Solar-powered UAV flies two days straight

Aug. 18, 2005
An electric-motor driven UAV (unmanned aerial vehicle) recently completed a 48-hr nonstop test flight using only solar energy.

The SoLong unmanned aerial vehicle from AC Propulsion recently flew over 48 hr nonstop fueled only by solar energy. The plane sports a wingspan of 4.75 m and weighs 12.6 kg.

The two-day test demonstrated the feasibility of sustainable flight using solar-electric power.

The test flight of the SoLong solar-powered aircraft by AC Propulsion Inc., San Dimas, Calif., was more a test of endurance for its seven ground-based pilots than the craft. The fatigued team, lead by AC Propulsion's chief engineer and SoLong's designer Alan Cocconi, brought the flight to a successful touchdown 48 hr and 11 min after takeoff. The 48-hr mark was mere formality as SoLong could stay flying indefinitely.

Power from 76 SunPower Corp. (Sunnyvale, Calif.) solar cells supply the plane's energy. Power distribution among the onboard systems is controlled by management software developed by Cocconi. During daylight flight the nominal 225-W solar array powers all systems and recharges 120 Li-ion cells from Sanyo Corp. The Li-ion cells fulfill the craft's energy demand at night. Propulsion comes from a high-efficiency electric motor driven by a split-phase power controller developed by AC Propulsion. A variable-pitch propeller fine-tunes thrust for different rpm and power settings using a load cell for in-flight thrust measurements.

An earlier 24-hr test flight showed the original battery reserve couldn't keep the craft airborne. "We split the first test flight's night in two, flying midnight to midnight," said Cocconi. "We were getting enough solar energy during the day but we didn't have quite enough battery to take us through the night." The Sanyo cells pack 220 W-hr/kg and have a charge-discharge efficiency of over 95%. "That made the difference," Cocconi stated, allowing the SoLong to pass the 48-hr mark.

Twelve PIC18 microcontrollers from Microchip Technology Inc., Chandler, Ariz., control and monitor all vehicle systems. Systems under control of the PICs include the autopilot, motor drive, power tracker, six servomotors, the battery monitor, and a tracking downlink antenna. For example, the autopilot controller decodes 13 PWM control signals from the uplink receiver, inputs serial data from the GPS module, and monitors 23 analog sensor channels.

Data from all systems and a live video feed from the "cockpit" telemeter to the pilots on the ground. Two servo positioners driven by another PIC18 microcontroller keep the telemetry-link antenna pointed towards the same ground position. The PIC18 computes servosettings using signals from the plane's autopilot and GPS.

Efficiency is the key to SoLong's success. Even so, the energy budget required riding thermals with the motor off as much as possible during the day. The variable-pitch propeller folds flat against the fuselage during motor-off flight to minimize drag. SoLong pilots must always try to bank energy, either in the form of stored solar-electric power or aircraft altitude. Pilots draw against either to keep the plane in the air when conditions turn sour.

AC Propulsion Inc.
, (909) 592-5399,
Microchip Technology Inc.
, (480) 792-7200,
Sanyo Corp.
, (619) 661-4888,
SunPower Corp.
, (408) 991-0900,

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