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### torque meters

Posted: March 11th, 2016, 11:08 am
My current analog torque meter varied too much for my liking so I made the decision to experiment with a digital torque meter. I was looking around online and found the Flitetork torque meter that some people had been using, and built a modified version of that with 3D printed parts replacing the aluminum casing. My question is this: when you measure torque, how far is the rubber band stretched? We find that the torque decreases as the rubber band is stretched less. An example of this is when I finish winding with the rubber band somewhat stretched I'll get a reading of say .41 in/oz, and when I move the winder closer to the torque meter when unhooking the rubber band the torque reading decreases. I've seen this on the analog torque meter as well and was wondering where you guys take your torque measurements from to ensure they're accurate.

Another question on the construction of the meter I built. I've seen some digital meters that just use a straight arm that comes down at an angle to apply pressure to the scale. I thought the arm needed to apply the force at a 90 degree angle to get accurate reading, so I made the arm L shaped. The torque equation is Force x distance from axis x cosine theta so this was my line of reasoning. Am I correct? And the arm that applies pressure needs to be 1 inch long to get accurate readings in in/oz right?

### Re: torque meters

Posted: March 11th, 2016, 11:19 am
Good observation with the torque meter. If possible, start winding on a stretched motor hooked onto your torque meter. Gradually move in and finish winding so your motor is the length it will be when on your plane.

### Re: torque meters

Posted: March 11th, 2016, 12:18 pm
For accurate, simple readings, yes your arm should be perpendicular to the sensor and a lenght that makes the math simple. You can use an angled arm, but the math to convert to actual torque gets messier and involves vector math and some assumptions about slip.

But as long as the setup is consistent you can use the meter for your planes, just makes it hard to compare to others results.

As to stretch, you monitor torque throughout the winding process. The changes in torque as you increase or relax stretch are normal. It will help tell you if you can wind your motor harder, or if it is time to move in, etc. For FINAL numbers, the rubber should be at hook to hook length for consistent comparison flight to flight. Part of the fun is learning how to pack in more turns for the same launch torque, and that's where noticing things like you are become useful clues.

Jeff Anderson
Livonia, MI