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Old Apr 09, 2005, 05:06 AM
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glen's Avatar
new zealand
Joined Sep 2003
1,039 Posts
Question
70 four stroke next prop down from 13*6

I recently tried a K series MA 13*6 prop on my OS 70 surpass. It was definitely just a titch over propped. On a MA normal series 13*6 the motor is sweet but I think jjjuuussst a tad over proped.
Question.
I've bought a 13*5 K series MA which Itís swing easily but to get the same air speed the motor will need to rev 2000 rpm faster.
Would I be better using a 12*8 prop. I donít particularly want the plane to go faster, I know the 13*5 will have more pull but Iím just a little worried it might not give the plane (P51 Mustang) enough air speed to respond solidly to control surfaces. Anyone tried any such combinations Or am I on my own here.

I intend to stick with MA or similar prop for the "look". I know APC are better and use them on other planes. NOT this one.
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Old Apr 10, 2005, 08:08 AM
Confederate AF Pilot
Hope Hull, Alabama
Joined Dec 2003
328 Posts
I am surprised that you're overpropped with a 13-6. My magnum 70 will turn a 14-6 at just over 10,000 rpm. It is on a fokker triplane, so it needs the extra length to get past the cowl. It will turn a 13-8 easily as well. I would try the 12-6 or maybe a 12-8. The only drawback to the higher pitch will be getting thew plane to slow down when landing, but you have to keep a mustang moving anyway. Just experiment, you will find something you like. My world models mustang has an old OS90 fourstroke turning a 14-8 prop. This is one of the old 4strokes with the cam and valves in the rear.
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Old Apr 10, 2005, 05:49 PM
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glen's Avatar
new zealand
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I need a tack meter!!!!!!
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Old Apr 11, 2005, 09:07 AM
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ivanc's Avatar
United States, TX, Round Rock
Joined Dec 2004
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My OS FS70II Surpass turns an APC 13x6 well over 10,000 rpm but I'm running it richer so the rpms stay below or at 10,000 on the ground. It doesn't seem overpropped - it can easily swing 13x7 around 10,000 rpm but that'd give too much speed to my plane - I like to fly it slow.
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Old Apr 11, 2005, 02:42 PM
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glen's Avatar
new zealand
Joined Sep 2003
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Guess I better recheck. Maybe I still need to get used to the 4st. This is my first one. the vertical performance on the 13*6 ma normal prop was definitely better than the vertical on the 13*6 K series MA and that was the comment by several people watching. It definitely SOUNDS a lot nicer on the standard prop.

What are you using your motor in ivanc??
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Old Apr 11, 2005, 03:08 PM
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Sparky Paul's Avatar
Palmdale, CA
Joined Oct 2000
13,469 Posts
You might try an APC 13x7. For speed, you need pitch.
Master props appear too flexible to me, watching the prop tip as it wiggles back and forth.
The APCs have less blade and are more pointy at the tips.
I don't use the OS 70 on anything fast, preferring the APC 14x5N for both the Kadet and the Wacky Waco. But I keep the APC props... 13x6 and x7 around...
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Old Apr 11, 2005, 03:25 PM
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ivanc's Avatar
United States, TX, Round Rock
Joined Dec 2004
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It WAS in a Sig Somethin' Extra. I crashed it last Saturday - I was practicing bombdropping and looked at the bomb (a big washer) after releasing it instead of at the plane which was flying almost stalled at 6'-7' above the runway. When I looked back at the plane it was nose down.

I moved the engine in my 4*40, so now I'm learning again how to land it - I had a TH 46 in it and it was doing pretty well in the 4*, but I wanted to fly a plane with the FS in it. The FS gives me totally wrong idea on how to land the 4* (compared to the SSE) - when it was in the SSE I would come in a nose high pitch really slow - just slightly above stall and would correct with its huge control surfaces. The noise of the FS in the 4* makes me feel she's slow when in fact she's not. Flying slower makes me shake as it's harder to control her with the smaller control surfaces. I'll probably get another SSE for my OS FS70II.

On a warbird however, you need to keep the plane pretty fast as they have higher stall speeds. So using a 14x4 will not provide you with enough speed. IMO 13x6 should be OK. 13x7 might even give you better results. Just keep the rpm in the 9,000 - 10,000 range. I wouldn't recommend you go below 9,000 as the engine will start to load up and the max torque will drop.
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