Solar energy density is relatively low, so the following two basic principles must be followed:
Low weight and advanced aerodynamics make for low energy requirements. To keep the Solair II aloft, only 755 Watts of net thrust power is needed. A 2 m/s climb requires 5071 Watts.
Solar radiation power received by the solar cells has to be transformed into propeller thrust power with minimal losses. Maximizing the efficiency of individual power train components is mandatory, but the main goal is to shoot for the highest possible total system efficiency.
Solar cell efficiency is (within limits of availability) mainly a matter of price - a few percents improvement might increase the cost to several times as much - so the real engineering challenge is to deliberately optimize and match the drive train components.
Two different drive trains were developed for the Solair II. Based upon commonly available and affordable permanent magnet DC motors, they reach a total efficiency of 79,4 % (direct drive) and 80,4 % (geared) without the generator. Both versions produce roughly similar output power - the geared drive has to cope with gearbox losses which are offset by a higher propeller efficiency because of lower rpms compared to direct drive. The geared drive features dynamic in-flight optimization of the propeller efficiency over a wide flight speed and power range with its variable pitch propellers.
Both drive train alternatives allow folding the propeller blades backwards to minimize drag in a glide. The mounts are compatible so drive trains can be changed in a short time. A special motor mount was designed which acoustically decouples the drive train from the airplane for silent operation. Both drive trains have built-in thrust sensors for in-flight optimization and getting the most out of available solar power under all operating conditions.