Flight in Ground Effect Dramatically Reduces Aerodynamic Costs in Bats

L Christoffer Johansson, Lasse Jakobsen, Anders Hedenström

Research output: Contribution to journalJournal articleResearchpeer-review

Abstract

Most flying animals, from insects to seabirds [1], perform flights close to ground or water when taking off or landing [2], drinking, and feeding [3–5] or when traveling near water surfaces [1, 6, 7]. When flying close to a surface within approximately one wingspan, the surface acts as an aerodynamic mirror, interrupting the downwash [8, 9], resulting in increased pressure underneath the wing and suppression of wingtip vortex development [10]. This aerodynamic interaction lowers the energy added to the air by the animal, reducing the cost of flying. Modeling suggests that flapping wings in ground effect can affect the expected power savings compared to gliding flight, either positively or negatively, depending on the wing motion [11–13]. Although aerodynamic theory predicts substantial power reductions when animals fly in ground effect [4–6, 9, 11, 12], quantitative measurements of savings are lacking. Here, we show, through wake-based power measurements, that Daubenton's bats utilize 29% less aerodynamic power when flying in compared to out of ground effect, which is twice the predicted savings. Contrary to theoretical predictions [4–6, 9, 11, 12] we find no variation in savings with distance above ground when in ground effect. Given alterations in kinematics with ground proximity, we hypothesize that modulation of wing kinematics raises the achievable benefit from ground effect relative to current model predictions. The savings from ground effect are comparable to formation flight [14, 15] but are not limited to large bird species. Instead, ground effect is experienced by most flying animals and may have facilitated the evolution of powered animal flight. Animals flying close to a surface may save energy, but measurements have been lacking. Johansson et al. find that energy savings for Daubenton's bats during flapping flight in ground effect are twice the model predictions. The bats may also vary the wing stroke to modulate savings, challenging our understanding of how animals use ground effect.

Original languageEnglish
JournalCurrent Biology
Volume28
Issue number21
Pages (from-to)3502-3507.e4
ISSN0960-9822
DOIs
Publication statusPublished - Nov 2018

Keywords

  • Chiroptera
  • aerodynamic power
  • aerodynamics
  • animal flight
  • bats
  • energy savings
  • evolution of flight
  • ground effect

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