Sometimes you just have to try it !
There's an electrified Sparky,at 64" that caught my eye,making me wish it was larger,though.
So I carried the plans to be enlarged to roughly 1/4 scale,if there had been a full sized one.
At 175% of the original model's size,this resulted in a 114" wingspan.
Another thread asked about scaling up a 24" model to 48",and the associated problems.
I should inject here,that I've been enlarging plans since 1977,beginning with the enlargement of a Sterling Piper Super Cruiser to 12',twice the size of the original.
Sterling's models were built with hard balsa,which allowed dull die-cutters to force their way thriugh the wood.(die crunched kits),we called 'em.
INSERT: WHEN I STARTED THIS THREAD,I THOUGHT SPARKY WAS ORIGINALLY 64"...NOT 32"....Sorry...now your math will work..

Sterling Models built strong,hefty models ideal for my flying style.I like scale flying,mostly 'circle-plane' stuff,just relaxing.
Enlarging those plans resulted in learning ways to reduce the size of the timbers that were presented....a 1/4 x 1/2 spar didn't need to be 1/2 x 1" now.
So in the case of the spars,they could remain at 1/4 x 1/2",but now with shear webbing added.....stronger,with less weight.
1/8" thick ribs could still be used in the wing and tail,and the sheeting didn't need to be thicker either.

ENTER SPARKY..The first thought I had,on seeing the larger plans,was that here was another model that would not need a lot of modifying.
Some of you may know that I collect wing spars from full size restorations,to use in constructing my babies.-Up to 16'of free Aircraft Grade Certified spruce.free.did I say it is free?

Sparky looks like an Anteater...He has a long snout,like me.
I believe it was originally designed for rubber power..Since elastic doesn't weigh so much,the aircraft nose needed to be 'way long to balance the tail assembly.

I'm leaving it long. Typically a 1/4 scale Cub's C.G.is 15-17" from the prop.This one is 25".But it ain't gonna be rubber powered.I want to use an O.S.91 4 cycle as the motive source.
A lack of engineering savvy requires me to arrive at solutions differently than most folks. I'm thinking,' if I hang an engine out there,it's going to be nose heavy'!
Much discussion ensued,and the idea of installing the engine in the cockpit originated,with an extension shaft to the prop.Roughly 25" of extension shaft!!!

Boy did the cages start rattling then ! -can't-won't-don't-go for it! I chose the last,for two reasons.I have a P-39/P-63 envisioned in future,so let's encounter the pitfalls now,and continue along,or scrap the whole idea.

Once I get the stab and rudder framed up,I can do a dry run to see what will be required in the way of balance compensation....If I can simply plop the engine in and go,fine.If not,we get to try my little experiment...-that's called waffling-!

SO!-does anyone here believe in fate? stevster has plans for sale for a 92" P-63
I'm going to PM him momentarily.

All I can say is....



I don't recall if I said much about the extension shaft in the other thread but I suspect that you're best off to go with a totally idependant shaft and use a high power RC power boat universal to connect it to the engine. Now boats use a shaft within a stuffing tube to keep things reasonably water tight. In the case of an airplane likely an exposed shaft supported by ball bearings would be a better option. For vibration and impact loading issues I'd suggest that some medium thickness chrome moly aircraft tubing would be a wise way to go. Something in the manner of 5/16 to 3/8 OD with a bearing at each end and one just slightly offset from the middle. At the forward end there should also be a thrust bearing so the radial bearings re not loaded axially. Or if you can find a suitable size angular contact ball bearing for the front then it would provide both the thrust and radial support you want.

Why do I say the middle support should be slightly offset? To avoid the risk of harmonics in the shaft causing runaway vibrations at some particular RPM. If you play guitar or know someone that does they can tell you about plucking harmonics from the string at specific frets. Pick the wrong fret and you just get a dull and short lasting "plump" sound instead of a high long lasting tone. The idea is you want that middle bearing at one of these "dead" points so the materials damp out any vibrations. It doesn't take much either. Something like a 54-46 split is enough.

The journey begins with the enlarged structure framed out,and the all important support for the proposed drive system.

The front bearing is an O.S.91 crankcase,with extraneous attachments surgically removed. Light aluminum covers will seal the holes to atmosphere,and the case will be partially filled with lubricant.the rear cover will accept an ordinary shaft seal,to contain fluid ..

