The mechanics of birds’ “suspension system” which enables them to fly in gusty winds has been at the forefront of a study which could have real implications on the design of bio-inspired aircraft.

Scientists from the University of Bristol and the Royal Veterinary College have discovered how birds are able to fly in challenging conditions. The ability to cope with strong and sudden changes in wind is essential for the bird’s survival as well as being able to land safely and capture prey.

The study was aided by Lily, a barn owl trained in falconry who was filmed gliding through a range of fan-generated vertical gusts, the strongest of which was as fast as her flight speed. Lily is a veteran of many nature documentaries, so wasn’t fazed by all the lights and cameras.

“Wind gusts as fast as their flight speeds”

Dr Shane Windsor from the Department of Aerospace Engineering at the University of Bristol explained: “I have always been fascinated by the ability of birds to deal with complex, kind of dynamic, situations. You watch them flying around on windy days and they make it look effortless.

But they’re dealing with wind gusts that are as fast as their own flight speed, yet they are quite happy to cruise along in these conditions. This is always something that’s really fascinated me but its been really hard in the past, to understand how birds are able to do this.”

The study, published in Proceedings of the Royal Society B, reveals how bird wings act as a suspension system to cope with changing wind conditions. The team used a combination of high-speed, video-based 3D surface reconstruction, computed tomography (CT) scans, and computational fluid dynamics (CFD) to understand how birds ‘reject’ gusts through wing morphing which they do by by changing the shape and posture of their wings.

Wings acts as “suspension systems”

Dr Windsor said the study had resutled in a better understanding of the mechanics of bird’s wings: “Essentially, the wings of the birds act as suspension system, so as the birds are flying along. It’s kind of got a steady state where it’s holding the load on wings balancing out its body weight, then, as it hits the gusts, the wings are suddenly elevated up. And this happens really quickly and you’ll see that the wings go up on the bird, but its body and head continues in a nice, straight line.”

“So the wings are acting like a suspension system to isolate the head and the body from these sudden motion. We discovered that mechanics of this are really interesting. It happens so quickly at the beginning of the gust, that it can’t be based on bird kind of sensing what’s happening and responding, it’s faster than its reaction time.”

Professor Richard Bomphrey of the Royal Veterinary College described the process of the experiment: “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.”

Body mass “absorbs gusts”

“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. 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 per cent of our body weight; for a bird it’s about double, and they use that mass to effectively absorb the gust,” said lead-author Dr Jorn Cheney from the Royal Veterinary College.

“Sweetspot” built into wing mechanics

“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. The mechanics are very elegant. When you strike a ball at the sweetspot 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 sweetspot, just like a bat.”

“Our analysis suggests that the force of the gust acts near this sweetspot 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,” added Dr Jonathan Stevenson from the University of Bristol.

The next step for the research, which was funded by the European Research Council (ERC), Air Force Office of Scientific Research and the Wellcome Trust, is to develop bio-inspired suspension systems for small-scale aircraft.

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