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.
Thanks for posting your experiments. I look forward to.learning more
Tim
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
Following with interest!
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.
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.
Great description Stephen, perhaps I should wait for your next post but do you remove the prop for winding.
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.
`I'm glad I didn't miss something obvious. That makes perfect sense.
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 (https://www.aliexpress.com/item/1005005485448461.html?spm=a2g0o.productlist.main.25.fa91163dCOgex6&utparam-url=scene%3Asearch%7Cquery_from%3Apc_back_same_best%7Cx_object_id%3A1005005485448461%7C_p_origin_prod%3A&algo_pvid=be141ee4-ce61-4aab-a698-a280cb7c8e82&algo_exp_id=be141ee4-ce61-4aab-a698-a280cb7c8e82&pdp_ext_f=%7B%22order%22%3A%2228%22%2C%22fromPage%22%3A%22search%22%7D&pdp_npi=6%40dis%21CAD%214.52%214.52%21%21%2121.26%2121.26%21%402103128817857039434378478e26d3%2112000033269979552%21sea%21CA%21127787166%21X%211%210%21n_tag%3A-29919%3Bd%3A606c47ca%3Bm03_new_user%3A-29895)
200PCS Mixed Plastic Gear Bag Motor Motor Transmission Gear (https://www.aliexpress.com/item/1005012511034728.html?spm=a2g0o.productlist.main.10.fa91163dCOgex6&algo_pvid=f213462f-9066-4688-85c0-0ed3fee5ecbb&algo_exp_id=f213462f-9066-4688-85c0-0ed3fee5ecbb-9&pdp_ext_f=%7B%22order%22%3A%222%22%2C%22eval%22%3A%221%22%2C%22fromPage%22%3A%22search%22%7D&pdp_npi=6%40dis%21CAD%2126.31%2112.89%21%21%21123.75%2160.63%21%4021030f4a17857039502388233e1230%2112000058587318182%21sea%21CA%21127787166%21X%211%210%21n_tag%3A-29919%3Bd%3A606c47ca%3Bm03_new_user%3A-29895&curPageLogUid=lWiUgBrYPUpX&utparam-url=scene%3Asearch%7Cquery_from%3A%7Cx_object_id%3A1005012511034728%7C_p_origin_prod%3A)
You can also buy short lengths of brass tubing
10Pcs Brass Round Tube 2-10mm OD 0.5mm/1mm Wall Thickness 30mm Length (https://www.aliexpress.com/item/1005001286622715.html?algo_exp_id=3bd46995-6611-4369-a984-cd887446eefe-6&pdp_ext_f=%7B%22order%22%3A%22155%22%2C%22eval%22%3A%221%22%2C%22fromPage%22%3A%22search%22%7D&utparam-url=scene%3Asearch%7Cquery_from%3A%7Cx_object_id%3A1005001286622715%7C_p_origin_prod%3A)
** 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.
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.
Quote from: ProsperI 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.
Thanks for all this detailed info Stephen. It doesn't seem to be all that much of a problem winding directly onto the main gear from the front. It would require a bit of redesign to wind the motor from the rear - not a great idea with Scale models.
John
Quote from: Prosper. . . The trouble with this is that it invites another difficulty - one to be addressed in the next post.
Okay, scrap that - the next post sfter this one . . . I think . . .
Another difficulty: the security of the gearshaft.
BTW I've taken photos of an actual assembly, so after a good deal more rambling I will post a practical, illustrated example.
The gearshaft connects the motor hook to a winding lug of some sort - with a gearwheel attached for extra fun. There must be a good few ways to achieve this. My method has a piano wire shaft glued into a brass tube. What if this joint fails?
At the very least, a right mess of the front end of one's painstakingly and lovingly constructed model fuselage, I should think. The whole point of this gearing is to have a short fat motor driving a small, whizzing propeller. So the motor sizes involved are abnormally large for the size of the model, and the tug required while stretch-winding is outsize too.
Initially I used long brass tubes for the winding lug - expressly to provide lots of gluing area for the wire/brass joint. The drawback is that the propeller has not to bind with the winding lug, so needs to be set well forward of the gears. So a long propshaft with the prop sticking out well ahead of the model's nose - not a good look, and making it more vulnerable to damage. I haven't tried boosting the joint by filing grooves in the wire shaft and crimping the brass tube in place - mainly because it's irreversible. At this experimental stage I like to be able to disassemble things.
I had thought of using a roller bearing for the gearshaft, and have some micro-bearings from Micronwings. But I was squeamish about using a side-bearing as a thrust bearing. When I finally tried it it worked fine. I've no idea what goes on inside these microbearings - whether they're roller or ball or what pre-lube they have or how durable they are, but they seem fine even with oversize rubber motors. They have less play in 'em than the play between my brass/wire plain bearings.
The other question about roller bearings was that although they allow a much shorter gear shaft, would the shorter length allow a
secure glued joint between winding lug and motor hook?
Again the answer seems to be yes, even for big motors (say 5/16" cross-section for example) in small models. Perhaps I was being overcautious. It's early days for all this experimenting tho'. We'll see.
Stephen
Okay, the last post where I moan about how difficult this geared biz is, and next will come some illustrated posts of a new unit I turned out. . .
None of this biz is truly concentric or linearly true, unless by happy chance. It's all Mk.1 eyeball and TLAR. Even at the very outset there are inaccuracies: the plastic cogwheeels aren't perfect. They have irregularly-moulded teeth, and traces of 'flash' between some teeth. The centre holes of most of the wheels I use are slightly over 2mm dia., so 2mm brass tube slops around unless the gap is filled. There's play between the brass 1mm I.D. and the 1mm piano wire . . .
Given the above it's not surprising that the spinning wheels often bind at some part of their orbit. An answer is to increase fractionally the distance between prop and gear shaft - but too much and the play between the brass bush/piano wire shaft can cause the gears to jump. Individual binding teeth can be traced and filed to mitigate matters. Some individual 'rogue' cogwheels can be discarded.
The upshot of all this is that each setup has its individual character in terms of noise, vibration and friction. This varies with the size of the setup, because the length of a brass-bush plain bearing dictates the angle of play between brass and wire. However, in my experience a decent setup has remarkably low overall friction.
https://youtu.be/4OZ4fcD5J-k
Here's my last 'archive' geared-rubber video - a basic, unfinished Miles Hawk Major 20" span, with very quiet gears - most of the noise seems to be clattering rubber. Sept 2023.
Stephen.
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
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.
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.
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.
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.
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.
Fascinating, but far too complicated for this chump.
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.
Hi Stephen. Yes, it is a fascinating build ... 8) And the pictures are more than just excellent. :o You obviously put a lot of work into those pics! Long live the "core" ! ;D
Lastwoodsman
Richard
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 :o 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.