Motor & Propeller


The solar airplane requires 55 to 62 watts of electrical power to sustain flight. The power produced by the solar cells is limited, so a motor of high conversion efficiency was required. Seven candidate outrunner motors were purchased and tested on an ad hoc dynamometer. Each motor was tested to determine its efficiency under various loading conditions and at a wide range of input power levels/throttle settings.

375 individual measurements of motor performance were collected. Data were entered into spreadsheets to facilitate calculations and plotting of results.

The motor dyno/test stand included instrumentation to measure:

● Motor input Voltage

● Motor Input Current

● Motor RPM

● Motor Output Torque

● Static Thrust

 The motor's input electrical power is Voltage x Current. Its output shaft power is RPM x Torque. For each combination of motor, shaft loading, and throttle setting, the efficiency of the motor is:


After testing all motors I chose to use an Axi 2820/14 Gold Line Kv=860RPM/Volt outrunner motor, based largely on its high tested efficiency of 77%, and loaded RPM when operated with the available input power of 50-80 watts at 7.4 to 8.4V.


Propeller Selection for Maximum Efficiency

The propeller converts the power of the rotating motor shaft (Angular Speed x Torque) into Thrust Power (Force x Airspeed). If properly selected, props can perform this task with high efficiency.

In selecting the solar plane's propeller, I referred to the excellent resources available on the web including:

Martin Hepperle's Airtools website

UIUC Propeller Data Website

APC Propeller Performance Data Website

Based on the target airspeed of 25MPH and the motor's optimum RPM and torque at the cruise power setting, I chose an AeroNaut CAM 12x8" folding prop for initial test flights. This prop favors a bit more thrust horsepower at low flight speeds. Later on, I'll switch to the optimum 11x8 prop for peak efficiency.

It was necessary to use a folding prop because the plane has no landing gear. Landings would likely damage a rigid propeller.


Motor & Prop Testing

















Motor Test Stand Video (MP4)




Dynamic Motor and Prop Testing









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