A prototype solar-powered airplane completed several important tests last Thursday and Friday.

Solar Impulse's HB-SIA, which was finished this past summer, taxied down a runway using power from the 11,000 solar cells covering its wings and did a series of acceleration and braking tests. The next test will be revving up the plane to its 35km/hour take-off speed.

Founder of Solar Impulse, Bertrand Piccard, a former astronaut and the first man to circle the world nonstop in a balloon, hopes to perform the same feet in a solar-powered plane derived from on the HB-SIA design. Solar Impulse aims to test the prototype in flight next year and to achieve a 36-hour flight without fuel shortly after that. Results from these tests will be used to build a solar-powered plane to will attempt a transcontinental flight sometime after 2012.

A number of solar-powered aircraft exist already, such NASA's Helios, the Solar Riser glider or the Sunseeker which flew across the US in 1990 using a mix of solar power and gliding.

The Solar Impulse prototype is made of lightweight materials, weighing only 3,500 pounds and it has a wingspan of 210 feet. It is intended to fly at only 28 miles per hour to keep energy consumption low. It will store solar energy for night flight.

The video below shows computer simulations of Solar Impulse's plane, and the real thing on the runway.

Carbon nanotubes, atom-thick sheets of carbon, are among the strongest known materials. They already add their strength and toughness to several products on the market, including many bicycle frames. It's not surprising, then, that carbon nanotubes can also improve the properties of advanced aerospace materials.

Airplane skins are composed of composite materials made up of layers of carbon fibers held together by polymer glue. They can fail when the glue cracks and the fibers come apart, and reinforcing them is tricky. Using pins and stitching might seem like a good idea, but this can pierce and weaken the carbon layers. Researchers led by Brian Wardle, an assistant professor of aeronautics and astronautics at MIT, have now strengthened these advanced aerospace materials with what they call "nanostitching." Rows of carbon nanotubes perpendicular to the carbon microfibers fill the spaces between them, reinforcing the fiber layers without piercing them.

According to theoretical work to be published by the MIT group in the Journal of Composite Materials, these materials are not only 10 times stronger than those that don't contain nanotubes, but they are also more than one million times more electrically conductive, which suggests that they might protect aircraft from lightning strikes.

MIT aeronautics and astronautics professor Brian Wardle shows a composite material strengthened by carbon nanotubes. Credit: Donna Coveney/MIT