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Calculating Propeller Pitch

jethro1

I'm New Here
Sensenich gives the formula for calculating propeller pitch as



But in an example of calculating propeller pitch they measure the blade angle at 75% of the radius of a blade. Not at 75% of the diameter.

Am I missing something or is the document incorrect?

 
Pi * .75 * D is the circumference at the 75% radius station. You could also express it as Pi * 2 * (.75 * R).
 
I believe the intent is to determine the blade angle at 75% of the blade length. Then convert that angle to pitch.

75% of the diameter of 70" propeller (tip to tip) is 52.5" outward from the propeller hub.

75% at one half of the diameter (the radius) is 26.25".

A big difference.
 
The document and the formula are correct.

When measuring the angle of the propeller blade, this is taken, as the document says at the 75% blade radius station. The blade radius station is denoted as 0% at the blade hub (an impracticality) to 100% at the blade tip. Remembering that each blade station describes a helix as the blade rotates, the blade pitch angle is larger near the hub and smallest at the tip. The convention is the blade angle for calculating geometric pitch is taken at the 75% blade radius station.

The geometric pitch is a measure of how far (in inches) the propeller would advance with no slip, thus it uses 0.75 (the blade radius station) x diameter to calculate the circumference and multiplies this by tan of the angle.
 
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