Solar Impulse: solar powered aeroplane prototype launched at Dubendorf airfield Switzerland

With the unveiling of the first prototype - the HB-SIA - the Solar Impulse environmentally friendly aeroplane project has entered its final test phase. E&T flew (on a fossil fuel powered plane) to Switzerland to find out more.

The curtains pull back to reveal the true scale of what it takes to build a long-range solar-powered aircraft. Bertrand Piccard and André Borschberg, the two main pilots of the Solar Impulse HB-SIA aircraft, embrace; thumbs up signs are given, arms held aloft. Meanwhile the curtains are still retracting to the walls of a hangar that could easily house a commercial passenger airliner.

The wings go on forever. And they need to: not only are they there to provide as much lift as possible, they also provide the largest possible surface on which the solar panels that will power the aircraft are mounted. Every spare centimetre is covered. To save weight the fuselage has been designed to be minuscule relative to the wingspan, hardly bigger than a conventional glider. Make no mistake: Solar Impulse HB-SIA is a huge presence. And it's not even the size of the aircraft that the team will use to fly around the world using nothing but the power of the Sun.

During Piccard's presentation, fact after startling fact emerges. With the wingspan of a Boeing 747-400, the Solar Impulse aeroplane weighs less than an average family car (1,600kg). Close to 12,000 wing-mounted solar panels supply renewable energy to four electric motor gondolas that propel the plane. During the day these panels will also charge the lithium-polymer batteries that will supply power for the night-phase. With the batteries weighing in at 400kg - a quarter of the plane's total weight - getting the balance right has been one of the key challenges.

Piccard's plan

Piccard explained that the HB-SIA is the first prototype in the Solar Impulse project. In order to save weight and space, the aircraft's cabin is unpressurised (restricting the maximum height to 8,500m), and this is where the test pilots will assess the feasibility of a complete day-night-day flight over 36 hours, propelled only by electricity generated on board by solar power technology. After fine-tuning, the aircraft is scheduled to make the first of a series of three types of test flights before the end of the year, cumulating in a maiden night flight in Switzerland in 2010.

The prototype has three main objectives. The first is to validate the results of the computer simulations and materials selection decisions. The flight will see how the aircraft performs in real-life. Attaining a 63m wingspan with the necessary rigidity, lightness and flight controllability with just 1,600kg take-off weight is an aeronautical challenge that has never been achieved to date. And the flight will show how efficient the energy capture and storage system really is.

The results from the test flights will be fed into specification changes for the second aircraft - the HB-SIB - that will carry out the actual project mission of circumnavigating the world in five stages, each lasting several days, in 2012.

Bertrand Piccard is one of the great explorers of the modern era, perhaps most famous for the first ever non-stop circumnavigation of the globe by balloon. Accompanied by aeronaut Brian Jones, Piccard's Breitling Orbiter 3 landed in Egypt after a 45,755km flight lasting 19 days, 21 hours and 47 minutes, prompting the pilots to co-write the best-selling book 'Around the World in 20 Days'. But it could have all gone so badly wrong. It was the realisation that the whole project could have been scuppered by lack of fuel that drove him to attempt a further circumnavigation flight - only this time without the use of fossil fuel or its attendant polluting emissions. Solar Impulse was born.

Piccard, who is not known for his ease with measured understatement, says: "If an aircraft is able to fly day and night without fuel, propelled by solar energy, let no one claim that it is impossible to do the same thing for motor vehicles, heating and air conditioning systems, and computers. We are proclaiming our conviction that a pioneering spirit and political vision can change society and put an end to fossil fuel dependency."

In summarising the achievement of the 50 staff employed by the project and the hundreds of experts and advisers who have co-ordinated the technology behind Solar Impulse, Piccard's colleague Borschberg kept his feet on more solid ground: "A challenge like Solar Impulse," he says, "can be met only by bringing together engineers from every background."

At the unveiling of the aircraft, the Solar Impulse company hosted a display of some of the components, materials and electronics that went into making the HB-SIA. These include carbon-fibre structural pieces such as the wing ribs that give the aerodynamic profile. Despite being so light it can be lifted with your little finger, perhaps the most interesting item is a cockpit instrumentation panel that is effectively a power status summary indicator. Parameters such as rpm and temperature are clearly shown for the four wing-mounted engines, but there is also a series of slider bars that show the condition of batteries or energy accumulators. The batteries are crucial to the success of the circumnavigation because this is where the surplus energy generated during the day will be stored to power the night-time flying.

