After several weather-related flight delays, Solar Impulse made a successful demonstration flight June 26, the last day of the Paris Air Show. The sun-powered four-engine aircraft is intended to shine a light on green technologies. It's backed by Bertrand Piccard, the Swiss adventurer best known for circumnavigating the world in the Breitling Orbiter 3, a propane balloon, in 1999.
The aircraft has the "wingspan of an A340, with the weight of a VW Golf." Pilot André Borschberg flew the plane to Le Bourget airport from Brussels and the intention was to fly for airshow audiences much earlier, but several windy days kept Solar Impulse grounded. Sunday morning provided the weather needed to show attendees this year's guest of honor.
What does it take to get Solar Impuse into the air? AVweb spoke with Flight Test Engineer Christoph Schlettig:
The aircraft, based in Payerne, Switzerland, has four custom-made lithium polymer batteries, each weighing around one hundred kilograms, or 220 pounds, located on linen-covered, carbon-fiber wings. The wings themselves house solar cells that charge throughout daylight hours in order to give the plane "seven to eight hours" of battery-powered flight at night. All told, the cells cover approximately 220 square meters, or 2,300 square feet, on top of both the wings and horizontal stabilizer of the aircraft.
As Schlettig explained, the challenge with extended battery-powered flight lies in the storage capacities of the batteries. The solar cells, on the other hand, are "perfect."
The plane stores both solar and potential energy. During flight, Solar Impulse climbs to cruising altitude, and at sundown begins a slow, gliding descent in order to provide added power to the engines, which are the basic equivalent of "four electric screwdrivers."
During level flight, Solar Impulse has a cruising speed of a modest 23 knots, with each engine providing 10 kilowatts of power. Referring to the persistent delays in planning the demonstration flight, Schlettig said that the plane needs "perfect weather" in order to fly. This means winds of less than five knots and sunny skies. More wind, and the super-light aircraft would be buffeted too much to fly safely.
Schlettig said the goal of Solar Impulse is not to gain commercial interest in solar-powered aircraft, but instead to convey the message that "even today we have the means and technology to fly around the world with solar energy." Schlettig hopes that this will encourage people to turn towards greener technologies.
Solar Impulse differs significantly from previous solar-powered aircraft, such as NASA's unmanned Helios aircraft. Schlettig said a manned aircraft presents myriad difficulties concerning weight distribution and overall design considerations. "In order to have a pilot, you need a fuselage." Instead of the single-wing design, Solar Impulse must incorporate all the requisites of manned flight, including controls and instruments, but also high-altitude survival gear. Helios is "a hell of a lot easier than what we did," Schlettig said.