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Old Nov 12, 2010, 11:58 PM
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Diesel tuining to prop size changes

Hello all

I am nearly finished building a Free Flight model which the plan says fits a 1 -1.5cc motor, however I am about to put a 2.5 cc diesel on board. I have converted the plane to three chanell being rudder elevator and fuel cut off.

I intended to go from a standard 9 X 6 prop to a 10 X 5 with the intention to wind out the compression lever and richen the fuel up to depower the operation to better suit the plane.

However, after bench testing, I discovered that compression lever change could only accommodate about 1/8 of a turn however needle valve went out almost two ful turns.

Question for the diesel experts.

1)For the same compression lever setting but with a prop change from 9*6 to 10*5, would the engine be actually going slower given the bigger prop?

2) If the engine revs are the same (ie same compression setting but different props) what would the effect on the plane be in the air?

Thanks
George
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Old Nov 13, 2010, 07:37 AM
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George, you would need to know the rpm difference between the two props at optimum fuel and compression settings for each. The settings for the 9x6 and 10x5 should be the same unless they are different brands with different load factors.
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Old Nov 13, 2010, 07:40 AM
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1) a 9x6 or 10x5 propeller yield approximately the same RPMs. Now because the 10x5 is larger if it has wider blades or a different shape it may result in a little RPM loss, but nothing to really get excited about.

2) It really depends on the airplane and how much aerodynamic drag it has. Thus different airplanes may result in one prop or the other being faster per se.
Now removing the plane from the figures, results in the 9x6 prop being faster as the extra prop pitch would result in a faster speed.
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Old Nov 13, 2010, 04:54 PM
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With a big prop, wanting to go slow, the trick is to run low compression and rich fuel. Adjust low compression to when the engine misses, but keeps running. Rich fuel will keep the engine running at lower compression.
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