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.

A variable pitch propeller allows tuning for maximum propulsion efficiency under varying flight conditions.

Solar cells that convert solar energy to electricity with an efficiency of 20% are controlled by proprietary peak power tracking software that makes best use of the photovoltaic energy. Much of that energy goes to charge the Li-ion battery pack that powers the motor and a separate pack for the controls and communications.

With a charge-discharge cycle efficiency over 95%, the Li-ion batteries do not squander the power from the solar cells, and at 220 Wh/kg, the Sanyo cells pack a lot of energy without much weight. Still, the battery makes up 44% of the aircraft’s total weight because the air frame uses efficient structure and composite materials so it is light but still strong enough to withstand 30 mph winds.

The 6 servos that move the control surfaces use special electronics that were developed by AC Propulsion to reduce power consumption.

The radio-controlled craft also sports 23 channels of telemetry, navigation data from a global positioning system, and even a live video downlink.

AC Propulsion claims that the SoLong could have remained flying indefinitely.