Very little effort has been made to utilize paddle wheels for aerial
propulsion; the screw is almost universally employed. Every one knows
that when a bolt turns in a stationary nut, it moves forward a distance
equal to the _pitch_ (lengthwise distance between two adjacent threads)
at every revolution. A screw propeller is a bolt partly cut away for
lightness, and the "nut" in which it works is water or air. It does
not move forward quite as much as its pitch, at each revolution,
because any fluid is more or less slippery as compared with a nut of
solid metal. The difference between the pitch and the actual forward
movement of the vessel at each revolution is called the "slip," or
"slip ratio." It is never less than ten or twelve per cent in marine
work, and with aerial screws is much greater. Within certain limits,
the less the slip, the greater the efficiency of the propeller. Small
screws have relatively greater slips and less efficiency, but are
lighter. The maximum efficiency of a screw propeller in water is under
80%. According to Langley's experiments, the usual efficiency in air
is only about 50%. This means that only half the power of the motor
will be actually available for producing forward movement--a conclusion
already foreshadowed.
In common practice, the pitch of aerial screws is not far from equal
to the diameter. The rate of forward movement, if there were no slip,
would be proportional to the pitch and the number of revolutions per
minute. If the latter be increased, the former may be decreased.
Screws direct-connected to the motors and running at high speeds will
therefore be of smaller pitch and diameter than those run at reduced
speed by gearing, as in the machine illustrated on page 134. The number
of blades is usually two, although this gives less perfect balance than
would a larger number. The propeller is in many monoplanes placed in
_front_: this interferes, unfortunately, with the air currents against
the supporting surfaces.
There is always some loss of power in the bearings and
power-transmitting devices between the motor and propeller. This may
decrease the power usefully exerted even to _less_ than half that
developed by the motor.