My friend, Rob Nevitt made a solar-powered glider in the 1990's. It was 110" span with cells under a clear covering in the wing. We then used 88 cells of 50 mm square that provided about 25 watts, no battery for motor power. The plane weighed 1kg, and the motor we geared a Maxon brushed moor, which was a bit heavy, but had an excellent (for the time) efficiency of near 90%. as we couldn't decide what the flying speed would be, we designed and made a 14" variable-pitch prop. This was controlled by a small servo with a push-rod through the tubular drive shaft. This could be manually controlled, or via an automatic system designed by Rob. The maximum power-point of solar cells tends to be at a constant voltage, so the controller adjusted the motor power so the cell output was constant. It would climb OK on solar-only power, and was tricky with manual control of the prop pitch. With the auto circuit in use, it was different. Because the motor was fed with near-constant voltage, it kept a constant RPM. uncontrolled motors tend to speed up in a dive and slow in a climb. Factor in the varying sunlight hitting the cells due to direction or roll or cloud and the usual clues to fly a plane were minimal. It took a lot of concentration to keep it flying well, but on a bright June day we managed a flight of just over 2 hours. BMFA records show:-
71 R/C AEROPLANE ELECTRIC MOTOR DURATION Robert Nevitt 121 minutes 18 seconds 30-06-1994
