Considerations of a dyno set-up to measure thrust, torque and power of electric motors

... or how to characterise those motors and props!

by Rod Badcock April 2005

Spurred on by the prop data measurements made by Hyperion for their Emeter and made available at Aircraft World and the success of ThrustXL ver 1.02, EffXL and the extensive liaison between the data contributors at RCGroups and Aircraft World I have started on a new measurement system for power, torque and thrust.

Essentially several key elements are required:

RPM of prop via tacho
Current being drawn by motor
Voltage at motor
Torque induced by prop
Measured thrust

At the moment I am in the building stage of this rig and I am outlining some of the design considerations here. After testing I'll also start to post results from the dyno testing.

Mechanical arrangement

How do you know what is the optimum prop selection for the motor you have? Traditionally you could either rely on manufacturers data for a small selection of props or invest substantial time and money in a motor dynamometer. Using this arrangement and simple (cheap) hand-held instruments you can characterise motor efficiency, thrust and power for most common propellers.

The mechanical arrangement and strain gauge installation is shown below:

The theoretical considerations of the torque are shown below for torque:

The theoretical considerations of the force are shown below for thrust:

So from measurements from these two bridges the force (P) in Newtons and torque (T) in Nm can be calculated. But this isn't the whole story; the measurements we really want are:

Power absorbed by propeller (watts)
Thrust from the motor / prop combination (kg)
Motor efficiency (%)

In order to make these measurements the following considerations must be made:

Rod