Rubber motor torque levels and rotor question

[bjt4888]

This is Coach Brian from the National Free Flight Society. I am posting a few of the questions and answers from the "Flight" discord channel. This forum is a better format for questions and answers that will help more students.

I'm making a helicopter presentation for Division B and have a couple questions:

Will general torque values remain consistent from last year? I'm guessing not because thicker rubber can probably sustain more torque, so maximum torques might go a bit higher this year. For last year's winding I noted a sharp increase in the first few winds to ~0.2 in oz, a plateau around 0.25, then a gradual increase to 0.55 followed by a spike that signals walk-in. Will this still happen (but with different values)? Will this be relatively consistent across different rubber masses?

Will propeller pitch be adjustable for a given set of rotors? Will the rotors be interchangeable to allow for pitch adjustment?

Feasibility of balsa rotors?

[bjt4888]

Reply from Coach Chuck:

1. Yes, substantially higher torque, but rubber is rubber. Same technique. See notes about the higher torque being a potential problem for precision winders and torque meters.
2. Depends on your design. The ffm rotors last time we an x-spar, which is not easily adjusted. Probably have to make new to try a different pitch. The j&h prototype this year appears to have a central spar, so may be adjustable depending how they mount into the hub. A similar design could be adjustable by twisting the spar. I think j&h uses carbon spar, would not put up with twisting. Interchangeability depends on how you mount them, I think most kit makers are using a ikara button, so they are removable.
3. Could do balsa rotors but I believe you would struggle to make weight

[coachchuckaahs]

Note also:

There are reports that some expensive winders (Geauga , IFFS) may not do well at torques over 2.0 oz-in, with some reports of broken gears. These precision winders are intended for typical indoor torques up to 1 or 1.5 oz-in. the yellow KP winder at 10:1 works fine. The 15:1 may be hard to turn at high torque.

In addition, many torque meters of the twisting-wire type are limited to under 2.0 torque. The Geauga torque meter has arbitrary numbers on its face, which must be converted using the chart that came with the meter, or is published on the IFFS web site. The actual torque depends on the element installed. I believe the strongest element goes to 1.55 oz-in torque. Winding a wire-element torque meter over 1 full turn may permanently damage the element!. Many other low-cost torque meters top out at 2.0 torque. Digital torque meters should be fine.

Coach Chuck