Solar Plane Breaks Two-Night Flight Barrier

Based in San Dimas, California, AC Propulsion is a research and development shop that specializes in high-efficiency electric propulsion. The company is also known for their T-Zero electric sports car which has shattered many preconceptions of the what electric cars can do. The SoLong is the company's latest foray into solar-electric powered UAVs (unmanned aerial vehicles).

The company incorporated lightweight Sanyo high-capacity Li Ion 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 225 watts of power, while the craft required only 95 watts for level flight.

SoLong took off at 4:08 PM, Wednesday, June 1 from the sun-baked runway at Desert Center Airport just east of Eagle Mountain in California's Colorado desert. It remained aloft until Friday when it skidded to a stop at 4:24 PM after 48 hours and 16 minutes in the air. During that time it had fully recharged its batteries during the day and then flown through the night on battery power. Twice. Nothing, save the flagging energy of its pilots on the ground, kept the SoLong from flying for another two days, or ten, or a whole month says the company.

Remaining aloft for two nights is the milestone for sustainable flight. One night is possible just by discharging the batteries, but two or more nights means that the plane has to fully recoup and store the energy used at night while flying in the sunlight the following day. Once that is achieved, the cycle can repeat continually, and keep the plane airborne indefinitely.

"We flew 24 hours in April", said Alan Cocconi, SoLong's creator and chief pilot, "but we split the night in two, flying midnight to midnight. That was a warm up for this flight. It showed us that we were getting enough solar energy during the day but we didn't have quite enough battery to take us through the night."

Cocconi said the addition of Sanyo high-capacity Li Ion cells made the difference for the craft. With a charge-discharge cycle efficiency over 95%, the Li-ion batteries do not squander the bounty 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. To make up for the inevitable weight of the batteries, the rest of the aircraft is as light and efficient as possible. The six servos that move the control surfaces use special electronics that were developed for this mission by AC Propulsion to reduce power consumption and to extend durability.

"Every system and part on the SoLong was designed to minimize weight and drag, and maximize efficiency" Cocconi said. "Of course that is true of just about every airplane, but with the SoLong the entire mission depends on efficiency. We had to push everything to the limit."

That included the pilots who flew SoLong from the 5 ft x 8 ft trailer that serves as SoLong's ground station. Led by Cocconi, the team of crack radio-control and hang glider pilots took turns monitoring flight conditions from the twenty three channels of telemetry plus GPS navigation and video downlink data available in the ground station. The pilot's job is to find updrafts, avoid downdrafts, and make judicious use of the battery power to maintain altitude and find "good air" that will lift the plane. The energy budget requires riding thermals with the motor off as much as possible during the day. With the motor off, the entire output from the solar wings goes into the battery. The energy margins are so thin, and the weather so unpredictable that the pilot must focus intently, always trying to bank energy, either as battery charge or altitude, that can be drawn upon to get the plane out of trouble when the air turns bad.

The balance points between strength and weight, between stability and drag, between energy and power make a very fine line for the aircraft. On one side of that line are airplanes that cannot fly through the night. On the other side are airplanes that fall from the sky.

"Many efforts, some extremely well-funded, have tried to find the balance that will keep a solar-powered airplane plane in the air for two consecutive nights. Until today, not one had found it," Cocconi said.