I am not really a beginner as I have been flying for some years now. However I have always used the "recommended" prop for each engine. My question is this; If I want to play with prop size, what do I need to know. How do I know how much is too much? I understand the basics. More pitch = more bite, more length = more thrust. I guess what I am looking for is a formula or some thing of the sort so I know what numbers to look for.

I would look up the typical max RPM for your engine then put a tach on it and size the propeller to get the engine speed you are looking for.

A a lot depends on the plane. A slow, fat airfoil has a lot of drag at high speeds.

Depends on what you are looking for.

Go to higher pitch if you want more speed.
Go to bigger diameter, lower pitch for more pull.

High pitch props tend to have less of a pitch-factor. Will require less rudder input on take-offs.

Try going one step in each direction. I.e from 10-6 try 10-7 and 11-6 or 11-5.

A tach is a good tip. You want to be within the range. For example the best prop on my 91FX gave 12400 rpm on the gound. It would hit close to the max (16k) at full speed. If RPM on the ground is too low, it may not reach the ideal power band. If too high, it will max out the engine in the air and will be hard on the engine.

The general rule of thumb is that if you go up one in pitch, go down one in length. If you go up one in length, go down one in pitch.

What is the pitch? Let's say I have an 11x4 prop. The pitch is 4...That means that if the propellor was 100% efficient it would go forward exactly 4 inches for each revolution. Obviously our airplane engines and props are not 100% efficient, which is why prop choice is such a big deal.

Think of a longer flatter prop as a lower gear in a car, first gear.

Think of a short curved prop (high pitch) as a very high gear in a car, 5th gear.

If you want an airplane that accelerates quickly, holds a constant speed, and gives you the most thrust you want to be in first gear. This means that you will reach a certain speed and just can't go any faster. Your acceleration will be great! For a .46 size engine this would be something like a 12.25x3.75 prop.

Now let's say all you care about is top end speed. You want to be in 5th gear of course...Your acceleration will be slow, you will lose speed in turns, gain speed with the nose down, and not be efficient at all when you're at slow speeds. For a .46 this would be something like a 9x8 prop.

Keep your engine happy and somewhere within the recommended prop ranges. If you ever experience overheating or have a hard time getting the high end/low end set, you've probably gone too far.

Pitch factor? I'll assume you're talking about propellor factor, or p-factor. This is a VERY VERY small effect regardless of what prop you use. This is nothing to worry about...A larger prop will have a bigger gyroscopic effect though causing gyroscopic precession. Again, that is not a big deal. A larger diameter prop also causes a larger slipstream which also causes a yaw to the right with high angle of attack. This sort of matters, but is nothing to worry about. Torque is something to worry about!

You make a good point about props unloading in the air... A longer flatter prop will not unload nearly as much as a short curved type...

That would mean using about a 9x6 on my Pro .46 to get it going at 16,000 RPM????

P-factor always referred to pitch factor AFAIK.

It was a huge factor on my GP Extra 300. A 12-6 vs a 11-7 made a bige difference. With the 12-6 it would require a considerable amount of rudder on takeoff and the climbout. You could ignor the rudder once in the air, but climbout was much faster with a bit of rudder as drag was less.

Once in the air and up to speed, it did not have a large AOA, it was not real noticable. But when I would do something like a harrier and hit the gas to pull out, there was a big difference.

It depends on the plane. Most trainers don't seem to care as they have more yaw stability. It will be noticable when you gun it on aborted landings.

When you start getting low-pitch props, the AOA effects the pitch more. Think of a prop with a 15 degree pitch verses a 25 degree pitch. If the plane reaches a 5 degree AOA:

15 degree prop will have 10 and 20 degree AOA on right/left sides.
25 degree will have 20 and 30 AOA.

10-20 is 2:1 20:30 is 2:3. These numbers are non-typical but it shows the effect.

You were most likely experiencing slipstream... P-Factor is a very minor effect. Even during take-off your AOA is not high enough with an Extra for p-factor to make that much of a difference. Slow speeds and high power settings is when slipstream is most pronounced.

I guess there is no way to know, huh?

Well, if the tail came up fast and oscillated it would yaw right left as the tail oscillated from center.

With the 12-6 you would see it do the first left turn+climb with the nose way out on the outside of the turn. Lots of drag. Right turns much easier. Probably made much more significant as it was set up tail heavy. The 11-7 was much easier to fly. Then again, the change from the 61FX to 91FX changed the takeoff completly. Roll a bit then straight up.

The plane was a bit under powered with the 61FX and the crappy pitts muffler. That made climb out slow.

Moot point as I flew the plane until it totalled itself. Guess the rudder had too much "Hanger rash" over the years.

My new GP PW Extra has little P-effect. Then again takeoff is again vertical.

I doubt you'd do very well sizing your prop for max rpm.

Most engines have a position considerably below peak rpm where they produce the most torque. For your 46, I'd guess it's in the 11,500 to 12,500 rpm range. (Perhaps lower)
Dennis-