Birds

Lily, the barn owl, shows how birds fly in gusty winds

Scientists from the University of Bristol and the Royal Veterinary College have discovered how birds can fly in gusty conditions – findings that could influence the development of bio-inspired small aircraft.

“Birds routinely fly near buildings and terrain in strong winds – often in gusts as fast as their airspeed. The ability to deal with strong and sudden wind changes is therefore vital to their survival, such as making land safe and catching prey, ”said Dr. Shane Windsor from the University of Bristol’s Department of Aerospace Engineering.

“We know that birds can do amazingly well in conditions that challenge technical aircraft of similar size, but until now we haven’t understood the mechanics behind it,” said Dr. Windsor.

The study, published in Proceedings of the Royal Society B, shows how bird wings act as a suspension system to cope with changing wind conditions. The team used an innovative combination of high-speed, video-based 3D surface reconstruction, computed tomography (CT) and computational fluid dynamics (CFD) to understand how birds “reject” gusts by morphing their wings, changing the shape and attitude of their wings.

Barn Owl, Copyright Glyn Sellors, from the Surfbirds Galleries

In the experiment, conducted in the Royal Veterinary College’s Structure and Motion Laboratory, the team filmed Lily, a barn owl, sliding through a series of fan-generated vertical gusts, the strongest of which was as fast as its airspeed. Lily is a trained falconry bird who is a veteran of many nature documentaries and was therefore not in the least impressed with all the lights and cameras.

“We started with very gentle gusts in case Lily had any trouble, but soon found that Lily was not bothered even at the highest gust speeds we could make. She flew straight through to receive the meal reward from her trainer, Lloyd Buck, ”commented Professor Richard Bomphrey of the Royal Veterinary College.

“Lily flew through the bumpy gusts, keeping her head and torso consistently amazingly stable over the trajectory, as if she were flying with a suspension system. When we analyzed it, we were surprised that the effect of the suspension system was not only due to the aerodynamics, but also benefited from the mass in their wings. For reference, each of our upper limbs makes up about 5% of our body weight; For a bird it is roughly twice that, and they use this mass to effectively absorb the gust, ”said lead author Dr. Jorn Cheney from the Royal Veterinary College.

“Perhaps the most exciting thing is to discover that the fastest part of the suspension effect is built into the mechanics of the wings so that birds don’t have to actively do anything to make it work. The mechanics are very elegant. If you hit a ball on the sweetspot of a racket or bat, your hand will not be shaken as the force there will decrease. Anyone who plays a racket-and-ball sport knows how effortless it 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 that this significantly reduces disruption to the body in the first fraction of a second. The process is automatic and leaves just enough time for other clever stabilization processes to begin, ”added Dr. Jonathan Stevenson from the University of Bristol added.

Dr. Windsor said the next step in research, funded by the European Research Council (ERC), the Air Force Scientific Research Bureau and the Wellcome Trust, is to develop bio-inspired suspension systems for small aircraft.

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