I 'normally' fly my 1/4 scale Cub,with a .91 4 stroke,at 4,200 rpm...I say 'normally',because we set the high idle trim to that on the ground,with a tach,and it will gain a few r's in the air,unloaded.

Once airborne,we pull the throttle to idle,increase the trim to 'full' and fly on the wing.

Not forgetting Sparky is a "whim" aircraft,I won't expect maximum performance from it.....but rather a trip to History when we see it fly.

Old eyes don't see as well from a distance,and the larger size will rectify the smaller image for those guys who actually flew the originals.

And I'm thinking using the original blade holders might allow folding prop blades if I became so daring. The clutch/reduction should allow some softening of the power impulses so as not to over-stress a well built prop which looks quasi scale.

A folder would be even more amazing. And with a centrifigal clutch I can't see why it could not work. But you'd want some spring give to the "prop stop" which traps the prop in the right position to allow the blades to fold correctly. But the hub and hinges WOULD be a big step up from what we get away with on a 13 inch rubber model. I'd suggest that this is something you could look into later on after the model is flying with a fixed freewheel prop. And to make your own freewheel prop would be structurally a lot easier to start with.

And if you don't mind lugging the .91 a little I don't see why the 8.5:1 reduction won't let you spin a 3 foot free wheeler at around 950 rpm. Which with the gearing ratio would be around 8000rpm. Not that you need to run it that high if you don't want to. But a burst of full power for the first 5 seconds COULD replicate the power burst of a fully wound rubber model version nicely. The small ones with contest like power climb for the first few seconds with the nose up around 60 degrees for a good circle or so before the power fades to more of a climbing cruise. The key would be to find out what sort of diameter and pitch the engine can tolerate to still run reasonably. And I don't see any way out of it other than to make a prop and try it then base the next one on the results of the first.

So to star what sort of rotor diameter did the copter have that the engine and gearing came from?

The 'waffleing' is behind us.....The decision has been made to use the Helicopter drive system,the automatic clutch,and a SHAFT to reach the .40 size crankcase in the nose,and the helicopter rotor as the basis for the large prop.
If the OS.61 can be made to run in reverse,we'll use it as motive power......if not,we'll mount the OS.91 4 stroke,(which I prefer) and build a proper BRITISH prop (left hand)...
The Heli had a 44" rotor,which rotated at 2k....I've shortened it to 34"-dictated by prop ground clearance on the model....I'm using the rotor as a basis for building a folding propellor directly on (around) it. In order to lighten and simplicate,I'm using ONLY the blade holders....they will be ground adjustable for pitch,and modified to fold at idle.
Yeah-I know -about the weight....I don't care-I'm doing this !
And I'll have more than enough thrust to haul it around.With the adjustable pitch,I can limit prop speed to the amount of thrust I need...8,000 eng rpms isn't lugging it down,since I normally fly it at around 4k. Onward and?upward?

Should be interesting!!! Have you considered making the shaft a larger diameter to hold up to torque and resist "whip" since the shaft will be so long? I relize the spool up of the prop will be slow but there is still a great deal of twisting forces placed on the shaft.

Thanks,RCRobby,we discussed that early on,weaving between the shaft and a cable....I have a 5/16 I.D. hollow shaft,that will rotate inside one with clearance for the OD. (a tunnel) that will also contain the cable,which I will test later in the program.
I have a universal on the rotor end of the heli shaft.,and the forward end will be inserted into a .40 size two cycle crankcase,milled so it's only two bearings and an oil reservoir.

There's always some need to experiment. As long as you've allowed for removal of the whole system changes can be made and tried without having to cut into the airframe.

I don't know about the rest of the folks but I'm pretty excited to (hopefully) see videos of how it runs regardless of it being a success or failure. Failures, after all, are simply an excuse to learn more stuff....

Oh-I NEVER have failures- I just 'gain experience' lol.-and then-get THIS- some guy says use an electric motor,and you can make it SOUND like rubber !

He's prolly right...

Or we could just lace a couple of motorcycle tire inner tubes together and get this really big winder to put in some turns....

