I could use some advice with picking props for my pylon planes.
Our club does more of a "fun-fly" race where planes are broken into classes based on engine size.
Since I race in 4 categories, I could use help picking good prop sizes for each engine, in each category.
Is there a basic rule of thumb for picking prop sizes that would work across the board?
I have heard (for example) that with a OS .46 FX, A 9X7 prop is ideal, but I get 15.8K rpm with 8.1 lbs thrust with a 10X7.

Im so confused.

Any help would be appreciated

Thanks
Craig

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If you are comparing props for the same engine/airplane setup the following formula is a good starting point.

MPH= RPM x Prop Pitch X 0.000947

This is course compares only one prop against another. Which is what you are trying to do. You will now have to match the prop to the airplane to determine the true airspeed. For that you have to try it and fly it in the time honoured fashion.

Ed S

Exactly what Ed said! No two engines are the same. No two props act exactlly the same. There is just to many factors that allow an absolute in this kinda of answer.
For Instance 2 Brand new Webra Q-500's bought the same day. Broke in the same way. Running the same exact props 8.75 X 7.75 NN APC's 15% Red Max. 1 run's 19+K Rpm's the Other one run's 18,5+K Rpm. The OS. 46FX runs very nice you will just have to experiment with different props. Try a smaller diameter with more Pitch, try a wide blade vs. a narrow blade there is so many ways to get the Most power to weight of an aircraft.

Sorry no definate answer.

AMA 6518

Where, if anywhere, does "felt thrust" (thrust measured on the bench) come into the equasion?
Eg. 9X8 prop at 15.9K rpm and 7.01 lbs thrust VS.
9X10 prop at 15.3K rpm and 4.14 lbs thrust

Thanks again
Craig

CrashBevil,

It sounds like you have a very strong O.S. Fx engine there. Our local races use that same engine, and very few, if any, of our engines would turn a 10x8 prop at the rpm you stated.

Some of the better props we have found for that engine would be:

Sport APC 9x7
Sport APC 10x7
Pylon APC 8 3/4" x 8.5 W
Pylon APC 8.8 x 8.25
Pylon APC 9 1/2 x 7 1/2 N (My Favorite)

We use these props on Quickee airplanes, which may be a bit more streamlined than what you may be flying. You need to do some experimenting, all of these props will run differently on different days based on Temperature, Humidity, and Pressure.

A radar gun will help with proper prop selection.

To answer your second question, static thrust, as measured on the ground, means next to nothing. It has been found that if you prop for the highest static thrust on the ground your airplane will be a dog in the air. As soon as your plane starts moving forward the thrust it's prop produces is not the same as when the plane is sitting still. You could consider yourself being on the "backside" of the thrust curve. What you need to prop for is maximum thrust at your slowest speed on the race course, which is generally in the middle of a turn. Hard to do without a datalink to your plane!

TRUTH IS, THERE IS NO SUBSTITUTE FOR GETTING OUT THERE AND TRYING EVERYTHING YOU CAN. TOO MANY "SUNDAY" RACERS JUST WANT TO GO OUT THERE AND BE GOOD AT IT. IT TAKES TIME, LOTS OF TIME. YOU HAVE TO TRY EVERYTHING, MAKE NOTES, AND BE READY TO ADJUST TO THE CONDITIONS OF THE DAY.

Thank you to all who have replied. You all have more or less told me what I thought I knew but wasn't sure about.
I finally just broke down and got a radar, which confirmed what everyone said. Now its just a matter of trial and error and practice, practice, practice.

Thanks again

Craig