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Thread: OS91 FX


  1. #1

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    OS91 FX

    The engine is a new OS 91 FX with about a quart through for break in.
    The engine started fine but needed the electric. I tried to back flip the prop to start but no hint of life. Glow plug is a OS #8 for the numbers. A fox standard long was tried and added a few hundred rpms but idle suffered a bit so it was not used in the test.

    The stock muffler’s baffle was not removed. For the tuned pipe I used a Macs 11cc muffled marine (#1369) because it’s what I had on hand. It’s a bit small on paper however I believe it was the optimum pipe. The header ended up 3 ¾” long from the cut to the flange face measured down the tubes centerline.
    The props are the dark grey Master Airscrew S-2 series formally known as Scimitar type.
    Power Master 15% nitro 18% oil was used. Power Master FAI fuel was tested and the numbers were down only 150 rpms from what you see below and the reliable idle speed had to be raised from 2000 to 2100 rpms. Fuel usage was very low, but lets face it the power output is pathetic.

    Throttle response was surprisingly lightning fast with the pipe no flat spots no loading up at any throttle position. The throttle position is the most sensitive I’ve come across with the 13750 rpm test, just close the throttle ever so slightly and the rpm drops, this leaves me to believe that that carb is the primary rpm limiting factor of perhaps many others. This engine is rated at 2.8 hp@15000 so in further test I will reduce the prop load to TRY to get this engine to 15k and find the true power at this rpm, muffled and piped.

    The engine was peeked just long enough to get the reading than richened and allowed to cool at part throttle.
    Air temp 75* Humidity 50%


    Prop…………Exhaust………....RPM…...Hp… …Pitch speed mph…. Thrust lbs.
    12X 8……….Stock Muff ……11500….1.28……….87.12……………. .8.9
    12X 8……….Tuned Pipe……13750…..2.11………104.17……… …12.5
    13X 8………..Stock Muff……10700….1.38………81.06………… . 9.18
    13X 8……….Tuned Pipe…….12500….2.19……….94.7………… ……12.5


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    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

  2. #2

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    RE: OS91 FX

    .
    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

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    RE: OS91 FX

    FF,


    There is another thread, regarding the exact same engine, further forward in this forum.


    You can see it right here.

    This would not require that you bump your thread, to keep it in focus...
    Dar Zeelon - ISRAEL - ddzeelon@gmail.com
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    RE: OS91 FX

    Thanks Dar.
    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

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    RE: OS91 FX

    Good info in both threads ( here and the link that DarZeelon mentioned ).

    Now , let me make a crazy question , do you think a shortened prop let's say an APC 12x14 to 9x14 will produce "similar" load than a stock 12x8 or 12x9 prop and run properly on the 91 FX ??? I'm considering 1"= 2pitch I know that the wider blade of the 12" prop would pruduce more load than a normal 9" blade stock prop due to the total area , that's why I'm saying also an 12x9.

    My thoughts are , if the power of the 91FX is able to move the 9x14 prop near to 12k (with tuned pipe of course) we would have a "cheap" engine doing 168,000 " per minute .... in theory , would be this kinf of "equivalent" to have an 8 pitch prop spinning at 21k ??? , 8 x 21,000 =168,000 ". Yes I'm thinking on SPEED .

    I'm losing my mind or I'm getting a chance ??? [&:]




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    RE: OS91 FX

    SpeedBoy,


    It looks more probably that you are loosing your mind...[&:]

    If you look at any prop blade, from any make, you will see that the blade tips have a reasonably smooth shape...

    The tip of the prop must promote smooth airflow, to produce good thrust, so it is shaped and if you often make 'kangaroo style' landings, which result in prop-ground clashes; the prop pulls less strongly afterward; even if you touch-up the stricken blades with a model-knife.


    There are no props around, which have 'chopped' tips...


    A chopped prop can be used to reduce the load on the engine, while raising the RPM... But it can be 'efficiently' used during break-in only (it will provide cooling), but never for flight.


