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

What prop for a .91 OS 2S?

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Old 01-18-2010, 12:11 PM
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microdon2
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Default What prop for a .91 OS 2S?


I'm about to build the new ARF GP 70" Revolver (looks beautiful...) and will be overpowering it with an OS 91 2s (instead of the 61 they recommend - I tend to overpower...), and I'm looking for prop recommendations. I'm seeing different "official" recommendations depending on where I look (Tower Hobbies is saying a 13x8, but the online OS docs say between a 13x8 and a 16x6 or so). I'm looking to fly sport - not 3D just yet - but would like to ring as much speed out of this setup as possible. My friend runs a 13x7 on his 55 OS 2S on his 59" Revolver and that thing moves! Tower says to use a 13x7 on the 91, but that seems too small to me. I just ordered some 14x7's, but I'm just guessing. Any recommendations?

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Old 01-18-2010, 12:33 PM
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Default RE: What prop for a .91 OS 2S?

I used a 12x6 apc, on my patriot, also an 11x8, IMHO for speed, the props you mentioned are not really, more for sport, scale, 3d flying.
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Old 01-18-2010, 08:30 PM
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Default RE: What prop for a .91 OS 2S?


ORIGINAL: microdon2


I'm about to build the new ARF GP 70'' Revolver (looks beautiful...) and will be overpowering it with an OS 91 2s (instead of the 61 they recommend - I tend to overpower...), and I'm looking for prop recommendations. I'm seeing different ''official'' recommendations depending on where I look (Tower Hobbies is saying a 13x8, but the online OS docs say between a 13x8 and a 16x6 or so). I'm looking to fly sport - not 3D just yet - but would like to ring as much speed out of this setup as possible. My friend runs a 13x7 on his 55 OS 2S on his 59'' Revolver and that thing moves! Tower says to use a 13x7 on the 91, but that seems too small to me. I just ordered some 14x7's, but I'm just guessing. Any recommendations?

For speed with a OS 91FX, APC 12X8,12X9,13X7 or 13X8 depending on drag and type of exhaust i.e Pitts, stock, Jett or tuned pipe.

Your friend is probably turning a 11X7, because a 13X7 on a 55AX would be a real dog.
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Old 02-19-2010, 03:27 PM
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microdon2
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Default RE: What prop for a .91 OS 2S?

Thanks for the suggestions. So far I've tested 14x7 (with 10% Nitro), 13x10 (w\ 15% Nitro), and 13x7 (15%). 13x10 has been the fastest so far. 13x7 allowed higher revs, but not much speed. I'm still not blown-away by the speed - could be my engine's not tuned properly. Working on it. Maybe a 12x10 next?

Think there's much of a performance difference between 10% and 15% Nitro? I've heard differing opionions. But I do like saiving the $3\gal on the 10%...
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Old 02-19-2010, 04:43 PM
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Default RE: What prop for a .91 OS 2S?

On the nitro - no, the difference is not worth noting.

I don't recommend jumping up to 10" pitch off the bat. FF probably nailed it with the 12-8 or 12-9. Don't lug it down too much - more speed requires more power, so you want to prop the engine so it unloads in the air closer to it's horsepower peak. OS rates it's peak horsepower output at 15k rpm, and others have quoted it at more like 12k.. so ?? But in any case, lugging it down to 10k on the ground is a waste of time if you want airspeed. Numbers I've seen on a 12-8 APC are up to about 12k on the ground with stock muffler.

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Old 02-19-2010, 11:15 PM
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Default RE: What prop for a .91 OS 2S?

Hi, I'll jump in and recommend you try an APC 12x8 also.
I have two OS 91s that Ive run on propjets over the last three years and I really like the APC 12x8. With a stock muffler (no baffle), 12x8, 15% CP both engines will spin 12k no prob. 20% CP will get you another 2-300rpm. I know about the 20% because I accidently bought a jug of it last year. The small gain certainly not worth the cost to IMO.
Anyway, I think the 12x8 will unload well in the air and pull the revolver to 100 mph, straight and level.
Cheers.
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Old 02-19-2010, 11:50 PM
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Default RE: What prop for a .91 OS 2S?

I've had at least (6) OS .91's and always flew them with 14 x8's, most recently XOAR. Will turn about 11 grand. I used both Wildcat and Byron fuel, both 10% nitro.
If you want to get a few more MPH moving to a 13x10 would proabbly do it
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Old 02-27-2010, 10:56 PM
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Default RE: What prop for a .91 OS 2S?

APC 11x10 or 12x10. You want less diameter and more pitch for speed. I love the 11x10 setup for my GP Little Toni. She is a very fast bird with that prop engine combo. Of course I am at 4600ft above sea level so you could probably turn a 12x10 the same way I turn a 11x10.
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Old 02-27-2010, 11:02 PM
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Default RE: What prop for a .91 OS 2S?

IMO a 13x10 will not let the engine rev to it's peak HP. A 12x10 will probably be your best prob for your needs. Byron 10% Nitro 20% castor/syn blend is what I run and it does great.
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Old 03-01-2010, 11:35 AM
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Default RE: What prop for a .91 OS 2S?

MJD and Fast are the ones to listen too. those are what I would suggest too. I have a the 120AX in my little revolver so I know the 90 in the larger one isnt going to be a speed plane. If you really want it to go fast run a YS 110 min or even better the ys 170, 120 AX
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Old 03-13-2010, 09:22 AM
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Default RE: What prop for a .91 OS 2S?

The best speed set up from my experience on the OS 91 motor will need muffler upgrade to Jettstream tuned muffler (jettengineering.com) for under 100 bucks and APC 11x9.
That's what I use in the Patriot XL and Cermark F16 propjet and got impressive speed runs.
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