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Old Apr 12, 2010, 10:09 PM
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Grand Rapids, MI
Joined Mar 2010
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Help!
OS 91FX Engine Prop question

I am getting some conflicting information and im hoping someone either with this engine or someone who knows can just help me...

I just got a OS 91FX ringed engine and the manual says for prop selection to be 15x8, 16x6-8, 17x6, or 14x7 (3-blade), and a friend of mine showed me the OS website and it says 13x8, 13x9. CONFUSED!! Anyone suggest a break-in prop? Its going on a Cessna 81" Top Flite from towerhobbies. Thanks guys
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Old Apr 13, 2010, 01:48 AM
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Everett Wa.
Joined Jun 2001
5,925 Posts
Please remember that prop selection is in fact more about the airframe than the engine. Are you wanting to prop for power or torque. Or speed verses climb.
But for your engine I would work with the 15 to 16 inch props unless you are looking for some extreme performance that is outside then norm for sport flying.

http://www.osengines.com/engines/eng...continued.html

There is a typo on this site. For break in use a prop that will be about 1 inch in diameter or 2 inches in pitch smaller that what you think you will need to fly. Actually it more about keeping the engine near the operating RPM point while she is running rich than it is about any flying prop. So what kind of RPM are you looking for?

Friends don't let friends fly nickel,
Konrad
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Old Apr 13, 2010, 04:46 AM
Ron - AMA 1025
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prop

14x6 15x6 14x7 13x7 13x8
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Old Apr 13, 2010, 09:56 AM
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Grand Rapids, MI
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Im basically looking at general scale flying. Not many loops or anything are going to be performed. I just want a solid looking plane with a real smooth flight. I guess if anything its going to be speed because climbing is not going to be an issue. It would be gradual with my airplane.

I currently dont have a tach to get my rpm readings from other planes but im assuming just basically a solid RPM right before the max out because I never like to run my engine totally maxed. Plus its a plane set for a .60 engine so a 91 2 stroke is going to be plenty of power anyway.

I do currently have a 15x8 because that's what i saw in the manual so I went and bought that. Konrad, am i going to be good with using this for break in's? it suggests a 16x14 for break in on the os page.!!!!!!!! I thought the point was just a prop to help it run, not appear to wanna kill it! haha any help? thanks!
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Old Apr 13, 2010, 10:46 AM
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Everett Wa.
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Quote:
Originally Posted by bnixy7 View Post
Im basically looking at general scale flying. Not many loops or anything are going to be performed. I just want a solid looking plane with a real smooth flight. I guess if anything its going to be speed because climbing is not going to be an issue. It would be gradual with my airplane.

I currently dont have a tach to get my rpm readings from other planes but im assuming just basically a solid RPM right before the max out because I never like to run my engine totally maxed. Plus its a plane set for a .60 engine so a 91 2 stroke is going to be plenty of power anyway.

I do currently have a 15x8 because that's what i saw in the manual so I went and bought that. Konrad, am i going to be good with using this for break in's? it suggests a 16x14 for break in on the os page.!!!!!!!! I thought the point was just a prop to help it run, not appear to wanna kill it! haha any help? thanks!
That is one of many typos on that site. I would look to use a little smaller prop to break in the engine, but it is not critical
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Old Apr 13, 2010, 11:08 AM
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14x6 for break in on that engine with stock muffler.
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Old Apr 13, 2010, 02:48 PM
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Grand Rapids, MI
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after break-in can i use the 15x8 or should i return it?
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Old Apr 13, 2010, 03:20 PM
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Everett Wa.
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The 15x8 is on the heavy side but recall that OS is trying to pull the RPM down to help make the engine quieter. If your ship is on the lighter side of the recommended range (for the engine) I would try the 15x8 if you don't need to find more power by going up the RPM band? I like to 8 inch pitch prop. As you will need pitch speed to make the wing generate lift.I recall that OS says max power is developed at 16K rpm. This would need something like a 12 inch prop to get the OS to spin that fast.
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Old Apr 13, 2010, 04:54 PM
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I started with an APC 15x6 on my Shoestring with OS 91FX but switched to an APC 16x6. I liked it better with the 16x6 so I left it there.

Ivan
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Old Apr 13, 2010, 10:54 PM
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I recently acquired a Sig 1/4 scale Cub that had an older 90 or 91 OS engine...maybe an FSR. Anyway the previous owner was running a Zinger 16X5 on it and I can tell you the Cub needs about 40% power to rotate and climb out. Full throttle pulls very hard with that prop.

Rube
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Old Apr 14, 2010, 04:20 PM
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Grand Rapids, MI
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im going to break-in with the 14x6 and hopefully it take off with it :P
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Old Apr 14, 2010, 06:44 PM
Ron - AMA 1025
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Quote:
Originally Posted by bnixy7 View Post
im going to break-in with the 14x6 and hopefully it take off with it :P
Very good choice -
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Old Jan 31, 2014, 12:35 PM
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United States, NH, Concord
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apc 15x4 works good with larger fun fly.
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