My guess is that the "true" pitch isn't what we typically measure, or what is on the prop.

When you use a pitch gauge, the underside of a prop is touched by a straight edge. That means the straight edge touches two points on the sometimes complicated underside profile. Of course why should you expect this "pitch" to be related to the speed that a plane may fly. Air isn't a piece of wood that a screw turns through.

Ok now that I set you up, imagine a different test. Mount the prop on a motor and measure how much thrust it makes at different airspeeds. If I had access to a wind tunnel I could try this. I also occasionally think about attaching a long boom on my car (where the prop sits out way forward of the front of the car, and run down the road at various speeds (calm air only!), measuring the thrust as a function of speed and rpm.

So assuming some predictive abilities, you could imagine at some speed and rpm line, the thrust of the prop will be zero. That means at that speed and rpm, the air flowing past the spinning prop makes no net forward thrust. That's what I'd call the "true" pitch speed. Some people have noted that for most of our props (including those with a Clark Y airfoil, the prop blade will actually have what looks like to be a slight negative angle of attack at this point.

For a fixed rpm, as a function of speed, the thrust will be positive for speeds less than this "true" pitch speed, and actually negative (braking) for speeds greater than this "true" pitch speed.

A real plane (to which the prop and motor is attached) has drag, and so will fly at an airspeed less than this "true" pitch speed. A draggier plane will have a slower airspeed, or conversely you may need to up the rpm to get to the lap time you like.

So the moral of this story, is that pitch should only be used as an approximation of speed. I will note that the pitch that you find on the prop, is also not necessarily even that what we measure. A good example of this is the APC 12-6 tractor and pusher props. You would think that simply interchanging the prop would give you the same lap speed (reversing the motor rotation too!). In my case I found the pusher to give significantly lower lap times--so low for me that the rpm that I used on the tractor prop was too low to even attempt maneuvers. So I say buyer beware! 

I am trying to set up an old 0.15 Flying Clown to fly on 50' lines.  Looking at the posts in the sucessful setups topic I can see that for the two detailed setups given, the plane flies only several mph slower than the pitch speed of the prop. Alan's setup calculates out to governor pitch speed of 55 mph with a plane speed of 52 mph.
Dennis's setup has pitch speed aboput 52 mph or so and 51 mph air speed.  I have no experience in this at all but I would expect at least a 10% difference. Since the governor is used is it possible that the governor has some sort of "elasticity" to the goverened speed.  Can someone explain what is going on?
Thanks
Jim

Just read your post a bit more carefully ( ).

One thing to consider is that for a 6"pitch prop, turning at 8000 rpm, I calculate the pitch speed to be 45 mph. If I am flying 5.1s laps on 62 foot center to pilot center, that calculates out to 52mph (I note that my numbers are fluctuating a bit over time as I adjust stuff). But anyway my point is that I am actually flying faster than my pitch speed.

So that is why I am somewhat skeptical of pitch speed--or at least using the values of pitch that you find on the prop.

Originally with the Nobler, I paid a lot of attention to it, but was really surprised by how fast the Nobler was flying when I was using rpm's that I thought would give me the lap time I wanted for the pitch of the prop.

Another detail is that pitch speed doesn't say anything about diameter. So 8000 rpm on an 11x6 has the same pitch speed as a 12x6 (lets assume at least both have the same airfoils and actual pitches). However the 12" prop will provide more thrust for airspeeds less than the pitch speed, so you might expect a faster lap time with the 12" prop than the 11" prop, even through a naive pitch speed calculation would give you the same airspeed for the same rpm.

It may also say something that I have a lot of props with lots of diameters and pitches (so I have 11x5.5, 11x7, 11x9, 12x6, 12x8, 12x10, plus 10" ones (and these are just the APC Thin Electric props!).