How a robot kestrel reveals the secrets of stable flight

The kestrel is a small falcon and keen-eyed hunter, commonly spotted beside open fields and roads.

This compact bird of prey can remain almost stationary in a strong wind. As it hovers with its eyes trained on prey below, gusts that could throw a small drone off course scarcely displace it.

For years, that skill has intrigued the people who design flying machines.

While small aircraft find turbulent, gusting air difficult to navigate, birds seem to cope with the same conditions effortlessly.

To discover how they do it, a research team built a robotic kestrel and put it through tests in a wind tunnel.

Holding still in moving air

The behaviour investigated in the study is known as wind hovering.

A kestrel faces into an upward-moving wind and holds its position above a single point on the ground, without beating its wings. The feat is more demanding than it might appear.

Wind speed and direction around the bird are continually shifting, so it must make constant, minute adjustments to avoid drifting away.

Those adjustments involve numerous small movements of its body.

How kestrels stay stable

The study drew on earlier tracking of live kestrels flying in the Royal Melbourne Institute of Technology (RMIT) wind tunnel facility, among the largest facilities of its kind in Australia.

“Birds don’t rely on a single response to wind gusts,” said RMIT researcher Matt Penn, who led part of the work on how birds cope with turbulence.

“They constantly adjust their wings and tails to stay balanced, while the natural flexibility of their feathers and joints helps absorb sudden changes in airflow.”

“They can also sense disruptions very quickly, which allows them to respond almost instantly and maintain control.”

Building a robotic kestrel

To establish which movements made a difference, the researchers focused on the nankeen kestrel (Falco cenchroides).

Its long, slender wings are adapted to open skies, making the species a useful model for the small aircraft engineers hope to improve.

The researchers used computed tomography (CT) to scan the bodies of two kestrels, mapping the shapes of their bones, muscles and feathers. They used the scans to guide the construction of a robot capable of reproducing the bird’s principal movements.

The robotic kestrel concentrated on three actions: extending and tucking its wings, opening and closing its tail in a fan shape, and angling the tail upwards or downwards.

Dr Mario Martinez Groves-Raines constructed and operated the model while studying at RMIT and the University of Bristol. He is now based at the Royal Veterinary College in London.

“By creating a robot replica, we were able to measure how specific movements were contributing to steadiness in flight,” said Groves-Raines.

Testing each movement separately

The team mounted the robotic kestrel in RMIT’s low-turbulence wind tunnel, facing a steady airflow of about 26 km/h.

That corresponds to the speeds at which real kestrels fly while hovering.

For each posture, a sensor measured lift, drag and twisting forces. The researchers altered one movement at a time and examined how those forces changed.

How the wings change forces

Extending and tucking the wings produced an unexpected result: the amount of lift generated by the wing changed very little.

The main effect was instead on pitch, or the balance between the nose pointing up and down. This provides the kestrel with a gentle means of remaining level over a range of flight angles.

Drawing in one wing while stretching out the other also rolled the body, rather like an aeroplane banking with its ailerons.

How the tail controls flight

The tail proved to be a powerful control tool. Spreading it out substantially increased both lift and drag at the same time.

Of all the individual movements, tilting the tail had the greatest effect on pitch. It acted like the elevator on an aeroplane’s tail.

Angling the tail sharply downwards could even serve as a brake during landing.

Wings and tail working together

The most intriguing result emerged when the wings and tail moved together. Kestrels often combine these actions while hovering, and the tests offered an explanation for that behaviour.

Extending the wing while spreading the tail increased lift more than either action on its own. At the same time, their effects on pitch almost cancelled each other out.

The kestrel could therefore gain or lose lift without its nose tilting upwards or downwards.

That allows the bird to cope with an upward gust without changing its heading, something that causes significant problems for small drones.

Staying stable without constant effort

Each posture tested by the researchers was naturally stable. Following a disturbance, the bird’s shape tended to return it towards level flight without further action.

A spread tail provided the greatest stabilising effect, while tucked wings also increased stability.

Using both movements together could make a kestrel more stable still, potentially allowing it to maintain its position with less muscular effort.

Why light bodies turn faster

The next finding brought a further twist. Compared with the control flaps on two agile model aircraft, the kestrel’s shape-changing movements did not generate greater forces.

Why, then, do kestrels appear to fly better than such machines? The explanation lay in their weight rather than their power.

Most of a kestrel’s mass is concentrated in its body, with relatively light wings and tail. Because its weight is not spread far from the body, it can rotate into a turn much faster than a drone subject to the same control forces.

The ability to turn quickly is central to agility in flight.

What this means for drones

For engineers, the finding offers two lessons: imitate the bird’s ability to change shape, but also reproduce its lightness.

“We uncovered several unique techniques behind the kestrel’s impressive stability. Many of these techniques have the potential to improve manoeuvrability of small aircraft, which encounter similar challenges to kestrels,” said Groves-Raines.

The results could help future drones operate in gusty conditions that keep current models grounded. That could be important for tasks such as search and rescue, crop monitoring and delivery.

“This research shows what’s possible when engineers look to nature for solutions,” said Professor Abdulghani Mohamed of RMIT.

“Our findings open new pathways for designing aircraft that can better handle turbulence.”

The researchers now plan to investigate how kestrels sense the air around them and adapt what they learn for larger aircraft.

As climate change is expected to increase turbulence, a more stable drone could be especially useful.