Dumas Zero (As a trainer)

[Jmk89]

6 strands motor continued:

4.  Put half the hank through the O-ring and fold over end to end so you have a 6 strand hank with both single loops at the same end (NB I used a fibre washer rather than a usual O-ring because I had it to hand and this is a demonstration, not an actual installation)


5.  Put the two single loops onto a bobbin


6.  Put the double loops at the same end onto the bobbin and gather the hank with a rubber band

[Konrad]

Jmk89, thank you!

Normally my motor strand count is 2 times an even number. As I fold over the loops to make my braided motor. The one exception is the 2 strand motor where I end up having to tie the ends of a preloaded single strand motor to make a single loop before allowing the loop to "unwind" into a single loop (2 strands) motor.

Some follow up. Do you not braid your motors? Have the threaded knots let go as the cross section of the rubber strands changes when the strands are stretched (wound)? Have you had any issue with the thread cutting into the strands?

Is there something in motor (rubber) theory that would explain why 6 strands of X total cross section (thickness) would have a different characteristic than an 8 strands motor with the same cross section.

[Jmk89]

Jmk89, thank you!

...
Some follow up. Do you not braid your motors? Have the threaded knots let go as the cross section of the rubber strands changes when the strands are stretched (wound)? Have you had any issue with the thread cutting into the strands?

Is there something in motor (rubber) theory that would explain why 6 strands of X total cross section (thickness) would have a different characteristic than an 8 strands motor with the same cross section.

No problems Konrad, happy to contribute.

Yes I do braid motors, depending on the situation.  Essentially if the motor is tight between the prop and the motor peg, I don't bother because there is no advantage.  That is usually for a duration model with a fixed motor weight (e.g Coupe d'Hiver, modern Wakefield, P-30) or where it is easier to just lengthen the fuselage (e.g Open rubber). 

However, if I have total freedom on motor weight and the fuselage length is limited (either by rule or in practice), such as if the model is a replica of an old model (e.g. vintage Wakefield) or a scale model and the best motor will be longer than the distance from prop to motor peg, then I do braid the motor.  If the motor isn't braided, when it unwinds the slack tends to form a lump of slack rubber at the nose end or the tail end and that puts the CG in the wrong place for a good glide. Or even worse the nose block falls out, which really destroys the glide! So I braid in those circumstances to get the right motor weight and run time and to keep the motor tidy so it doesn't alter the model's trim.

Theoretically, there is a slight loss of turns available when a motor is braided - https://www.endlesslift.com/maximum-turns-formula-for-braided-rubber-motors/ - but since the point is to get more rubber into the space allowed, thee loss from braiding is much less than the gain in available rubber from braiding, if you get what I mean.

As for the second question, as far as I know, there is no significant difference between motors of the same length and weight, regardless of the dimensions of each strand or the number of strands.  So 2 strands of 1/4" rubber will be the same as 4 strands of 1/8", so long as they weigh the same and are the same length.  I have often wondered if the smaller strands may lose something in friction between the strands (either in winding or in unwinding), but I don't think that anyone has found any significant difference, at least if the motor is properly lubricated.  The formulae for calculating maximum turns or breaking torque use total cross section of the motor (i.e number of strands x width of each strand x thickness of each strand) and don't adjust for number of strands - https://www.endlesslift.com/rubber-motor-formulas/

[Konrad]

Ok, moving forward ever so slowly!

Here I've ditched the S hook and now have the motor going through a 3/32" (labled) O-ring. Still using Glycerine. I pre-loaded the loop with 100 turns before folding the loop over and letting the motor braid itself.

I think I'll try 50 turns when I fashion a rear support for the prop shaft.

[Piecost]

Thanks for the 6 strand tutorial Jeremy and for the progress on the zero.