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Laser pulses show how birds fly

October 03, 2003

We all know that birds can fly. But no one has ever been able to explain how they can produce enough lift to neutralize their body weight. Researchers at Lund University in Sweden have now discovered where the missing modicum of momentum is to be found. Using laser technology they have studied a nightingale in flight in a wind tunnel.

In the 1980s scientists studied the air vortexes generated in the wake of flying bats and birds. They photographed helium-filled soap bubbles illuminated with light from a stroboscope. They calculated the amount of movement needed to correspond to the elevating force in the air vortexes, but it was only possible to account for half of what would be necessary for flight. In scientific circles "the paradox of wake momentum" is a well-known concept. But the paradox of wake momentum has just been solved. The new findings were made by Anders Hedenström and Mikael Rosén at the Section for Zooecology at Lund University and by G.R. Spredding at the University of Southern California, Los Angeles. They are being presented in the prestigious Journal of Experimental Biology. The experiments were carried out in a wind tunnel that was constructed in Lund for the express purpose of studying birds in flight. There are only a few such facilities in the world.

"We have further developed a technique that is used for testing cars and airplanes in wind tunnels," says Anders Hedenström, and he continues: "The wind tunnel is filled with steam from water. A nightingale flies in the tunnel. It flies into the wind and, owing to the velocity of the wind, it gets nowhere. In the space behind the bird pulses of light are transmitted from a YAG-laser through the fog. The set-up presents no danger to the bird.

"In this way, we get a series of pictures of how the particles of fog move. The laser transmits two pulses at a time at a very short interval, so it's possible to distinguish the movements of the bird from the background, that is, the uninterrupted free flow of air. This is repeated at the rate of ten times per second; in principle, all we need to do is measure what happens during one single flap of the wings.

"We found that the air vortexes in the wake of the bird are far more complicated that we thought. Air can flow in opposite directions in one and the same vortex. And when we calculated the total amount of kinetic energy in these vortexes, the sum was roughly 100% of what it should be.

"We hope to be able to repeat the experiment with other birds that fly with different techniques . For example, we can study soaring in our wind tunnel since the flow of air can be directed at different angles. This may result in a general theory of how birds fly. The current theory of flight was posited in the 1960s," concludes Anders Hedenström.

Vetenskapsrådet (The Swedish Research Council)