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OS .91FX with Turbo-Jett & 15x8

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Old 06-01-2008, 09:20 PM
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CustomPC
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Default OS .91FX with Turbo-Jett & 15x8

Hi,

I'm looking for a 2-stroke engine combination that matches a Saito 125 which is turning an APC 15x8 @ 8,800rpm (still running rich for break-in).

I want to get away from 4-strokes so i can save on engine costs and run cheaper fuel (5% Nitro). Rising mortgage interest, fuel and baby number 3 is killing the hobby budget.

Looking at the figures on your web site for the OS .91FX it says that you've achieved 9,600rpm but my question is how responsive is the engine with the 15x8.
The model is a Super Skybolt which is fly as a sport-pattern. I don't fly 3D and i expect the engine to be responsive enough to go from level cruise to full vertical climb with no lag or sag.

I really like the way it flys with the saito turning that prop at 8,800 so want the same performance.

I'm trying to choose between the .91 FX and 1.20AX. I know the 120 can do it at the expense of about 4oz in weight gain but was wondering how the 91 would be.



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Old 06-02-2008, 10:22 AM
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Default RE: OS .91FX with Turbo-Jett & 15x8

Hi....

I do understand about the baby/house/mortgage/$$$$$$$$$$$ stuff ! trust me [&:]

Unless CG is a concern, id gladly give up 3-4 oz between the 91FX and 120AX engines. I have to check as well, but from what I saw the OS120 was not that much heavier and is about the same physical size. I had in my notes that the OS91FX weighs about 19.5oz and about 25oz with the stock muffler. The new 120 weighs in just at 22oz engine only, and muffler is a bit over 7 oz.

I will tell you the 120AX would be my personal first choice here. Its an animal. Runs nicely on a turbo-jett as well not a bit rpm gain, but it is noticable.

THe 91FX runs great with the 20cc turbo-jett on it. Performs about the same as the BSE-100 does. 15x8 at 9000+ is not a problem with 10% nitro - I would imagine the lower nitro would not cause it to drop significantly.

Nice thing a bout the turbojett is its a pipe that doesnt fly like a pipe. Very smooth on transition excellent midrange torque

If you pursue the OS91 option, you might want to focus an OS91/turbojett combo on a 14x6 or 15x6 and to try some other brands of props. I ran an Menz 15x6 and 15x8 on my BSE-100 and both out-performed the APC props of the same size in all measurable aspects. The OS91/turbo liked the Menz 15x6 most of all. The throttle response and "spool-up" has to do with the prop. Light-weight props respond very quickly. APC props are heavy and spool very slowly.

Bob
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Old 06-02-2008, 11:32 AM
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Default RE: OS .91FX with Turbo-Jett & 15x8

bob, where do you purchase the menz props? just swaped engines on my toni. took my 91fx w/ a jett muffler and put it on my revolver. took my 120ax off my minnow and put it on my toni. wow. bob is right, the 120 gets my vote aswell. the toni is now evil, mean, wicked and nasty.
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Old 06-02-2008, 12:10 PM
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Default RE: OS .91FX with Turbo-Jett & 15x8

I know Desert Aircraft, 3W folks, and some other giant scale/gasser folks sold them. Finding the 15" size was a trick though.

Vess also makes the 15" size now. So far the smaller Vess props have REALLY impressed me.

Bob
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Old 06-02-2008, 05:27 PM
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Default RE: OS .91FX with Turbo-Jett & 15x8

Thanks Bob,

You pretty much summed up where i was heading. Assuming both engines were fitted with a Turbo-Jett, the .91FX would be an all up weight of 24.3oz Vs the 1.20AX at 28.5oz. The difference would be even less if i used the 20cc turbo-jett on the .91 like you say.

I'm not restricting it to 15x8 props but that is the maximum size of prop that can physically fit on the model. It's a light load for the 120AX but with less nitro it will probably be on the money. A .91 might struggle to deliver the performance i desire on 5% but the overkill 120 will be de-tuned to be just right.

I flew the model with an OS .75AX and APC 14x6 @ 10,000 on 15% nitro which was adequate but i was still wanting more for the verticals. A .91 with a 15x6 might be the ticket if it doesn't lose too much on 5%.

I'm not a huge fan of APC props myself. I use them when recording and discussing performance data because they are a known quantity and the defacto industry standard.

I really like the Graupner Super Nylon prop because they pull like a tractor. They typically deliver about 1,000rpm less than the same size APC but in the air they feel like they are pulling harder. Unfortunately the max rpm limits in the larger sizes is getting too close to what the .91 and 120 can achieve so i keep them for 4-strokes.

I've just got hold of some Xoar wood props and these look very nice. I hope to fly one this weekend to compare.

P.S. Decision made. 120AX will be on the way. I'll be ordering the Turbo-Jett later this week.
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Old 06-03-2008, 07:57 AM
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Default RE: OS .91FX with Turbo-Jett & 15x8

great....

let me know how things work out.....

Bob
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