The Solar Impulse has made significant progress toward its goal of being the first solar-powered aircraft to fly at night. Led by psychiatrist and accomplished aeronaut Bertrand Piccard, the team began construction of the 200-foot-wingspan prototype in late April. Flight tests are scheduled to start next year.

The Solar Impulse concept, at least, was tested last month in Geneva in a simulated virtual flight. It took off early on Monday, May 21 from Honolulu, and landed late on May 24 in Phoenix, rather than 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 virtual pilots said.

The design has been changed significantly since a presentation at the 2005 Paris Air Show. First, the Solar Impulse 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 across the wing,” Solar Impulse CEO Andre Borschberg explained.

Safety is another benefit of the change. Failure of one of two motors would have made the aircraft uncontrollable. It would thus have called for shutting down the second engine and entering a glide. If one engine of four fails, the opposite one can be shut down, and thrust remains symmetrical, Borschberg said. The combined power of the four motors is 11 horsepower, close to that of the Wright brothers’ Flyer in 1903.

The prototype will be rolled out next summer. Its first flight through the night is pegged for the following year. In the history of solar-powered aircraft, only
a remote-controlled model has flown more than 24 hours; it stayed 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 is intended to fly around the world in four or five legs, needs an estimated 260-foot wingspan. But this is close to a physical limit. A smaller wingspan would be beneficial, he explained, and the Solar Impulse team is still hoping to limit the span to, say, 230 feet. The round-the-world flight has already been postponed 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 advances in battery technology are boosting the team’s optimism, but further improvements are needed, Borschberg said. Another way to store energy during the 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 loading is close to that of a hang glider, at 1.6 pounds per sq ft.

The Solar Impulse is touted as showing the way to use new technologies to benefit the environment. Project promoters also hope to gather enough enthusiasm to promote renewable energy sources. Piccard believes that if his team succeeds in flying its solar-powered airplane through the night, it will lend credibility to renewable energy.

Piccard made it clear that he does not foresee solar-powered commercial airplanes flying at high speed with heavy payloads. But he suggested that biofuel could be produced on the ground using solar energy. The process would not emit any CO2.
The Swiss pioneer sees the Solar Impulse project as a symbol. “If the pilot wastes his energy, he will crash before sunrise,” he said. Piccard noted that “aviation still has to lead the game in the future.”

The Solar Impulse is an $82 million project, two-thirds of which has been raised already. Dassault is participating in the project as an aeronautical advisor.