Study Reveals ‘Elegant’ Mechanics of How Birds Fly in Gusty Winds

(CN) — Stabilizing an aircraft in gusty conditions has perplexed aerospace engineers since the beginning of human flight. Even in the age of supercomputers, scientists have struggled to develop features that extend the range of safe air travel in difficult conditions.

But sometimes answers to complex questions can be staring us in the face — or at least from a distant height.

Scientists from the University of Bristol and the Royal Veterinary College say they have unlocked the secrets to how birds fly in gusty conditions by studying the flight of a special barn owl named Lily.

“We know birds cope amazingly well in conditions which challenge engineered air vehicles of a similar size but, until now, we didn't understand the mechanics behind it,” said Shane Windsor from the Department of Aerospace Engineering at the University of Bristol.

The scientist’s findings, published in Proceedings of the Royal Society B, demonstrate how bird wings act as a suspension system to cope with wind fluctuations, a discovery that could alter the development of bio-inspired small-scale aircraft.

“Birds routinely fly in high winds close to buildings and terrain — often in gusts as fast as their flight speed,” Windsor said. “So the ability to cope with strong and sudden changes in wind is essential for their survival and to be able to do things like land safely and capture prey.”

Using a combination of high-speed, video-based 3-D surface reconstruction, computed tomography (CT) scans, and computational fluid dynamics, researchers filmed Lily gliding through a range of fan-generated vertical gusts, including gusts as fast as her flight speed. The experiment was conducted in the Structure and Motion Laboratory at the Royal Veterinary College in London. 

“We began with very gentle gusts in case Lily had any difficulties, but soon found that - even at the highest gust speeds we could make — Lily was unperturbed; she flew straight through to get the food reward being held by her trainer,” said Richard Bomphrey, a professor of comparative biomechanics  at the Royal Veterinary College.

It takes a special bird to perform naturally in a giant lab surrounded by lights and cameras, and Lily, a trained falconry bird who is a veteran of many nature documentaries, was unfazed by all the commotion.

“Lily flew through the bumpy gusts and consistently kept her head and torso amazingly stable over the trajectory, as if she was flying with a suspension system,” explained the study’s lead author Jorn Cheney, a postdoctoral research associate at the Royal Veterinary College.

Analyzing Lily’s technique, researchers were surprised to discover that the “suspension-system effect” was due to more than aerodynamics. The barn owl also benefited from the mass in her wings, which is double the mass of arms in a typical human, allowing her to absorb the gust.

What she demonstrated time and again helped scientists understand how birds change the shape and posture of their wings to absorb gusts through a technique known as wing morphing.

A wing has a “sweetspot,” just like a baseball bat. Researchers realized that the force of the gusts acts near this spot, automatically reducing disturbance to the body during the first fraction of a second.

“Perhaps most exciting is the discovery that the very fastest part of the suspension effect is built into the mechanics of the wings, so birds don't actively need to do anything for it to work,” Windsor said. “The mechanics are very elegant.”

Windsor hopes to use the research, which was funded by the European Research Council, Air Force Office of Scientific Research and the Wellcome Trust, to develop bio-inspired suspension systems for small-scale aircraft.

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