Low Cost Flights

Daubenton's bat

Jens Rydell

When animals fly low to the ground or water, the surface blocks the downwash of air, acting as an aerodynamic mirror to increase air pressure under their wings and increase lift. The energy saved from this ground effect has long been predicted by computer simulations, but actual measurements of the savings have been lacking. Scandinavian scientists recently took up the challenge by putting bats in a wind tunnel. They discovered the savings were larger than expected.

Christoffer Johansson and Anders Hedenström of Lund University in Sweden, and Lasse Jakobsen of the University of Southern Denmark in Odense used low-placed mealworms as incentives to train three wild-captured Daubenton’s bats, Myotis daubentoniid, to fly close to the ground. They used a technique called particle image velocimetry to measure the bats’ energy expenditures. Filling the wind tunnel with tiny particles that move at the same speed as the air, the researchers lit up the particles with a powerful laser and filmed them with high speed video to reveal the three-dimensional velocity of air. This allowed calculation of energy added to the air by the flying bat. They then compared flight costs in and out of ground effect, finding that bats required 29 percent less aerodynamic power when flying in ground effect—twice that previously predicted by mathematical modeling. Contrary to expectations, however, the study found no variation in energy savings at different distances above the ground.


Animals are capable of using the ground effect in ways that we don’t fully understand yet,” Johansson said, adding that, “we think they might be changing the way they are flapping their wings relative to the way we have modeled them before.” These findings support the hypothesis that the evolution of flight was preceded by running and jumping up, rather than gliding from trees [see “Evidence of Early Takeoff,” 3/16]. (Current Biology)