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Author Topic: Engine run questions.  (Read 1245 times)

Offline Tim Wescott

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Engine run questions.
« on: July 31, 2011, 08:06:37 PM »
My #2 plane is a Sig Skyray.  It currently has an old OS 25S in it, and I'm having trouble getting it to run right.  I understand that it's not really enough engine for the plane, but my aim isn't to win stunt contests -- it's to have a relaxing 'after practice' plane, and something that I can use to teach a beginner, if I should ever get one in my sights.

It seems like it wants to run at 9500 - 10000 RPM on the ground, but if I launch at that speed it leans out.  If I launch rich, it either never speeds up, dies in the air, or (every once in a great while) gives me a good run.

I'm running 15% nitro, about 22% total oil content.  I'd tell you the venturi size, but I don't know -- its either 3/16 or 11/64, with a 1/8" diameter spray bar (I'll post dimensions the next time I'm in the same building as the plane).  Prop is a Top Flight 9-5 wood ('cause that's what I have lots of!).

So -- what should I be looking at changing, to make this run right?  Is the above set of behaviors a sign that it needs a smaller venturi?  More oil in the fuel?  A smaller prop?  Something else?
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The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Steve Helmick

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Re: Engine run questions.
« Reply #1 on: July 31, 2011, 08:32:45 PM »
More castor oil for sure. I think the prop is about right, tho I've not been impressed with any TF paddle blades that I've ever used. How about a 9-4 or 9.5 x 4.5 APC?

The venturi & spraybar dimensions seem wrong. I would expect the venturi bore to be around .250" or even larger, and the spraybar is probably bigger than .125".  H^^ Steve
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Offline Tim Wescott

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Re: Engine run questions.
« Reply #2 on: July 31, 2011, 08:38:34 PM »
Quote from: Steve Helmick on July 31, 2011, 08:32:45 PM
More castor oil for sure. I think the prop is about right, tho I've not been impressed with any TF paddle blades that I've ever used. How about a 9-4 or 9.5 x 4.5 APC?

The venturi & spraybar dimensions seem wrong. I would expect the venturi bore to be around .250" or even larger, and the spraybar is probably bigger than .125".  H^^ Steve
I made the venturi, but I can't remember what drill I used.  I know that the spraybar is 1/8" diameter (with a #5-44 thread) because I made that, too.

I was out flying yesterday with my other plane that has an FP-20 on it.  I have a pile of 9-4 APC props and I almost slapped one on just to see what happened.  Maybe next time...
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Online Brett Buck

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Re: Engine run questions.
« Reply #3 on: July 31, 2011, 08:51:05 PM »
Quote from: Steve Helmick on July 31, 2011, 08:32:45 PM
More castor oil for sure. I think the prop is about right, tho I've not been impressed with any TF paddle blades that I've ever used. How about a 9-4 or 9.5 x 4.5 APC?

   25S? I don't think it's going to spin nearly fast enough to get an acceptable flying speed, at least without just gutting it out. I would try a 9-6 and start cutting it down until you can get decent lap times (maybe 4.5-4.6 seconds on 55-56 foot lines).

    The oil is certainly a good idea; that may be the entire problem right there. Run it with lots of good old fashioned castor oil. Overheating would certainly explain the observed issues - set rich it stays cool and holds the setting, lean and it overheats and goes leaner.

     This is not a good performance combination.

    Brett
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Offline Tim Wescott

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Re: Engine run questions.
« Reply #4 on: July 31, 2011, 09:00:22 PM »
Quote from: Brett Buck on July 31, 2011, 08:51:05 PM
This is not a good performance combination.
That's why it's a trainer.  But I want to make the most of what I have.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

Offline Tim Wescott

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Re: Engine run questions.
« Reply #5 on: July 31, 2011, 09:20:07 PM »
Quote from: Tim Wescott on July 31, 2011, 08:06:37 PM
I'd tell you the venturi size, but I don't know --
13/64, with a 1/8" spray bar.  That works out to an area of about 0.016 square inches.
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AMA 64232

The problem with electric is that once you get the smoke generator and sound system installed, the plane is too heavy.

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