Babbage | Solar-powered flight

The green machine flies in

A solar-powered aircraft arrives at the Paris Air Show

By P.M.

THE Paris Air Show, one of the oldest and biggest events of its kind, gets under way at Le Bourget on June 20th with many of the exhibitors keen to promote greener aviation. Besides aircraft powered by biofuel and new lightweight fuel-saving designs, such as Boeing's 787 Dreamliner, there is plenty of other kit promising to help make aircraft more efficient. But nothing could match the rather ungainly bird that touched down on runway 21 in the evening on June 14th.

The aircraft, known as HB-SIA, has the wingspan of a jumbo jet but just a tiny single-person cockpit. It is designed to be powered only by sunlight. HB-SIA was built by a Swiss-based project called Solar Impulse, which was set up by Bertrand Piccard, an adventurer who helped to pilot the first balloon to fly around the world, and André Borschberg, a fighter pilot and engineer. Their ultimate aim with Solar Impulse is to build an aircraft that can circle the world using only the power of the sun.

HB-SIA almost never made it for the start of the show. Mr Borschberg had taken off from Brussels on June 11th but was forced to turn back because cloudy conditions meant there was not enough sunlight for the 11,628 solar cells covering the upper surfaces of the aircraft's wings and tailplane to charge its lithium-ion batteries fully. Headwinds and downdrafts meant flying at a lower altitude than planned, which used up power too quickly. This time, however, the team decided to cheat a bit and topped up HB-SIA's batteries before take-off by plugging it into the mains.

In their eagerness to get the aircraft to the show this can be forgiven (the team say they will not count it as a solar flight). Nevertheless, it was a 16-hour marathon. Although the distance between Brussels and Paris is only about 300 kilometres (186 miles) as the crow flies, Mr Borschberg had to dodge weather fronts and avoid lots of commercial aviation, which meant meandering over France at an average altitude of 2,500 metres before passing over Paris and being allowed to land at Le Bourget. The plan is for HB-SIA to give demonstration flights during the show, subject, of course, to the weather.

HB-SIA uses four electric motors turning propellers. These can produce a maximum of 9 kilowatts, or 12 horsepower—which is about the same amount of power that the Wright brothers had. With all four engines at full power, HB-SIA is only as powerful as a motor scooter. The series of test flights which it is undertaking are being used to set the final design for a second aircraft, HB-SIB, which will be used to make the global flight. Construction of the second plane has already begun. It will be about the same size as the prototype but will have a larger cockpit to provide more creature comforts.

They will be needed. Mr Borschberg has already shown the aircraft can be flown through the night. The flight around the world, however, will be attempted in five stages, following a route which is at least as long as the Tropic of Cancer—a requirement to qualify as a round-the-world flight—and landing on each continent. This means piloting stints of some four to five days at a time. Mr Borschberg will spend all day climbing as the aircraft's batteries are recharged and will then descend slowly under power throughout the night, taking care to conserve energy. He will have to keep a close eye on the weather and navigate around windy areas. Encountering a headwind at night is Mr Borschberg's biggest worry because that would make the night much longer, so he would risk running out of energy before sunrise.

The global flight will push solar power to its limits. Airliners capable of carrying passengers and which rely only on energy from the sun are far into the future, if they ever appear. But lightweight construction techniques, solar cells and battery technologies are constantly advancing. An unmanned solar-powered drone has already stayed aloft for two weeks and some battery-powered electric trainer aircraft are also being developed. Just as the Wright brothers showed, no one really knows where such adventures will lead.

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