    Also, a fixed-pitch prop with a very high pitch-to-diameter ratio, will take 'ages' to build a 'vortex' and produce effective thrust...

    So, with a 9x14 prop, as you suggested, a sleek plane may eventually fly very fast, but it will take it eons to reach about 65% of that high speed, for the prop to make effective thrust that would enable it to reach that speed.

    This is the very reason all but the most basic aircraft, use variable pitch, or constant-speed propellers...


    I believe the most 'radical' props that can be purchased are from APC... They offer a 7x10, 8x10, 9x10, 10x10 and 11x10, 11 and 12 sport props...

    But even their ultra high-speed carbon fiber pylon racing props, are 7.4x8 at most...
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    RE: OS91 FX

    Hi Dar ,

    Mmmm interesting points you say ..... I have some questions though

    You say that the tips of the prop must promote/have smooth shape to produce good thrust right .... and what if I modify the airfoil trying to leave a smooth shape ( as the pic below ) including the original shape of the tips ?? will work ???

    What exactly you mean with chopped prop ? .... to shortened .... ???

    Why you say that a shortened prop will take ages to produce effective thrust ???

    I have reduced one inch a prop and also reduced its airfoil with good results and another props with only reduced airfoil with awesome results , this is what makes me think that "probably" my crazy thoughts about an aggresive pitch with 9" could work , not sure at all as I only have reduced one inch and that's the reason why I'm making a lot of questions and I like to learn from others .

    Ahh and I suggested 12x14 because I saw a guy in the thread you mentioned using that prop and that made me think that exist .

    Thanks in advance

    P.S. Sorry to kidnap the thread freakingfast .

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  8. #8
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    RE: OS91 FX


    ORIGINAL: SpeedBoy

    You say that the tips of the prop must promote/have smooth shape to produce good thrust right .... and what if I modify the airfoil trying to leave a smooth shape (as the pic below) including the original shape of the tips ??? does it will work ???

    What exactly you mean with chopped prop ? .... to shortened .... ???

    Why you say that a shortened prop will take ages to produce effective thrust ???
    SB,


    I hope the prop images below clear up what I meant by 'chopped'...

    If the blade tips are 'reworked' well enough, the prop should behave like a production prop, with a high pitch-to-diameter ratio.


    As to the last statement; I did not mean a shortened prop. Not necessarily.

    I meant any prop that has a high pitch-to-diameter ratio; whether a production unit, or one like you had modified.


    Until the 'vortex' builds up, the blades in their entirety are stalled; producing a lot of turbulent air and noise, but very little thrust...
    With props that have a pitch-to-diameter ratio, which is high enough, this vortex may never build up in static, or slow flight conditions.


    Only as flight speed is increased dramatically, does the prop begin to 'blend in' with its surroundings, becoming much more efficient.

    See the full size Mustang, Hercules and Bear and notice the dramatic pitch at their blade tips, in high-speed flight...


    At take-off, the pitch they all use is much, much flatter...

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    RE: OS91 FX


    ORIGINAL: SpeedBoy


    P.S. Sorry to kidnap the thread freakingfast .

    No problem Speed. Good info going on here, thats what this place is all about.

    I've seen what happens with "square" and "over square" props. In two cases they tached well on the ground (stalled blade tips) but once it got up to speed it hooked up and realy pulled the engine rpm down and the plane had too much drag to go any faster with the power on hand. I ended up droping back one inch on pitch and it was fine.

    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

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    RE: OS91 FX

    Thanks for the pics and the answer Dar , now I get anothers points to keep on mind . [sm=thumbup.gif]

    freakinfast thanks for the answer too and to be cool

    I'll do kind of "experiment" with a prop (11x12 shortened to 9x12 ) and see how it work on a OS 61 RF and on a sleek airframe , just to have another own experience and I'll report it later .



    P.S. the pic below encourage me to do it .



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    DarZeelon's Avatar
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    RE: OS91 FX

    We'll await your report...

