The Aeroplane Speaks. Fifth Edition

In order to test the pitch angle, the propeller must be mounted upon a shaft at right angles to a beam the face of which must be perfectly level, thus: First select a point on the blade at some distance (say about 2 feet) from the centre of the propeller. At that point find, by means of a protractor, the angle a projection of the chord makes with the face of the beam. That angle is the pitch angle of the blade at that point. Now lay out the angle on paper, thus: The line above and parallel to the circumference line must be placed in a position making the distance between the two lines equal to the specified pitch, which is, or should be, marked upon the boss of the propeller. Now find the circumference of the propeller where the pitch angle is being tested. For example, if that place is 2 feet radius from the centre, then the circumference will be 2 feet x 2 = 4 feet diameter, which, if multiplied by 3.1416 = 15.56 feet circumference. Now mark off the circumference distance, which is represented above by A--B, and reduce it in scale for convenience. The distance a vertical line makes between B and the chord line is the pitch at the point where the angle is being tested, and it should coincide with the specified pitch. You will note, from the above illustration, that the actual pitch line should meet the junction of the chord line and top line.