The dream of solar-powered, long-distance flight is taking shape. Bertrand Piccard, one of the two pilots who became famous with the first round-the-world balloon flight, yesterday introduced a model of a sun-powered, single-pilot airplane that could fly in 2008. The latest design update of the Solar Impulse aircraft, shown here at the Paris Air Show, included noticeable design changes since program launch late in 2003. The main challenge is to be able to fly at night for intercontinental travel at low speed. The ultimate aim is to fly around the world with one stopover per continent in 2010.

A major scientific partner in the team Piccard has gathered is the Ecole Polytechnique Fédérale de Lausanne in Switzerland. Its students and researchers are no strangers to the new architecture of the aircraft, now at the end of the concept phase. For example, both propellers have been moved forward from the empennage (where they were after the feasibility study) to a more conventional location on the wing. This is to reduce the length of the cables–and the resulting losses–between the cells, the batteries and the electrical engines.

The cockpit is now under the wing, adding ground clearance to the propellers. The latter need to get maximum diameter for high efficiency. The shape of the wing, too, has evolved. It is now rectangular to provide maximum surface to the solar cells. “However, it could still be refined for aerodynamic reasons,” André Borschberg, Solar Impulse’s CEO and future pilot, said.

There are 2,700 square feet of monocrystalline solar cells on the wing. With a 20-percent efficiency, they feed 880 pounds of batteries. The latter drive two engines that have a combined power of 40 horsepower.

Storing electricity in batteries is one way to store energy. Climbing to high altitudes at day is another, providing you allow the aircraft to descend at night. The flight profile therefore involves altitudes of around 39,000 feet at maximum and 10,000 at minimum.

Cruise speed is planned to be approximately 55 knots. With only one pilot onboard, Piccard and his team believe five days is the maximum reasonable duration for a leg. Thanks to electrodes on the pilot’s head, the electronic pilot assistant will be able to determine whether the pilot is sleeping and wake him, if necessary.

The Solar Impulse aircraft is valued at $49 million. Piccard and his team said they have gathered 25 percent of that so far. Several partners are providing both financing and technology expertise, including chemicals specialist Solvay, aircraft manufacturer Dassault Aviation and consulting firm Altran.