    Good luck![sm=thumbup.gif]
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    RE: OS91 FX

    Well I’m finally getting around to posting some more numbers.

    Air temp 43*F, Humidity 98%, Bar. 29.8, Elev. 200’.
    OS#8 plug Powermaster 15/18

    Tuned pipe as before //stock muffler

    11X8 MA, S-2 @ 14,700+ ended test due to extreme prop noise!
    10X10APC @ 14,950*
    11X8APC @ 14,870*//13,000
    11X9APC @ 14,250*//12,600 (12,500 on FAI fuel to run out nitro)
    12X8 MA, S-2 @13,650//12,000 (repeat test)
    13X8 MA, S-2 @13,060//11500 (repeat test)

    *Perhaps better performance could be found if I shortened the pipe for these props, the pipe was tuned for 13,650.

    In the 15,800 to 13,000 range the engine’s output was flat at about 2.4 hp if you use the “Prop Power Calculator”. The “ThrustHp Calculator shows 2.7 to 3.6 in that range, but it doesn’t take in to account for the type of prop’s efficiency.
    Note that the prop’s listed pitch was used in factoring Hp, not the actual pitch and the13X8 MA showed the highest Hp but the actual pitch measured was less than 7”.

    The engine exhibited rattling/knocking noises with the stock muffler while running the smaller props at the peak rpms so the engine was richened until it went away and the numbers were than recorded, typically 300 below peak. Less nitro or a colder plug may eliminate that sound. I would have heard the sound if it had occurred with the tuned pipe as it has an effective muffler.

    I was getting cold, so I ended my testing before I did something stupid.
    I will do one more test with a shorter header with the 11X8 and 11X9 in the hunt to fined the peak Hp and call it good.
    Than I’ll mount wings on it.
    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

  13. #13
    DarZeelon's Avatar
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    RE: OS91 FX


    ORIGINAL: freakingfast

    The engine exhibited rattling/knocking noises, with the stock muffler, while running the smaller props at the peak RPM. So the mixture was richened until it went away and the numbers were than recorded, typically 300 below peak. Less nitro, or a colder plug may eliminate that sound.
    FF,


    This knocking was probably a result of too hot a glow plug...

    Please see what the OS-Japan web site says about it.


    The recommended plug for this engine is the colder OS A5, not the medium-hot OS #8 (there isn't an OS38...).

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    RE: OS91 FX

    I agree with you Dar, and the web site, but those turkeys supplied it with a #8 and the manual calls for a #8. That’s about what I’ve come to expect from OS’s documentation.

    Pulling large props at sub 11,000 rpm it may not be a problem with the #8.
    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

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    RE: OS91 FX

    More props & numbers, basically the same setup as above. All APC props this time:

    11X 12 at 12,750 and the 11X14 at 11,700 on 15% and a few hundred less rpm on FAI fuel. I could tell the engine was working hard on the 11X14 by the richer needle setting and the extra smoke, it didn't seem like it was static stalled like I have seen the 10X10 props do on other engines.
    The pipe is a bit short for these prop to peak on the ground but it may right where I want it in the air. Testing-testing!

    An 11X11 or 11X12 may be the best prop to run on my Phoenix Strega if it will hold or unload rpm as the plane moves faster and the prop hooks up. Testing testing.

    Other props I tried to answer some questions: APC Sport 13X8 @ 12500 and APC Pattern 13X9 @ 11700.
    An A5 glow plug was tried but it lost 200 rpms with these props, went back to the OS #8.
    Oooh eee oooh ah ah(the crowd) ting tang walla walla bing bang!(the plane crash)

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    RE: OS91 FX

    Excellent info freakingfast ,

    Just wonder where did you find the 11x14 APC ??? I want one of those props.

    I'm running the O.S. 91 with 12x10 APC on a F-4 Phantom and seems OK , I'll try to borrow a tach to get the reading and share it here .
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    RE: OS91 FX

    APC 12x10 , 10% nitro , A8 plug , stock muffler 11,500 RPM.
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