Bat flight: aerodynamics, kinematics and flight morphology

Abstract

Bats evolved the ability of powered flight more than 50 million years ago. The modern bat is an efficient flyer and recent research on bat flight has revealed many intriguing facts. By using particle image velocimetry to visualize wake vortices, both the magnitude and timehistory of aerodynamic forces can be estimated. At most speeds the downstroke generates both lift and thrust, whereas the function of the upstroke changes with forward flight speed. At hovering and slow speed bats use a leading edge vortex to enhance the lift beyond that allowed by steady aerodynamics and an inverted wing during the upstroke to further aid weight support. The bat wing and its skeleton exhibit many features and control mechanisms that are presumed to improve flight performance. Whereas bats appear aerodynamically less efficient than birds when it comes to cruising flight, they have the edge over birds when it comes to manoeuvring. There is a direct relationship between kinematics and the aerodynamic performance, but there is still a lack of knowledge about how (and if ) the bat controls the movements and shape ( planform and camber) of the wing. Considering the relatively few bat species whose aerodynamic tracks have been characterized, there is scope for new discoveries and a need to study species representing more extreme positions in the bat morphospace.

Inspiration for wing design: how forelimb specialization enables active flight in modern vertebrates

“…At high speeds, lower lift coefficients are sufficient, so decreasing camber becomes more advantageous for decreasing profile drag. Accordingly, bats decrease their wing camber as flight speed increases [87,114] (figure 7d).…”

“…Although their ability to vary wing span may be more limited, bats increase the camber of their wing more than any other extant flying taxa [114]. This is done by lowering the membranes between their digits and by deflecting the legs [59,84,87,114].…”

“…Wingbeat kinematics further diverge between birds and bats; for birds, the path traced out by the wingtip during downstrokes is anterior to that of upstrokes, whereas the opposite holds true for bats (figure 7e). Bats adjust the camber and angle of attack of their wing membrane to generate weight support as they invert their hand-wing for the upstroke [84,114]. At low speeds, the rotation of the hand-wing manifests as a backward flick generating thrust and weight support [118,131,132], but at high speeds, the upstroke generates only weight support [84,132].…”

“…How the wingbeat frequency depends on flight speed varies more among birds; the frequency is either constant [135], increases linearly [136] or in a parabolic (U-shaped) fashion with speed [137]. Wingbeat frequency among bats generally decreases with increasing flight speed until cruising speeds are reached, above which it remains relatively constant [84,114,138]. While frequency scales similarly with body mass for both birds and bats, birds have a higher wingbeat frequency than bats of the same mass (figure 7f ), and they also have proportionally more flight muscle mass [106].…”

“…Bats, on the other hand, are comparatively less aerodynamic due to a less well-shaped aerofoil and appendages needed for echolocation. In particular, the ears and nose leaf of bats disturb flow over the body, which increases parasite drag [114,126,140]. Whereas large ears may contribute some lift, it likely reduces their inner wing performance [133].…”