Part 15 (1/2)

This angle secures the ”pitch,” which is the distance the propeller advances during one revolution, supposing the air to be solid The air, as a ives back to the thrust of the blades just as the pebbles slip back as one ascends a shi+ngle 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 pitchspeed of the aeroplane plus the slip of the propeller For exaiven the following conditions:

Flying speed70 miles per hour

Propeller revolutions1,200 per minute

Slip15 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 = 5903 feet

1,200

In order to secure a constant pitch frole 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 exa 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 le of path (pitch angle) le of path taken by the sloalker Their pitch angles are different, but their pitch (in this case altitude reached in a given time) is the same

[Illustration]

In order to test the pitch angle, the propeller les to a beam the face of which must be perfectly level, thus:

[Illustration]

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 le a projection of the chord le of the blade at that point

Now lay out the angle on paper, thus:

[Illustration]

The line above and parallel to the circu 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 circu 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 31416 = 1556 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

The propeller should be tested at several points, about a foot apart, on each blade; and the diagram, provided the propeller is not faulty, will then look like this:

[Illustration: A, B, C, and D, Actual pitch at points tested

I, Pitch angle at point tested nearest to centre of propeller