Geared motors for rubber scale models

[Anyun]

Thanks for the interesting write-up about gear drives. Especially the Miles in the last video is very impressive. I know you like to keep the original wing profiles on your scale models, is that true for this one as well? The pretty Miles monoplanes do tend to have very fat profiles, not the best for small models. /Andrea

[Prosper]

Hullo Andrea, yes, the wing is 'full fat' - 19 or 20% thick at the centre section. It's a Clark YH foil if I remember rightly. But the model was in such a basic state that the low weight allowed for good duration. If I'd added all the details and painted it, things might have been different! The model still exists, but it's so beaten up it's only good for work as a testbed. I'm intending to try it with some different gear ratios and prop sizes.

I tried a small Miles Libellula once - Peanut size or smaller. With the 20% thick aerofoil it couldn't fly at all. Miles exceeded themselves with the Master trainer of the WWII period - that had a 24% root thickness!

Stephen.

[Prosper]

Picture 1 shows the bits which make up the motor shaft. left to right: a T-hook. I nearly always make a T-hook whatever the size of model, but any hook will do. I've inset an enlargement of the T-hook to show the fairly coarse roughing-up I've given the shaft with a metal file.

Next a micro-bearing - 5mm diameter; 2.5mm deep and 2mm centre hole. A plain brass bush will do, but would (for a setup like this) probably need to be 4-6 mm long. The micro bearing makes for a more compact unit. Next, a 32 teeth gearwheel. This started as a double gear, with a small (10 teeth IIRC) gear as part of the same casting. I've shaved that into a cone. Next, the 2mm O.D, 1mm I.D. winding lug.

Below the lug is a curled strip of paper. This has the curled end sanded down to a feather-edge.

Pic. 2 shows that same feathered-edge stuck to the brass winding lug with a tiny drop of thin CA, applied with the red-handled applicator. The paper was pinched to the brass between finger and thumb (thumbnail in this case) and the drop applied. Now the strip of paper is anchored it can be tugged tightly round the diameter of the brass tube - just one turn, whereupon it's torn off and the collar of paper is wetted out with CA. Pic. 3 shows the result - I should've mentioned that I used a bit of 1mm piano wire shoved into the brass tube to provide a handle for all the finicky manoeuvring. The gearwheel has its bore roughened slightly with a diamond file.

[Prosper]

4: the all-important gearplate, of balsa ply. The hole for the propshaft bush is straightforward but the large 5mm hole for the micro bearing takes some care. Note the circular abrasive thingy - wet-and-dry paper wound round a bamboo skewer. Note that the propshaft bush is not fixed in place yet: the exact distance between the prop and motor shaft axes is critical . . . might need adjustment. Pic. 5 - the bearing is pressed into place using the handle of a pin chuck. A good, firm fit in this case. I don't think there's any need to glue the bearing into place - I have used a bit of CA to secure a bearing when the fit has been less tight than in this case. Pic. 6, the bearing is checked for alignment with some brass tube and a Mk.1 eyeball.

7: the back of the bearing is retained by a bit of plastic card glued to the gearplate. This prevents the tension of the fully-wound rubber motor from pulling the bearing backward out of place. Unlikely but possible.

[Prosper]

I had already glued the large gearwheel onto the winding lug before taking picture 7. That's one of the tricky jobs - obtaining a concentric and rectilinear result. The dry-fit of the gearwheel onto the paper-wound lug is snug, so after adjustments until everything looks as square and true as possible, thin CA is run into the joint. Now that's a solid joint - not one of those which is likely to slip. Next, the projecting end of the winding lug can be pushed into the bearing race. For once, the 2mm brass tube I use is a perfect fit into the bearing! A firm pressure gets it in place. Again, no locking with glue needed here.

