Solar Impulse Crosses America

Who needs fuel when the sun can keep you aloft?

Solar Impulse over San Francisco. What it lacks in speed, it gains in energy-efficiency. Solar Impulse/ J. Revillard

On May 3, Solar Impulse, a single-seat, solar-powered airplane capable of flying day and night, began a cross-country journey across the United States. The one-of-a-kind aircraft is being flown by Bertrand Piccard, who made the world’s first nonstop, round-the-world balloon flight in 1999, and André Borschberg, an entrepreneur and former fighter pilot in the Swiss air force.

Proving that unfueled, clean flight is possible, Piccard flew the mission’s first leg from San Francisco to Phoenix. Borschberg picked up the second flight from Phoenix to Dallas, and Piccard completed the third and fourth legs, from Dallas to St. Louis, and from St. Louis to Washington, D.C., where Solar Impulse arrived on June 16 (it will be on display at Dulles International Airport on Saturday, June 22). In early July Borschberg will complete the final leg, from Washington to New York City. Air & Space associate editor Diane Tedeschi interviewed the busy pilots by email.

Air & Space: Solar Impulse looks delicate. What are your takeoff and landing speeds, and how does the aircraft handle during these parts of the flight? Does the 208-foot wingspan generate so much lift that landing is difficult?

Borschberg: It’s delicate to handle on the ground, but it can sustain loads in flight like any transport airplane. Takeoff and landing speed is about 30 miles per hour. You are right: In some ways, the very low wing loading makes the airplane very sensitive to turbulence and makes it difficult to land in gusty weather.

Are the wings designed to flex?

Borschberg: The airplane is designed to sustain turbulence, but in such conditions the aircraft is difficult to handle. The wing structure is designed to be very stiff, and bending at the wing tip is small (1.5 feet difference between the aircraft on the ground and flying). We plan flights in such a way that we keep the aircraft out of heavy turbulence.

What was the average cruising speed and altitude during the flight from San Francisco to Phoenix?

Piccard: I flew at an average speed of 40.6 mph and a cruising altitude of 20,000 feet. The goal is not to go fast, but to fly almost forever as the sun gives enough energy during the day to run the four electric motors and load the batteries, which will allow the plane to fly through the night.

What was the view like during the San Francisco-to-Phoenix leg? What natural land formations were visible from the airplane?

Piccard: The most memorable vision was Edwards Air Force Base [in California], where so many great firsts of aviation happened. While overflying it, I was thinking about all the pioneers who wrote the history of aviation in the 20th century. They inspired me to also try to achieve the impossible. And today Solar Impulse is starting a new cycle by flying day and night without fuel. I hope we will inspire the next generation to use clean technologies.

Did you ever sleep during any of your flights? If so, is there a place to rest inside the airplane?

Piccard: There’s no way to sleep, as we have no autopilot. But when you have the privilege of flying the most incredible airplane in the world, one that flies day and night with no fuel, the last of your wishes is to waste your time by sleeping. It’s also a big responsibility to carry the hope of so many thousands of people who have signed up on our website to support the introduction of clean technologies in our world.

Is pilot fatigue an issue during these long flights?

Borschberg: To fly the aircraft is so beautiful and special that you don’t feel the fatigue and want to take advantage of this fabulous moment. Our passion for the project and for flying certainly play an important role. Interestingly, we learn to focus and concentrate on the present moment instead of thinking how many hours remain until landing. We end up by creating our small world in the cockpit in which we feel very comfortable.

What food and beverages have you consumed in the air?

Piccard: Since I had to concentrate on the flying and not on eating, I had some energy food and beverages.

How long did it take you to learn to fly the single-seat Solar Impulse? Is there any aircraft type that you would compare it to in terms of flight characteristics?

Borschberg: I am a professional pilot who has flown many different aircraft since I was 17 years old, but I tell you, Solar Impulse is a special airplane. Its lightness and slowness make it very sensible to the slightest turbulence. It behaves like a large microlight, except that the disproportionate scale creates a lot of inertia. The reaction time for controls is therefore very long, with the risk of over-correcting and inducing oscillations. A former NASA chief pilot tried our simulator; he could not land the airplane. He crashed it on every landing and was furious.

Can Solar Impulse fly during cloudy weather? Or does the charging of the aircraft’s solar cells require full sun?

Borschberg: Dense clouds above the airplane will certainly reduce the amount of energy we can collect. This would not affect the day flight but could limit the possibility of flying through the following night. That’s the reason why we have a team of weather specialists on the ground, planning each flight very thoroughly in order to ensure the airplane is always flying in sunny weather, especially in the morning.

Solar Impulse has demonstrated that it is possible to fly cleanly for long distances, but how practical is solar-powered flight for general aviation?

Piccard: Today the technology does not allow a solar airplane to transport many passengers, but don’t forget that was also the case when the Wright brothers and Charles Lindbergh were flying. Anyway, our goal is not to make a revolution in air transport. Our goal is to make a revolution in the way people think about clean technologies, renewable energy, and energy conservation. We need to show how the new technology can create jobs, make a profit, sustain growth, and at the same time protect the environment. This is the goal of Solar Impulse.

Would you like to see Solar Impulse end up someday at the National Air and Space Museum?

Piccard: It was one of the most beautiful days of my life when the Breitling Orbiter 3 capsule from my nonstop round-the-world balloon flight was brought to the Smithsonian Air and Space Museum. I think for me the only way to experience a greater emotion would be to see Solar Impulse also in this museum, where I was dreaming about exploration when I was 12 visiting it with my father.

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