The lofty ambitions of the Solar Impulse project are approaching reality with a fully assembled prototype set to be unveiled to the public for the first time on June 26th. Entirely powered by the sun, the aircraft designated HB-SIA has a giant wingspan of 61 meters and is covered in almost 12,000 solar cells.

Founded by Betrand Piccard, who was also involved in the first non-stop circumnavigation of the earth in a balloon back in 1999, the Solar Impulse project has encountered a series of headache-inducing technical challenges to overcome in its quest to fly entirely on solar power for 36 hours non-stop. The question of energy, for example, determines everything from the size of the aircraft to the amount of weight that can be carried.

Even with 200 square meters of photovoltaic cells, the plane’s four motors each achieve no more than 8hp (6kW), which is roughly what the Wright Brothers had available to them in 1903. As a result, the Solar Impulse can only average speeds of around 43.5mph (70kmh). And the only way a plane that slow will stay in the air is to have an enormous wingspan, comparable to that of an Airbus A340.

Weight is another critical issue. The four lithium batteries used to power the engines each weighs 220lbs (100k). Yet, by using carbon fiber construction, the entire weight of the plane has been kept to only around 3,300lbs (1500kgs) or roughly that of a car.

It’s expected that the Solar Impulse’s first test flight will take place towards the end of this year. The solar cells will be used to charge the batteries on the day before the flight so that the aircraft can takeoff without using any external power sources.

After a further series of tests the project team hopes that, in 2010, the aircraft will be able to achieve a maximum cruising height of 28000ft (8500m) and fly continuously, day and night, for 36 hours.

Source: Solar Impulse.

View gallery - 4 images