What has a wider wingspan than a 747, weighs the same as a car, and can fly almost forever without a drop of fuel? If you were in Payerne, Switzerland on Wednesday, you would have seen the answer as psychiatrist and explorer Bertrand Piccard and engineer and entrepreneur André Borschberg unveiled the Solar Impulse 2. The result of 12 years work, the ultra-light, solar-powered airplane will attempt to fly around the world next year relying exclusively on solar power to keep it aloft for days at a time.

Attended by luminaries such as Swiss Councilor Ueli Maurer and Prince Albert of Monaco, and representatives of the various companies that contributed sponsorship and technology to the project, the début of the Solar Impulse 2 marks the culmination of 12 years of effort by 80 technicians. It builds on the success of the previous Solar Impulse, which set eight world records, including a flight across the United States that saw the first day/night flight by a solar-powered plane.

The Solar Power 2 is a single-seater aircraft built of carbon composites with a 72-m (236-ft) wing span, which is larger than that of a Boeing 747-8 and 8 m (26 ft) wider than the previous Solar Impulse. Despite this huge expanse, it weighs only 2,300 kg (5,070 lb), of which 633 kg (1,395 lb) are lithium-ion batteries used to power the plane's four 17.5 hp electric motors that spin the propellers with an efficiency of 94 percent.

During daylight hours, the power to run the plane comes from the top of the wing, which is covered with flexible solar panels that conform to the wing’s curve. A central truss makes up the fuselage, and the wings are formed from a complex latticework of composites. According to Piccard, this new construction is much lighter than the first Solar Impulse with the composite fabric cover weighing a mere 25 g/sq m. The entire plane was also designed for a high degree of reliability to allow it to remain aloft for 12 hours without maintenance, yet keeping the weight trimmed to a minimum.

The solo pilot for the round-the-world attempt will sit in the 3.8 sq-m (40.9 sq-ft) cockpit, which was designed using computer-aided ergonomic simulations and is described as a "business class seat" for the circumnavigator, complete with lumbar massage and the ability to convert into a bunk so the pilot can catch a nap during the five-day ocean crossings. Everything from using an oxygen supply to eating, and even sleeping were tested on the ground using a flight simulator while the pilot’s vital signs were monitored.

To save weight, the cockpit isn't pressurized, nor is it heated. Instead, the pilot relies on an oxygen supply stowed in the cabin area for high altitude flight and both the pilot and batteries are protected against the subzero temperatures by a new insulating foam developed by Bayer.

However, the cockpit is hardly a return to the days of Alcock and Brown. There’s an autopilot that the designers refer to as a "virtual co-pilot" that can alert the pilot in an emergency via a wrist-mounted alert buzzer, and the flight will be followed by a ground-based mission control that measures 50,000 parameters to modify the flight plan while under way. Communications with control is by means of a satellite communications system by Swisscom, which weighs less than 5 kg (11 lb), and is connected to what the team says is the lightest satellite antenna in the world – it's so light that a Swisscom logo serves as the counterweight.

The Solar Impulse 2 isn't fast, with a top speed of 45 mph (72 km/h), but Piccard says that its endurance is that if its pilot. It operates at 28,000 ft (8,500 m) during the day as it stores solar energy, then descends during the night, using the drop in altitude to maintain airspeed without drawing on the batteries, until it reaches a second cruising altitude between 6,000 and 9,000 ft (1,800 and 2,700 m) and goes back on battery power.

Despite all this, the plane is very heavy to control in turbulence. Taking off and landing are scheduled for early morning and after dark to avoid turbulence and heavy air traffic. This restricts takeoff and landings to very calm conditions, and landing and takeoff need to be carefully planned in advance because the plane has a minimal undercarriage. This means that to keep the wings stable on takeoff and landing, the ground crew use electric bicycles to hold the wings up on takeoff and catch them again on landing like a very low budget version of Thunderbirds. They grab on to posts suspended from the plane, which are also equipped with small wheels, so the Solar Impulse 2 can make emergency landings.

Piccard, compares this to conditions of the earliest heavier-than-air aviators and calls the Solar Impulse 2 the start of a new cycle of aviation.

As to the future, ground tests are under way, which will be followed by flight tests in Switzerland in May, a public air show debut later this year, and the round-the-world attempt in March 2015. If all goes according to schedule, the Solar Impulse 2 will fly over the Arabian Sea, India, Burma, China, the Pacific Ocean, the United States, the Atlantic Ocean and Southern Europe or North Africa before returning to its starting point in the United Arab Emirates. Along the way, the plane will land periodically to change pilots and participate in public events.

'A vision counts for nothing unless it is backed up by action. With eight world records for Solar Impulse 1, the first solar aircraft capable of flying during the night, crossing two continents and flying over the United States, we have shown that clean technologies and renewable energies can accomplish the impossible,” says Piccard.

The video below shows Solar Impuse 2's début.

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