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The Engineer – Kestrel’s flight stability could inform drone design

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The Engineer – Kestrel’s flight stability could inform drone design

The study conducted at RMIT’s Industrial Wind Tunnel facility is the first to accurately measure the stability of a Nankeen Kestrel’s head during hovering flight, finding movement of less than 5 mm when exhibiting hunting behaviour. The team’s findings are detailed in the Journal of Experimental Biology.

https://www.youtube.com/watch?v=JMkilRpEhjU

“Typically, aircraft use flap movements for stabilization to achieve stability during flight,” RMIT lead researcher Dr. Abdulghani Mohamed said in a statement. “Our results acquired over several years show that raptors rely more on changes in surface area, which is crucial as it may be a more efficient way to achieve stable flight in fixed-wing aircraft as well.” Falcons and other raptors can keep their heads and bodies extremely still while hunting. This hovering allows the birds to ‘hang’ in place under the right wind conditions without flapping their wings. They achieve stability by making small adjustments to the shape of their wings and tail.

Using camera and motion capture technology, the research team observed two Nankeen Kestrels, trained by the Leigh Valley Hawk and Owl Sanctuary, in high resolution. Fitted with reflective markers, the birds’ precise movements and flight control techniques during flapping flight were tracked in detail.

“Previous studies have involved birds casually flying through turbulence and gusts inside wind tunnels; in our study, we tracked a unique hovering flight behavior in the wind, where birds actively maintain extreme stability, allowing us to study the pure control response without flapping their wings,” said Dr. Mohamed.

By mapping these movements, researchers gained insights that could be used to achieve more stable flight for fixed-wing aircraft.

“The wind-hovering behaviour we observed in falcons is the closest representation in the avian world of fixed-wing aircraft,” said Dr Mohamed. “Our findings around the changes in wing surface area can be applied to the design of morphing wings in drones, increasing their stability and making them safer in adverse weather conditions.”

Associate professor of bioinspired aerodynamics at the University of Bristol and co-author Dr Shane Windsor said the usefulness of current fixed-wing unmanned aerial vehicles (UAVs) was significantly reduced by their inability to operate in gusty wind conditions.

“UAVs are being used in the UK to deliver mail to remote islands, but their operating time is limited due to regular gusty conditions. Current commercial fixed-wing aircraft need to be designed with a fixed geometry and optimised to operate in a flight condition.”

Dr Windsor continued: “The advantage of morphing wings is that they can be continually optimised during flight for a variety of conditions, making the aircraft much more manoeuvrable and efficient.”

The team now plans to further their research by examining the birds in turbulent and gusty wind conditions, which would provide further learnings on stable flight, with the aim of enabling UAVs to operate more safely and frequently.

Looking ahead, the team hopes to simplify the data collected so it can be adapted for larger aircraft.

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