A coating of “feathers” on this cylinder in an airflow modifies the vortical structures behind it (shown in orange and blue), cutting aerodynamic drag by about 15%. The solid line in the graph shows the drag on the coated cylinder, the dashed line that of one with a smooth surface (Image: Julien Favier)

Coating the rigid wings of airplanes with artificial bristles that mimic feathers could make them more efficient, according to engineers. An Italian team has demonstrated how feather-like structures help reduce drag on a cylinder and says they could have the same effect on underwater and aerial vehicles.

Birds use long, stiff flight feathers to help generate the lift and thrust needed to get off the ground and to stay aloft. But Alessandro Bottaro at the University of Genoa is more interested in how a set of smaller feathers – called coverts – keep birds flying efficiently.

Although they may not look like they can have much of an effect, during gliding some covert feathers stick up at right angles to the wing’s surface and vibrate in the airflow. To test whether this has any effect on flight performance Bottaro and Julien Favier, also at Genoa, added synthetic coverts to a computer model of a 20-centimetre-diameter cylinder and put it in a virtual wind tunnel.


Drag race

Their synthetic feathers are modelled as rigid keratin bristles 4 to 6 centimetres long and 0.5 millimetres in diameter, coating the cylinder at a density of around three fibres per square centimetre. The cylinder was orientated with its long axis perpendicular to the air flow, placing the synthetic feathers parallel to the wind.

As the wind speed increased the bristles started to vibrate in a similar way to real covert feathers, reducing the drag on the cylinder by 15%.

The researchers say that’s because the fibres help to cushion the effects of the air flow on the cylinder itself. Normally the air flows rapidly across the cylinder and creates an area of low pressure behind it. This encourages the formation of strong vortices, creating turbulence and increasing the drag on the cylinder. Racing car drivers exploit that area of low pressure – the slipstream – to stay close to their rivals without having to combat the drag experienced by the car in front.

With the feathers, the low-pressure slipstream does not form, and the vortices affecting the cylinder are weaker. A similar process explains why fresh, fuzzy tennis balls can speed through the air faster than worn ones, says Bottaro.

Preening problem

Bottaro thinks artificial feathers could be added to aircraft or underwater vehicles to improve their efficiency. Though, he adds, they might need a self-cleaning system to mimic the way birds preen their feathers to ensure efficient performance.

“I think the [new] finding has relevance for the field of fluid mechanics, but not so much for our understanding of bird wing aerodynamics,” says David Lentink from University of Wageningen, in the Netherlands, who leads a team that developed “RoboSwift” – a remote-controlled small aircraft that changes the shape of its wings like an acrobatic bird to improve its aerodynamic performance.

Other recent lessons on fluid mechanics from nature include a study last year that suggested the way dolphins wrinkle their skin when swimming at high speed could provide a new tactic to reduce drag on submarines.

Journal reference: Journal of Fluid Mechanics (DOI: 10.1017/S0022112009006119)