Solar Impulse Plans U.S. Flights, Builds Bigger Airplane

 - May 2, 2013, 2:55 AM

Switzerland-based Solar Impulse is planning “Across America” flights this spring to showcase its sun-powered aircraft to the U.S. public and demonstrate and develop the possibilities of solar energy. Meanwhile, in Switzerland, the company is developing a second, larger aircraft that it hopes to fly around the world in 2015.

The “Across America” itinerary brings the aircraft from San Francisco to Phoenix and Dallas, then on to Saint Louis, Mo. or Atlanta, Ga., before Washington, D.C., and finally New York. The aircraft, registered HB-SIA, was scheduled to take off from California on May 1. The exact dates for each leg are fluid, as local weather conditions will determine when the aircraft can fly. HB-SIA has already flown in Europe and Morocco, and it achieved its first night flight when it flew for more than 26 hours in July 2010.

While HB-SIA is demonstrating its capabilities in the U.S., the engineering teams in Payerne and Dübendorf, Switzerland, are working on the project’s second aircraft. The team plans to fly HB-SIB around the world (in multiple legs) in 2015. Because it will have to fly for days at a time, in sometimes less-than-ideal weather conditions, “we need a higher-performance aircraft, and HB-SIB will be larger,” CEO, co-founder and pilot André Borschberg told AIN. Its external dimensions will be 11 percent bigger than those of the original aircraft, providing greater surface area for photovoltaic cells.

Proportionally to its size, HB-SIB will be slightly heavier than HB-SIA because of the demands on payload: the pilot will need food and water for five days and some safety equipment, including a life raft.

One weight-saving effort–an ultra-light carbon-fiber wing spar–resulted in a serious setback. Lacking sufficient torsion, the spar failed during a test in July last year, pushing the round-the-world flight from next year to 2015. Engineers have redesigned the spar, using the same materials as those used on the failed spar but this time incorporating them into a design similar to the proven one on HB-SIA.

HB-SIB is making the most of several technology advances. For example, the solar cells will be thinner and lighter, and the propulsion system will be more efficient. From the battery output to the propeller shaft, efficiency will be 94 percent–a gain of five or six percentage points, Borschberg said.

Beyond the technological challenges, Solar Impulse is a human challenge, too. The longest flight, at 38 knots over the Pacific Ocean, is predicted to last 120 hours. Yet the pilot will be allowed to take naps of no more than 20 minutes.

HB-SIB will not be equipped with an autopilot. Instead, a stability augmentation system will serve as a “virtual copilot.” When the pilot is resting, the “copilot” will compare the aircraft’s flight path to the plan and warn the pilot of deviations. In the absence of a warning, the pilot will be awakened after 20 minutes, check the aircraft status and maybe catch another nap if all is well.

Borschberg has already experimented with such a schedule, under medical supervision, for 72 hours. He and cofounder Bertrand Piccard also count on meditation, relaxation and breathing techniques.