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Hovering flight in bats

Nectar feeding bats need to be able to stand still in the air, or hover, in front of flowers while feeding. Hovering and very slow forward flight is very demanding for an animal since it does not get any help from the forward flight speed to generate flow over the wings to generate lift. Instead the bat depends only on the flapping motion of the wings and the wings’ ability to generate lift, expressed in the lift coefficient.

During our first wake measurements we showed that the lift coefficient during slow flight in bats was exceptionally high. The values were well above the maximum steady state coefficients, suggesting that the bats required unsteady aerodynamic mechanisms to generate the amount of force we measured. Although hinted at from kinematic studies, this was the first evidence that bats were actually able to reach these high lift coefficients. 

To determine the mechanism behind these exceptionally high values we visualized the airflow directly above the wing of Glossophaga soricinaand found that they make use of a leading edge vortex (LEV). The LEV enhance lift up to 40% of the total lift and explains most of the additional lift required for hovering. Recently, we have shown that the bigger nectar feeding species, Leptonycteris yerbabuenae,also uses LEVs to enhance lift production during hovering and at slow flight speed.

In addition to the elevated ability to generate lift by using LEVs, bats supplement their lift production by turning the outer wing up-side down during the upstroke (similar to hovering insects and hummingbirds). The result is a forwards and upwards directed force being generated during the upstroke, which also turns out to be enhanced by a separate LEV generated on the outer wing. However, the relative contribution of the upstroke to the lift generation has not yet been determined. This and several other exciting topics regarding hovering in bats are now being explored by the AFL team.

Leading edge vortices on bat wings
LEV above the wing of a slowly flying bat. The colors show vorticity strength, the top panel shows velocity vectors, and the zoomed in panel shows streamlines. At the bottom the measurement plane compared to the bat is shown.
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A hovering bat
A bat hovering in the Lund wind tunnel.