How do you find the pitch of a propeller in which the pitch is not known or labeled on the prop. Do you have a product or method I could use?

From : Don Stackhouse

There are a number of pitch gages on the market, and you might find one of those at a really good hobby shop or advertised in one of the magazines.

The pitch of a prop is typically defined as the distance it would screw itself forward through the air if there was no slippage, like a screw going through wood. Typically it's specified at the 75% radius of the prop. The actual pitch is usually not constant along the blade (for the highest efficiency, it usually should not be constant). Since the aerodynamic center of the blade is at about the 75% radius, we use that location to specify the overall pitch.

To measure the pitch at the 75% radius in inches, first measure the diameter and multiply by 0.75. Measure from the center of the prop and mark this distance on the blade. Now measure the blade angle of the underside (the flat or concave side) of the blade at that point. Hobbico's "Builder's Protractor" (part no. HCAR0490) can be used for at least an approximate measurement of this. Find the tangent of that angle, then multiply that tangent times the radius in inches at that point times two and times Pi. The result is your pitch in inches.

For example, I just checked the pitch of an unmarked Peck Polymers free flight rubber prop this way.

The actual measured diameter diameter of this was 7.812, so the 75% radius was 0.75 * 0.5 * 7.812, which is 2.930".

I marked this distance from the center on the blade's underside (be careful not to use anything that could cause a scratch, resulting in a stress concentration and later in a fatigue crack!) and measured the pitch in degrees at that point. It was approximately 19 degrees (note the "approximately"; unless you use a very big, very accurate protractor and a special jig to hold the protractor square with the propeller hub, your measurement will only be "approximately", although probably still good enough for rough estimates).

The tangent of 19 degrees is 0.3443 . The circumference at the 75% radius is 2.930 * 2 * 3.141592, which is 18.410". That, times the tangent of 19 degrees, is 18.410 * 0.3443, which is a pitch of 6.338". It takes longer to explain than to actually measure and calculate.

If you want to know precisely, such as for carving a prop accurately, you need a very accurate and precise protractor and a good fixture to hold it and the prop square with each other. A propeller pitch gage is just such a device. It's typically a block of wood with a peg to hold the prop and a series of slots in the face of the block to hold the protractor at various radii along the blade. There's also often a chart for calculating the pitch in inches from the measured angle. If you dont' mid a little math and you're good with precise woodworking, you should be able to make one yourself. Your gage's accuracy will depend on the quality of your workmanship, but the work invoved is not difficult.

Don Stackhouse
DJ Aerotech