Gulls’ wings offer inspiration for variable-geometry aircraft

Research revealing how seagulls stabilise their flight by changing the shape of their wings could influence future aircraft design

A young glaucous gull gliding over British Columbia. To stabilise its flight in turbulence, it will pull its wingtips in and back. Image: Christina Harvey, University of British Columbia, University of Michigan

Birds have always been an obvious inspiration for humanity’s attempts to design aircraft, but despite centuries of invention, we have never been able to directly utilise the techniques or material adaptations which birds use to fly. Zoologists from the University of British Columbia (UBC) in Vancouver have now discovered how the relatively-simple wing joint in seagulls helps the birds to stabilise their flight, and they believe that this could be mimicked by engineers to design aircraft that can dynamically adjust their wing shape to cope with challenging flying conditions.

“It’s not enough that those two seem to produce sufficient lift and thrust,” explained researcher Christina Harvey, who has now moved from UBC to the University of Michigan. “They must also control and stabilise their flight paths to be able to successfully forage and migrate in the natural habitat.” Seagulls – which contrary to popular belief are not seabirds as they live on land, albeit on the coast, seabirds spend most of their time in the open ocean – often encounter turbulent air flying near buildings or cliffs, or fly through convective airflows over open water. To cope with this, Harvey and research leader Douglas Altshuler explain in a paper in the Journal of the Royal Society Interface, the birds alter the angle of the elbow joint to pull the tips of their wings in and back. This flexed shape gives them more control over flight through turbulent air.

The researchers first photographed freely gliding gulls from below to determine which wing shapes they used to cope with different wind conditions, then studied the wing bones and joints to derive the relationship between wing shape and joint movement. They then studied the behaviour of gull wings posed into different shapes by flexing the elbow joint in a wind tunnel by placing the shoulder joint on a six-axis force sensor that allowed them to measure the twisting force on the wing – known as the pitching moment – as well as the lift and drag produced by each configuration. The turbulence generated by the tunnel was not as great as that encountered by gulls in their normal flight.

The researchers studied how different flexing of the elbow joint affects the in-flight characteristics of the gulls’ wings. Image: Douglas Altshuler, Christina Harvey, UBC/Journal of the Royal Society Interface

“The gull’s wing design points to a novel, and fairly simple, avian-inspired joint that may enable aircraft to adjust dynamically,” said Altshuler. “This is the first empirical evidence demonstrating that wing morphing affects avian stability.”

Wing morphing has a long history in engineered flight. The use of movable control surfaces and extending flaps is the current embodiment of the technique, but in the earliest days of powered flight, the wings themselves were engineered to twist by altering the tension on cables; the Wright brothers’ aircraft used this technique. “The Wright brothers weren’t the first to design an aircraft that was able to fly, but they were the first to successfully control and stabilise a powered aircraft in flight,” Harvey said. The most familiar use of variable geometry in aircraft is the “swing wing” system pioneered by the renowned wartime aerospace engineer Barnes Wallis, and used by aircraft such as the Tornado, F-14 Tomcat and B-2 bomber, which spread their wings at low airspeeds and pull them in to create a more delta wing-like configuration at high speeds.