Solar Impulse

A new solar-powered aircraft attempts to fly around the world with zeroemissions

WHEN an airliner takes off for a transatlantic flight it needs to carry some 80 tonnes of fuel, which accounts for around one-fifth of its weight. On really long flights, fuel can account for 40% of a plane's take-off weight,so that around 20% of the fuel is used to carry the rest of the fuel.

Each tonne of fuel burned also produces 3.2 tonnes of carbon dioxide. Yet inside a hangar at a Swiss airfield is the prototype of an aircraft that does not use any fuel at all. The wings of this aircraft are almost as big as those of an airliner, but they are covered in a film of solar cells that convert sunlight into electricity to drive its engines.

Solar-powered aircraft have flown before. The pioneer was Paul MacCready,whose Gossamer Penguin made the first manned flight in 1980 in California,with his then 13-year-old son at the controls. A derivative, Solar Challenger, crossed the English Channel in 1981.

But nothing like HB-SIA, as the Swiss aircraft is known, has ever taken to the air.

If it works as expected, another version will be built and this will take off, climb to 10,000 metres and, by storing some of the electricity generated during theday, continue flying through the night. Its pilots, Bertrand Piccard andAndré Borschberg, plan to cross the Atlantic in it and later to fly itaround the world.The prototype will be unveiled on June 26th by Solar Impulse, a project the aviators run.

Mr Piccard helped pilot Orbiter 3, the first balloon to flynon-stop around the world, and comes from a family of adventurers: hisgrandfather, Auguste, was the first to fly a balloon into the stratosphere and his father, Jacques, plunged to record depths in a bathyscaphe. MrBorschberg is an engineer and fighter pilot.

Testing to the limit

Although he has flown HB-SIA in a simulator, Mr Borschberg says he will not really know how it performs until the first test flight later this year.

The prototype pushes some technologies to their limits, especially in the trade-off between weight and performance. So, although it has a wing span of 61 metres, HB-SIA has room only for a pilot.

It weighs just 1,500 kilograms,making it five times lighter than a high-performance glider would be if madethat big.

The complex skeleton of HB-SIA is constructed from carbon-fibre composites formed into honeycomb and sandwich structures. This is covered in plastic film

- Economist