Hi.

I purchased a tachometer (GloBee IntelliTach) to check my prop's RPM. I had a Top Flight 12 x 6 Power Point prop on an OS .65 LA engine. Running full throttle, the RPM topped out around 11500.

This weekend, I changed props to a Master Airscrew 11x6 Beechwood Propeller. I anticipated the RPM to increase, but it top out around 11500 also.

Anyone have a quick tutorial on RPM and prop sizes?

1) Is there a way to calculate the power of the prop size/pitch combined with the RPM? (for example: a 12 x 6 prop and 11500 RPM equal XX amount of thrust)
2) I would expect a smaller prop's RPM to be higher. Should I be looking for 100 RPM variations or will the variations be as small as 10 to 20 RPM?
3) Or does anyone know the normal range of RPM on different motors or prop sizes? Is 11500 normal?
4) Will fuel have a significant impact on RPMs? More nitro equals more RPM?

Etc. etc.....


Thanks.

Several pay-ware and free-ware programs out there will approximate thrust, rpm etc. Here is a thrust calculator to get you started...

http://www.bmaps.net/

Look under "Goodies"

Theoretically you should see several hundreds rpm increase moving to a smaller diameter but keep in mind that that other factors like prop brand, fuel content (more nitro CAN increase power/rpm), ambient temperature, barometric pressure, needle setting and more all will affect engine performance and thus prop speed. It may vary from day to day. There is just experience and 'rule-of-thumb' to guide your prop selection. Try several different props to see what fits you and your plane best.

Here's a chart I use to get me started on prop selection.

The Thrust HP calculator is a fantastic tool. There is a huge difference in thrust between a 12x6 and 11x6 when running at the same RPM. 7.76 lbs vs 5.48.

Thanks.

That thrust hp calculator which is freeware is not even close. don't use it.

I try not to put too much into the RPM and thrust calculations the tachs are a good reference point for tuning, finding max rpm and backing off the needle for proper tune settings other than that if you are running the recomended prop and your plane seems to be bogging through the air try another prop until you find one that gives you the best flyable performance, diamiter does have alot to do with RPM but so does the Pitch of a particular prop and how much thrust the prop produces in the real life situations in a particular airframe.

The best tool for finding the best prop for your aplication is experimentation. Try different props untill you find one that meets the needs of YOUR flying style. Sometimes the most thrust is not what you are going to want. And what you want your plane to fly like may not be what the next guy wants the same plane to fly like. In general smaller prop diameters with increased pitch will fly faster and larger diameter with reduced pitch will give you more pulling power w/ a lower top speed. Also the larger diameter props create a larger thrust cone enveloping a greater amount of your main wing thus making your ailerons as well as other control surfaces more effective at slower speeds, such as thrust vectored manouvers performed in 3D flying. Small control surfaces that are common on most basic sport models will benifit less from this than the 3D planes with "over sized" control surfaces. Another important thing to know is that the RPM you see on the ground is not even close to the RPM you will reach while flying. The reading you get on the ground is refered to as "static" rpm as the aircraft is restrained and pulling air from in front of it. When the aircraft is flying it allows the prop to "unload" and actually spin at a higher RPM.