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New study shows the benefit of flying close to a surface

That animals flying close to a surface may save energy is well known, but confirming measurements have been lacking. In this week’s issue of Current Biology, CAnMove scientists show that energy savings for Daubenton’s bats during flapping flight in ground effect are twice the model predictions. The large savings challenge our understanding of how animals use ground effect.
Daubetons bat
Daubetons bat. Photo: Jens Rydell

Christoffer Johansson and Anders Hedenström at LU have, together with Lasse Jakobsen at the University of Southern Denmark, successfully managed to measure the ground effect when Daubenton’s bats fly in a wind tunnel. The challenging study involved training the bats to capture mealworms in a copy of the wind tunnel in Denmark and replacing the bottom of the wind tunnel with a treadmill to simulate the aerodynamic conditions of bats flying in the wild.
“Not only do our measurements show that animals save twice as much energy as models have suggested, but the bats also seem to modulate their saving by changing how they flap their wings.”, says Christoffer Johansson.
The ground effect, which is effective within a distance of a wing span from the ground, means that a surface, ground or water, acts as an aerodynamic mirror that increases the air pressure under the wings decreasing the costs of generating lift.
Although the study was performed on bats, it has implications for other flying animals. One of the theories of the origin of flight in birds is that flying began on the ground – the ground up theory. By running and jumping, proto-wings could have allowed the animals to run faster and jump higher and eventually flight evolved. The corresponding theory behind today’s flying insects is that they moved on the water surface and eventually evolved wings as a means of propelling themselves across the surface. If flapping animals save more energy than we previously believed by flying close to the ground, then the ground up theory becomes more probable.

For more information:
Christoffer Johansson, senior lecturer
Department of Biology, Lund University
+46 46 222 49 55
christoffer [dot] johansson [at] biol [dot] lu [dot] se

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