What do the number for prop sizes mean? I figure the first number was length. This is easy enough. I know the second number has to do with pitch, but what does it mean exactly? Looked around trying to find this out and didn't come up with anything.
TIA
Brandon

In the number 10 x 6, the 10 is length, and the 6 means that the pitch is such that if you rotated the prop one revolution, it would (theoretically) move forward 6 inches.

Props are like the gears in a car engine.

If we look at a typical .40 two stroke engine, a 11x4 is like first gear in a car. It will pull and climb well, but top speed is limited. Still it might be enough speed for trainers and slow airplanes. A 10x7 or 9x8 is more like fifth gear in a car. The plane won't accelerate as fast or climb nearly as steeply as with the 11x4, but level flight top speed will be much higher.

How I choose props:
1) Pitch. What kind of plane is it, and what type of flying will I be doing?

2) Diameter. Higher diameter will be a heavier load and reduce rpms. Choose a diameter that lets the engine run at "suitable" rpms.

This was probably more (and other) information than you were looking for, but I decided to post it in case you wanted a "link" between the theory and how it can be done.

Some props do not have the "X" and just go with a 4 digit designation. 1080 would be 10" diameter and 8 pitch.

Just remember the simple proverb: Pitch=Speed , Diameter=Power.

Mark Shuman