Solar plane on trans-US flight: It's about 'pioneering spirit,' pilot says

Solar Impulse, the first human-piloted solar aircraft able to operate day or night, was flying from Dallas to St. Louis Monday, aiming to make the trip in about 21 hours.

Somewhere in the skies between Dallas and St. Louis, a spindly aircraft fueled by photons from the sun was winging its way Monday eastward on a transcontinental flight and into aeronautical history.

The plane, Solar Impulse, has the wingspan of a Boeing 747, the weight of a Chevy Volt, 10-horsepower electric motors to spin its four props, and nearly 12,000 solar cells providing the juice.

Although Solar Impulse is not the first solar-powered aircraft to carry a human, it's the most ambitious. With batteries that store power for night flight, the craft is the first human-piloted solar aircraft able to operate day or night.

The team's ultimate aim is a 2015 flight around the world.

Indeed, this mission is giving ground crews, pilots, the team's weather forecasters, and mission control in Switzerland experience in dealing with complex flight conditions over complex terrain at continental scales – the kind of conditions they will face as they try to circumnavigate the globe around the Northern Hemisphere, says Andre Borschberg, one of the mission's two pilots and chief executive officer of Solar Impulse.

"It's training for everyone," he says.

But the project's overall goal is not to find new ways to transport people.

"The goal is really to transport a message about [the] pioneering spirit, about innovation, about the clean technologies our world so badly needs today," said Bertrand Piccard, pilot of Monday's leg of the trip, in an in-flight interview with the St. Louis Public Radio Monday morning.

It's the third of five legs in a cross-country odyssey that began May 3 at NASA's Moffett Field in Mountain View, Calif., at the southern tip of San Francisco Bay. The trip is scheduled to make an additional stop at Dulles International Airport outside of Washington in mid-June before its final leg takes it to New York's JFK Airport in early July.

Solar Impulse is not for the impatient. It's the kind of aircraft that would earn you frequent-flier hours, rather than frequent-flier miles.

A non-stop commercial airliner typically makes the Dallas-to-St. Louis run in just under 2 hours. Solar Impulse's cruising speed is about 37 knots, or about 43 miles an hour. If all goes well, the craft will have reached St. Louis in about 21 hours, landing sometime after midnight Central Daylight Time at Lambert Field.

In 1999, Mr. Piccard and co-adventurer Brian Jones became the first people to travel around the world in a balloon – a hybrid hot-air and gas balloon dubbed the Breitling Orbiter 3.

Asked Monday about what it was like to pilot Solar Impulse, Mr. Piccard told St. Louis Public Radio, "Absolutely fabulous."

Mr. Borschberg elaborates. During the day, especially at high altitudes, fuel is not a problem. The solar cells are getting more sunlight, and the frigid outside temperatures keep them cool – a combination that increases the efficiency with which they convert sunlight to electricity for powering the plane and keeping its batteries topped off for flying after dark.

But Solar Impulse also presents unique challenges.

Night flying is "a bit more suspenseful," Borschberg adds, since one needs to ensure the batteries don't run low before sun-up.

The plane lacks the power to buck headwinds, which explains the west-to-east trip. For much of the first half of Monday's flight, Piccard had to keep the plane at around 5,000 feet because high-altitude winds at the time were blowing out of the northwest – the wrong direction if you're bound for the Gateway Arch.

Later in the day, he booked it – to the extent 25 knots is booking it – for 27,000 feet to ride the same winds after he reached a point along his flight path where they'd looped back in a more favorable direction.

The craft's light weight also has its pluses and minuses.

Solar Impulse can take off at very slow speeds, thanks to its ultra-light-weight construction and its large wing area. And it needs only a few hundred feet of runway to do so, although the enormous wingspan limits it to major airports.

But the light weight doesn't serve it well if the air gets turbulent. The craft takes off early in the morning and lands late in the evening to reduce the likelihood it will encounter turbulence during these crucial phases of flight. The team's meteorologists also go to great lengths to help Solar Impulse's pilots avoid thunderstorms, turbulence around mountains, and even intense thermal updrafts that can build up over deserts.

Indeed, foul weather has forced the team to test ahead of schedule an inflatable hangar engineers had designed for the global flight in 2015. Severe storms that raced across the St. Louis area last Friday heavily damaged a temporary, tent-like hangar officials at Lambert Field had set up for Solar Impulse.

"There will be a lot of suspense this afternoon and this evening" as the plane's ground crew tries to inflate the hangar for the first time, Piccard told St. Louis Public Radio.

And for all it's apparent grace in the skies, the aircraft can handle like a supertanker, adds Borschberg, who as a former military pilot has racked up flight time in a wide range of aircraft. Solar Impulse's slow response to a pilot's touch on the controls may not be much of a problem at 15,000 feet. But it can add some excitement to landings.

"It took us quite a long time to understand how to fly this airplane," he says.

The project is the fruit of 10 years of design, testing, then gradually exercising the aircraft over ever longer distances. The first version of Solar Impulse, version A, went through its initial test flights and certification for night flight in 2010 – setting several world records in the process.

Its first international flight took place in 2011, when it flew from Switzerland to Brussels and on to Paris. A year later, the team got more ambitious, making the craft's first transcontinental, round trip in seven segments from Payerne, Switzerland, to Rabat, Morocco, and back – setting additional world records for a solar craft.

With its Solar Impulse version B, the team is now embarked on its first major transcontinental flight.

Meanwhile, the team also is building version C for the global effort. That plane will have to be far more efficient to ensure it can make the trans-oceanic crossings along its route. Traversing the Pacific will take about four or five days and nights, Borschberg says.

The 140-million Swiss francs ($148 million US) to underwrite the 12-year project has come from private and corporate sponsors and supports a team of 80.