Willdo

During the discussion earlier on the MVVS watercooled engines, reduction gearing was mentioned, where the prop shaft was geared down to make the use of larger more efficient props possible.

Someone mentioned the ill fated BOMO boxer twin made in germany, and some OS single cylinder models with geared shafts were also available.
These were smaller glow motors of course, produced in the eighties and early nineties, but they all seem to have disappeared.

I wonder if this arrangement could be of any benefit to gas engines, (both aircooled and liquid cooled), - there are certainly possibilities of getting a much smaller frontal area by positioning the prop shaft in the right place, and of course there is the possibility of belts being used instead of gears.

Has anyone had any experience with these geared engines? - would the benefits outweigh the disadvantages (like cost) ?

Diablo-RCU

My Feedback: (1)

Geared reduction isn't a very good choice. A gearbox adds weight, complexity, and requires maintenance. The simple solution is to increase engine displacement to increase the torque to drive a larger prop at low rpm. The porting and exhaust system can be designed to put the torque peak at the lower rpm band.

w8ye

My Feedback: (16)

The gear reduction units have not worked very well, Besides the reasons mentioned above, The pinions on the gear box prematurely fail because of the singular power pulses of the engine.

Enjoy,

Jim

saramos

For models in the 5 to 12 lb range, you could go with one of the RCV SP series engines. These engines are 4 stroke and geared internally at a 2:1 reduction. They come in three sizes, .60, .90, and 1.20. I have the RCV .90 SP engine, and will be installing it in the Top Flite Spitfire that I have under construction. The recommended prop for the .90 is an 18 x 12, 2 blade, or a 15.5 x 12, 4 blade with max RPMs around 5700 - 6000.
They also have the RCV CD series engines. These engines are geared, but the gear ratio is 1:1. It allows them to use their unique porting which eliminates poppet valves on their 4 stroke engines. This makes for a more compact form factor for their engines.

Scott

tkg

My Feedback: (1)

Mick Reeves Models has a belt drive reducer for the G-62. He designed it for use in scale WWi planes where he needed a big slow turning prop. He has sold a bunch.

pe reivers

He still makes them, and as far as I know, the toothed belt gearing works very well, especially with multiple narrow belts, which distribute the load better, and hardly will fail simultaneously.

Willdo

DIABLO,
Increasing the engine displacement and playing with the porting is certainly one answer, but in order to increase the displacement, the engine will probably be physically bigger and heavier too.
With these chainsaw type engines, just increasing the bore is not really an option, as the bore/stroke ratio is already very much oversquare ( at least for two strokes).

I was thinking more on the (well proven) possibilities of using smaller and lighter engines producing real horsepower at higher revs, as opposed to torque at low revs.
- horses for courses I guess.

WY8,
Why are the gears that unreliable? must be bad design, materials, heat treatment, or lack of lubrication, - gears normally work well in most applications ( eg competition motorcycles), if properly done.

SCOTT,
The RCV engines are certainly unique and interesting, and can turn large props, but marketing them seems to be a little slow, I heard someone mentioning that it was difficult to get a reliable idle. I find them very interesting though.

BTW, does anyone have any experience with the Ericksson ? - It's unique and interesting too, and the shaft is also in the middle!

TJK & PREIVERS,
I think belts are a good idea too, - less precision engineering than gears, no lubrication required, and easy to replace, - probably the best answer!

rmh

You really have to experience the problems with reduction setups to appreciate what a pain in the arse they can be
I have done the following types:

OS 60w/ gear (a half dozen of those wavering wild screamin banshees.
They used a hunting tooth arrangement -which means the gears constantly cycle thru all the teeth
This means the prop is in a different position each time the engine fires -so it sets up a horrific undulating noise

A pair of Wankels on Gilmer belts -- Unreal performance 2-1 gearing -drank glo fuel like it was a dehydrated elephant.

A horrible steel geared Webra setup .60 - not worth mentioning ---

The weight to power is simply dreadful on most of the gas setups I have seen - but at least you may get to spend a lot of time screwing with one -------------

Willdo

DICK
Those are good answers , straight and to the point! I really needed to hear what the downside was to all these different arrangements, and I got my answer, ( and I believe it ).
- Still no mention of belts on gas piston engines though, sounds like my only hope!

Pelle Gris

There are several manufactures that markets belt drives for gas engines. And as far as I know they work quite OK.

http://www.warbirdpropdrives.com/ 2,6:1 gearing using a 26" constant speed Mustang prop.
http://www.modellbau-schlundt.de/news.php 2,5:1 drives for G45 and G62. Using 32x18 and 34x20 props.
http://www.mickreevesmodels.co.uk/ 1,75:1 drive for G62.
http://www.toni-clark.com 2,8:1 drive for G38. Using a 32x18 prop.
Ralf Borchers (or something like that) from Austria Several as far as I know. One of them a 33" 4-blade Mustang prop for a 60-100cc engine.
Byron/Iron Bay 24" "rubber" Mustang prop...

The biggest problem is finding suitable props, as most props have way too little pitch to be used effectively. I´m building a 3,5:1 drive for my G-62, and I have also started making my own prop, as I can´t buy a suitable prop anywhere. The prop is a 33x35" 3-blade... Try finding that at your LHS..


I guess you are all right when you say that the power to weight goes to h*** with these drives, but the scale realism goes WAY up. And for a big warbird/WWI fighter they should work very well.

ZAGNUT

My Feedback: (1)

a very good reason for using a redrive is to let the engine spin and make power...something that most of our current gassers weren't designed for. rethinking the engine choice is in order, maybe take a look at some of the hotter 50cc moped engines that come close to 10 HP

a hot .40 pylon engine can match a G62 when it comes to power...so theoretically we could actually get a much better power to weight ratio if designing the redrive correctly


dave

FenceMagnet

My Feedback: (2)

of course, then we can also look at 4 strokes and run the prop off the already-slower-turning camshaft....

[:-]

Willdo

Good to see that the idea hasn't totally died, I guess in bigger gas engines the power to weight ratio wouldn't be as much of a problem as on small glow engines.

Belts I believe are a little more efficient than gears, - not as compact, but that big pulley up front could be responsible for shrouding the engine and starving it of air! - how about a spoked pulley?

It's all a little more complicated of course, ( so is a fourstroke engine!) but the bigger prop is a bonus as opposed to a four stroke designed to produce torque at lower revs, ( they then run out of power at high revs - a two stroke can be designed to give power at low revs too you know - for less weight.)
Dick has had a lot of practical experience, - I wouldn't want to argue with any of those guys - (not in their league ) but I'm just exploring all the possibilities - no doubt it has all been discussed before somewhere. .

Update - Jim, that was tried by Bomo in Germany but on a boxer twin, a good idea but for a single it may be more practical, ( no long spindly pushrods).