Its moment in the sun

A REVOLUTIONARY solar-powered aircraft touched down recently at Moffett Airfield, in the heart of Silicon Valley. No champagne corks were popped, however, for it arrived disassembled in the belly of a 747 cargo jet. The aircraft will be reassembled by the end of March and then begin flight tests. If all goes well, by May it should be ready to fly across America, stopping in four cities before landing in New York. However, this aircraft is just an experimental prototype for a much bigger exploit.

The team behind the project, called Solar Impulse, are using their prototype to learn what will be required to build a second aircraft capable of circumnavigating the globe using only the power of the sun. After carrying out a number of successful test flights of the prototype from their base at an airfield in Payerne, Switzerland, construction of their second aircraft began. But last July the Solar Impulse project suffered a big setback. The second aircraft failed a critical safety check. Its main wing spar, the backbone of any aeroplane, broke during structural tests.

There is always a risk of pushing technology too far when doing something new. Bertrand Piccard, one of Solar Impulse’s founders, had been in the same situation before and knew that success comes by learning from mistakes and moving on. In 1999 he co-piloted Orbiter 3, the first balloon to circumnavigate the globe (the other two Orbiters failed to complete the trip). It was when he landed in the Egyptian desert with just 40kg of propane left from the 3.7 tonnes he had taken off with 20 days earlier that Mr Piccard decided his next challenge would be to repeat the flight using no fuel at all. He teamed up with André Borschberg, a fighter pilot and engineer, to form the Solar Impulse project. Mr Piccard comes from a line of adventurers. His grandfather, Auguste, was the first to fly a balloon into the stratosphere. His father, Jacques, plunged to record depths in a deep-sea submersible.

Making the most of it

The problem with the wing spar has set back the team a year. Making a new one, completing the second aircraft and waiting for suitable weather means that the round-the-world flight is now scheduled for 2015. In the meantime, the team decided to make the best of their enforced delay by flying their prototype across America. This will provide more valuable operating experience and help with the development of the technologies they will need. Plus, if anything goes wrong, it is easier to land on dry land than the ocean.

Solar-powered aircraft are not new. One of the earliest, Solar Challenger, flew across the English Channel in 1981. It was built by the late Paul MacCready, an American aeronautical engineer. Its 14.3-metre wing was covered in photovoltaic cells. These powered two electric motors, which in turn drove a single propeller.

The Solar Impulse project is a very different beast. The prototype aircraft (pictured above) which will fly across America has a wingspan of 63.4 metres, which is as big as a jumbo jet’s. Yet its fuselage is as slender as a glider’s and its single-person cockpit is cramped. The wings are covered in almost 12,000 photovoltaic cells, which can simultaneously run its four electrically driven propellers while charging four packs of lithium-polymer batteries. The batteries are needed because the aircraft has to be able to fly through the night.

The second aircraft (the one being built to circumnavigate the globe) will have to do that non-stop for five-to-six days at a time. The plan is to take off in an easterly direction and land on every continent that touches the Tropic of Cancer. This will involve long flights across oceans. With only as much power as a motor scooter, the planned aircraft will cruise at just 70kph (43mph). Its ultimate range will be limited by the physical ability of the pilot to remain alert, with little room to move or to store much food and water. With current technology, the team reckon, a two-person solar plane would be too heavy.

To give Mr Piccard and Mr Borschberg (who take turns as pilot) room to exercise and lie down, the next aircraft will be about 15% bigger than the prototype, which tips the scales at just 1,600kg (3,527lb). Weight is the critical factor. The wing spar that broke had been redesigned with an ultralight carbon-fibre process to shed the grams. “But we went too close to the limits,” confesses Mr Borschberg.

There is little scope for a full autopilot system, and it would weigh too much. However, Altran, an engineering consultancy based in France and one of the project’s supporters, is developing a partial system. In calm weather, it will keep the aircraft pointing in the right direction. And if turbulence causes a wing to dip by more than five degrees, a cuff on the pilot’s right or left arm will vibrate to tell him which way to correct course. He must react quickly to keep control. This system will be tried out on the prototype flight in America.

The ground crew can monitor both the flight and the pilot with telemetry. When circumnavigating the globe the pilot will be able to lower a seat (which also doubles as a toilet) to lie down and take catnaps of up to 20 minutes. That is enough, the team calculate, to ward off some of the effects of sleep deprivation. The pilot must not be too groggy if he has to swing suddenly into action. This procedure has been tested in a flight simulator for 72 hours non-stop, and seems to work.

A typical flight involves taking off in the early morning, when winds are light, and ascending to 10,000 metres to stay above any stormclouds. At this altitude, though, the air is thin and an oxygen supply is needed. The pilot has to wear an oxygen mask because pressurising the cockpit, as an airliner does with air from its jet engines, is not possible. Nor do the team want to carry heavy oxygen cylinders. As an alternative Air Liquide, an industrial-gases firm, is developing a solar-powered system to generate oxygen.

At night the pilot descends slowly, carefully using up the power until dawn. Once the sun returns the batteries can recharge in three hours as the plane ascends again. Landings are also left until the early evening, when winds are light.

The flight across America will give the team’s meteorologists an opportunity to see how their weather models stand up. Sometimes it is necessary for the pilot to delay a landing to wait for optimum conditions. Mr Piccard and Mr Borschberg have learned an interesting technique to do that. Flying slowly, they turn the nose into a headwind, which can make the aircraft fly backwards. It is not the sort of thing to try in a jumbo jet.