Home-Brew Motor/Prop Tester

As a means to “de-mystify” what motor/prop combinations yield, a few hours effort can pay big dividends

One of the areas that I see many forum posts has to do with prop selection – “Will this work OK with this motor?” is quite common. I have the same questions and decided to build a simple test rig to test gear I have on hand. The pic above shows what I built.

I recently bought a digital postal scale from Staples (Pelouze – about $60) and its accuracy and range fits quite well – it maxes out at 5 pounds and reads to 0.1 ounces – it also can switch between ounces and grams, which I used in the tests. I built a very simple stand to hold the meter in a fixed position:

The base is 20″ x 7½”; two side stanchions are 10″ x 3″ with the holes for the positioning rod 8 7/8″ from the bottom – the holes were drilled so that the post on which the motor is attached contacts the digital scale in the middle of its platform.

The rod was something I scrounged in my workshop – 11″ x ¼” diameter. I drilled a hole for the rod (free floating) in the middle of the motor mounting arm:

I then positioned the center of the motor 4″ from the center of the rod and positioned a plastic foot the same distance in the other direction:

I used a hefty block on the back to make sure the scale does not move:

The setup involves the following gear:

• Watt Meter to measure energy throughput
• Servo Tester to act as the ESC’s throttle input
• Esc
• Lipol Battery
• Motor / Prop to be tested
• Tachometer

When I first started it up, it became quickly apparent that the spinning prop could do some real damage if it let loose or a stray finger got in the way. I scrounged around the shop and found this grill from an electric heater:

I NEVER put myself in front of this when testing! I also measure rpms from the BACK of the prop.

To ascertain how this rig compares to published data, I used my Scorpion 2215-22 as the test motor with two GWS SF props I have – these were used in the Scorpion website on the Prop Data Chart for this motor.

Prop – Tester	Total Watts In	Thrust – grams	Prop RPM	grams / 1000 rpm
GWS SF 9 x 7 – Scorpion	151	581	6450	90
GWS SF 9 x 7 – Joe	143	650	6834	95
GWS SF 10 x 4.7 – Scorpion	144	830	6575	126
GWS SF 10 x 4.7 – Joe	157	858	6609	130

I am very pleased with how my test results track with Scorpion’s test data – close enough to give me some confidence that I can rely on results when configuring a model; my results are marginally higher and I’ll take that into consideration. The design shown here is really very simple and can be assembled and ready for testing in a few hours.

As a means to “de-mystify” what motor/prop combinations yield, a few hours effort can pay big dividends – I can more easily see “sweet spots” where I get plenty of thrust for a model’s weight and yet maximize flight times with a given motor/prop/Lipo configuration.