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Giant solar-powered aircraft to begin cross-country flight

Flyweight jumbo to soar from Silicon Valley to New York City at 70km/h, tops

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The photovoltaics-powered Solar Impulse HB-SIA aircraft has arrived at the NASA Ames Research Center in Silicon Valley's Moffett Field to prepare for a flight across the US.

After it has been offloaded in pieces from its Boeing 747-100 transporter, the Swiss-made HB-SIA will be reassembled in a Moffett Field hanger, then embark on a series of test flights around the San Francisco Bay Area before its cross-country jaunt.

That flight is scheduled to begin on May 1 – weather permitting – and to include stops in Phoenix and Dallas, plus Atlanta, Nashville, or St. Louis, depending upon weather conditions. The delicate craft will then fly on to Washington DC and finally settle down at New York City's Kennedy Airport in early July.

As to when each of those seven- to 10-day stopovers will occur, it's impossible to say. "The exact dates are of course undefined as every leg is dependent on local meteorological conditions," the Solar Impulse team says on its website.

"For those of you who don't live in the United States or near the cities on our itinerary, don't despair," they add, "because as usual, all the legs – from San Francisco to New York City – will be filmed and streamed LIVE on our website with the hottest news posted on Twitter or Facebook."

The Solar Impulse HB-SIA flies over Switzerland during an April 2011 test flight

The Solar Impulse HB-SIA flies over Switzerland during an April 2011 test flight (click to enlarge)

Weather conditions are critical to the flight's success in large part because of the HB-SIA's fragility. Despite its enormous size, it weighs a mere 1,600 kilograms (~3,500 pounds), just a bit more than another all-electric vehicle, the Nissan Leaf.

And when we say "enormous size", we're referring to its wingspan of 63.4 meters (208 feet). A Boeing 747-100, in comparison, has a wingspan of 59.6 meters (196 feet) and a 747-400 a wingspan of 64.5 meters (211.4 feet). The smaller of those two jumbos, by the way, has a maximum takeoff weight of 333,400 kilograms (735,000 pounds) – that'd be equal to over 200 HB-SIAs.

Another rather notable contrast between those two jumbo jets and the HB-SIA is their carrying capacity. When set up for two levels of service, the 747-100 can carry 496 passengers and the 747-400, 524. The hopefully transcontinental Solar Impulse aircraft carries but one: pilot and Solar Impulse cofounder André Borschberg.

That solo operation, in fact, is one of the main reasons that the longest flight that the HB-SIA has achieved lasted 26 hours, 10 minutes, and 19 seconds. "What is most incredible is that this revolutionary aircraft could practically fly perpetually into infinity if it weren't for the human side of the pilots," the team writes. "So how do we make humankind perpetual? Well, I think that's another story."

The ability to fly "perpetually into infinity" (and beyond...?) is due to both the HB-SIA's propulsion system and its style of flight. Its electrical power comes from 11,628 exceptionally thin (135-145 microns) SunPower Maxeon solar cells, 10,748 on top of the wing and 880 on the rear horizontal stabilizer. These cells feed power into 400 kilograms (~880 pounds) of lithium polymer batteries – about one-fourth of the HB-SIA's total weight.

Battery performance being a function of temperature, those batteries are heavily insulated to allow them to function at the -40°C (-40F°) temperatures at the craft's maximum cruising altitude of 8,500 meters (27,900 feet).

Before takeoff, the batteries are charged by the solar cells to a minimum of 50 per cent capacity "for pilot safety." The HB-SIA then fires up its four 10-horsepower brushless and sensorless electric motors, which each spin a twin-bladed, 3.5-meter (11.5-foot) propeller up to 400rpm for takeoff – which occurs when the HB-SIA reaches a leisurely 44km/h (27mph).

Another 747-400 comparison: fully loaded, that jumbo lifts off at around 300km/h (186mph).

Once airborne – in daylight – the HB-SIA cruises at an average speed of 70km/h (43mph) as the pilot gently guides it upward to around 8000 meters (about 26,250 feet) above clouds, into thinner air, and where its solar cells will do their electricity-producing and battery-recharging business with more efficiency.

When the sun goes down, the pilot throttles back the motors – well, there are no throttles per se, but you get the idea – and starts a slow descent down to a nighttime altitude of around 1,000 to 1,500 meters (3,280 to 4,920 feet). And when we say slow, we mean slow: the descent is at about 0.4 meters per second, meaning that the craft can glide for around four to five hours using almost no battery power.

When it reaches that low altitude – what the Solar Impulse team calls its "loitering point" – the engines are switched back on using battery power, and the craft creeps along at a sedate 46km/h (29mph) until the sun has risen far enough to get the solar cells sufficiently awake to begin the up-down cycle again.

At these blinding speeds, you can better imagine why the HB-SIA's cross-country flight, if it begins on schedule, won't be completed until early July.

As mentioned above, those up-down cycles could continue indefinitely – and that's one of the design goals of Solar Impulse's next craft, the not terribly imaginatively named HB-SIB.

That solar soarer is being designed to achieve Solar Impulse's long-term goal: a flight around the world powered by sunlight alone. The HB-SIB is still under development, with test flights scheduled for 2014 and an attempt at a global circumnavigation hoped for between April and July 2015, with individual legs lasting between four and six days each.

The HB-SIB will have more-efficient solar cells and batteries, its electronics will be rainproofed, and its construction will include, Solar Impulse promises, "carbon fibers that are lighter in weight than any previously seen."

Also importantly, that tired pilot will be looked after as well: the cockpit will include a reclining seat that will presumably give the poor devil a chance to grab some shut-eye while the HB-SIB takes over cruising duties.

But before the HB-SIB can make it around the world, the HB-SIA will have to make it across the US – and we here in San Francisco's Vulture Annex will be scanning the skies to see if we can spot it during its final shakedown cruises. ®

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