"1/2 A" & "1/8 A" airplanes These are the small ones...more popular now than ever.

Shaft driven props

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Old 07-14-2012, 07:52 AM
  #1
Max_Power
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Default Shaft driven props

I'm not sure this would be something best scaled to 1/2a..., But this is one of those ideas that has been in my head since about 12 years old. Has anyone ever seen someone try to make a twin with a single engine? My mind's picture has always been to have a engine mounted in the middle of the wing with a carbon or aluminum tube going from the mount out to where wing mounted "propshafts/engines" would be right behind the spar . seems like about12-14 inches between props or less. ...idler prop shafts with bearings and pullies at the ends and all be connected by vacuum cleaner or small cogged hobby belts (aka micro-heli tail drive belt) You would just start the engine in the center and let it drive the props at the ends. A variant of the idea would be to use it as a way to have a mid engine front propeller model like a P-39 Airacobra? Seems like it could be built fairly light. Its hard for me to imagine this has never been done in a model before. What were the failures? I guess I'd be thinking something like a torquey old .15 pulling a pair of 6x4s? on a airframe like a twin otter or something not too complex. If I thought it wasn't a loonie idea I would say using a pair of Trex 450 tail rotor gearboxes and belts would be cool and actually have variable pitch as a bonus. Aim and Shoot![8D] Todd
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Old 07-14-2012, 09:05 AM
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Default RE: Shaft driven props

I an not sure why you would want to make the power system so complicated when there are so many other more simple and reliable alternatives?
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Old 07-14-2012, 09:29 AM
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Default RE: Shaft driven props

I think a few of the pluses of these ideas if workable would be: For the twin....one engine, one fuel tank and throttle servo, and all the plumbing/linkages for throttle. No "one engine out" scenarios. mass of weight in center of plane.

For the "Mid-Engine" the main advantage would be being able to model long nosed models as a option besides building a pusher. Especially on making something with a long skinny nose like a F-5 or something.

So we can satisfy your sureness of why I would want to do this, what are the simpler and more reliable alternatives in these cases? And just in case its what you are thinking....I am not interested in electric solutions. A lot of the models I would consider this for are already ARFs in foam with EDF,etc but its just not cool enough for me, LOL Like I said...most of these ideas have been lingering in my head for 25 years back when electrics werent a option really and the availability and cost of engines was a different situation for a daydreaming kid, So I'm just trying to see if anyone else has thought about it. Again I'm interested in the simpler and more reliable alternatives that perhaps I hadn't thought of? please expand Todd
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Old 07-14-2012, 11:26 AM
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Default RE: Shaft driven props

15 or 20 years ago I had a M.A.N. magazine and in the "Small Steps" article was a picture of a B-17, I believe it was built by Bob Kress. The caption said it had a belt drive to pull the 4 propellers with a single .049 T.D. (maybe .09) in the fuselage if I am not mistaken. Anybody else remember that? There was only one picture, if I remember right it showed the bottom of the B-17 with the T.D. head sticking out of the bottom of the fuse and virtually no other information. I hate when they tease like that, I want pictures and details.

I also have another magazine somewhere with a C.L. P-39 powered by a .15 size boat motor that was water cooled, the model was complete with a home made radiator and cooling fan. It was mid engine with shaft drive to the prop.


Austin
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Old 07-14-2012, 11:30 AM
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Default RE: Shaft driven props

An 'Aeromodeller' article in the 1950s did something along these lines-it used a flexible drive from the engine backplate to the other nacelle-so was a 'twin' in that sense but did not use a central engine position. I have the AM issue somewhere and will see if I can find it. Again-I'd have to agree with skyhawkjock-why go to the trouble when there are simpler arrangements-for a start did you give any consideration of how you would cool the engine?.............

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Old 07-14-2012, 11:57 AM
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Default RE: Shaft driven props

Most of this is thinking out loud like I said. The twin"otter" I had daydreamed about when I was young was a fire up/toss/ fly 1/2a type job so I figured the engine sticking out the top in the wind would keep it cool. I think I was going to build it right after my F4-U corsair Guillows/Cox model with throttle, flaps,retracts, tx-retractable canopy and firing bb-machine guns.

I guess what re-sparked my imagination today was people talking about long nosed fighters like the F-4 in which case I was just assuming a engine 4" behind the prop would still get enough blast to cool it. Austin, The B-17 sounds neat....I'll have to look around and see if I can find that picture. ffkiwi, what would a simpler arraingement be if someone wanted to build say a .049 tractor long nose prop jet with a long slender nose? I'll reiterate ...I'm looking less for people to try to talk me out of what is likely a hair-brained plan(I realize it isnt normally done for a reason...I'm not currently planning a build on these ideas), and more just for conversation/curiousity on if anyone has seen or tried these setups and besides for what SEEMS like it would be a issue...What actually WERE the results and problems? I've just never seen mention of it, yet am sure mine isnt the first mind it has crossed.
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Old 07-14-2012, 12:19 PM
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Default RE: Shaft driven props

Give me a bit of time and I'll locate and scan the AM article...............the other issue I can see would be loading the engine-I'm not sure how well it might run-(regardless of cooling issues!) without a prop directly attached-some form of flywheel might be required........which I suppose on reflection-could be in the form of a fan to assist cooling.........

