Build an Inexpensive Motor Thrust Calculator

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Build an Inexpensive Motor Thrust Calculator

Post by Neons » Sun Feb 07, 2016 11:34 pm

This is something that you can accumulate the parts piecemeal as needed. The main thing is to make a plywood base about a foot square. Then you make a 90 degree arm with a shaft through the bottom corner. You have 2 short pieces of vertical stand to pass the shaft through secured to the base plywood. I used a pair of ball bearing races on the shaft. It is better but still ok without them. Remember this is a static reading and not a true reading of the thrust. It is all relative when you make changes in batteries or props. I shows the changes as you write down notes of each modification.
You will need a Whattmeter for the motor digital readings of the volts, amps, and watts of power. You will need a digital postal scale to read the static thrust also. I bought my well used one at Walmart. The height of the arm pin should be elevated to about the height of the scale. For motor mounts you can use a stick as one of the mounts. You can make a flat mount for motors with the X back mounts. Screw the motor onto he mount. I ran an old ESC down the angled arm to the Whattmeter and my battery goes also to the input on the Whattmeter.

There is no need to run the motor with a receiver and your transmitter if you buy a servo checker. You can if you want to do that but it is more cumbersome. I prefer this. Find them in Hobby King or eBay. A RPM meter is another option available. I have one but rarely have used it. Select a prop that is recommended for the motor and the battery you want to use to fly on the plane. Plug the battery into the meter. Make sure you have all 3 wires correct on the motor so it does not start backwards. Slow throttle spool up and holding the arm will tell. When this is all set to go you turn on the postal scale and wait for it to register 0.00 lb. on the screen. Ready to go.

Get a pad and pencil handy and make a few columns. Mainly for this test make a volts/amps/watts for a starter. What you want to do is bring the rpm on the motor up to full power and get the Whattmeter readouts of each column with the specified prop. Did the amps fall safely into the range of your ESC so it will not burn up? How many watts did it produce with this prop? How is the volts reading of the battery holding up? Now try a few different make props and sizes. Make another row of results on your pad. After running a few props compare the results on the pad. Try a 3 blade one diameter down in size and compare. Maybe you want a 4s battery over a 3s one. Try it. Always drop pitch or diameter as you go with more voltage lipo batteries. This is a basic setup but very informative and rewarding with the power findings.

As a rule of thumb using a bigger prop will us more amps and produce a shorter flight time. The more watts you produce is translated as horsepower. Adjust to what your needs are.

Click the picture to enlarge. Click again to zoom in.
Motor  Test Stand (1).jpg
Motor  Test Stand.jpg
Motor  Test Stand (notes).jpg
Bob Pacheco

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