Babbage | Aviation technology

Flying with the sun

By P.M.

IT LOOKS like being one of the most uncomfortable long-haul flights ever. This much is obvious from the unveiling in Switzerland on April 9th of Solar Impulse 2, an aircraft ungainly in appearance with a 72 metre (236 foot) wingspan, bigger than that of some jumbo jets, but offering space inside for just one person: the pilot. Starting in March 2015, the intention is to fly this plane around the world using only the power of the sun.

With 17,248 solar cells on its wings driving four electric motors that turn its propellers, the aircraft will take off from the Gulf and head in an easterly direction over India and China, and then cross the Pacific, the United States, the Atlantic, Southern Europe and north Africa. Although it will land along the way to change pilots, with a cruising speed of just 70kph (43mph), the flights across the oceans mean having to stay airborne, day and night, for five to six days at a time.

To do that, the pilot will climb during the day to 8,500 metres, topping up the aircraft’s lithium-polymer batteries as he goes. At night, he will slowly descend to some 1,500 metres, eking out the power to the engines. As the solar cells are energised again by the rising sun, the aircraft climbs again.

Keeping the weight of Solar Impulse 2 to a bare minimum has been critical: it turns the scales at just 2,300kg (5,000 pounds). Much of the structure is built from extremely light composite materials, including novel carbon-fibre components. Some of the production techniques involved are expected to find commercial uses.

To save weight, the cockpit is somewhat spartan. There is enough space on board for oxygen and food supplies, but not for heating or air-conditioning to cope with temperatures that will range from +40° to -40°C. A specially developed high-density foam surrounds the cockpit to provide some protection. A full autopilot system would also weigh too much, so in calm weather a partial system will keep the aircraft pointing in the right direction. But if a wing dips by more than five degrees, cuffs on the pilot’s left or right arms will vibrate to tell him which way to correct course. He must react quickly to keep control, especially if having a nap. There is, at least, a bit of room to stretch out on a reclining seat (which also doubles up as a toilet).

The flying will be shared by the two men who have set up the project, Bertrand Piccard, who in 1999 co-piloted Orbiter 3, the first balloon to circumnavigate the globe, and André Borschberg, a fighter pilot and engineer. They are backed by a number of companies, including Solvay, Omega, Schindler and ABB, who have also worked on the many technological challenges involved in building Solar Impulse 2.

The first Solar Impulse was smaller and flown as a prototype to test the feasibility of circumnavigating the globe with solar power. Mr Piccard and Mr Borschberg used it to set a number of new records, including last year flying the prototype across the United States. Solar Impulse 2 will undergo a series of test flights in May. These will be followed by a number of longer training missions before the big adventure really begins.

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