Part 16 (1/2)

6. CAMBER.--The camber (curvature) of the blades should be (1) equal, (2) decrease evenly towards the tips of the blades, and (3) the greatest depth of the curve should, at any point of the blade, be approximately at the same percentage of the chord from the leading edge as at other points.

It is difficult to test the top camber without a set of templates,[18]

but a fairly accurate idea of the concave camber can be secured by slowly pa.s.sing a straight-edge along the blade, thus:

[Ill.u.s.tration]

The camber can now be easily seen, and as the straight-edge is pa.s.sed along the blade, the observer should look for any irregularities of the curvature, which should gradually and evenly decrease towards the tip of the blade.

7. THE JOINTS.--The usual method for testing the glued joints is by revolving the propeller at greater speed than it will be called upon to make during flight, and then carefully examining the joints to see if they have opened. It is not likely, however, that the reader will have the opportunity of making this test. He should, however, examine all the joints very carefully, trying by hand to see if they are quite sound.

Suspect a propeller of which the joints appear to hold any thickness of glue. Sometimes the joints in the boss open a little, but this is not dangerous unless they extend to the blades, as the bolts will hold the laminations together.

8. CONDITION OF SURFACE.--The surface should be very smooth, especially towards the tips of the blades. Some propeller tips have a speed of over 30,000 feet a minute, and any roughness will produce a bad drift or resistance and lower the efficiency.

9. MOUNTING.--Great care should be taken to see that the propeller is mounted quite straight on its shaft. Test in the same way as for straightness. If it is not straight, it is possibly due to some of the propeller bolts being too slack or to others having been pulled up too tightly.

FLUTTER.--Propeller ”flutter,” or vibration, may be due to faulty pitch angle, balance, camber, surface area, or to bad mounting. It causes a condition sometimes mistaken for engine trouble, and one which may easily lead to the collapse of the propeller.

CARE OF PROPELLERS.--The care of propellers is of the greatest importance, as they become distorted very easily.

1. Do not store them in a very damp or a very dry place.

2. Do not store them where the sun will s.h.i.+ne upon them.

3. Never leave them long in a horizontal position or leaning up against a wall.

4. They should be hung on horizontal pegs, and the position of the propellers should be vertical.

If the points I have impressed upon you in these notes are not attended to, you may be sure of the following results:

1. Lack of efficiency, resulting in less aeroplane speed and climb than would otherwise be the case.

2. Propeller ”flutter” and possible collapse.

3. A bad stress upon the propeller shaft and its bearings.

TRACTOR.--A propeller mounted in front of the main surface.

PUSHER.--A propeller mounted behind the main surface.

FOUR-BLADED PROPELLERS.--Four-bladed propellers are suitable only when the pitch is comparatively large. For a given pitch, and having regard to ”interference,” they are not so efficient as two-bladed propellers.

[Ill.u.s.tration: SPIRAL COURSES OF TWO-BLADE TIPS.

SPIRAL COURSES OF FOUR-BLADE TIPS.

Pitch the same in each case.]

The smaller the pitch, the less the ”gap,” _i.e._, the distance, measured in the direction of the thrust, between the spiral courses of the blades (see ill.u.s.tration on preceding page).