Estimates of aerodynamic efficiency suggest hovering flight is less efficient than forward flapping flight, while the overall energy conversion efficiency (mechanical power output/metabolic power) was estimated at 13%.

Aerodynamics of manoeuvring flight in brown long-eared bats (Plecotus auritus)

“…Over the complete manoeuvre, from stable forward flight over initiation, displacement and termination of the manoeuvre back to stable forward flight, we found general wake elements: wingtip vortices, root vortices and reversed vortex loops typical for bat flight as shown in previous studies (e.g. [14,[20][21][22][23][24][25]). The overall structure and shape of the wake throughout a complete sideways manoeuvre is illustrated in figure are symmetrical before the bat starts turning, followed by a single distinctly asymmetrical wingbeat, which represents the start of the manoeuvre.…”

Ear-body lift and a novel thrust generating mechanism revealed by the complex wake of brown long-eared bats (Plecotus auritus)

“…Despite the novel features of the wake, the overall wake structure is consistent with that found in previous studies 10 11 12 13 17 30 , and the characteristic features attributed to bat wakes are also found here; distinct wing root vortices and end of upstroke reversed vortex loops at cruising speed 12 . Also at the lowest speed, the wake of P. auritus resembles the hovering wake of other bat species 31 . However, our results show that the wake of bats is even more complex than previously recognized and calls for revisiting previously studied species, as well as other species, to determine how general the novel wake structures found here are.…”

Wings as inertial appendages: how bats recover from aerial stumbles

“…Flight in bats differs from that in other taxa in several fundamental ways. In particular, many bats use substantial flexion at numerous joints to fold their wings as an integral aspect of wingbeat kinematics, especially during upstroke (Håkansson et al, 2015;Iriarte-Diaz et al, 2011;Riskin et al, 2012Riskin et al, , 2010Riskin et al, , 2008Vejdani et al, 2019), which contrasts with the relatively extended wing postures that insects and hummingbirds maintain during flight (Altshuler et al, 2010;Dudley, 2018;Kruyt et al, 2014;Warrick et al, 2005;Weis-Fogh, 1973). In bats, the highly articulated wing skeleton enables substantial wing flexion during upstroke, and most of these degrees of freedom are directly actuated by muscles in the forelimb (Bahlman et al, 2016).…”