PARIS — Keeping any airplane aloft requires a delicate balance of four forces: lift, drag, weight and thrust. André Borschberg’s requires a fifth factor to be in perfect equilibrium as well.
During the day, his slender, propeller-driven plane is powered in flight by the energy it collects from the sun through an array of solar cells
atop its 64-meter, or 208-foot, wingspan. For the plane to continue flying through the night, however, Mr. Borschberg must carefully ration that energy, which is stored in the aircraft’s 450 kilograms, or 990 pounds, of lithium-ion batteries. In this way, he said, the plane could theoretically fly indefinitely.
“It has become a game,” Mr. Borschberg, a former Swiss Air Force fighter pilot, said of his aircraft, the Solar Impulse. “It’s not a question of flying fast. It’s how do you manage to keep flying until the sun rises the following day.”
Taking wing from Brussels without burning fuel, emitting noise or trailing pollution, the experimental plane arrived at Le Bourget airport, north of Paris, last week — although it took nearly 16 hours for the craft, the guest of honor at Paris Air Show this year, set to open Monday.
The show is a global coming-out party for a project that has won admiration from many in the aviation world for its promotion of renewable energy.
“Presenting this technology to the public and showing that you can fly an aircraft with no fuel — that shows the progress this industry has made in its environmental consciousness,��� said Damien Lasou, a managing director and aerospace analyst at Accenture in Paris.
Nonetheless, some industry executives privately dismiss the plane as little more than a vanity project of Mr. Borschberg and his business partner, Dr. Bertrand Piccard, with scant commercial prospects, at least for the foreseeable future.
The Solar Impulse, which has been in development since 2005, attracted worldwide attention last July when it completed its first test flight of more than 24 hours, circling high above the Jura Mountains of northern Switzerland and relying on the power generated by the 11,628 solar cells across the upper surface of its wingspan, which is just a touch wider than that of a Boeing 747.
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