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 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. = 5.903 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 alt.i.tude reached in a given time) is the same.

[Ill.u.s.tration]

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:

[Ill.u.s.tration]

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:

[Ill.u.s.tration]

The line above and parallel to the circ.u.mference 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 circ.u.mference of the propeller where the pitch angle is being tested. For example, if that place is 2 feet radius from the centre, then the circ.u.mference will be 2 feet x 2 = 4 feet diameter, which, if multiplied by 3.1416 = 15.56 feet circ.u.mference.

Now mark off the circ.u.mference 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 ill.u.s.tration, 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:

[Ill.u.s.tration: A, B, C, and D, Actual pitch at points tested.

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

E, Circ.u.mference at I.

J, Pitch angle at point tested nearest to I.

F, Circ.u.mference at J.

K, Pitch angle at next point tested.

G, Circ.u.mference at K.

L, Pitch angle tested at point nearest tip of blade.

H, Circ.u.mference at L.]

At each point tested the actual pitch coincides with the specified pitch: a satisfactory condition.

A faulty propeller will produce a diagram something like this:

[Ill.u.s.tration]

At every point tested the pitch angle is wrong, for nowhere does the actual pitch coincide with the specified pitch. Angles A, C, and D, are too large, and B is too small. The angle should be correct to half a degree if reasonable efficiency is to be maintained.

A fault in the pitch angle may be due to (1) faulty manufacture, (2) distortion, or (3) the shaft hole through the boss being out of position.

2. STRAIGHTNESS.--To test for straightness the propeller must be mounted upon a shaft. Now bring the tip of one blade round to graze some fixed object. Mark the point it grazes. Now bring the other tip round, and it should come within 1/8 inch of the mark. If it does not do so, it is due to (1) faulty manufacture, (2) distortion, or (3) to the hole through the boss being out of position.

3. LENGTH.--The blades should be of equal length to 1/16 inch.

4. BALANCE.--The usual method of testing a propeller for balance is as follows: Mount it upon a shaft, which must be on ball-bearings. Place the propeller in a horizontal position, and it should remain in that position. If a weight of a trifle over an ounce placed in a bolt-hole on one side of the boss fails to disturb the balance, then the propeller is usually regarded as unfit for use.

[Ill.u.s.tration]

The above method is rather futile, as it does not test for the balance of centrifugal force, which comes into play as soon as the propeller revolves. It can be tested as follows:

[Ill.u.s.tration]

The propeller must be in a horizontal position, and then weighed at fixed points, such as A, B, C, D, E, and F, and the weights noted. The points A, B, and C must, of course, be at the same fixed distances from the centre of the propeller as the points D, E, and F. Now reverse the propeller and weigh at each point again. Note the results. The first series of weights should correspond to the second series, thus:

Weight A should equal weight F.

Weight B should equal weight E.

Weight C should equal weight D.

There is no standard practice as to the degree of error permissible, but if there are any appreciable differences the propeller is unfit for use.

5. SURFACE AREA.--The surface area of the blades should be equal. Test with calipers thus:

[Ill.u.s.tration]

The distance A--B should equal K--L.

The distance C--D should equal I--J.

The distance E--F should equal G--H.

The points between which the distances are taken must, of course, be at the same distance from the centre in the case of each blade.

There is no standard practice as to the degree of error permissible. If, however, there is an error of over 1/8 inch, the propeller is really unfit for use.