Solar equipment

Beneath the wings are four gondolas, each containing a 10HP motor, a lithium-polymer battery set and a management system controlling charge/discharge and temperature (represented in the cockpit on the instrument display). The thermal insulation has been designed to conserve the heat radiated by the batteries and to keep them functioning despite the outside air temperature of -40°C at 8,500m (roughly the height of Mount Everest). Each engine is fitted with a reducer that limits the rotation of each of the 3.5m diameter, twin-bladed propellers to within 200-4,000rpm (another parameter displayed inside the cockpit).

The energy is gathered by 11,628 monocrystalline silicon cells plastered all over the upper surfaces of the wings and horizontal stabiliser at the rear of the plane. Each cell is 150μm thick, and has been selected for being lightweight and flexible. But not, it would seem, for its efficiency. At 22 per cent, the Solar Impulse technical documentation is first to admit, these are nowhere near the most efficient available, but the additional weight required to improve efficiency would have thrown out the mathematical balancing act and the less efficient option won out on other considerations. The designers say that the maximum energy density for the aircraft prototype is 220 watt-hours per kilogram and only the test flights will be able to provide clues as to whether this needs to be improved upon.

There is only a small part of the day when the solar panels are illuminated at such an angle that they are at full efficiency. At midday, each square metre of the wing surface receives the equivalent of 1,000 watts of light power. Over the day, this averages out at just 250W/m2. With 200m2 of photovoltaic cells and with 12 per cent total efficiency of the propulsion chain, the aircraft's engines achieve, even after extreme optimisation of the energy chain, an average of just 8HP, which is about the same power as a 50cc motorcycle. Or, in aeronautical terms, roughly the same power the Wright brothers had available to them in 1903 when they made their first powered flight. The difference is that Solar Impulse is generating its own power on board from renewable resources.

Energy management aside, one of the most critical developments has been the electronic instrumentation panel in the cockpit. This allows the pilot to monitor the condition of the flight in two key parameters - 'bank-angle' and 'side-slip'. The Omega instrument panel was the brainchild of Claude Nicollier, former European Space Agency astronaut, with four space flights under his belt as well as an eight-hour EVA (spacewalk) to his name. Nicollier also sits on the Swatch Group board of directors that has provided financial assistance for the project, as well as the technical expertise to develop a performance simulation and testing system for the aeroplane's propulsion chain. Omega already had the technical experience in the field of hybrid propulsion, but, more importantly, was in a position to align its own reputation for engineering excellence in the field of high-end horology with an environmentally-friendly sustainable energy project.

"I came up with the idea for what we needed from the instrumentation and I made a drawing to show how it could be implemented," says Nicollier, demonstrating a prototype schematic at the Dübendorf airfield launch. According to Nicollier, there are two fundamental aspects to the instrument. First, there is a precise indication to the pilot of the bank-angle. This is a critical parameter on Solar Impulse, because, according to Nicollier: "99 per cent of the turns will be made with a bank-angle of less than five degrees. If you go beyond 10 degrees it becomes a little more difficult to recover. From our simulations we know that if you go beyond 15 or 20 degrees then you cannot recover. You end up in a spiral dive and have to jump out."

Second, because of Solar Impulse's large wing-span to length of fuselage ratio, there is the tendency of pronounced sideslip, an error where the plane drifts off course relative to the direction in which it is pointing (it won't go in the direction it's being steered). Nicollier, who will be taking part in the later phases of the test flights, says that in the simulator runs there was sideslip of up to 15 degrees, "which means that, because you are flying slowly, as you approach a runway, you will not immediately be able to figure out which direction the aeroplane's flying". To indicate sideslip, Nicollier has devised an array of blue LEDs with a green light superimposed that tells the pilot at a glance whether he is good to land.

Symbolic power

The wider environmental implications of a flagship technical challenge like Solar Impulse is largely symbolic, giving bodies such as the European Commission a platform to display their green credentials. There's no doubt that Bertrand Piccard has made the most of the opportunity to display technology as a force for environmental sustainability. Piccard and Borschberg are travelling the world spreading the word. At the Beijing Olympics they presented Solar Impulse, and they have taken models to India and the UAE. Along the way they have been helped by a group of high profile ambassadors, including Prince Albert II of Monaco, Buzz Aldrin, Yann Arthus-Bertrand, Paulo Coelho and Al Gore.

It is quietly spoken Nicollier who makes the message clearest. "We have not really made any quantum jumps in technology here today," he says, "but what we have done is used technology at the limit of what is do-able. Ten years ago it was impossible. In ten years it will be much easier. If we can use stronger, lighter materials with more efficient energy management systems drawing on renewable resources, we are simply engineering for a better future."

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