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Old Nov 24, 2012, 07:21 AM
Marty M
Joined Nov 2012
5 Posts
Discussion
Great Planes P51 Mustang engine. Diesel?

Hi all.

I am new to IC engined planes so please bear this in mind when getting technical!
Basically I have just purchased a Great Planes P51 Mustang kit and very good it is too. As to the engine, the manual says either a .4 - .46 2-stroke or .5 - .7 4-stroke. Will someone please explain the pros and cons of 2-stroke vs 4-stroke? I have already noted that the former is somewhat cheaper to buy. What about a diesel in this plane? I plan to fly it in a 'scale' manner on a four blade prop for realism.
I think the current recommended engine for this kit is the OS 46AX II.

All comments gratefully received.

Mart.
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Old Nov 24, 2012, 07:40 AM
Complete RC Idiot Savant
The Netherlands
Joined Nov 2009
2,617 Posts
If you want to use a fourbladed prop, you only stand a chance when using a fourstroke, because most likely a 2 stroke cannot deliver the torque needed to spin that fourbladed prop, at the relatively low RPM it wants to be spun. But even with a fourstroke I doubt if it is possible.

But altogether, I think 4 bladed props are a bit too much asked for IC engines, you probably would have more succes with a geared electric if the 4-blade is a must.

If it was my choice, I would at all times choose a 4-stroke engine, just because the sound is better fitting to the character of the model.

Whether Diesel or glow, that is a choice. I would even go for spark ignition (since I discovered that, and how easy it actually is, that is for me the next best thing since the invention of sliced bread)

Brgds, Bert
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Old Nov 24, 2012, 07:42 AM
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United States, NY, Eldred
Joined Jun 2012
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If you are going for a scale warbird then you absolutely want a four stroke. A four stroke sounds more like a real airplane. A two stroke sounds like a ticked off bumble bee in a tin can. A four stroke generally has more torque as a two stroke gets its power through speed. As far as using a four blade prop, just know that the more blade the less efficient the prop. If all you want is a scale look while displaying then just get a static four blade prop. When the engine is running no one will know how many blades anyway, albeit a four blade prop may produce a different sound than a two blade. Generally you will get much more performance from a two blade prop, plus they are less expensive.
Also, four strokes are usually noticeably more fuel efficient...not a huge deal on smaller power plants, but when you start getting into larger displacement glow engines the fuel just seems to disappear in the two strokers.
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Old Nov 24, 2012, 07:51 AM
Marty M
Joined Nov 2012
5 Posts
I really would like to go with a four blade prop so it sounds like it will have to be a 4-stroke engine. Any recommendations? Also a suitable prop? Model is .40 scale.

Mart.
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Old Nov 24, 2012, 08:02 AM
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United States, NY, Eldred
Joined Jun 2012
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I'm gonna let some people chime in that either have tried a four blade prop on a 40 size model but I strongly agree with Brutus. You really may have a tough time getting a power plant in a 40 size engine, whether 2 or 4 stroke that will have enough authority to swing the prop. Also take into consideration that even giant scale war birds generally use two blade props...it is for the reasons previously mentioned as far as performance. In all honestly I may even be surprised if a 40 sized warbird would even get off the ground with this setup unless maybe you over power it...but then there goes the "scaleness" you are looking for.
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Old Nov 24, 2012, 08:20 AM
Marty M
Joined Nov 2012
5 Posts
This is the beauty of forums, experience is everything. I will take your advice and go for a two blade prop for flying and a four blade for display. Same question then, any recommendations as to an engine / prop combination? The manual just concentrates on a 2-stroke, no recommendation is made for a 4-stroke at all except up to a .7.

In addaition I have seen scale 'real' exhaust manifolds ie the model's engine sends its exhaust through the twelve 'real' RR Merlin exits. These are expensive I believe but look awesome. But, do they work efficiently?!

Mart.
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Old Nov 24, 2012, 08:29 AM
Complete RC Idiot Savant
The Netherlands
Joined Nov 2009
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Oh, it will get off the ground, IF you manage to build a reduction gearbox for it.
The fourblade being less efficient is basically a myth, it is a matter of the most efficient RPM of prop and engine being too far apart. At the correct RPM, a fourblade will most likely even be more efficient because it calls for lower RPMs and thus less friction losses between blade and air will be less.
Most people that have tried multiblade props have done so on normal IC engines, with small props in order to allow the RPM's required by these engines OR with "scale sized" props forcing the engine to run at far too low RPM, and wrongfully concluded that multibladed props are less efficient. They are not: the set-up as a whole is less efficient....
Let's put it this way: if the model calls for a .40 2 stroke, and you put a .60 on it that is only running 8000 RPM, you are not overpowering the model, because that .60 will have approx the same output at 8000 RPM as a .40 at 12000 RPM.
With an electric motor and gearbox, it will most definitely work.

But fourbladed props are more expensive and more critical in balance, and one more thing: the 4 blade prop will NOT make your model sound more "fourstroke-like".... Only a fourstroke will make your plane sound like a fourstroke....

So... if this is your first venture in IC power, best keep to proven engine/prop combinations: just go with what the model manufacturer recommends for engine size, and go with the props, the engine manufacturer recommends.
Clean and simple, and much more chance for success....

The same goes for those beautiful scale exhaust manifolds: better leave them alone, until you have some IC-experience. Even if that means you are proficient at starting and adjusting your engine and nothing more, that will be a very big advantage over having beautiful dreams but not a clue as to how to solve the problems that might arise from them.

Brgds, Bert
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Old Nov 24, 2012, 11:09 AM
Hutch
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United States, Mt, Bozeman
Joined Jan 2012
313 Posts
Saito .82 if you are going with a 4 blade, weighs less than the .72. Lots of power & should fit just as well as the 46 ax or the like.
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Old Nov 24, 2012, 11:26 AM
Marty M
Joined Nov 2012
5 Posts
Thank you, extremely informative! To ask is to learn.
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Old Nov 24, 2012, 12:19 PM
Marty M
Joined Nov 2012
5 Posts
RCV Engines do a four stroke with a 2:1 reduction gear which, they say, is specifically for scale warbirds on, if required, a four blade prop. There are videos on their website displaying the performance of this installation and they look pretty good. The SP 60 is around two hundred quid.
By the way the Solo Props look pretty good too. Possibly a good combination there?
Regarding the scale exhausts I am going to give it a go. These would be easy to make and if they don't work well they can come off again.

Mart.
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Old Nov 24, 2012, 12:55 PM
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Quote:
Originally Posted by kimchiyuk View Post
A two stroke sounds like a ticked off bumble bee in a tin can.
I couldn't have said it better myself!
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Old Nov 24, 2012, 07:00 PM
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United States, MD, Elkton
Joined Oct 2011
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How about a compromise ? Use a Saito .82,which is about as heavy as you can get away with, and a 3 blade prop. Master airscrew makes some very affordable ones,and they have a large selection to choose from.
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