Solar aircraft have crossed the continent, soared almost three times higher than jet airliners and even flown through the night. The last great test is circumnavigating the globe, and a Swiss team is building a plane it says will accomplish that feat in five years.
Solar Impulse plans to begin flight testing its eponymous aircraft next fall and hopes to make a transatlantic flight in 2011, followed by the first flight around the world in a solar plane.
It's an ambitious timeline, but then, the Solar Impulse is an ambitious airplane. Find out why after the jump...
Never before has a manned aircraft flown around the clock, let alone around the world, powered only by the sun. That's an amazingly difficult feat because the photovoltaic cells must power the craft during the day while simultaneously supplying batteries that will keep the props spinning through the night.
Solar cells also don't produce a whole lot of power -- about 28 watts per square meter, or enough to illuminate an electric bulb. So there's only one way to make an airplane fly long distances with so little power -- make it light, and make it big.
"Anything that doesn't break is potentially too heavy," said project CEO Andre Borshberg.
The Solar Impulse weighs about 3,300 pounds and has a wingspan of 201 feet -- just 7 feet shorter than that of the Airbus A340 airliner. It has a carbon fiber skin that is just a few tenths of a millimeter thick in places, and about 2,690 square feet of solar panels on its wings and tail.
The prototype currently under construction in Lausanne, Switzerland is a bare-bones model - only the most basic instrumentation and an unpressurized cockpit - meant to find the optimal balance of energy consumption, performance and maneuverability. Making something so big stay aloft with so little power, and having it responsive enough to fly safely, is a huge challenge for the 50 engineers and designers who have spent four years designing the prototype.
According to Flight Global, the Solar Impulse will carry 880 pounds of lithium batteries. It will be powered by four electric motors producing a maximum of 10 horsepower apiece. Borschberg said that should be sufficient to cruise along at 28 mph.
Solar Impulse hopes to have the craft airborne buy by this time next year and says the first flight, which it expects to be just a few yards above the ground, will be a "moment of truth."
The earliest flights will be made under battery power without solar cells, but later tests in Bienne, Switzerland will put that technology to the test. Solar Impulse It hopes to make several 36-hour flights in 2009 to demonstrate the ability to fly a complete day-night-day cycle powered only by the sun.
To do that, the pilot -- the craft carries one person -- will take off with charged batteries, cruise at 28,000 feet during the day under solar power and descend to about 3,800 feet to fly through the night using battery power. The goal is to have enough power at dawn to climb back to cruising altitude and recharge the batteries, Borschberg told Flight Global.
Solar aviation traces its roots to the early 1970s, when hobbyists and engineers began using solar cells to power model aircraft. The first manned flight came in 1980, when the Gossamer Penguin flew near Bakersfield, California. The Solar Challenger crossed the English Channel one year later and made several long-distance flights throughout Europe.
American Eric Raymond flew Sunseeker across the United States, hopping across the country in 21 stages during two months in 1990. His longest lap was 250 miles.
The most famous solar aircraft might be Helios, the unmanned remote control aircraft with a wingspan of 247 feet. It made several flights between 1999 and 2003, when it crashed, and reached an altitude of 98,863 feet on Aug. 13, 2001.
The first solar night flight occurred two years ago when Alan Cocconi, founder of AC Propulsion - maker of the T-Zero electric sports car - flew an unmanned craft with a wingspan of 15 feet for 48 hours nonstop.
No one at Solar Impulse holds any illusion that solar planes are going to replace jetliners, or even Cessnas. But that's not the point of solar flight. By simply reaching for the sky in so ambitious a manner, Solar Impulse says it is helping advance new technology in aerospace, solar energy and other fields. But more than that, it says, the plane is an "ambassador of renewable energies" that can show people what is possible.
"The public is excited about great adventures and relating to the dreams of pioneers and explorers," it says on its website. "Solar Impulse wants to mobilise this enthusiasm in favour of the technologies, which will generate sustainable development and positive emotions about renewable energies."
