Conference vmszoo::rc

    My Kadet Sr. flew great with an OS .40 4 Stroke swinging a 12 X 5
    prop if thats any help.
     

    I believe during break-in, OS recommends an 11 X 4 or 11 X 5.  I
    swing a 10 X 6 during normal sport type flying.  I think the 12
    inch might be a tad big...
    

    I don't know if this is the best place for this, but I need some
    advice on what props to try on my soon to be completed Sportster
    90.  I am going to run an OS 1.2 Surpass 4-stroker in it, and I
    have no experience with this motor.  Does anyone have any idea where
    I should start with a prop?  The only advice I have so far was that
    a 16-8 should work well.  This seems in the large side acording
    to the stuff with the engine.  They suggest mostly 14 and 15 dia.
    props with about 8 inch pitch.  I'm confused.
    
    Charlie
    

    Charlie,
    		My experience with four strokers and from what I've
    read about their use in pattern indicates that the larger four strokers
    don't pull diameters like you would think. They do however pull
    pitch. I'd try props in the 13-14 inch range with pitches from 8
    to 11. Best prop engine combo I heard of was the enya 120r with
    a 14/10 cut down to a 13. This way you actually have the blade tip
    with some pitch in it rather than at zero. I'd go with a 14/8 or
    13/9 13/10. For the first test stand runs I'd use a 13/6 or 7 to
    keep the heat down.
    
    
    							Tom

    Tom,
    	Thanks for the advice.  I'm going to order some props, so I
    think I will try a 14/8 for break-in.  That shouldn't load it too
    bad.  I called Great Planes, and they suggested a 15/8 or 16/8.
    The guy I talked to thought that I should stay with 8 inch pitch.
    
    Charlie
    

    Charlie, 
    	They also suggest that you only buy wooden props vs. plastic
    ones on anything larger than a .60!  I didn't realize this until
    I read it this morning in the safety guidelines for my .90FS.
    
    Ken

    Ken,
    	I've seen similar recommendations to stick with wood on larger
    engines, but I've seen lots of 90 4-strokes run with Fiberglass
    props.  I've never seen one come apart, but I've seen them come
    off the shaft due to backfiring.  I'll try to get some wood props.
    
    Charlie
    

Provided you use a continuous fiber prop (ie: glass or carbon) not a straight 
plastic (ie: Master Airscrew) they will have more strength than wood. However, 
as they tend to be more resistant to breaking they also can do more damage if 
you put any part of your anatomy in it. Read the safety note and you will see 
what I mean.

The backfiring you refer to is caused by pre-ignition induced by a too hot plug 
or ignitor (eg acetone) in the fuel. Nothing to do with the prop.

John

    re .18
    	The prop can affect pre-ignition.  If you put too big a prop
    on the 4-strokes, it causes them to run with more load and this
    tends to make pre-ignition worse.  The lower RPM's with more load
    tend to make a glow engine fire earlier.  Many people are converting
    the big 4-strokes to ignition to help this and improve idle.  I
    think that most of the big 4-strokes will be run on ignition when
    more better units are available that don't add too much weight.
    The guys that I have seen having the most trouble with thrown props
    are running large diameter props on big scale planes.
    
    Charlie
    

    
    My intent was more to the safety aspect.  I like the Masterscrew
    props, but they don't come apart when you put your finger in them.
    Supposedly, wood props break and do less damage to your body!
    
    I've got a .90FS and was also thinking about using a fiberglass
    prop, but I have a tendency to test my luck....better get a wood
    one and save what fingers I have left.
    
    Ken

    
>   Supposedly, wood props break and do less damage to your body!
    
    I'd be very surprised if a wooden prop would break before inflicting
    major damage to a hand, possibly to the extreme of removing a finger.
    Therefore I don't think safety comes into the wood/plastic decision
    at all.  In fact, maybe if you use plastic or fiberglass it will
    give you more incentive to avoid sticking your finger in the way :-)!
    
    Bill

Agreed Charlie, Prop, fuel, plugs, compression ratio and even valve clearance
will all effect pre-ignition. However the material the prop is made off is not a
factor; to a point... 

A consideration is the prop weight, some glass props are very heavy. If we have
a very heavy prop the inertia can reduce the effect of pre-ignition provided the
prop is pinned. Without pinning it can accentuate the problem. I am not in
favor of pinning, it can too easily weaken the hub of the prop, besides it is
not curing the problem of pre-ignition only hiding it. Really who wants to hear 
an engine detonating, it not doing it any good only harm.

The spark ignition system is the ultimate way to prevent pre-ignition and
improve idle and a bonus is you can eliminate nitro, and for the SAM freaks it
gives them better economy in Texaco. However, it adds weight, complexity, a
potential radio problem; although it is not such an issue today, and is
something else to go wrong. 

The whole issue of pre-ignition is complex, it has no single answer, every 
occurrence must be examined for the correct solution.

