It has been known that birds use a small group of feathers, called “the alula”, a thumb-like structure that is present at the bend of the wing, in slow and steep flight such as landing.
Researchers at the Seoul National University in South Korea have found that a small vortex of air that is formed at the tip of the alula feathers helps the birds in flight.
The researchers aim to apply the way that the alula works in designing a device that enables the air vehicles to turn better and more efficiently.
In the study, researchers first observed magpies in the aviary when the birds moulted the alula feathers.
“It’s not that they cannot fly without the alula. But with the alula they seem to turn more easily,” said Dr Sang-im Lee, the first author of the study.
Then the researchers conducted wind tunnel experiments where they visualised the movement of tiny particles around magpie wings.
They observed that a tiny vortex from the alula tip presses the air flow over the wing and makes the air flow better attached to the wing surface.
“For the wing to fly better, the air has to move closer to the wing. Although the alula is small, it creates enough vortex to prevent the air flow from being detached from the wing,” said researcher Dr Jooha Kim.
Measurements of the forces acting on wings in the wind tunnel proved that indeed the presence of the vortex from the alula helps the birds in flight, researchers said.
This is the first evidence that the effect of the alula is due to a small vortex formed at the tip of the alula feathers, researchers said.
The study was published in the journal Scientific Reports.
PTI