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

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Old 01-25-2008, 06:51 PM
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Erik R
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Hi Bob,
I'm going to run an OS 91FX with a Mac's 15cc nitro pipe in a tiporare.Planning on using an apc 11x8.Figure 14-15k.I'm looking for a starting length for the pipe(measured from the glow plug to the widest part of the pipe).I've been thinking about putting a Jett 90lx in there,but I have this OS sitting around.Thanks in advance for your help.I will be out of the country,and computerless for the next week,so if I don't respond right away,it's not because I'm unappreciative of your help.Thank you,and stay well,
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Old 01-28-2008, 12:06 PM
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bob27s
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ORIGINAL: Erik R

Hi Bob,
I'm going to run an OS 91FX with a Mac's 15cc nitro pipe in a tiporare.Planning on using an apc 11x8.Figure 14-15k.I'm looking for a starting length for the pipe(measured from the glow plug to the widest part of the pipe).I've been thinking about putting a Jett 90lx in there,but I have this OS sitting around.Thanks in advance for your help.I will be out of the country,and computerless for the next week,so if I don't respond right away,it's not because I'm unappreciative of your help.Thank you,and stay well,
Erik
Nothing at all wrong with the OS91 engine.... its runs great. Really likes the tuned exhaust
But its timing is a bit mellow - it has good torque .... try targeting the low end of your range, closer to 13-14K --- maybe a bit more prop like 11x9 or 11x10.

Going by how I set up the .61 engines years ago....

Start with the pipe set at 13.25" This will likely be a little bit long, but its easier to keep cutting than swear at a short header.

Measure from the glow plug, along the CL of the header - up to the weld in the pipe (end of divergent section - max diameter).

That will be a good starting point. You will probably end up around 12.75 when done, so plan mounting accordingly. And of course, do the tuning on a test bench first.

Id suggest running it at that initial pipe length, then going up and down in prop size - its easier than cutting the pipe initially - just to give you an idea of how much too long the pipe is. When you get to a big prop where the engine does not run right, or rpm starts falling rapidly, that is the tuning length rpm. Who knows, you might find a prop/pipe set up that surprises you

Enjoy the trip!

Bob
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Old 02-03-2008, 08:11 AM
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Bob,
Thank you very much.
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