Impulse flight

The first solar aircraft aiming to fly night and day without fuel will be unveiled in Switzerland on 26 June.

The prototype aircraft, called the HB-SIA, has been developed through Solar Impulse, a project run by Swiss aviators Bertrand Piccard and André Borschberg.

The aircraft, which has been under construction since 2007, combines a lightweight 1,500kg structure with a 61m wingspan and a length resembling that of an Airbus 340. The difference is these wings are covered in a thin sheet of solar cells that convert the sun’s rays into electricity that drives the machine’s engines.

The Solar Impulse team hopes its aircraft will make its first test flight in October of this year. If all goes according to plan, it could make a 36-hour flight in 2010 without fuel.

The team also hope to build another aircraft to fly several consecutive 24-hour cycles. This will lead to the first transatlantic flight in 2012 and then an around-the-world flight.

Borschberg, chief executive officer of Solar Impulse, said there are still several tests that need to be performed on the aircraft before the maiden flight in October. Those tests include checking to make sure the highly electronic aircraft would not come under any risk of electromagnetic interference.

‘This aeroplane is in a domain that has never been explored,’ he said. ‘There’s no aeroplane of this size and this weight that has been flown in the past. We really need to go step by step and make sure we are fully ready before we attempt the first flight.’

There have been other solar-powered flights in the past. The first manned flight occurred in 1980 in California and another aircraft, the Solar Challenger, crossed the English Channel in 1981.

Borschberg said the difference with the Solar Impulse aircraft is it will have the capability of flying through the night. This means, he added, that his group needed to find a way to store sufficient solar energy during the daytime.

The team decided to use batteries to store energy and figured the amount of energy consumed could be affected by adjusting the altitude of the plane.

‘We understood one way to store energy was to store energy in altitude,’ he said. ‘The aeroplane climbs during the day and flies down early night.

'It is of course easier and needs less energy to fly at low altitude compared to flying at high altitude because of the air density.’

The aeroplane’s cockpit is non-pressurised and therefore unable to fly above 8,500m.

Borschberg said the Solar Impulse team briefly considered using a fuel cell to provide energy at night, ‘If you electrolyse water during the day you could store energy, hydrogen and oxygen, and use it in a fuel cell at night.’ he said, ‘but the solution was much less efficient than using batteries.’

The main objective for the Solar Impulse team was saving energy with their aircraft design. Borschberg said flying only with solar energy means the aeroplane does not need fuel to carry fuel, which happens with large transport aeroplanes.

‘A big part of the fuel taken on board of an Airbus or Boeing is required to transport the fuel to propel the aeroplane itself,’ he said. ‘We were able to get out of that. To fly only with the energy collected by the aeroplane itself we had to be very efficient and save as much energy as possible.’

The maiden flight scheduled in October will be flown by a test pilot, but Piccard and Borschberg will alternate piloting all flights beyond then.

Borschberg said he is looking forward to taking off in the aircraft.

‘I’ve flown for 40 years. I fly every week,’ he said. ‘I think I feel more comfortable to be in the air than on the ground in some ways.

Borschberg added that the rest of the Solar Impulse team is also excited to start.

‘Of course there will be some nervousness but this is good because it makes you prepare very well,’ he said. ‘If you’re overly confident that’s where you may face difficulties.’

Siobhan Wagner