Heck78

Is there someone who has build or maybe tried to build a model with middle engine..? maybe it´s stupid but i´m going to do it, one way or another. There MUST be someone who has tried!

adamtina6

I have never done it, nor have i seen it in anything other than Ducted fans or Jets. I applaud you innovativeness but I am curious as to why you would want to???
Keep posting pictures and info as your project progresses please.

Heck78

Maybe I only want to try something weird...[&:]
It's going to be kind of jetplane and the nose is coming to be quite long... want to try with quite big engine and otherwise it's going to be nose heavy, badly... The plane is my own design and I hope it will be able to fly too...
I will add some pictures when I get something finished... Thanks for your interest!

Tall Paul

I believe most attempts at this fail because structural difficulties keeping the prop shaft aligned all along its path. Any flex creates whipping in the shaft, and that will yank the front bearing out of its mount.
There may be more material needed to keep the shaft in place than the project can handle as extra weight/complexity

Heck78

The shaft is going to be 445 mm long (17,5") and has 2 bearings in it's way, but you might be right. First I tried with carbonfiber shaft but it didn't work really... I have a few troubles to solve... [>:] But I'm going to do it before I die!!! [8D] maybe I have only 50 years left...

Boomerang1

I saw a similar system on a scale pilatus porter, a high wing beast with a long thin nose. The shaft was much shorter than yours and only supported at the prop end, seemed to work fine. Cooling may be an issue for you, the porter had an intake directly in front of the engine & an air duct about 200mm long. - John.

Heck78

Thanx boomerang! Yes, it's going to be some kind of intake, but not in front... Do you know what was the shaft made of?

Heck78

I have been thinking a while that cooling system, but maybe it won't be so easy... But the most worried I'm for the shaft...
If you know some more troubles witch I can get so please write it. I haven't think clear all... I'm thinking and building at the same time...
C.G is going to be quite problem, I think.

greenbaron

Do a search for Shinden in Rcscalebuider.com
There is a similar arrangement well documented there. I hope this helps.

Heck78

I will. Thanks for hint.

krossk

My Feedback: (27)

Take a look at the drive systems for nitro race boats. The engine is mounted mid-hull with a flex-shaft running through a brass tube and connected to a stub-shaft riding in bushings or needle bearings in the strut. It's not uncommon to spin a metal prop at 20K+ RPM. I see no reason that something like that wouldn't work for this.

Heck78

I'll begin the search of boats immediatly!!! If boats use this method it must been working on the plane too, somehow. I don't want to finish the fuse before I'm sure it will hold all this mechanism. It will hold much more when it's finished so if it holds like half finished it will be no problem later...
Thank you krossk, I believe this will be something I can use!

krossk

My Feedback: (27)

No problem - One more thing, though.
I presume this is a "pusher" configuration? The key to boat drives is that the strut bears the load on a thrust bearing from the rear.
If this is a conventional "pull" setup, you will most certainly have to factor in the load on the front. Most high-performance boat systems utilize a "square drive" mechanism where the flex shaft floats in a drive collet on the motor. A fixed collet is a bit more sensative in that you ahve to factor in the changing length of the flex cable. That type of hardware is very much a cottage industry, so you won't find many of those parts in a LHS or Tower Hobbies.

Heck78

It won't be a "pusher"... I'll try one more time with my own way and then I'll use somekind of flex shaft. You don't have any good pictures of that kind of shaft? I think I must do the parts myself... Maybe you can guess that parts are sure much more difficult to get in here, in Sweden...

Tall Paul

There's a lot more load with an aerodynamic prop than a boat prop.
Gyroscopic forces will be higher.
The flex shaft probably won't handle an airplane propellor.

