Owl wings may hold the key to turbulence-proof planes

The ability of birds to react to choppy gusts is leading aeronautical engineers to explore the concept of hinged wings on planes and drones.

Published 5 Mar 2021, 10:03 GMT
The responses to wind of a barn owl named Lily were studied in a purpose-built indoor ...

The responses to wind of a barn owl named Lily were studied in a purpose-built indoor flight corridor at the Royal Veterinary College (Hatfield, UK) using high-speed photography of 1000 frames per second. 

Photograph by Jorn Cheney / Structure and Motion Lab, Royal Veterinary College

The instant adjustments of a barn owl’s wings in response to gusts of wind could have a major influence on the future design of small-scale aircraft, according to new research by British scientists.

The team of veterinary and aerospace experts scrutinised every aspect of the bird of prey as it flew over a series of vertical updrafts created by powerful fans in a specially designed hangar. Using specialised monitoring equipment the scientists recorded in minute detail how the barn owl rejected sudden gusts of wind by changing the position, shape and angle of its wings.

Their findings, published in the Proceedings of the Royal Society B, reveal that the owl reacted virtually instantaneously to sudden blasts of wind, keeping its head and torso stable as if they were on a suspension system.

The secret to this immediate response lies in the bird’s hinged shoulder joints, which allow its wings to rise instantly when buffeted by gusts, says Professor Richard Bomphrey, professor of comparative biomechanics at the Royal Veterinary College and one of the authors of the study.

 

Graphic demonstrating the response of the owl wings to various degrees of gusting. The paper's findings show the wings reject gusts of wind almost instantaneously without neural response – suggesting the design of the wings alone is intrinsic in their ability to cope with choppy conditions during hunting, accessing nests and and landing.

Photograph by Jorn A Cheney et al / Royal Society

“It’s a mechanical linkage that doesn’t require any of the bird’s brain to engage; there’s no sensory feedback required, or motor neurone output,” he explains. “It’s operating at its peak within 80 milliseconds. The bird hasn’t got time to look around and notice that it is being knocked up into the air and respond to that by changing its wing position.”

In a two-stage process, this first passive reaction rejects about one-third (32%) of the vertical impulse of the wind, says Bomphrey, effectively buying the bird time before it can respond actively by adjusting the angle of its wings and tail to counteract the rest of the gust. The ability to remain stable in the teeth of strong and sudden changes in wind is vital for them to perform key tasks, such as landing safely or catching prey, he says.

‘Preflex’ reactions

For the experiment, Lily, a captive-bred trained barn owl, flew from a perch to her trainer’s glove over vertical jets created by a bank of 0.5m diameter fans, operating at low, medium and high intensities. The fastest wind speed created an updraft equivalent to the forward speed of the bird, yet the owl’s flight continued unperturbed. The research team deployed high-speed, video-based 3D surface reconstruction, computational fluid dynamics (CFD) and computerised x-ray imaging (known as computed tomography or CT scans) to monitor the flight path of the owl as it ran the gauntlet of the powerful updrafts and to compare this real-life performance with a computer-simulated bird with fixed, rigid wings.

“In a conventional aircraft, wings transmit enormous loads to the fuselage during turbulence, forcing aircraft manufacturers to heavily reinforce the point where the two meet.”

Bomphrey is also part of a team that has created a mechanical model that replicates the hinged wing action of the owl, although the results are yet to be published. The aircraft in question is a glider that mimics birds by allowing its wings to rise instantly and automatically when struck by updrafts, although its design falls well short of the natural world’s more sophisticated ability to change the shape and angle of a wing.

“It works remarkably well and is very bird-like,” says Bomphrey.

From an engineering perspective, the action of a bird’s wing hinge is analogous to a car’s suspension, which allows the wheel assembly to move up and down in relation to bumps in the road, protecting passengers inside from jarring and jolts, says Dr Jonathan Stevenson, research associate in the Faculty of Engineering at Bristol University, who was also involved in the study.

The owl's ability to adjust its flight to winds instantaneously – almost pre-emptively – has been coined by researchers as a 'preflex' reaction.

Photograph by Jorn Cheney, Structure and Motion Lab, Royal Veterinary College

He likens the owl’s ‘preflex’ reaction to a ball striking the sweet spot of a bat or racquet, an action that creates no jarring.

“The centre of percussion, or sweet-spot, where the wind hits the wing is key to our understanding. If load acts through that sweetspot the bird gets pure rotation around its shoulder, with no upward or downward jolt,” says Stevenson.

Owl or albatross?

The ability of birds to cope effortlessly with the turbulent impact of wind at low altitude flight holds exciting prospects for engineers designing low-speed, unmanned aerial vehicles (UAVs), better known as drones, which face unpredictable, swirling updrafts when flying between and above urban buildings, says Stevenson.

Watch: Owl 'Swims' for Its Life in Rare Video

But whether the theory will scale up to passenger aircraft remains an open question. The Reynolds number, which represents the ratio of inertial forces (those resistant to motion) to the viscous (sticky) forces of air molecules, ranges from about 100,000 for an owl to 30 million plus for an airliner in cruise, so principles that work at small scale may not translate to airliners, he cautions.

Nevertheless, the concept of hinged aircraft wings is not new – back in 1937 Rolland Sabins patented a system to reduce or absorb shocks in the air and when landing, where “the wings are arranged for deflection upwardly or downwardly from their normal positions relative to the other parts of the structure in response to changes in load on the wings.”

Moreover, the practice of biomimicry, whereby engineers and designers examine what they can learn and apply from nature, is well established. Airbus, for example, has experimented with copying the small, tooth-like riblets of sharkskin as a coating for plane wings and fuselages to improve aerodynamics, and the same aircraft manufacturer is also behind AlbatrossOne, a prototype which features hinged wingtips similar in function to the barn owl studied by the British scientists. Ironically, albatrosses are one of very few birds (another is the petrel) that have the ability to ‘lock’ their wings at the shoulder for long-distance soaring, although they can unlock them swiftly to deal with sudden blasts of wind.

The hinged wing AlbatrossONE prototype – one of the designs incorporating flexible, wind-responsive wings into its design to combat forces exerted on the craft during flight.  

Photograph by Airbus

AlbatrossOne, created where Concorde was developed in Filton, Bristol, deploys hinged wingtips of up to one-third of the length of its wings. The hinges allow the outer section of the wings to flex in response to gusts of wind, significantly reducing the load on the aircraft and cutting the impact of turbulence. In a conventional aircraft, wings transmit enormous loads to the fuselage during turbulence, forcing aircraft manufacturers to heavily reinforce the point where the two meet.

The AlbatrossOne prototype, based on 1:14 scale model of an A321 airliner, has already undergone test flights and achieved proof-of-concept in July last year.

In an online briefing, Tom Wilson, Airbus semi-aeroelastic hinge project leader, said hinged wing-tips could pave the way for a step change in aircraft performance.

“A major increase in wingspan with minimal impact on wing weight would reduce drag, leading to significant reductions in fuel burn and CO2 emissions,” he says. “Lift-induced drag accounts for about 40% of a large aircraft’s drag. But this figure falls as the wing span increases. The semi-aeroelastic hinged wing-tips’ span could potentially be increased beyond 50 metres without increasing wing weight.”

Fixed wings of this length would either risk transferring unbearable loads to the aircraft in high winds or demand excessively heavy extra reinforcement. So could lessons from the feathered albatross actually make it into production?

“There’s still a lot of engineering work required before we can prove it’s a viable product,” says James Kirk, AlbatrossONE chief engineer. “But the project team is motivated to achieve this goal and to inspire other engineers to think ambitiously about future aircraft.”

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