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Mechanical 'skin' reduces aeroplane drag

A new "skin" that changes shape at the push of a button could reduce the drag forces on aeroplanes or ships, making them faster and more fuel efficient, say US researchers.

Dr Othon Rediniotis and colleagues at Texas A and M University say the active coating can reduce the drag forces on an object in motion up to 40%.

Dr Dennis Bushnell, chief scientist at the NASA Langley Research Center, who was not involved in the research, says such research is key to cutting airline costs.

"The portion of airline direct operating costs attributable to fuel has increased from some 15% to 35%, heading to 50%-plus," he says.

"Aircraft drag reduction is becoming, again, a serious research issue."

As air or water flows over an object, it creates tiny pockets of turbulence that slow the object down.

Even the flattest surfaces, the creation of which is a traditional engineering goal, suffer spontaneous turbulence.

In recent years, designers have roughed things up, adding tiny, immobile riblets to skins.

The riblets break apart the random formation of turbulent pockets and create specific, controlled areas of turbulence that engineers can use to their advantage.

Passive riblets, which can be found in the full-body swimsuits worn by professional swimmers, use the same principle to reduce drag by 5 to 7%.

Shape-shifting system

Rediniotis and colleagues have developed and tested an active shape-shifting system and found it can reduce drag up to 40%.

The scientists created two 15 by 15-centimetre patches of artificial skin and placed them in a wind tunnel at Virginia Tech, one patch on top of the tunnel, one on the bottom.

The patch on top served as the control, while the patch on the bottom was equipped with a tiny 'riblet' mechanical system embedded under it.

Through a camshaft design, the 'riblets' under the skin extend or retreat ever so slightly, by less than a millimetre, creating tiny waves that undulate down the material.

"They are travelling bumps and valleys," says Rediniotis. "A good analogy is a travelling ocean wave."

NASA recently contacted Rediniotis and his colleagues about using their research to reduce drag on aircraft.

"We will study this option, along with many other options, over the next few years," says Bushnell.

While it's clear that an active skin reduces drag, saving energy, it also takes energy to activate the skin.

When the energy it takes to activate the skin is subtracted from the energy saved, the net energy savings only amount to about 7.5%, says Bushnell.

"We probably can improve this, but there are also worrisome but not yet quantified systems penalties such as cost, weight, complexity, reliability and maintainability," he says.