Wind? That’s barely a breeze

Birds can fly easily in winds that would challenge engineered air vehicles of a similar size, and now Lily the British barn owl has revealed how they do it.

When scientists from the University of Bristol and the Royal Veterinary College in the UK put her through her paces in the College’s Structure and Motion Laboratory, she comfortably coped with fan-generated vertical gusts that got up to as fast as her flight speed.

Using high-speed, video-based 3D surface reconstruction, computed tomography scans, and computational fluid dynamics, the team of engineers and veterinarians discovered that she “rejected” gusts through wing morphing – changing the shape and posture of her wings.

The performance was to be expected – Lily is a trained falconry bird and a veteran of nature documentaries – but the results were surprising

“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,” says the College’s Jorn Cheney, lead author of a paper in Proceedings of the Royal Society B.

“When we analysed it, what surprised us was that the suspension-system effect wasn’t just due to aerodynamics but benefited from the mass in her wings. For reference, each of our upper limbs is about 5% of our body weight; for a bird it’s about double, and they use that mass to effectively absorb the gust.”

Perhaps the most exciting discovery, according to Bristol’s Jonathan Stevenson, is that the 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.

“The mechanics are very elegant,” he says. “When you strike a ball at the sweet spot of a bat or racquet, your hand is not jarred because the force there cancels out. Anyone who plays a bat-and-ball sport knows how effortless this feels.

“A wing has a sweet spot, just like a bat. Our analysis suggests that the force of the gust acts near this sweet spot and this markedly reduces the disturbance to the body during the first fraction of a second.

“The process is automatic and buys just enough time for other clever stabilising processes to kick in.”

The aim now is to try to transfer this ability to the aforementioned engineered air vehicles of a similar size.

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