By Steve Gorman
(Reuters) - An experimental airplane powered solely by energy from the sun stored in batteries took off from Arizona early on Thursday on the eleventh leg of an historic bid by its pilots and developers to fly around the globe without a drop of fuel.
The single-seat aircraft dubbed Solar Impulse 2 departed Phoenix Goodyear Airport before dawn on an 18-hour flight scheduled to arrive at Tulsa International Airport in Oklahoma, 1,000 miles (1,609 km) to the east, at 11 p.m. local time (0400GMT).
The long hours required for covering relatively short distances is a condition of how slowly the plane flies compared with conventional aircraft.
With a wingspan exceeding that of a Boeing 747 but an ultra-light carbon-fiber skin and overall weight of a car, the Solar Impulse cruises at speeds ranging from just 34 to 62 miles per hour (55 to 100 kmh).
The four engines of the propeller-driven aircraft are powered exclusively by energy collected from more than 17,000 solar cells built into its wings. Excess energy is stored in four batteries during daylight hours to keep the plane flying after dark.
The plane can climb to 28,000 feet (8,500 meters) but generally flies at lower altitudes at night to conserve energy.
Swiss aviators Bertrand Piccard and Andre Borschberg have been taking turns piloting the plane on each leg of the journey, with Piccard at the controls for Thursday's flight. Both men have trained to stay alert for long stretches of time by practicing meditation and hypnosis.
Borschberg set a new endurance record for the longest non-stop solo flight last July during a 118-hour trans-Pacific crossing, over five days and five nights, from Japan to Hawaii. He also set new duration and distance records for solar-powered flight.
But battery damage sustained during the crossing kept the aircraft grounded for nine months.
The Swiss team's ultimate goal is to achieve the first round-the-world solar-powered flight, part of its campaign to bolster support for clean-energy technologies.
The team hopes to eventually complete its circumnavigation in Abu Dhabi, the starting point for the journey in March 2015.
The two men completed an earlier multi-flight crossing of the United States in a prototype of the solar plane in 2013 as a precursor to their globe-circling quest.
(By Steve Gorman)