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Old Oct 03, 2012, 03:50 PM
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Quote:
Originally Posted by gkamysz View Post
Elevation has nothing to do with engine RPM on a given prop. As elevation increases the air density decreases both the engine and prop work with the reduced density. Engine power output does drop, but so does the required torque to drive the prop. The result is virtually no change in RPM with air density.
For an electric motor, this is true. Fans (props) are constant volume devices. Not for internal combustion engines.
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Old Oct 03, 2012, 04:22 PM
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gkamysz's Avatar
Chicagoland
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The prop does not care what is driving it. As density decreases the torque required of the motor or engine to turn the prop a given RPM decreases.

Greg
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Old Oct 03, 2012, 10:03 PM
mailman
United States, TX, La Marque
Joined Aug 2012
68 Posts
all good ideas. thanks to all. i will get her back out tomarrow and see what happens. if it has abent con ron, would it affect how it runs? mine runs great. smooth running,good transition smooth idle, just low on top end.
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Old Oct 03, 2012, 10:05 PM
mailman
United States, TX, La Marque
Joined Aug 2012
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alt. is sea level. live 10 miles from gulf coast
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Old Oct 04, 2012, 01:17 AM
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United States, MI, Commerce Charter Township
Joined Dec 2000
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[QUOTE=OkiThumper;22902791]As you say, it is possible for hidden damage or correctable defect along the way. The tightness issue sort of reminds me of the old days, where one would run their piston / sleeve lapped motors, running rich (then called "4-cycle") accumulating at least 1/2 hour run time prior to flying. Otherwise, one could not maintain full speed by leaning out the needle.

If no damage or leak, perhaps the solution is to fly, running slightly rich, accumulate some time on it then recheck RPM?
/QUOTE]

I don't think cool, rich running is the way to go-you might want that for the first run or 3 to help flush metal particles out that come from parts bedding in, but the engine isn't going to develop proper fits if it is not running at in it's normal temperature range. You want some amount of friction for parts to wear in to one another, running cool and sloppy wet is counter to that.
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Old Oct 04, 2012, 01:21 AM
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United States, MI, Commerce Charter Township
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Quote:
Originally Posted by surfer_kris View Post
You might have to much nitro in the fuel, mine does +17000rpm on that prop and 10% nitro. Check the number of shims and optimize for top top revs, staying on the side of slightly too low compression.

Another thing to look for is the throttle barrel, it is important that it lines up with the carburator bore at full throttle. It can be off sideways, as it rotates on it's threads, or it might not open fully if you have the throttle stop screw still installed (just remove it, it is not needed).

A cox babe bee will do more that 12000rpm on a 6x3, there is certainly something seriously wrong...
Take off the engine from the plane and run it in on bench before you put it in a plane.
I can certainly drop down in nitro to see if it makes a difference, but I don't think that's the problem. First reason is that the engine needles beautifully-engines that are high on nitro tend to have a very narrow needle range where they are happy to run at peak power. The second is that Norvels are typically very undercompressed from the factory, or at least the .049 and .061's are so it follows that the .074 is too given the target of being a milder tuned sport motor.

Makes me wish Norvel had made an AME version available.....
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Old Oct 04, 2012, 07:01 AM
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The .074 might be mild compared to the .061, but compared to other engines it is not. The Norvel .074 needs to do above 17000rpm to be happy, that's my felling at least. The engine comes with three head shims and it will run fine like that on 10% nitro. There are many ways to run in an engine but I prefer to do it on the bench where everything is easily accessible. They can take some time to run in, but they are screamers from day one, in my experience.
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