I queried the engine masters on reversing the rotation of the engine, to get the correct information....Don't want to waste time or energy doing the wrong thing.

On engines with three piece crankcases, the carb mount can be rotated anti-clockwise 90 Degrees and it will run fine the opposite direction..That allows me to use a standard prop,in this case a 32" x 14" pitch....or the built - up scaley looking one.
The second (lower) shaft from the drive unit is the starter shaft..I'll assemble a cone to fit over the probe,and I can start from the front of the model.
I just can't get a good picture of the set-up yet..this one's better,but foggy.
The unit will install from below,and be covered with a hatch,allowing a seamless fuselage top.I'm installing a rounded forward deck,rather than the flat faced original..I wanna do some 'strip' planking there.

I'm not a big heli builder. But if you can find a belt drive setup for the .91 four stroke you'd be able to let the engine turn in the original direction. Gearing reverses the rotation but a belt drive keeps the directions of rotation the same.

You betcha' ! I ain't done yet,just running with what I have.
I NEED the 4 stroke..We'll get it eventually,but I think the muffled 4 cycle will better serve my purpose.
I have some toothed belts off my Eastcraft Starters,with about the same reduction....
Now I've just learned there is a sprag clutch that will have to be locked,if I reverse the rotation.
I'm getting through the 'walls' but man there's a bunch of them.Blood,sweat, and tears.

Originally Posted by epoxyearl

You betcha' ! I ain't done yet,just running with what I have.
I NEED the 4 stroke..We'll get it eventually,but I think the muffled 4 cycle will better serve my purpose.
I have some toothed belts off my Eastcraft Starters,with about the same reduction....
Now I've just learned there is a sprag clutch that will have to be locked,if I reverse the rotation.
I'm getting through the 'walls' but man there's a bunch of them.Blood,sweat, and tears.

Can you flip the clutch upside down so the directions match the engine rotation?

No- 2 gears meshing will always go opposite rotations.Unless the drive gear is INSIDE the driven gear,which is not the case here.Reversing the engine rotation is the most reasonable choice with the equipment I have on hand.
Much too much work is involved for a more exotic result,until I have some numbers in front of me.....Engine revs,prop revs,generated thrust,and total weight.
We'll stay inside my comfort zone for a while.
The objective is to set up a running system on a test stand and learn as much as possible.I believe testing it in place is faster,without working in a finished airframe.
I'm prepared for failure-the engine mount is in place if we need to fly it direct drive with the .91....At present that spot is occupied by the milled down crankcase for the propellor mount.

The one way bearing can be easily reversed. Carefully drive the bearing out of the block it is in. NOTE which way it is installed now. Swap it end for end and reinstall. The end that came out first should be the end to go back in first. If per chance you ruin the bearing, do not dis-pare. It is not a "special" item. Go to a local bearing supply house and purchase a new one. There should be some ID numbers on one end or the other.

If you "forget" which way the bearing goes on. Just put the bearing only on the shaft and turn the bearing. If it grabs in the wrong direction, just turn it around. Sometimes theses bearing need a good cleaning and re-greasing to function properly.

With the stock setup I would set the pitch of the main blades at about 4 degrees to hover. That is with a 7 1/2 to 8 pound heli. Rotor RPM was about 1500 or 1600. Max pitch was around 8 degrees and it would climb like a home sick angel. At least for back in the day

For what it's worth. Century helicopters use a main gear with the drive on the inside of the gear.

We've traded the heli drive for a BELT drive. Getting the final drive to turn a prop the "scale" direction was hurtful with the heli parts.

The Engine Masters,whom I hold in high regard,indicated that I had 'way too much power for my requirements.-They said a .46 would turn that gianormous prop at the speeds I require.(32 x 14)...

I think 1500 to 1750 rpms should be fine for 10 lbs of thrust,and 25 mph flight speeds.

So here we are at the beginning-it's like deja-vu all over again! The cooling fan needs some thought,but that's a minor problem.I need to make a set of mounts to locate the out put shaft with the engine,and circumvent the fan. Hoo ruh !