Gallery: Solar Airplane Almost Ready for Its First Transcontinental Flight
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The Solar Impulse crew is assembling its massive solar airplane in a hangar south of San Francisco, where it will conduct several test flights before embarking on a transcontinental flight. The massive hangar originally housed dirigibles and remains one of the largest wooden structures in the world.
Fred Merz / Rezo.ch022013-03-04-AcrossAmerica-reassembling-in-Moffett-Merz-035
A technician safety wires a connector atop a wing. The plane, known by its utilitarian name HB-SIA, has a wingspan of 208 feet.
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The Solar Impulse team spent much of January and February disassembling the aircraft and packing it for the journey from Switzerland aboard a Boeing 747. But they only needed a few weeks reassembling the prototype airplane at Moffett Field south of San Francisco.
Image: Mozilla042013-03-06-AcrossAmerica-reassembling-in-Moffett-Merz-066
HB-SIA features four brushless electric motors, each good for 10 horsepower. Flying under solar power alone, the engines provide an average of 8 horsepower, enough to keep the propellers spinning at 400 rpm and the airplane cruising at a leisurely 43 mph. The Wright brothers made their first flight in 1903 using a 12 horsepower motor of their own design.
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The airplane has a wingspan of 208 feet, but the wing has a high-aspect ratio, meaning the chord — the length from the front to back — is shorter than a typical wing of that span. In other words, the wing is very similar to an efficient glider wing, allowing it to stay aloft with minimal thrust.
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As impressive as the Solar Impulse effort is, it won't be the first transcontinental solar flight. That honor belongs to Eric Raymond, who in 1990 flew [Sunseeker across the country](http://solar-flight.com/sunseeker/index.html) in 21 flights totaling 121 hours in the air. Raymond, a pioneer and expert in solar flight, helped build the photovoltaic panels that power HB-SIA. HB-SIA is expected to be the first solar powered airplane that will be able to complete the trip flying both day and night.
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With the plane once again reassembled, Solar Impulse will use Moffett Field as home base for a series of test flights to ensure everything is in order. Keeping an eye on the region's famously foggy weather, the team spend April shaking the plane down as the team prepares for its longest flight yet.
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The airplane has 11,628 solar cells atop its wings and horizontal stabilizer. They cover about 2,150 square feet, providing enough flowing electrons to keep the motors turning during the day while charging the lithium-ion battery pack that keeps the plane going at night. The pack weighs 880 pounds, roughly one-quarter the total weight of the aircraft.
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HB-SIA uses traditional control surfaces such as ailerons, though it features four ailerons, on each wing. Here a technician adjusts one of the spoilers on the tip of a wing. The spoiler's leading edge dips into the airflow, helping counter some of the "adverse yaw" that pushes the nose in the opposite direction of a turn when the eight ailerons are deflected to bank the airplane.
Image: Mozilla102013-03-08-AcrossAmerica-reassembling-in-Moffett-Merz-086
The cockpit is unpressurized and relatively small. André Borschberg has flown the airplane for more than 26 hours straight, but going any longer than that isn't terribly practical, in part because the cockpit is so cramped. The plane's successor, HB-SIB, will feature a pressurized cabin with more room to stretch out and sleep during the around-the-world flight planed for 2015.
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Technicians make adjustments to the wing as a MC-130 from the California Air National Guard departs Moffett Field.
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A technician attaches the vertical stabilizer and rudder assembly to the airplane.
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Each wing has a carbon-fiber spar and 120 ribs that give the wing its shape. Here, technicians attach a wingtip. This photo provides another view of the spoiler at the end of the wing, and part of one of the ailerons.
Image: Mozilla142013-03-08-AcrossAmerica-reassembling-in-Moffett-Merz-093
HB-SIA, assembled and ready for flight testing. Photo: Solar Impulse/ Fred Merz
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The crew, with the plane, ready to fly. Photo: Solar Impulse/ Fred Merz