John

    From the back of the 1991 Tower [big] catalog, page 203:
    
    Engine cu in	prop, 2 stroke		prop, 4 stroke
    
     .020		4.5x2
     .049-.051		6x3, 6x4, 5.5x4
     .09-.10		7x3, 7x4, 7x6
     .15		8x4, 8x5, 8x6
     .19-.25		9x5, 9x6, 8x6
     .20-.21 4s					9x6, 10x4
     .29		10x5, 10x6, 9x7
     .30-.35		10x6, 10x7, 9x7
     .40		10x6, 10x7, 10x8
     .40-.45 4s					11x6, 12x5
     .45-.50		11x5, 11x6, 10x8
     .60-.61		11x7, 11x7.5, 11x8	12x6, 13x5
     .71-80		12x4 through 14x6
     .90		14x4, 14x6		11x10, 12x7, 12x10, 14x6
    1.08		15x8, 16x6
    1.2			14x6 through 16x4	16x8, 15x6, 15x7
    1.6 4s					16x8, 18x6
    2.0			18x6, 20x6
    2.4			18x10 through 22x20	18x10, 20x8
    2.7	4s					20x8, 20x10
    3.0	4s					20x10

    Resurrecting a *really* stale thread...
    
    I got my Fun-One completed Friday nite and was anticipating bringing it
    down to the field Saturday, hoping to get it inspected and test flown.
    Best laid plans and all that fell by the wayside with chores and a
    rescue mission to recover our teenagers with my SO's stalled car a 
    "mall away" from home (with a tank of crappy gas)...Sigh...
    
    With the crapola weather Sunday I was reduced to going about the task
    of running in new engines, starting with the OS32F/abc. Before I lit
    this thing up for the first time I pulled the glowplug and backplate
    off and flushed  the crankcase and cylinder with clean fuel, checked
    that everything was put together tight, and since then  I've run three
    full tanks (~36oz) of fuel through it.
    
    Noting that I have zero experience with this engine, I have a few
    questions for the group. Fwiw, I'm using fuel with 10% nitro, 18%
    synth, and 2% castor, and running a KB1L plug w/idle bar. The tank on
    my home-brewed test stand is damped with sufficient foam to make sure
    the fuel is running clear (no bubbles) through the lines - which are
    reasonably short, kink-free, and equipped with a new craptrap. The tank
    center is also vertically aligned with the spraybar. I installed a piece
    of tubing around the needle to help block any air leaks as well...
     
    Things I've noticed when taching this engine with an APC 9/7 on it:
    
    - Allowing for a reasonably juicy mixture (a few clicks richer than
    peak rpms) it's running right around 12.9K rpm (peaked it'll turn
    13.3K). I could set the screw anywhere between but I'm in no hurry to
    toast the cylinder, but in general, does this sound like a reasonable
    engine speed with this prop? Is there enough guardband to allow for
    unwinding in a dive? (spec allows for 16K). Also, I could easily run a 
    longer prop (eg: a 10/6 or even 11/6) - would either of those make more
    sense than the 9/7?
    
    - Spinning along at 12.9K, I've noticed the cylinder head will get
    *damn* hot to the touch - much hotter than my TTPro40 or 46 do (and
    they're turning 14~14.5K with a slightly bigger prop - 10/7). The
    number and height of the fins on the OS head are both fewer and smaller
    than the TT's, which undoubtably has some impact. 
    
    Even though the  oil is running clear out of the exhaust, it concerned
    me enough that I've pulled the silencer off the OS and examined the
    exhaust port - not a trace of toast to be seen. Should I be concerned,
    or is this just a fact of squeezing power out of a "small-block"? 
    
    - I've noticed that there is a slight variation in rpm - in the
    neighborhood of 100 rpm +/- around 12.9K - while running in a steady
    state. It's not rhythmic or pronounced, just a bit of wandering. I also
    noted a bit of what appears to be blow-by around the front bearing
    (this is a dual BB engine). It's a bit more than I see on the TT BB
    engines. 
    
    Other than the drooling from the front bearing, no other leaks are
    apparent. So does this slight rpm "wandering" sound like normal
    break-in behavior, or is it something worth investigating further?
    (and if so, what to look for?)
    
    Thanks for any advise,
    
    /dave

    
     Dave,
    
      My .02 cents:
    
        I think the 9/7 is fine for starters with the new .32 OS. It will
     be spinning fast, but for break-in it doesn't hurt to lighten the
     prop load a bit.
    
        I have been using a 9/8 APC on my "old version" .32 in my "Mini-
    Gremlin" and it really likes it. I could probably drop to a 9/7 or even 
    a 9/6 if I wanted to get the rev's up. The "new" .32 is significantly 
    more powerful than the old version ( I have the "new" OS heli .32 on my 
    Shuttle ). My guess is a 9/8 will turn out to be a good prop for the
    new engine. A 10/6 will give roughly the same prop load as a 9/8, so
    it would probably be fine too. I would stay away from a 10/7, etc
    because of the increased load.
    
       I am using ALL sythetic fuel in my .32 with no problems. You have
    the added insurance of 2% castor, so you should be fine. I have used
    KB-1l plugs in all my .32's with good success.
    
       It obviously will hotter during break-in. Just keep an eye on it
    and richen it a click or two if needed.
    
                                                       Good luck,