The Solar Impulse project has made significant progress toward its aim of being the first solar-powered aircraft flying at night. The team initiated by psychiatrist and famous aeronaut Bertrand Piccard began construction of the 200-foot-wingspan prototype late in April. Flight tests should start next year.

The company tested the Solar Impulse concept in a virtual flight in Geneva last month. It took off early on May 21 from Honolulu, Hawaii, and landed late on May 24 in Phoenix, Arizona–in lieu of the originally planned destination in Florida. “The experience gained in being forced to carry out a significant diversion has been invaluable in the training of the mission team,” the program’s virtual pilots said.
They might have looked for convection to gain altitude; however, those vertical winds are synonyms for turbulence and the aircraft must avoid them as the airframe is fragile.

The design has changed significantly since the company presented it at the 2005 Paris Air Show. It now has four electric motors instead of two. “One motor would be better in terms of efficiency, but two, as we were planning in 2005, and now four allow for better weight distribution over the wing,” Solar Impulse CEO Andre Borschberg explained last month in Geneva.

Another benefit is safety. A motor failure with a two-motor design would have made the aircraft too asymmetric to keep controllable. It would thus have called for shutting down the second motor and going for a glide. With four motors, if one fails, you can still shut down the opposite one and fly symmetrically, Borschberg said. The combined power of the four motors is 11 horsepower, close to that of the Wright brothers’ Flyer in 1903.

The company plans to roll out a prototype during the summer of 2008. It pegs the first flight through the night for 2009. In the history of solar-powered aircraft, only a remote-controlled model has flown more than 24 hours–a U.S. design, it kept aloft for 48 hours in 2005.

The Solar Impulse prototype will help refine the final design. According to Borschberg, Solar Impulse’s second aircraft, which the company intends to fly around the world in four or five legs, needs an estimated 260-foot wingspan–but this is close to a physical limit, so a smaller wingspan would be appreciated, he explained. The Solar Impulse team is still hoping to limit the wingspan to around 230 feet. It has already postponed the round-the-world flight from 2010 to 2011.

The single-seat Solar Impulse will fly at an average speed of 38 knots. During the day, it will store energy from 2,700 sq ft of solar cells on the wing. Recent progress in electrical batteries is making the team optimistic but the design needs further advancement, Borschberg said. Another way to store energy at day will be to climb at altitude, allowing for night descents. A low sink rate has been a design driver. The Solar Impulse’s wing load is close to that of a hang glider, at 1.6 pounds per square foot.

The company touts the Solar Impulse as showing the way to use new technologies in the service of environment. Project promoters also hope to gather enough enthusiasm to promote renewable energies. “Let’s get rid of the common assumptions,” Piccard urged an aviation audience in May. He reminded listeners that his father discovered that there was some fish life seven miles deep in the ocean.
This was evidence of the presence of oxygen–hence the existence of vertical currents. This stalled plans for using deep ocean trenches as the mankind’s rubbish bins.

Similarly, Piccard believes that if his team succeeds in having its solar-powered airplane flying overnight, it will give renewable energies rock-solid credibility. “No one will be able to say, ‘No, renewable energies do not work or could work only for a marginal percentage of human needs,” he emphasized.

Piccard made it clear that he does not foresee solar-powered commercial airplanes flying fast with heavy payloads. But he suggested that biofuel could be produced on the ground using solar energy. The process would not emit any CO2, which is the main contributor to global warming.

The Swiss pioneer definitely sees the Solar Impulse project as a symbol. “If the pilot wastes his energy, he will crash before sunrise,” he said. He also noted that General Motors has become a junk bond on the stock market, while Toyota is now the number-one car maker, notably thanks to its hybrid models. Nevertheless, Piccard stated that “aviation still has to lead the game in future.”

The Solar Impulse is an $82 million project, two thirds of which has been raised already. Business jet manufacturer Dassault is an aeronautical advisor on the project.