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
●
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
Back to the Solar Plane Main Page