Kestrel flight studied to develop safe drones

Researchers at Melbourne’s RMIT University have put the aerodynamics of kestrels to the test in the hope of gaining useful insight in the development of safer small aircraft design.

The research is published in the Journal of Experimental Biology.

Kestrels are a small bird of prey with a unique hunting method. They hover almost completely still in the sky at heights of about 10–20 metres to detect subtle movements on the ground before diving down and capturing their prey which includes small mammals like mice.

Cosmos spoke with RMIT’s Abdulghani Mohamed, senior author of the new paper.

“Kestrels are one of the species of bird that can ‘wind-hover’. In fact, you’ve probably seen them frequently on the side of highways while driving, this bird that seems to be glued in midair with very minimal movement,” Mohamed says.

“Even with high levels of turbulence, their head movement stays within millimetres, which is quite the discovery.”

Mohamed and his team tracked the birds’ movements in a wind tunnel using reflective markers. Similar technology is used in the film and video game industry to develop realistic movements of CGI characters.

He says that the birds are able to stay so still thanks to slight muscular adjustments which change the area of their wings.

“That is a bit of a discovery compared to what we are accustomed to in the aerospace world,” Mohamed notes.

“If you look out the window of a passenger aircraft, you’ll find that the aircraft has a lot of flaps and ailerons and all these control surfaces,” he says. “The way the aircraft controls its attitude (angle of flight relative to the Earth) is by deflecting those surfaces.”

Kestrels’ focus on wing area, according to Mohamed, “seems to be a more effective, efficient way to do things”.

“Their wings have a similar anatomical structure to an arm. They’ve got a shoulder, an elbow and a wrist. We attached the markers to some of those key joints, and we checked the movements of that.”

Mohamed notes that a lot of work has previously been done in understanding the “passive stability” provided by wing shape and feathers of birds. “This work is unique because it focuses on the active side,” Mohamed notes. This allowed the researchers to examine how kestrels are able to stay extremely steady even with turbulence and changing wind conditions.

Because hovering kestrels aren’t flapping their wings, studying their flight could have implications for the fixed wing of a human-designed aircraft.

“We’re most likely not going to see passenger aircraft with wings similar to a kestrel,” Mohamed laughs. “That would be quite a bit of a challenge. But where this is most applicable is smaller aircraft or UAVs.”

UAVs – unmanned aerial vehicles – includes drones. Mohamed says there is a growing push for “urban air mobility” where drones can deliver food and drink or parcels.

Some developers are even looking at potential air taxi services and medical drones.

“As they fly over urban environments, it becomes critical that they can handle large levels of turbulence, because the last thing that you want when operating a fleet is any accidents or to be grounded for different weather or wind conditions.”

Information gathered about kestrel hovering could be applied to make urban drone flights safer, says Mohamed.

“Such technologies can be adapted for those aircraft that operate in these environments to extend the number of years of operation.”