Motor Thrust Measurement Tool
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Home > Articles & Tips Index > Tools and Electronics > Motor Thrust Measurement Tool

[Courtesy of peterb "at" home.informi.com, July 1999]

If you want to get practical data as opposed to scientifically true data, there is a very simple method which I use myself. Basically i'ts a small waggon with the fan mounted pulling a suspension scale.

I have a hard and plain plywood board, 10x20", on which there is a bracket for mounting a fan. The board is striped with double side tape to hold things like accus, a reciever/ESC and the watts-meter. At the opposite end of the fan bracket, there is a screw for attaching the scale. I use a fishermans one going to 10 pounds.

Now the trick is: The board is laid on 3 round harwood sticks (eg broom shaft type) and on a hard surface, then the board becomes a kind of waggon.

Now mount the fan, connect the things, hook the fishermans scale on the screw, fire up the fan, pull slightly backwards in the scale and read volts and amps from the wats meter and thrust from the scale. Do it on peak accu, after 30 seconds and finally do a 75% throttle stick reading allso (cruise data).

By the way: Couldnt we teach the fanmakers to use gramms or ounces instead of Newtons? It is very hard to get a Newton scale and most of us are ordinary people and not engineers.

It's a very simple construction and data is reliable (it produces the same results at the same conditions, eg if you do the same a second time).

If you need higher precision, you can use a lab suspension scale instead of the fishermans scale. But honestly, it does not matter if true static thrust is 25 or 27 ounces. What matters is, that when you get a higher thrust at the same watts usage, you know the fan is better. Better or worse, thats the question. When i compare my readings against wemotec tables, they are within +- 5%.

The static thrust is only valid for the acceleration from zero, eg how fast your model gets airborne. When your model increases the speed, the dynamic thrust and power usage is more interesting put harder to measure.

So measuring static thrust on fans is very simple:

1. 1 ply board (10x20") 2. 1 bracket to hold fan (the simplest form: just double sided tape) 3. 1 screw 4. 3 harwood rollers (the very original "first wheels of mankind" type) 5. 1 10 punds fishermans scale 6. 1 piece of string to tie the fishermans scale to a table leg 7. 1 roll of double sided tape 8. 1 Astro watts meter if volts amd amps are important to you allso

Hilsen Peter Bech


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