By strapping a flight recorder onto the back of a steppe eagle, Aquila nipalensis, researchers have figured out the mechanical reason behind “wing tucks” in soaring birds: Collapsible wings are their answer to turbulence. The findings were published in the Journal of the Royal Society Interface this month.
Blustery, turbulent flight conditions often keeps aircrafts grounded. Yet, birds like eagles, vultures, and kites keep on soaring, and stably too. “Soaring flight may appear effortless but it isn't a free ride,” Oxford’s Graham Taylor explains in a news release. “Soaring may enable a bird to travel long distances but it also puts an enormous strain on its flight muscles. The nature of rising air masses, such as thermals, is that they create lots of turbulence and buffeting that jolts a bird’s wings and could knock it out of the sky.”
Borrowing from aeronautics, Graham and colleagues trained a steppe eagle named Cossack (pictured above and below) to carry its own “black box” flight recorder while soaring through the Brecon Beacons in Wales. The 75-gram onboard gizmo has GPS and measures acceleration, rotation rate, and airspeed. The trio of researchers also used ground-based videos.
By logging Cossack’s movements from 45 flights, the team found that the eagle collapsed its wings as a response to particularly strong, headwind gusts -- rather than hold them stiffly in position like the unyielding wings of a plane. During these 2,594 documented “wing tucks,” the bird drops its wings and folds them beneath its body briefly -- for about 0.35 seconds -- so that it’s effectively falling in a nose-down motion. You can watch a quick clip of this wing tuck maneuver here (don't blink, you'll miss it).
“Wing-tucking is a direct response to a substantial loss of lift that occurs when a bird flies through a pocket of atmospheric turbulence,” Taylor explains. “Birds use this technique to damp the potentially damaging jolting caused by turbulence.” Collapsing wings likely help eagles avoid extreme loads, similar to the suspension on a car or shock absorbers on a bicycle.
Here’s Cossack wearing his black box flight recorder backpack:
It’s unlikely that airliners will be using collapsible wings to alleviate turbulence any time soon, though a technique like this one could help keep micro air vehicles aloft even in turbulent atmospheric conditions.
Images: Simon Walker (top), Graham Taylor/University of Oxford (middle)