engine rpms

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engine rpms

Postby 97corvette » Sun Sep 23, 2007 7:57 am

i have a question about engine rpms. using a fox 35 as the test engine, if you try various size props, i.e. a 10/6, a 10/4, an 11/4, a 9/6, and a 9/8, what changes in rpms would be expected? of course, rpms can be adjusted with the needle valve, but the different props should have different results. or do the rpms stay about the same? i am trying to slow down a stunt plane but maintain an engine speed that is reliable.
regards, marv (still uses fox 35s)
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rpm

Postby chiefss » Sun Sep 30, 2007 1:31 pm

Every prop will have a different rpm. Even the same size from different manufacturers. Even two the same from the same manufacturer ocassionally.

Rule of thumb: decrease pitch, decrease speed and increase pull. Increase pitch, increase speed. Go from a 10x6 to an 11x5 for less speed or a 10x6 to a 9x7 for more speed.
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rpms

Postby 97corvette » Sun Sep 30, 2007 1:45 pm

i understand that rpms vary from prop to prop even if the props are the same size. what i am asking is if an engine turns 10k with a 10/6 and the plane does a lap in 6 sec; what is the best approach in changing prop size to slow the airplane down without changing the engine rpm. remembering that a 10/4 will push less air behind it, and the plane will slow some; will the engine, with less load, turn a higher rpm. slowing the engine rpms by richening the needle could cause a cooler running engine that could lose the fire prematurely.
regards, marvin
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Postby chiefss » Sun Sep 30, 2007 2:17 pm

You set the engine for the highest rpm for a give prop. By definition the prop with the same diameter but lower pitch will always turn faster. Will the plane go faster? Not necessarily. A lot depends on the type of plane

In control line stunt, the object was to get an engine that would break from a rich "4 cycle" to a two cycle when climbing and the opposite when diving. This maintained a constant speed. The Fox Stunt was one of the great engines in this repect. That's why it lasted so long. Been available almost 60 years now

The only true way to find out what prop is the best is to use several deffierent props sizes and makes to find out.

Again you want the engine tuned to almost max rpm for any prop you are going to use. Richening to keep a given rpm is not the way to go. Cold engines aren't real efficient and quit.
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