May 1, 2008 -- A new "skin" that changes shape at the push of a button could reduce the drag forces on an object in motion from 20 to 40 percent.
When applied to ships or airplanes, the active coating could make them faster and more fuel efficient, saving millions of dollars each year.
"The portion of airline direct operating costs attributable to fuel has increased from some 15 percent to 35 percent, heading to 50 percent-plus," said Dennis Bushnell, chief scientist at the NASA Langley Research Center, who was not involved in the research.
"Aircraft drag reduction is becoming, again, a serious research issue," he added.
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 percent.
An active shape-shifting system, like the one developed by Texas A & M researchers in the new study, could reduce drag up to 40 percent.
To test their idea, the scientists created two 6-inch by 6-inch 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 millimeter, creating tiny waves that undulate down the material.
"They are traveling bumps and valleys," explained Othon Rediniotis, a researcher at Texas A & M University and study co-author. "A good analogy is a traveling 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," said 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 percent, said 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 said.
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