Solar Impulse Crosses America | Flight Today | Air & Space Magazine
Solar Impulse over San Francisco. What it lacks in speed, it gains in energy-efficiency. (Solar Impulse/ J. Revillard)

Solar Impulse Crosses America

Who needs fuel when the sun can keep you aloft?

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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.

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