A close look at the first picture reveals an O.S.91 mounted there temporarily.Notice the size of the muffler,compared to the fuse......I don't think "overpowered " applies here necessarily.

Ol' Earl posted on the wrong thread again.....hopefully that won't happen again.
Sorry.
Since this is a whole package,we're dealing with enlarging the structure as well.

The fuselage longerons were 3/16 " sq.,on the original...We just went to 3/8" sq spruce on this one,with a saw kerf groove down two sides of them.The groove accepts 3/32 x 5/16" spruce diagonals and uprights.The kerf locates these smaller strips,and allows a three sided glue joint.

So you are going to use the crank case as the forward bearing? Isn't the crank unbalanced without the connecting rod?

Bmatthews.would it matter which way I biased the support bearing ? Closer to the engine,or indifferent ? Plucking my Guitar string didn't teach me much about a shaft I don't have yet.
Further,I know a hollow shaft,versus a solid one is better for most variables involved in these dynamics..
My intent was to press the forward end of the hollow shaft into an interference fit hole in the prop support ,and forget about it..We intend to turn a 14 x 6 prop so the proper fit will be sufficient....
At the engine end ,I want an adapter that will screw on the crankshaft,and be locked by a nut screwed AWAY from the cylinder,toward the prop,to lock the adapter from coming loose with the starting force.
I envisioned the shaft sliding into the adapter,with set screws on opposite sides,to lock it down.
Is a universal joint necessary? I'd prefer not to use one.

I just realised I need to at least static balance the forward crankshaft,since I no longer have a piston and rod assembly rockin' around in there.

I think we may be up to the challenge....learning, learning.

The fuselage structure will have some flexibility to it so the shaft system NEEDS a universal joint that not only has some angular play but also some lateral and vertical play. The optimum setup for this is the model powerboat "dogbone" style univeral joint setup. Otherwise you need to ensure that the whole system from prop to backplate is held very rigidly in alignment with a degree of accuracy for side displacement and angular run out of any one part of at most a thou or two for lateral displacement and down to a few seconds of one degree for any angular mis-alignments.

It should be a good example that the RC racing power boats choose to use the dogbone style univeral setup because it works and avoids meticulous alignment issues and at the same time delivers good power. It's still best to try to align all the stuff from prop to backplate as well as you can. But the dogbone style universal setup will avoid the need for the alignment and nose structure to be inhumanly in alignment and centered while at the same time being able to handle the power well. And then add in a nose structure that is far less rigid than an all plywood boat hull and I think you will have to agree that "floating" the parts from each other by using at least ONE universal at the engine is a very good idea.

Frankly my own feeling is to use one at the nose and one at the engine so the shaft in between is a whole separate portion of the drive.

Okay a single universal it is,at the engine.I'll check into one today.
If my machine shop can get that shaft properly centered into the crankshaft,and limit run-out to a couple thousandths at the engine end,that should suffice.

I looked at the flexibility issue from a different view point......that same lack of ridgity in the structure should also be tolerant of a little misalignment.

I used to install 440 Chrysler engines in Greenwich Cabin Cruisers 43' long,built of wood.The propellor shaft had a 5" dia. flange,which mated to the flange on the engine crankshaft.The engines were mounted on articulated mounts ,which allowed adjustment three ways.You could raise,lower,tilt or move fore and aft with those mounts.The flange to flange mating would be adjusted to a tight fit for a .002 feeler gauge,in any direction.
After a season's use,reinspection could reveal a shift of up to .012,without noticeable vibration.I assumed the shaft could tolerate that misalignment,in it's 9' length. My eventual solution was to install metal plates to prevent the engine mount foot from crushing the oak wooden beams the mounts sat on.

This looks to be an interesting plane.
I enlarged a Sparky 10%, I thought that was big.

Originally Posted by pd1

This looks to be an interesting plane.
I enlarged a Sparky 10%, I thought that was big.

It is big-it's at 114"..It's something I wanted for an 'oldtimer'look and some serious relaxation.

Originally Posted by aerofly0610

So you are going to use the crank case as the forward bearing? Isn't the crank unbalanced without the connecting rod?

You betcha' ! we think removing weight from the counter balance is the better choice,rather than welding on a weight. We don't need the pin or any thing,just enough to shoulder the bearing..I believe the rear bearing is the thrust one.