The air,
as a matter of fact, gives back to the thrust of the blades just as the
pebbles slip back as one ascends a shingle beach. Such "give-back" is
known as Slip. If a propeller has a pitch of, say, 10 feet, but actually
advances, say, only 8 feet owing to slip, then it will be said to
possess 20 per cent. slip.

Thus, the pitch must equal the flying speed of the aeroplane plus
the slip of the propeller. For example, let us find the pitch of a
propeller, given the following conditions:

Flying speed.............. 70 miles per hour.
Propeller revolutions..... 1,200 per minute.
Slip...................... 15 per cent.

First find the distance in feet the aeroplane will travel forward in one
minute. That is--

369,600 feet (70 miles)
------------------------ = 6,160 feet per minute.
60 " (minutes)

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 feet 1 3/5 inches.
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 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.

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 t