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Old 07-14-2012, 01:07 PM
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Default RE: Shaft driven props

Yeah, the "flywheel effect" needed, even on a engine running WOT is one of my concerns. How far one could go with the drive pulley/starter cone maybe,fan could quickly approach being the weight penalty of the second engine with the drive system included. I think on the "Mid engine" setup you could get away without it if the prop shaft was rigid enough and there was no transfer or reduction gear. It would be great to see the article if you come across it. No hurry for sure as I'm just kinda pondering most of this. Thanks, Todd
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Old 07-14-2012, 01:35 PM
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Default RE: Shaft driven props

The main problem is weight and cost. there isnt any "propeller shafts" made this small. regular carbon and aluminum shafts are not balanced and true as needed. take one of these shafts put a beaing on one end then hook the other end up to a electric motor. now spin it up to 15,000 or more rpm and see how bad it is.

now you are close to the right track with the Tail rotor idea and belt drives. this set up would work the best as you could run it in a 1-1 ratio off the engine and a short shaft extension to the prop. this setup is best for a twin engine plane.

now if you have loads of money then anything can be done. The main thing it just isnt feasable.
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Old 07-14-2012, 03:17 PM
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Default RE: Shaft driven props

Quote:
ORIGINAL: airraptor

................there isnt any ''propeller shafts'' made this small..................
Why not model boat running hardware? Dumas still offers the .049 powered SK Daddle Jr kit.
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Old 07-14-2012, 03:50 PM
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Default RE: Shaft driven props

If I do it I would end up with just a straight aluminum shaft with a dogbone on the engine end....and there would be no angle really anyway. Just a step on the prop end and a small thrust bearing to take the thrust load. I think for the 4 or 6 inches I'm thinking of(from nose to about wingspar on a .049-25 plane) it would be workable as far as balance...as RC helis run dogboned aluminum shafts at 10K for 2-3 feet with only 1 mid-support, and dremel flex drives seem to work OK at 30k. I have the machines to manufacture most of this hardware so I really would'nt have to locate things or spend a ton of money. If I did a twin with tail rotor boxes I would just use the tail rotors for props, and have a servo to manipulate pitch. I definitely have a few projects in front of even making plans. I'm trying to finish my SWR tonight, So maybe by the time I'd be building one a few folks will have already done one, LOL Todd
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Old 07-14-2012, 08:42 PM
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Default RE: Shaft driven props

It’s been done boys!
I have seen a scale Bell Boeing V-22 Osprey with one turbine in the middle.
I think its on you tube.
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Old 07-15-2012, 07:52 AM
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Default RE: Shaft driven props

as I recal, not to long ago the idea was used in a pby with a 60 size engine. I believe it was in RCM.
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Old 07-15-2012, 09:30 AM
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Default RE: Shaft driven props

Here's the front end of a Heinkel-112 by Walt Musciano from a 1947 issue of "The Model Craftsman":

George

Edit: Forgot to mention, look how they misspelled his name on the article. They had it right on the plans.

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Old 07-15-2012, 05:01 PM
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Default RE: Shaft driven props


Quote:
ORIGINAL: nekked_man_2000

15 or 20 years ago I had a M.A.N. magazine and in the ''Small Steps'' article was a picture of a B-17, I believe it was built by Bob Kress. The caption said it had a belt drive to pull the 4 propellers with a single .049 T.D. (maybe .09) in the fuselage if I am not mistaken. Anybody else remember that? There was only one picture, if I remember right it showed the bottom of the B-17 with the T.D. head sticking out of the bottom of the fuse and virtually no other information. I hate when they tease like that, I want pictures and details.

I also have another magazine somewhere with a C.L. P-39 powered by a .15 size boat motor that was water cooled, the model was complete with a home made radiator and cooling fan. It was mid engine with shaft drive to the prop.


Austin

I remember that.... Don't remember the engine used, but IIRC, it used pulleys and long O-rings to transmit the power.
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Old 07-15-2012, 05:17 PM
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Default RE: Shaft driven props

I saw Dan Banjock had a semi scale dh2 control line stunter. The motor was in the nose and about a 10 inch shaft turned the prop, at the rear ofthe nacell. He could do the whole stunt program wiht it. possible but impractical. Had to launch quickly ot hte motor would overheat.
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Old 07-15-2012, 10:14 PM
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Default RE: Shaft driven props

I recall an article about a 1/2a proto that had a central engine and the prop shaft was a slot-car axle. Smart!
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Old 07-19-2012, 08:09 PM
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Default RE: Shaft driven props

