Intrigued by the remarkable ability of birds to remain stable in the face of powerful winds, aerospace engineers in the UK have carried out a study to examine the phenomenon in great detail. By observing a barn owl in flight as it contended with blustery conditions, the team discovered how its wings can act as a suspension system to absorb the forces in mid-air, lessons they hope to apply to the development of small-scale aircraft.
“Birds routinely fly in high winds close to buildings and terrain – often in gusts as fast as their flight speed,” says Dr Shane Windsor from the Department of Aerospace Engineering at the University of Bristol. “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. 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.”
To learn more, Windsor and his team, which also included scientists from the Royal Veterinary College, enlisted the help of a barn owl named Lily, who is a trained falconry bird that has featured in nature documentaries. Inside the laboratory, Lily was made to fly in the face of fan-generated vertical gusts at a range of forces, the strongest of which was made to match her flight speed.
“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, Lloyd Buck,” says Professor Richard Bomphrey of the Royal Veterinary College.
The team used a combination of high-speed video, 3D surface reconstruction techniques, computational fluid dynamics and CT scans to observe Lily in action. This showed the owl keeping its head and torso “amazingly stable” as it encountered the gusts of wind, with her wings acting as a natural suspension system to absorb the impact. What surprised the researchers most was just how effortless this was, with the bird able to counter the gusts automatically thanks to a type of sweet spot in its wings.
“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.,” says Dr Jonathan Stevenson from the University of Bristol. “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 stabilizing processes to kick in.”
Aerospace engineering research is littered with countless examples of avian-inspired technologies that could improve aerodynamics, lead to shapeshifting wings, prevent mid-air collisions or even help aircraft recover from them. Similarly, the team behind this new research hopes the lessons learnt from Lily the barn owl can help them develop bio-inspired suspension systems for small-scale aircraft.
The research was published in the journal Proceedings of the Royal Society B.
Source: University of Bristol
