Geared motors for rubber scale models

[Prosper]

I was tempted to put this in the 'techniques' forum, but since - apart from initial experimentation - I'll be using this for scale models only . . . we-ell, I dunno. But if it's in the wrong forum, then George K, who has the same omnipotence on this site as the Gods on Mt Olympus - will surely move the topic to where it belongs best.

My interest in this subject reawoke because my flying field is off-limits until at least late autumn, and probably until next Spring for models needing some grass cushioning to end their flights.

First of all, an example of what this is all about.

https://youtu.be/L6eHNjIhpbw

This balsa-sheet model has an 11" 280mm wingspan and a 3-blade prop with 3" 76mm disc diameter. Thanks to some gusty flurries of wind yesterday, it was dislodged from high in a tree where I think it was getting broody. No damage except one elevator knocked loose. The wind died completely at evening, and I got a full-turns video'd flight. Unfortunately the ground is very rough and every time I tried to move to acquire the model on camera I I lost my balance. Not so this morning, when the wind was still zero - I was careful to find a level footing amid the tussocks - so a better-recorded flight. Note how high the model is when the turns run out. The motor run equates to an average prop rpm throughout the run of 3440. The motor is two 6" 150mm braided strands of 3/32" rubber. The model has a very high (in my opinion, for a model this size) wingloading. The geared unit is the one in the picture with the 3-blade prop. The gearing is 2.5:1. The prop blade pitch at 75% of disc diameter is 25°.

Over coming days I hope to post about the basic ideas and issues, my earlier experiments, thoughts and findings. Doing this may well repeat things I already said (wrote) on the vanished HPA1 or HPA2.

Stephen.

[Piecost]

Thanks for posting your experiments. I look forward to.learning more

Tim

[OZPAF]

That model flies well with geared motor - the gearing appears to even out the thrust delivery through the motor run with a more constant and lower torque.

With a free wheeling prop that would have been a long flight - err or ended high up in a tree 

it's good to see this experimentation continuing.

John

[malc]

Following with interest!

[Prosper]

Thanks for the interest gents. John, judging by ear, the motor torque curve is the same as un-geared motors - I hear the gears' initial high pitch screech rapidly falling then declining more slowly. However that's just my estimation. You may be onto something.

A bit of background . . .

I started mucking around with geared rubber motors in Spring 2023. Being almost exclusively a scale modeller the attractions for me are:

   -A smaller prop can be used, for a more scale look. Or perhaps I should say 'less unscale look', since in most cases the prop diameter is still more than scale diameter.
   -A geared setup will use a shorter, fatter motor, meaning a more forward CG, reducing the need for noseweight.

Other factors emerging are that the rubber motor in most cases will be mounted lower than the propshaft; typically about 7 to 10mm - an aid to stability. Another likely aid to stability is that the prop and the motor spin in opposite directions. I'm sure this must reduce overall torque, though happy to be corrected if this is 'bad science'. Most models I make need added noseweight, even though the motor peg is well forward. The weight of the plastic gears and the metalwork provide some or all of this necessary weight. It's well understood that the larger the prop, the more efficient it is - however, I wonder about this, based on observation. Some of these geared models fly for longer than I'd imagined they would, given friction losses in the gears and given the smaller prop. My idea is that the prop, though smaller, will have a raised Reynolds number because it spins faster, making it more efficient (or again, 'less inefficient' ). Another possible stability gain is that the smaller prop's wash doesn't smother the tail surfaces so much - although possibly its faster spinning negates this advantage.

https://youtu.be/LKYrDP-XFgE

https://youtu.be/sdochFRhA0A

https://youtu.be/827rFGQXwS0

These are three videos of the first test model, Spring/summer '23. This model was all-sheet, kind of a larger version of the 11" job pictured above. 20" span; 2.4:1 gearing.

Prior to posting this, I've been trying to write up the construction of a typical geared unit. I've found it quite hard to make a clear description, so bear with me.

Stephen.