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

OS 120 AX vs. the OS 91 FX for speed

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Old 03-12-2006, 07:57 AM
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JetKing340
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Default OS 120 AX vs. the OS 91 FX for speed

I know the 120AX swings a bigger prop - so I guess its a torque beast and not a revving out engine for rpms... Is the 91 fx better for speed than the 120 or can you stick a smaller prop on the 120AX? I have three planes that need power - two prop jets - one vmar f-4 and one cermark f-16....

Please tell me your thoughts on these two engines... - please only speak from experience...

Helimanik
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Old 03-12-2006, 11:10 AM
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Cyclic Hardover
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Default RE: OS 120 AX vs. the OS 91 FX for speed

Experience? Okay, I have several OS engines along with many others and like them all. I returned two of the 91fx's in a months time. I would not mount one in a plane if it were free. if I won it, I would sell it . Thats experience. As far as the new 120, Have not heard a word on them yet, But I would not touch it yet until we here back from those have them.
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Old 03-12-2006, 12:00 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

Impressive quote according to the OS ads in the magazines:

The 1.20 AX boasts 25% more displacement than a .91
Who did that math?
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Old 03-12-2006, 03:49 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

os 91 for both of the planes. the 1.20 would require to large of a diameter prop for those planes. use the 91 with a 12x8 or 13x8 narrow prop.
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Old 03-12-2006, 06:32 PM
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Cyclic Hardover
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Default RE: OS 120 AX vs. the OS 91 FX for speed

They also claimed the same size as a .90. I held up the two together and not even close.
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Old 03-13-2006, 12:11 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

You can forget the OS .91 FX for speed....it is a dog. []

You want speed, get an OS 1.08 FSR, and put a tuned pipe on it. [sm=thumbup.gif]

FBD.
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Old 03-13-2006, 04:56 PM
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daven
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Default RE: OS 120 AX vs. the OS 91 FX for speed

That would be closer to 32% more displacement.
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Old 03-13-2006, 05:21 PM
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HighPlains
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Default RE: OS 120 AX vs. the OS 91 FX for speed

It's hard to say really what is true here. If you use the bore and stroke off of Tower's site, the displacement of the .91 is different from what they say. If you trust their bore and stroke numbers, the "1.20" is actually 33.8% larger than the .91 engine. Marketing..........
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Old 03-14-2006, 10:27 AM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

Ask Crazy4Flight about putting a 1.20 in a prop-jet.

He has a Great Planes F-4 Phantom with one of the "older" jett SJ-120 engines up front.

The new OS 120 is similar to this engine, an is more similar to the BSE-120

Although you can load the engine down with enough prop to make it run where it is most happy, you will not get that real 'unload' in the air as you would with a good "90" engine. Has a great deal to do with internal passages and porting.

Bob
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Old 05-16-2006, 05:15 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

The 1.20 AX boasts 25% more displacement than a .91

Who did that math?
The 120 is 33.5% larger than the 91, but the 91 is about 25% smaller than the 120

Joseph
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Old 05-16-2006, 09:00 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

The 120 OS is a good eng. I would give it a 4 star out of five. It was pushing the Toni around with authority. Nice vertical to. I gave its first flight two week ago, to check it out. I will give the specs if he brings it down this week.
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Old 05-16-2006, 09:17 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

the os 120 fits right in the same mounting holes that my os 91 was in and fits fine,,,its in a vmar f4 phantom,,,not flown yet with that engine,,,the 120 that is..
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Old 05-19-2006, 12:02 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed


ORIGINAL: Flyboy Dave

You can forget the OS .91 FX for speed....it is a dog. []

You want speed, get an OS 1.08 FSR, and put a tuned pipe on it. [sm=thumbup.gif]

FBD.
I just bought a (very) old OS 1.08 FSR, but it has never run...

I have to deside on a model for it. Either a TVM Groovy 90 F3A, where I want to run a high pitch prop for vertical speed. Or a CMPro Zero, for scale like performance.

What is this engine best for ????
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Old 06-06-2006, 08:44 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

The OS advertisement for the 120AX says 3.1 hp at 9,000 rpm but it recommends a 15X6 prop.

3.1 hp at 9,000 rpms have to be more like an APC 17X6 prop I expect.

My saito 100 can turn an APC 15X6 well over 9,000 rpms.

I wonder what gives with that.

Larry

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Old 06-07-2006, 07:59 AM
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Default RE: OS 120 AX vs. the OS 91 FX for speed


ORIGINAL: gplarry

The OS advertisement for the 120AX says 3.1 hp at 9,000 rpm but it recommends a 15X6 prop.

3.1 hp at 9,000 rpms have to be more like an APC 17X6 prop I expect.

My saito 100 can turn an APC 15X6 well over 9,000 rpms.

I wonder what gives with that.

Larry

What gives is that

1) you are confusing the HP rating and a recommened prop. They have absolutely no correlation. HP rating is always published along with a relavent rpm, since they are directly related. And, as documented by many, published HP is nearly a meaningless number since all engine manufacturers do not use the same method or standards to derive that number. At best, its a reference.

2) the mfg documentation on several OS engines is sorta goofy based on real-world experience and operation.

3) They are trying to illustrate that this is a low-rpm/big prop/torque engine, not intended for high rpm.

The 15x6 recommended prop will turn around 10,500 rpm on the engine. The engine runs well with that. The 16x6 16x8 are good choices too, and some are running 17x6 props as well. Any prop load resulting in the engine turning between 9000 and 11,000 rpm is fair game for the engine. You select the prop based on your application and what the engine is actually capable of.
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Old 06-07-2006, 06:28 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

Bob

I appreciate the explination.

What I was questioning is the fact that a standard 15X6 prop will not produce 3.1 hp at 9,500 rpm.

If the 120AX produces 3.1 hp at 9,000 rpm as the add states which prop is being used?

I would expect that it would take an APC 17X6 or bigger prop to generate 3.1 hp at 9,000 rpm.


I was questioning the recommended prop size and wondering which prop they used to get 3.1 hp at 9,000 rpm.




Larry
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Old 06-07-2006, 06:59 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

Jet-King: "Please tell me your thoughts on these two engines... - please only speak from experience..."

I have not owned either of these, does that mean I cannot comment or make a recommendation on this matter? You are looking for speed, yes? Yet you feel that if I have not owned these engines that I cannot recommend what may be fast? Seems tacky...


17x6 @9K? That is a LOT of pulling power...
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Old 06-08-2006, 09:56 AM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

No idea what prop they used to get 9000 rpm. Chances are it was no prop at all, only some form of dyno load or dynamic load simulation.

That is one of the reasons you will see on the Jettengineering.com web site, the Jett engines are specified by performance rpm and a "known" and reproducable load - in other words, a standard prop. Jett does not publish the HP, simply the real world performance.
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Old 06-08-2006, 09:52 PM
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Default RE: OS 120 AX vs. the OS 91 FX for speed

I think that the claim of 3.1 hp at 9,000 hp is pretty optimistic for a normally aspirated 120 2 stroke.

I did some checking and it sounds like it would take an APC 18X6 to generate the necessary torque to get 3.1hp at 9,000 rpm.

I have not seen posts of rpms anything near that.

I am not knocking the engine just the advertisement.

In fact I am thinking about buying one.


I think I will visit the Jettengineering site first.

Larry

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