Solar flight

Last month, San Dimas, CA-based AC Propulsion successfully flew an unmanned aircraft for more than two days using only solar energy.

Dubbed the "SoLong," the craft stores solar energy in a lithium-ion battery pack during the day to keep it flying at night. The company incorporated the lightweight batteries into an energy-efficient craft made of composite materials weighing only 28 pounds with a wingspan of slightly more than 15 feet.

Along its wing are 76 Sunpower solar cells that could produce 225W of power, while the craft required only 95W for level flight.

According to the company, a critical factor in the SoLong's success was its high-efficiency electric motor, driven by the company's patented power controller.

The power system includes a high efficiency electric motor driven by a patented split-phase power controller developed by AC Propulsion. The controller gives high power for takeoff and maintains high efficiency even at the low power levels used in steady flight.