Prop Thrust Meter

4-22-2015 Version 2 is here. See Thing: http://www.thingiverse.com/thing:770637 If you like, please follow the link to the instructables contest and give me a vote. Thank you.

2-28-2015 Version 2 is coming. Modeled after the Turnigy thrust stand, I will be hacking a ordinary kitchen scale for $10 and building a stationary all in one thrust scale that can measure up to 11 lbs of thrust. Check back or simply follow-me and you will see my next posting.

I developed this double arm equal arm balance so that I could measure the amount of thrust any particular motor and prop combo can produce. This way I can tell how heavy my model can be and still stay 3D or maintain flight. All parts are equivalent for the left and right sides so the scale components will not skew (add or subtract from) the measured force. The double arms keep the motor and prop parallel to the measurement plain and the force transfer paddle always flat on the scale. I use a regular kitchen scale to measure the force / amount of weight lifting capability. Add a current meter inline with the battery and you have a pretty nice setup ready to find the best power to thrust combination. I was able to measure on the motor shown that a 6x4 prop produced more thrust than a 6x5.5 prop. For a given torque, exceeding the ideal prop pitch actual reduces the amount of static thrust produced because the motor becomes inefficient.

As aways, please be careful with rotating props. Proceed at your own risk....

Instructions

As aways, please be careful with rotating props.

I printed with 40% fill (by accident) and suggest 100% fill to make the arms stiffer. My current setup can measure up to 5lbs of thrust before the arm flex starts to exceed the structural capability (with the 40% fill).

Also, make sure you mount the scale to a heavy board so the whole mechanism does not take-off and cause injury or property damage. Use a clamp to mount the setup to your workbench if need be.

The two main arms are made of 3 parts that are glued together (I used CA on ABS). The height of the center post is set to work with my Polder kitchen scale with the whole setup level; adjust if needed. I included the Inventor Fusion files for you to mod.

Hardware is M3 screws. Clear the holes in the arms so the screws can freely spin (but not sloppy looseness). The idea is that you want to reduce the friction to as low as possible so that the arms float and should basically sit level when nothing is attached. Friction from tight screws will steal from your lifting force.

It all worked great; have fun and be safe.

Why such long arms, to keep the movement of the air produced by the prop from influencing the scale and so you can hang that side of the scale over the edge of the workbench so the air flow is not restricted.

If you adjust the pivot point location you can measure larger forces with the same limited scale by setting up a force ratio (lever). Test the strength of the arms first before going forward so you don't brake something and cause further damage to the surrounding area.

I plan to design and print a cradle that the motor side of the arm can sit in and keep the whole thing from dropping to low when not operating. I'll share when done.

Category: R/C Vehicles