Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

props size , what to choose ?

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Old 05-02-2008, 06:27 AM
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flying dutchmen
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Default props size , what to choose ?

Hello RC fans.

I hope that somebody out there can give me an easy an good explanation about prop sizes.
I have for example a Tucano /wingspan 1500 mm. with an OS 46 AX. The prop i have on there now is a 11 /7.
It flys pretty fast ( for me as a beginner anyway ) but if i go fertical up i stops pretty quick.
What size of prop can i try for some more pull. ??
What does a bigger pitch do ?? And what does a bigger prop ( longer ) prop do.
I hope that some of you can explaine me a few things about props.


Thanks a lot and have rc fun.

Regards Flying dutchmen.[)]
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Old 05-02-2008, 09:18 AM
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bob27s
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Default RE: props size , what to choose ?

Quick lesson..... [8D]

The prop selection has to be a balance of the needs of the aircraft and the needs of the engine, and also balances your desired performance.

Unless you are under noise restrictions an 11x7 is WAYYYYYYYYY too big for the OS .46 AX engine. It will run with that much load, but it is nowhere near producing its best power and is not likely allowing the aircraft to perform at its best either.

(If you are under noise restriction, perhaps back off to an 11x6 - not that much louder, engine will be happier. Otherwise, read on)

This is a 2-c glow engine. RPM = power. The balance comes from allowing the engine to develop its power using a prop that works well on the airframe.

Addressing the OS 46AX engine (stock, using the stock muffler or any other muffler):

For best engine performance, select a prop that allows the engine to turn a minimum of 13,000 ground peak rpm.

Selecting a prop that permits the engine to perform between 14,500 and 15,000 ground peak rpm is your best target.

Is there a maximum rpm? Not really, the engine is not going to break. But experience has shown that 16,500 peak rpm is a good top limit when selecting a prop.

Needs of the aircraft:

You have a 1500mm (59 inch) span model, probably around 5 lbs - typical 46-50 size aircraft. Semi scale, but generally a clean design.

For speed, you want to use a smaller diameter, higher pitch prop - again keeping in mind the rpm targets above.
A good choice for your OS 46AX in this application would be a 9x7 or 9x8 for this approach.
Down side here may be a slightly longer take-off run, and perhaps reduced vertical (but not by much)

For vertical, you want larger diameter, lower pitch - again keeping in mind the rpm targets above.
A good choice for your OS 46AX in this application would be an 11x5 or 12x4
Down side here is reduced top end speed, but you will get GREAT take-off performance and more consistant airspeed.

My suggestion for an over-all balance of performance - leaning toward a little more speed - would be to use a 10x7 prop.

I hope this is helpful !

Bob Brassell
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Old 05-02-2008, 10:09 AM
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flying dutchmen
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Default RE: props size , what to choose ?

Thanks Bob ,

Great explanation. Thanks for your answer.
I am going to try a few different props in the range you sugested.


Have RC fun.

Regards, flying dutchmen
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Old 05-02-2008, 10:18 AM
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ChrisAttebery
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Default RE: props size , what to choose ?

My favorite prop for vertical performace on the 46AX is the APC 11.5x4W. You could also try the APC 12x4.

Good luck,


Chris
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Old 05-02-2008, 10:39 AM
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Default RE: props size , what to choose ?

My favorite prop for thrust on this size engine is the APC 12.25x3.75. It will keep the R's up and is not as heavy as some of the other "wide" props. Worth a try.

Blessings, Terry
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Old 05-02-2008, 11:45 AM
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Default RE: props size , what to choose ?

My experience is that the 12.25x3.75 is slower to spool up but has slightly more pull. The 11.5x4 spools up quicker and has is slighty faster.
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