Now divide the feet per minute by the propeller revolutions per minute, add 15 per cent. for the slip, and the result will be the propeller pitch:
| 6,160 | |
| | + 15 per cent. = 5.903 feet. |
| 1,200 |
In order to secure a constant pitch from root to tip of blade, the pitch angle decreases towards the tip. This is necessary, since the end of the blade travels faster than its root, and yet must advance forward at the same speed as the rest of the propeller. For example, two men ascending a hill. One prefers to walk fast and the other slowly, but they wish to arrive at the top of the hill simultaneously. Then the fast walker must travel a farther distance than the slow one, and his angle of path (pitch angle) must then be smaller than the angle of path taken by the slow walker. Their pitch angles are different, but their pitch (in this case altitude reached in a given time) is the same.
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.