Heck78

It was a good point, and thanx for that.
No, it can't be only flex shaft, it must have some "hard" shaft nearly prop, with bearings. But maybe there's something good with flex shaft. First I'm going to try one more time with my "carbonfiberpatent"... [:@]
Now I'll add some metal too...[&:]

Hardy_H

Have you thought about using helicopter gear parts? Just a quick impromptu idea, put the prop on a short axle with a smaller toothed wheel, put a wheel of the same size on the engine. Connect these using a longer axle with two bigger wheels at the ends. All of these parts would have their own vibration, but maybe it woulsd sum up to less than the vibration a long high-rpm axle. Maybe the play between the toothed wheels can swallow some of the vibration. Or maybe you could use belts instead of toothed wheels?

[edit] Talking about engine cooling, maybe you could include a heli cooling fan also.

Heck78

When I got this idea to try the middle engine I thought if I could do it with toothed wheel and change the gear ratio at the same time for more rpm but prop doesn't need or hold much more rpm... And now I trying to keep it so simple as possible... That cooling fan begins to sound interesting, going to check how these looks like. I know nothing about Helis...
Thanks for ideas, it's not impossible these heli gear parts either...

rctrax

I saw a model of a p-39 at the AMA museum that had a center mounted engine with a long shaft running to the front to drive the prop. if I remember right thru a gearbox.

BMatthews

I agree that a boat shaft won't work. For starters the prop has far more flywheel effect and that alone would put a lot of bending loads into the shaft. There's other loads as well that make a slender boat powershaft less than ideal.

A better option would be a frame with a tubular drive shaft and probably three bearing points. The tube would be something like 3/8 or even 1/2 inch thinwall steel with pressed in machined ends pinned and silver soldered. A center bearing would help avoid any whip from developing. Also the middle bearing should be very slighty to the front so that any chances of harmonics are damped. The whole propshaft and bearing assembly should be mounted in a metal frame that holds the bearings in fine alignment to ensure the least possible vibration.

At least that's how I'd approach it.

And of course there would be lots of ground tests prior to installation to test it out. And I'd be standing WELL back .... just in case.... and with a good faceshield on.... and body armor....

BWooster

What about reducing the shaft speed through a reduction gear, and turning a bigger prop?

Tall Paul

Bruce, you'll be safe. It's in Sweden!
This is something that should be accomplished in stages, lengthening the shaft a bit at a time, and accumulating some runtime on each to see how the support structure is working.

Heck78

If you would know what I'm wearing when I'm trying to start the engine...
It really won't start with boat shaft...
I can't put so big prop, the plane has only 100 cm span and 120 cm as lenght, don't want at rectracts are 35 cm high... It could look like...something funny...
The engine is going to be Webra .61 Racing... quite heavy for so small plane...

BMatthews

You're right to make it direct drive. Geared props are great for climbing but this is obviously supposed to be a speed model and for that you need high RPM's and small props with lots of pitch.

There's enough torque in this that I think you need to treat the engine, shaft and support tray as a sub unit separate from the model itself. You may want to consider an arrangement similar to the speed pans used in the old control line speed models. But in this case the engine mount and shaft supports would be molded in glass and carbon fiber and form the lower half of the fuselage from the prop back to the engine. Doing it this way you could use the outer curves as a good basis for a stiff frame. Then a lot of stiffener bulkheads of aircraft ply with carbon rod or tube stiffeners to support the shaft bearing mounts. The bearing mounts should be made from aluminium for durability and the mounts should have flanges or fingers that allow lots of screws to bond them to the plywood pan.

Another option would be a slightly lighter pan with a machined out aluminium chasis to hold the shaft and bearings that then screws into the lower pan. This way the frame would ensure good bearing alignment and having the fram mount in the pan would help support the bearing frame and soak up a lot of the vibration.

At least that's what comes to mind at the moment.

Hydro Junkie

BMatthews is right on the flex shaft and bearings. A flex won't work since it will unwind when you try to start the engine. On a solid shaft, you need to keep bearings no further apart than 5 or 6 inches. Any further apart and the shaft will whip on you and destroy the bearings and the shaft(been there done that). I would recommend going with somewhere in the 8-10mm range on the shaft, as you will be putting a lot of torque on a 17.5" shaft when spinning an 11" prop with a .61.