Top of the pops

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The dimples, top, are due to be wind tunnel tested on a cylinder, above, this week..

Aircraft wings could be covered in a responsive surface that adapts to the airflow around it to reduce drag, slashing carbon dioxide emissions and potentially eliminating the need for flaps.

Aeronautics engineers at Imperial College, led by Dr Jonathan Morrison, are working with the Rutherford Appleton Laboratory (RAL) to develop a surface made up of small ‘dimples’ like those on a golf ball, capable of controlling the flow of air around the wings.

The dimples are made of electroactive polymer (EAP). When a charge is applied to the devices they pop down and up again, creating a vortex of air that injects energy into the boundary layer of slower moving air molecules around the wing that cause drag.

Morrison and his colleague Dr Beverley McKeon have developed dimples that can be set to pop up and down at fixed intervals, but their ultimate aim is to integrate pressure sensors into the all-polymer surface, to activate the dimples when needed to improve the airflow. Their work has attracted the attention of Airbus and BAE Systems, plus EPSRC funding.

Although the devices are at a very early stage, computer modelling has suggested they could reduce drag by between 30–40 per cent, said Morrison. And two demonstrator experiments will begin this week, including wind tunnel tests of the dimples on a cylinder to monitor their effect on fluid flow using a laser and camera.

‘Companies such as Airbus have signed up to the 2020 Vision of a 50 per cent cut in CO2 emissions per passenger/km. That is a huge challenge,’ he said.

Of this, at least 30 per cent will have to be achieved by the airframe design. But modern aircraft are already designed to minimise drag during cruise, so Morrison believes the target will not be met without fluid control technologies.

Dimples could also improve manoeuvrability by controlling the air emerging from the jet engine, thus controlling the direction of thrust. It could eliminate the need for control surfaces such as flaps, which would make wing design easier and reduce maintenance.

This could be of use to the military, where there is a move to all-electric, flapless stealth aircraft.

The dimples, top, are due to be wind tunnel tested on a cylinder, above, this week.

In unmanned aerial vehicles the devices could be used to increase lift by preventing wing stall. This occurs when the wing is at a steep angle, causing the boundary layer of slower moving air to ‘separate’ from the wing, reducing lift.

Researchers have for some time been investigating other vortex generators known as synthetic jets — small cavities with a membrane at their base that flex and contract to draw in air and push it out again. However, these cavities can get blocked by dust making them difficult to control, said Morrison.

The polymers can withstand temperatures of between –50 to 200º C. They are also compatible with body tissue, so in the longer term could be used for devices such as electro-activated artificial joints or bypasses in arteries.

The team is working with RAL’s Central Microstructure Facility to study techniques to cut the cost of fabricating the polymer surfaces, and is considering using nanoparticles to improve performance.