A groundbreaking collaborative study by RMIT and the University of Bristol has revealed the secrets behind kestrels’ exceptional flight stability. These findings hold great promise for revolutionising future drone designs and flight control strategies.
By improving drone stability, especially in turbulent urban environments with challenging wind conditions, this research has the potential to significantly advance important applications such as package delivery, food delivery and environmental monitoring.
The study, conducted at RMIT’s state-of-the-art industrial wind tunnel facility, provided the first precise measurements of a kestrel’s head stability during hovering flight, revealing astonishingly low movement of less than 5 mm during hunting behaviour.
“Aircraft typically use flap movements to stabilize the aircraft in order to achieve stability during flight,” said RMIT lead researcher Dr Abdulghani Mohamed. “Our results, collected over several years, show that birds of prey rely more heavily on changes in surface area, which is crucial as this could also be a more efficient way for fixed-wing aircraft to achieve stable flight.”
Kestrels and other birds of prey demonstrate remarkable abilities to remain still while hunting. This special flight behavior, known as wind hovering, allows the birds to remain still without flapping their wings in suitable wind conditions. Through subtle adjustments of their wings and tail, they achieve exceptional stability.
Advances in camera and motion capture technology enabled the research team to closely observe two Nankeen Kestrels trained at the Leigh Valley Hawk and Owl Sanctuary in high resolution.
Equipped with reflective markings, the birds’ precise movements and flight control techniques could be carefully tracked for the first time during flight without wing flapping.
“Previous studies involved birds flying leisurely through turbulence and gusts in wind tunnels. In our study, we followed a unique hovering behavior in the wind, where the birds actively maintain extreme stability. This allowed us to study the pure control response without wing flapping,” said Dr. Mohamed.
By analyzing these movements, the researchers gained valuable insights that could be used to improve the stability of fixed-wing aircraft during flight.
“The hovering behaviour in the wind that we observed in kestrels is the most comparable in the bird world to fixed-wing aircraft,” added Dr. Mohamed. “Our findings on wing surface changes could be used to design morphing wings in drones to improve their stability and make them safer in bad weather.”
Dr Shane Windsor, Associate Professor of Bio-Inspired Aerodynamics at the University of Bristol and co-author of the study, stressed that the current limitations of fixed-wing unmanned aerial vehicles (UAVs) in gusty winds significantly reduce their effectiveness.
“In the UK, drones are used to deliver mail to remote islands, but their operational time is limited due to regular gusts. Current commercial fixed-wing aircraft must be designed with a fixed geometry and optimised to operate in one flight condition.
“The advantage of morphing wings is that they can be continuously adapted to different conditions during flight, making the aircraft much more maneuverable and efficient.”
The team’s next step is to expand their research and study bird behavior in gusty and turbulent weather. They hope to gain a deeper understanding of stable flight, which could improve the safety and reliability of UAV operations.
Although their initial focus is on small aircraft, they aim to optimize the data collected for possible adaptation to larger aircraft.
Journal reference:
- Mario Martinez Groves-Raines, George Yi, Matthew Penn, Simon Watkins, Shane Windsor, Abdulghani Mohamed. Calm in hover: Kinematics of kestrel wing and tail deformation during hovering. Journal of Experimental Biology, 2024; DOI: 10.1242/jeb.247305
