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.

[Prosper]

Right, let me attempt some kind of cogent explanation of my gear system.

The plastic gearwheels I have come from two sources: Micronwings.com in Australia, and generic Chinese ones from Amazon. I just looked on Amazon and whereas there used to be page after page of all sizes, there are now hardly any, and mostly they're metal. I hope they haven't disappeared for good - most useful IMO were the jamboree bags of assorted gears, including double gears. Anyway, they're 'Modulus 0.3'; the smallest is 8 teeth and I think I have some of 70 teeth. The rest is piano wire and brass tube - and recently, since I re-evaluated the business, small roller bearings.

What I call the gearplate or backplate, is made of 3-ply hard or hardish balsa. Either 0.8mm or 0.5mm laminatons depending on the job. The 3 laminations are glued with aliphatic glue - the hard-drying type not the rubbery type. There are two lengths of 2mm outside diameter brass tube glued firmly into the plate. 1mm Piano wire runs thru each tube; the top tube being the propshaft and the lower one the large gear shaft with motor hook and winding lug.

The gears have 2mm centre holes. The propshaft is 1mm piano wire, and the gear on this shaft (usually 8 teeth) has to be fixed to the propshaft. So, a short length of 2mm O.D. brass tube is glued in the gear centre-hole, and in turn the propshaft is glued to this short tube. The two tubes glued into the backplate are bearings: the propshaft can be slid through the top bearing and secured behind with yet another short length of 2mm brass tube. This retaining collar only has to deal with the thrust of the propeller.

The winding lug is a length of 2mm brass tube flattened at one end with pliers, and with a hole drilled thru the flat to accept the hook on my winder. The lower, large gearwheel is glued to the winding lug. The motor hook is 1mm piano wire. This hook has a long stem, which passes thru the lower bearing fixed to the backplate. The winding lug, with its large gear attached, is glued to the long stem of the winding hook.

I hope the photos help. Now this is just a bare description of the setup - but there's more: how to do it. That's what I'll tackle in the next post or posts.

Meantime,

https://youtu.be/YTdOQEM_PqA

https://youtu.be/827rFGQXwS0

Two more Youtube vids 'from the archive' - this time a Peanut Comper Swift - very basic again, all-sheet Eezebilt style construction. I see that I wrote in the description boxes that it had a 2.5:1 gearing and then later a 2.75:1 ratio. I wish I could remember more about my experimants back then.

Stephen.

[dputt7]

Great description Stephen, perhaps I should wait for your next post but do you remove the prop for winding.

[Prosper]

No, Dave, the prop is fixed. When I wrote about these geared motors on HPA2, OZPAF John asked how the act of winding the motor didn't foul the prop. You seem to have spotted this too. The answer is that the winder hook does foul the prop. The whole assembly spins round. That may seem combersome or clumsy but it doesn't bother me. I'll take a photo if I can. The alternative of winding the prop or propshaft is daunting - you're winding say 2.5 times the number of turns the motor needs, because of the gear ratio. Also it would require some special jig because when you stretched the motor for winding everything would go skew-whiff.

Stephen.

[dputt7]

`I'm glad I didn't miss something obvious. That makes perfect sense.

[efudd]

Hi Stephen,

I found assortments of small plastic gears on Aliexpress.com** for example

58 Styles Plastic All Module 05 Robot Parts Reduction Gear Bag

200PCS Mixed Plastic Gear Bag Motor Motor Transmission Gear

You can also buy short lengths of brass tubing

10Pcs Brass Round Tube 2-10mm OD 0.5mm/1mm Wall Thickness 30mm Length



** I have been a customer for many years, but there are limitations.  Shipping is more reliable and swift if using the "Choice" items, and is free over a monetary minimum, usually $10.

[Prosper]

Thanks for the info efudd - a useful resource.

There are several complications dogging this geared rubber motor malarkey. Number one must be the slippage of gears and of prop on their respective shafts. Typically this occurs as a result of the prop being stopped abruptly whilst the rubber motor is still providing torque, as in crashing or even normal landing depending on where the prop is situated. I use thin CA to seize a prop or a gearwheel to a piano wire shaft. I roughen the shaft very slightly with a file, clean the bore of the brass tube bush in prop hub or gearwheel (thoroughly, using acestone and twizzling cocktail sticks - it's surprising how much black stuff sometimes comes out); position the item, and let a drop of thin CA run in by capillary action. This usually works, but it can be hard to judge how much CA has penetrated, and how far. I usually grip the shaft with pliers and give the cogwheel or prop a determined twist to see if it's secure. If the step has failed then I get everything lined up, but with the prop or gear displaced slightly along the shaft; put a dot of thin CA on the shaft and instantly run the prop/gear over it, twisitng the prop/gear one way and the other while doing so. 

When discussing this on Old HPA, threadlock was mentioned. I have tried this and it's OK up to a point but not nearly so secure as CA.

To dismantle all this I have a little pill container half-filled with acetone, and with a hermetic lid seal (the black and white tub in the photo). I dunk the parts (or even a whole assembled unit, minus prop). The plastic of the container is a polythene type which is immune to acetone. The gears are I believe made from 'POM', and are untouched by acetone too. But the CA will melt. Plastic card bits will melt and balsa ply will fall apart.

On this subject, there is the worry that the way the unit is assembled, CA can run from where you want it - a retaining or bearing collar, say - to where you don't: the bearing itself. I have had this happen, and the acetone bath was invoked, with cursing. I use a sliver of polythene sheet between two brass tubes to prevent this, as seen in the photo. Note that this particular gear setup uses a roller bearing and not a plain bearing for the main (motor) shaft. More on that later.

The other photo is of winding the motor: see how the winder hook snags the prop and so the whole unit spins round and round during winding.

One way to assure CA penetration where it's needed to lock a gear or propeller to its shaft, is to file a shallow but definite flat on the shaft, rather than slightly roughening the shaft all round. This ensures that the CA will flow along the flat, and no doubt to either side of the flat too. The trouble with this is that it invites another difficulty - one to be addressed in the next post.

Stephen.

[Prosper]

I use a sliver of polythene sheet between two brass tubes to prevent this, as seen in the photo.

In fact it's almost invisible in the photo. I took a close-up but forgot to edit it for HPA. So here's a blow-up of the above picture. Hope that's clearer. I forgot to say that the polythene rectangle had a hole twizzled thru it smaller that the piano wire diameter; it's then forced onto the wire so there's a good seal; the next tube or collar is slid onto the piano wire and butted against the polythene. A few seconds after applying the CA to the collar the polythene collar is cut and/or tugged away.