"The Sun Also Raises," Input/Output, ME Departments, October 2007

input/output

The Sun Also Raises

by Michael Abrams

The history of human flight with wings is the story of man separating himself from the surface of the Earth by deflecting air beneath him. Rarely has the flight-minded engineer turned to what's on the other side to power an aircraft.

The late Paul MacCready and his company, AeroVironment Inc., developed a variety of solar-powered aircraft. Perhaps his most famous, Solar Challenger, flew across the English Channel in 1981. MacCready, a former ASME member and a winner of the Society's Ralph Coats Roe Medal, died on August 28.

Now a team based in Switzerland wants to fly a solar-powered plane of its own even farther. The group is hoping to put the sun's energy to use to fly a plane around the world.

Sun and wing: Because of the limitations of available battery technology, a round-the-world solar-powered flight will not be nonstop.

Pilot Bertrand Piccard conceived the project during his nonstop balloon flight around the world in Breitling Orbiter 3 in 1999. Breaking flight records seems to run in Piccard's family. He is a grandson of Auguste Piccard, who made pioneering balloon ascents into the stratosphere in the 1930s.

He has teamed up with a mechanical engineer, André Borschberg, an aircraft designer and pilot, to devise a solar airplane.

"The first thing we did, in 2003, is see what technology could work," Borschberg said. "And we came to the conclusion that, with what we could expect from new technology in the next five years, flying around the world was feasible, but with stopovers. Nonstop would be possible, but in the future."

An airplane capable of flying around the globe powered by the light of the sun alone must also be able to stay aloft at night. Keeping up with the sun would be impossible, so the plane will need batteries to store some of the electricity generated by solar cells. Weight, then, is the primary challenge in Borschberg's aircraft, Solar Impulse.

Battery capacity per pound is on the increase and may be high enough in the next few years to work with Borschberg's design. Solar cells, on the other hand, although they've certainly improved in recent years, are not yet light enough. Borschberg has been working to get them right, and sacrificing efficiency to do so.

"We need thin, light, UV-resistant cells, offering a good barrier against humidity, with appropriate mechanical strength, and transparent materials," Borschberg said. Thin to him means 250 micrometers, half the thickness of current solar cells. Thinness means reducing power output by as much as a third.

To load the wings with plenty of solar cells, and to achieve a high lift-to-drag ratio and consume as little energy as possible, a Solar Impulse test prototype has a wingspan of 61 feet; the round-the-world version, 80. "Sixty-one feet is what we need to fly through the day, recharge the batteries, fly through the whole night, and fly the rest of the day," Borschberg said.

With a wingspan like that, the Solar Impulse will fly more or less like a giant hang glider. The pilots want as little turbulence as possible, and avoiding it is half the challenge. A team of aircraft controllers, meteorologists, engineers, and other pilots will help guide the plane.

Such a team gathered in May of this year for a "virtual flight" and simulated a journey from Hawaii to Florida. Winds over Mexico and storms in the Gulf diverted the flight to Phoenix, Ariz. A nonstop flight of 6,120 km took three days, seven hours, and 26 minutes.

Borschberg has called in a professional design team to review his plans for the plane. Since it is fully electric and meant for a long high-altitude journey, electromagnetic interference issues that wouldn't cross the mind of a small-plane designer become crucial. Another difficulty is that even when wings are rigid, there's still flexibility between them and the body of the aircraft.

"You may need to consider this deformation to understand what happens aerodynamically to the airplane," Borschberg said. "And you do not always think about the changing environment—going from very high-temperature, humid environments to very low temperatures can lead to delamination, things you don't experience in other light designs."

Borschberg sees the flight as largely symbolic. "It's to show that if you want to keep living the way we have and keep the environment, we have to reduce energy consumption by increasing efficiency," he said. "And we can even fly around the world with solar energy."