Another thing I was unable to photograph - fixing the T-hook with its coarsely filed shaft into the brass winding lug. What should happen is that you push the T-hook home, whistling cheerfully, and place a large blob of cyano onto the shaft just where it goes into the tube, whereupon the blob is instantly drawn into the joint by capillary action, and it's time for a nice mug of tea. But! note the immediate presence of the micro bearing. You can't have large blobs of CA sloshing around right next to that. So what really happens is that I insert the shaft just a few millimetres into the tube, then apply a large drop of CA to the exposed shaft and quickly shove the shaft home, whereupon the CA 'wicks' into the tube and the joint is made. This makes for a very fraught few seconds of action, because: 1) the CA can cure so quickly that the shaft locks before it's fully home; or 2) the quick sliding-home of the shaft can cause the fat blob of CA not to 'wick' into the tube, but instead to bulge around the tube entrance, where it could flow over the bearing. So far, 2) has never happened to me, but I've seen the bulge begin to build, and rapidly pulled the T-hook out.

Then the security of the joint can be tested (up to a point). If this joint fails during stretch-winding then your model may be trashed. I have considerd other approaches such as soldering, but haven't found a sounder and more practical idea yet.

[Prosper]

Et finalement, mes braves, the propshaft with its small 8-teeth gear glued in place, can be fitted. It's slid thru the brass bush from the front and tested thoroughly to see how smoothly the gears engage and whether there's excessive play. Then - only then - is the brass propshaft bush glued in place with lots of thin CA run around it and soaking into the balsa 3-ply. If the gears don't run well because they're too close together or too far apart, then some enlargement of the hole in the gearplate is needed and some shimming with slivers of balsa to fill gaps, before gluing takes place. Then the retaining collar is glued in place at the back of the propshaft.

The propeller seen in the photos is the very same 3" 76mm disc-diameter one that flew the little 11" span test model , with 2.5:1 gearing. It appeared in these pics largely 'for illustrative purposes' . . . but, since taking the photos some days ago, I did in fact fix that same prop on the shaft of this 4:1 gear ratio unit, and have indeed made another simple all-sheet test model - this one bigger, Peanut scale size, which flew for the first time on Sunday morning. Initial results look promising.

Stephen.

[TheLurker]

Fascinating, but far too complicated for this chump.

[Prosper]

We-ell, I dunno. If you can manipulate aluminium leaf . . .

I hope I succeeded in giving the (correct) impression that all this is just bits of wire and tube and the aforementioned Mk.1 eyeball . . . I don't see it as complicated - the thing that makes it difficult is the poor compatibility of parts, which calls for annoying workaraounds like wrapping a single thickness of notepaper round a brass tube. Oh and of course all my method is predicated on the use of thin cyano - some people won't like that.

I imagine there might be a completely different approach to this, perhaps winding the motor independently of a very differently-conceived gear unit - in fact even as I write this a possible simplifying wheeze has occurred to me . . .

Stephen.

[Lastwoodsman]

Hi Stephen.   Yes,  it is a fascinating build ...      And the pictures are more than just excellent.     You obviously put a lot of work into those pics!   Long live the  "core" !   

Lastwoodsman
Richard

[Prosper]

This is the model that roosted in a tree yesterday.

It's a 13" span, all-sheet EezeBilt-style job made to test the same geared unit (4:1 ratio) detailed in the above posts. I added some flanges to the gearplate in order to have it fit into the model's nose former. There is a small piece of balsa glued to the plastic card flange to provide left-thrust (it needs a lot of left thrust).

It uses the same 3" prop that flew the non-scale 11" span test model shown at the start of this thread. 3" is slightly over-scale for a Bonanza. Incidentally, the average prop RPM throughout the video flight is 4800 I'm too lazy or thick to calculate the Reynolds number indicated, but I think this RPM gives strength to my unsupported notion that these small, geared props might be surprisingly efficient due to a higher Rn than a bigger, direct-drive prop would have.

https://youtu.be/rkVTotVVeJc

The video illustrates the exact point I was making about concentricity and inaccurately-made gearwheels. The horrid knocking or clicking noise is due to the gears binding once per revolution. I was aware of this at the time of construction, and remodelled several teeth slightly. That seemed to remove the binding, but obviously not fully, as I soon found out.

I'm sure that a greater duration could be achieved with a different motor and slightly different trim. Because of the single-surface wings this is probably less draggy than a scale Bonanza would be, and it's lighter too I'm sure. but I think I'd be prepared to make a scale Bonanza based on this result.

I'll be trying to squeeze a bit more out of this test model.

Stephen.