A quantitative comparison of bird and bat wakes

Abstract

Qualitative comparison of bird and bat wakes has demonstrated significant differences in the structure of the far wake. Birds have been found to have a unified vortex wake of the two wings, while bats have a more complex wake with gradients in the circulation along the wingspan, and with each wing generating its own vortex structure. Here, we compare quantitative measures of the circulation in the far wake of three bird and one bat species. We find that bats have a significantly stronger normalized circulation of the start vortex than birds. We also find differences in how the circulation develops during the wingbeat as demonstrated by the ratio of the circulation of the dominant start vortex and the total circulation of the same sense. Birds show a more prominent change with changing flight speed and a relatively weaker start vortex at minimum power speed than bats. We also find that bats have a higher normalized wake loading based on the start vortex, indicating higher relative induced drag and therefore less efficient lift generation than birds. Our results thus indicate fundamental differences in the aerodynamics of bird and bat flight that will further our understanding of the evolution of vertebrate flight.

Aerodynamics of the flying snake Chrysopelea paradisi: how a bluff body cross-sectional shape contributes to gliding performance

“…Justification for 2D and quasi-steady modeling Unsteady 3D effects are well known to affect the aerodynamics of flapping flyers (Wang et al, 2004;Wang, 2005;Warrick et al, 2005;Johansson et al, 2010), and conventional, steady-state aerodynamics are insufficient for fully explaining how insects, birds and bats fly. A useful index for determining whether a quasi-steady approach can be used for studying animal flight is the advance ratio, J, which compares the animal's forward motion with the reciprocating motion of its wings.…”

“…Although many open questions remain in the study of animal flight, the aerodynamic basis of flight in birds, bats and insects has received focused attention, leading to extensive understanding of kinematics, steady and unsteady effects, 2D and 3D dynamics, wake patterns and fluid-structure interactions (Lehmann, 2004;Wang, 2005;Tobalske, 2007;Hedenström and Spedding, 2008;Song et al, 2008;Usherwood and Lehmann, 2008;Lehmann, 2009;Spedding, 2009;Johansson et al, 2010). In comparison, there have been far fewer studies on the aerodynamics of animals that can only glide (Emerson and Koehl, 1990;McCay, 2001;Bishop, 2007;Alexander et al, 2010;Miklasz et al, 2010;Park and Choi, 2010;Bahlman et al, 2013), despite their large morphological and taxonomic diversity and the far greater number of independent evolutionary origins of gliding flight (Dudley et al, 2007;Dudley and Yanoviak, 2011).…”

Mechanical power curve measured in the wake of pied flycatchers indicates modulation of parasite power across flight speeds

“…Here, we test if the power curve is U-shaped from time resolved wake measurements to determine the mechanical power output by freely flying pied flycatchers, Ficedula hypoleuca, a species feeding on aerial insects, and capable of flying at a wide range of speeds. When comparing how efficient birds and bats are at generating lift, it has been found that birds outperform bats [8,9]. The reasons for this have been speculated on, with a focus on what makes the bats less efficient than birds [8][9][10].…”

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

“…Birds generate lift more efficiently, due in part to a more aerodynamic body shape throughout the stroke cycle and the ability to generate body lift during inactive upstrokes [108,126,140]. Bats, on the other hand, are comparatively less aerodynamic due to a less well-shaped aerofoil and appendages needed for echolocation.…”

“…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].…”