er 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. To take out dirt or oily patches, try acetone. If that will
not remedy matters, then try petrol, but use it sparingly, as otherwise
it will take off an unnecessary amount of dope. If that will not remove
the dirt, then hot water and soap will do so, but, in that case, be
sure to use soap having no alkali in it, as otherwise it may injure the
fabric. Use the water sparingly, or it may get inside the planes and
rust the internal bracing wires, or cause some of the wooden framework
to swell.

The wheels of the undercarriage have a way of throwing up mud on to
the lower surface. This should, if possible, be taken off while wet. It
should never be scraped off when dry, as that may injure the fabric. If
dry, then it should be moistened before being removed.

Measures should be taken to prevent dirt from collecting upon any
part of the aeroplane, as, otherwise, excessive skin-friction will be
produced with resultant loss of flight speed. The wires, being greasy,
collect dirt very easily.

CONTROL CABLES.--After every flight the rigger should pass his hand over
the control cables and carefully examine them near pulleys. Removal of
grease may be necessary to make a close inspection possible. If only one
strand is broken the wire should be replaced. Do not forget the aileron
balance wire on the top surface.

Once a day try the tension of the control cables by smartly moving the
control levers about as explained elsewhere.

WIRES.--All the wires should be kept well greased or oiled, and in the
correct tension. When examining the wires, it is necessary to place the
aeroplane on level ground, as otherwise it may be twisted, thus throwing
some wires into undue tension and slackening others. The best way, if
there is time, is to pack the machine up into its "flying position."

If you see a slack wire, do not jump to the conclusion that it must
be tensioned. Perhaps its