The pitch readings for each "station" are different from each other, generally more pitch nearer the hub and less as you move towards the tip. I assume the prop blade is supposed to contact the pivoting arm at about 90 degrees, as shown on the instructions. Said instructions are essentially opaque as to how to actually use the tool. So, where along the blade is the "nominal" pitch? Do you average the readings of all the stations? Is there any general agreement on how to interpret the readings as you progress from station to station?

I appreciate that the blade pitch changes as you get farther from the tip to account for the increase in speed of each station along the blade as you progress towards the tip (I calculated tip speed on the 14-foot diameter prop on a B-29 as supersonic even at 2500 prop rpm), but we purchase and discuss props as being of a certain diameter and pitch, maybe just as a convention. The Mejzlik 3-blade I tried today on the Enya 61CXS is sold as a "12.5 x 5.2." The diameter is 12.5, but how does the maker decide the pitch is "5.2" rather than one of the 10 other values I measured along the blade? It is slightly over "5" in the fat part of the blade about 2/3 of the way to the tip. Sticking with the Mejzlik line, I also have what he calls a 12.5 x 4.5, which does guage out slightly flatter along its blade length. But how the heck do we decide what prop we are talking about, especially from one maker to another? And still the question, how do I interpret the readings on the Prather guage?

simple answer, you cant,

Absolutely right!

the pitch gauge gives you a reference to get a feel for comparative differences. but Ultimatly it is a guide and not absolute. This is because the airfoil each manufacture uses is different and will create a different amount of lift .

Again, right on.

The pitch is supposed to be roughly the same number at each station as it moves out on the blade.

Uh, no.  (See your own comment below.)

The pitch is whatever the prop designer decides it should be. Varying the pitch as a function of radius is a tuning activity that can give a prop significantly different performance characteristics. As an example, tweaking the last inch or two of a carbon prop with heat is often done by stunt flyers to make the prop do something different at one point or another in the pattern. (When the flyer does this, he becomes 'the prop designer'.)

If you had said "commonly" instead of "supposed to be", I would have no quibbles.

there is some people that mess with variances in the pitch to creat special effects, IE the tips have a flatter pitch than the root so they dont pull until the plane slows down.

Some would say "mess with", others would call it "tuning".

The pitch is a function of the distance the chord travels with each revolution. It equates in theory to the forward helical distance the blade travels per revolution. Normally stated in inches. So a 12 inch diameter prop, the tip will have less apparant pitch when you look at it because it travels a significantly farther distance per revolution than the root does. Hope this helps?

Pitch IS the helical distance the blade section would travel per revolution if operated in an incompressible medium. (Think "screw thread".) But it is only a geometrical number. The actual distance the blade moves is a function of a lot of parameters, usually lumped into what folks call "the efficiency" of the prop.

Regards,

Bill Lee