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.
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.
If the gap is too small, then the following blade will engage air which the preceding blade has put into motion, with the result that the following blade will not secure as good a reaction as would otherwise be the case. It is very much the same as in the case of the aeroplane gap.
For a given pitch, the gap of a four-bladed propeller is only half that of a two-bladed one. Therefore the four-bladed propeller is only suitable for large pitch, as such pitch produces spirals with a large gap, thus offsetting the decrease in gap caused by the numerous blades.
The greater the speed of rotation, the less the pitch for a given aeroplane speed. Then, in order to secure a large pitch and consequently a good gap, the four-bladed propeller is usually geared to rotate at a lower speed than would be the case if directly attached to the engine crank-shaft.