Hi Folks,
Actually, I have a model under construction that uses one motor to drive 3 props. I'm building a scale Caproni CA 42 tri-plane bomber for free flight Flying Aces power scale competition. Internal combustion engines get big bonus points in this class and multi-prop units even more. Now I know that it would be simpler to have three engines but in free flight, you want all of them running the same all the time and stopping at the same time. I have decided that running 3 props from one engine would be more reliable but it is also more demanding to build.
Before I started the project, I had to make sure that the power set-up would work so I built a test unit with the same dimensions as I would need for a 36" span model. I also wanted it diesel powered so I didn't have to use fuel proof finishes. The engine I'm using is a scaled down repo of the famous Mill .75 cc. The engine shown in the photos has a displacement of .020 cu. in.
The photos show my first version which I could not make run. These long stroke diesels need a lot of positive flywheel effect and my rather flexible universal joint and thin drive shaft of the main pusher drive did not provide this. The engine would run the tank dry oscillating back and forth on the compression stroke CW and CCW. I subsequently stiffened up the universal joint ( you need a universal joint if you don't want to wear out your engine's crank shaft bearing) and made a new 1/4" dia. hollow drive shaft. The engine now runs well and the set-up produces good thrust but you cannot hand start it, I have to use an electric starter.
I'm happy with the unit and can't wait until I have it in the airframe.
I hope this will encourage you to try this scheme. Contact me if you need more info.

Orv.
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Old 07-19-2012, 08:40 PM
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Default RE: Shaft driven props

Now there's a challenge for a F/F scale model!................actually great effort-but you picked the worst of the Mills to try it on! The 0.25cc replicas have a reputation for being cranky and uncooperative. The slightly larger 0.4cc is a much better choice-though if you really want to keep to this size the Schlosser 0.25cc would be the bees knees-as well as being ball raced which would help the shaft loading issues a bit.......

ChrisM
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Old 07-19-2012, 09:06 PM
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Default RE: Shaft driven props

so which prop do you flick to start it?
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Old 07-20-2012, 03:08 PM
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Default RE: Shaft driven props

Hi Chris & Rendegade,

Chris:
I wanted a long-stroke-side port diesel because I need the torque to drive the three props. I also wanted an engine that would run CW (viewed from front) so that I could use standard Cox 4.5 by 2.5 tractor props.
The engine I actually have is the .4 cc Mills Repo (which is close to .020 cu. in.). With a 6 X 3 prop mounted, this engine starts first flip every time and runs in a very reliable manner. It doesn't however like the compliance in the drive system so I have to use an electric starter to get the rotational inertia up to a value that will keep the engine turning over, then it is quite happy.
Because the props are so far away from the engine, I have a cooling problem which needs to be sorted out. I may have to machine a larger finned cylinder head or some other style of heat sink... any ideas?

Renegade:
The engine can't be flip started as I said above and have added a starting cone to the main pusher prop so that I can use an electric starter.
You will also notice that the two drive belts are fairly loose so as to lower the running friction and will slip cogs if you stall the tractor props.

Orv.
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Old 07-20-2012, 04:19 PM
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Default RE: Shaft driven props

Orv! Thanks a ton for sharing that! Awesome work, it seems you have accomplished both of the ideas floating around in my head at once! This is exactly what I wanted to see. So do you think the airflow from flight will help cool it any ?(I'm assuming it has only been benchrun?) Or a beer can "Shroud"? I'm definitely interested in following this project. You don't happen to have any video runs do you? AWESOME MODELING!!! Todd
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Old 07-20-2012, 04:57 PM
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Default RE: Shaft driven props


Quote:
ORIGINAL: ggeezer

Chris:

The engine I actually have is the .4 cc Mills Repo (which is close to .020 cu. in.). With a 6 X 3 prop mounted, this engine starts first flip every time and runs in a very reliable manner. It doesn't however like the compliance in the drive system so I have to use an electric starter to get the rotational inertia up to a value that will keep the engine turning over, then it is quite happy.
Because the props are so far away from the engine, I have a cooling problem which needs to be sorted out. I may have to machine a larger finned cylinder head or some other style of heat sink... any ideas?


Orv.
OK-you have the .4cc -which works out at .0244 cu in (nominal) [I always think of 020 and the like as being the upper limit-so assumed you meant the smaller 0.25cc Mills replica.] The 0.4cc one is a real gem-I have two of them-and they're both great on 6x3 and 6x4 props. Your idea of a larger finned head should work fine-but you might have to try several sizes to get the optimum cooling-and you'd probly have to try it out on the actual model, because bench tests would not replicate the in flight airflow. You might also have to consider making it asymmetric in shape (more finning at the front) so it doesn't prevent access to the needle.........

ChrisM
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Old 07-21-2012, 04:19 AM
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Default RE: Shaft driven props

Interesting project Orv. I was wondering how true-to-scale your Caproni will be. Do you intend to add some dihedral to it? An RC model wouldn't need any.
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Old 07-21-2012, 05:56 AM
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This is a very interesting thread! I'd like extensions for some of the Menasco powered race planes of the '30's. They had such long noses and are difficult to balance because of it.
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