AIN Blog: Solar Impulse Impresses, Even When Not Flying

 - July 19, 2011, 6:22 AM

The electrically powered Solar Impulse (HB-SIA) is an amazing airplane and I was anxious to see it fly during the Paris Air Show. Unfortunately for me and probably many others who attended the biennial event, the weather did not cooperate until the last day of the show, June 26.

By then I had already returned to the States after being gone from June 14 to 24. I had spent most of my time at Le Bourget Airport in our company “chalet,” editing manuscripts and checking pages for AIN’s Paris Airshow News. I put “chalet” in quotes, because ours was a four-walled, one-story temporary building that looked nothing like the picturesque Swiss mountain cabins that the word brings to mind.

Rain, wind and turbulence are problems for the big, single-pilot airplane. This I learned from editing an article about the Solar Impulse that we published on June 23, the fourth day of the show. As Thierry Dubois reported, “André Borschberg, company CEO and pilot, said that the Solar Impulse’s flying display [at the Paris Air Show] is planned to last about 20 minutes. But he admitted that the aircraft would stay grounded in the event of rain and convective turbulence. On the runway, a seven-knot wind is acceptable, but crosswind is limited to four knots.”

The aircraft, which does not have an autopilot, is so unstable that it can’t be flown easily in anything above a calm breeze. “When the plane is flying in calm conditions,” Borschberg said, “it’s a real pleasure to pilot it. She’s docile and gentle. But when she gets into turbulence, she’s something of a shrew, and much harder to control.” He made these comments after flying a two-hour sortie earlier this year in Switzerland, with the main objective of flying HB-SIA in turbulence.

The Solar Impulse, of course, gets its energy from the sun, via 11,628 solar cells on its wings and horizontal stabilizer. Its four 10-hp electric motors run off batteries, which are charged by the solar cells. The batteries (weighing some 900 pounds) can store enough energy to keep the airplane aloft overnight, if the pilot climbs it high enough in daylight and then maintains a slow, unpowered glide at night. Borschberg managed to stay in the air for 26 hours and nine minutes in July last year.

Thierry’s article piqued my interest in the Solar Impulse so much so that on my last day at the airshow, I went looking for it. I found it housed in a huge, rectangular tent, which had one clear-plastic sidewall. A long line of people waited to pass slowly through one corner of the tent to see the airplane up close. I dislike standing in lines and wanted to see other aircraft, too. So I walked up to the clear-plastic wall and peaked inside.

The airplane’s right wing tip was close to the wall, high over my head. A group of people standing under the narrow fuselage looked far away. The top of the vertical stabilizer was 21 feet above the ground. I had seen photos of the airplane and had watched videos of it flying. As good as these videos are, it was still difficult for me to comprehend the size of the Solar Impulse, which weighs only 3,500 pounds, but has a surprisingly wide wingspan of 208 feet.

That’s the same wingspan as the 811,000-pound Airbus A340-600 and nine feet wider than the wingspan of the 656,000-pound Boeing 777-200ER. However, the length of the Solar Impulse is only 72 feet, while the A340-600 and 777-200ER are 247 and 209 feet long, respecitvely. No wonder the Solar Impulse is squirrely in even light turbulence.

The objective of the Solar Impulse program is to prove that an airplane can fly without fuel–fossil, bio or anything else. It has already proved this can be done, even overnight. The next step is to build and fly a larger, two-pilot, long-duration airplane (HB-SIB) and attempt a non-stop, round-the-world flight in 2014. (That will be an exceptionally long endurance flight, if HB-SIB can’t cruise a whole lot faster than HB-SIA’s 38-knot average flying speed.)

Regardless of one’s position on the causes and effects, the consequences and politics of global warming, one can’t help be impressed with the accomplishments of the Solar Impulse team to date. Even if the round-the-world flight does not succeed, the program is already a success.

And everyone with any interest at all in aviation can appreciate the large, ungainly Solar Impulse airplane, which, I think you’ll agree, appears amazingly graceful in flight, even if you, like I, can watch it fly only on video.