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.
CHAPTER V
MAINTENANCE
CLEANLINESS.--The fabric must be kept clean and free
from oil, as that will rot it.
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