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Difference in performace with 3 blade prop?

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Old 12-11-2008, 08:37 PM
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blhollo2
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Default Difference in performace with 3 blade prop?

hello, i have a great planes cherokee with a OS 55.AX engine slinging a 12x7 prop, is there a performace difference in a 3 bladed prop? Im asking because the cherokee sits low to the ground and a 12x7 or larger prop tends to hit the ground on landing and taxing Thnaks for your input!!
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Old 12-11-2008, 09:53 PM
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Default RE: Difference in performace with 3 blade prop?

Slower top speed, better climb if you pick the right prop. go down 1" in diameter, up 1" in pitch.
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Old 12-11-2008, 10:07 PM
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Default RE: Difference in performace with 3 blade prop?

If you like it for scale looks..... thats no problem

But prop wise, that 12x7 is a bit too much prop for the .55AX. Read around the forums a bit.

My experience with the .55 engine is that it runs its very best with the stock muffler (and most aftermarket mufflers) propped to turn appx 13,000-14,000 rpm ground peak. An 11x5, 11x6, 12x5 are good places to start. A 3-blade is fair game too - just keep it in that same peak rpm band. Back off on the needle about 300-400 rpm from that peak for flight, and you should be good to go.

If ground clearance is an issue, consider going with the 11" props. You will get better engine performance, and no doubt the plane will perform a bit better than it does with the 3-blade.

The engine does run well with larger props .... so anything down to about 12,000 ground peak rpm is ok. An 11x7 or 12x6 is worth trying. But try not to load more prop on it beyond that.
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Old 12-15-2008, 10:21 AM
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Default RE: Difference in performace with 3 blade prop?

From Tower web site: Recommended Props: 12x7-8, 13x6-7


An inexperienced modeler came to our field with a new OS 55 with a 13-6 on it. It would barely run, but sure was quiet. Needle setting very touchy, but sure was quiet. Hard to keep rich enough to keep running, but sure was quiet.

Finally, I gave him an 11-6 to try. It ran pretty well, easy to needle, and it made some engine noises.
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Old 12-15-2008, 12:59 PM
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Default RE: Difference in performace with 3 blade prop?

Yeah... it is pretty quiet huh?
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Old 12-15-2008, 02:22 PM
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Default RE: Difference in performace with 3 blade prop?


ORIGINAL: HighPlains

From Tower web site: Recommended Props: 12x7-8, 13x6-7


An inexperienced modeler came to our field with a new OS 55 with a 13-6 on it. It would barely run, but sure was quiet. Needle setting very touchy, but sure was quiet. Hard to keep rich enough to keep running, but sure was quiet.

Finally, I gave him an 11-6 to try. It ran pretty well, easy to needle, and it made some engine noises.
A 13-6, 13-7 on an overgrown .46? Yowzers. The only thing quieter would be a dead engine, which is near what you get when you strap pieces of lumber like that to a .55 anyway. The ol' boys that tell you torque is all that matters and horsepower means nothing would probably nod their heads knowingly and salivate at those recommendations. I almost want to buy one so I can run it fast at the field to mess with their heads.

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Old 12-15-2008, 02:59 PM
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Default RE: Difference in performace with 3 blade prop?

I had no problems strapping a 13x6 or 14x6 onto a ST G60 bluehead - that engine would turn those no problem. Even an older ST56 could make use of a 12x6 or on a good day a 13x6.

The OS55 can run on bigger lumber too. The engine itself does not mind being propped for about 10,000 rpm.

But as well balanced as the OS55 engine is, the carb is a little large to draw fuel at those reallllly low rpms (at full throttle position). And with lower rpm comes reduced muffler/tank pressure too. A bit of a double whammy on fuel delivery.

Explains the sensitive needle noted above.

If you consider the OS55 a replacement for some of the vintage .60 and .61 engines and prop it the same way (10x7, 11x6, 11x7, 12x6) it runs great, performs as well as those older (smaller, ligher weight) .6o engines did, and it fits that nitch perfectly. The SPA guys are starting to find that out.

Bob
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Old 12-15-2008, 04:06 PM
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Default RE: Difference in performace with 3 blade prop?

Great, I can dig out all the Rev-Up 11-7 3/4, 11-7 1/2, ....., props for it.

As far as prop load Vs. needle sensitivity, it is a large V diagram.

At the top of the V, where the prop load is light, the needle setting is very forgiving and broad. The number of "clicks" between rich and lean is great.

But as you load it down with bigger props, you drop further down the V. Until rich and lean meet at a single point.

I think some engine manufacturers now use a Y graph.
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Old 12-18-2008, 12:59 PM
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Default RE: Difference in performace with 3 blade prop?

I run an 11x8 on my 55 with a pipe tuned for that prop at 14050 rpms.
With the stock muffler, I was running a 12x7 at 11500 rpms and an 11x8 at 12000 rpms.
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