By far the simplest and most efficient form of power which could be
installed in a model aeroplane to drive the propeller is a twisted skein
of rubber. Nothing is lighter, or more easily handled and repaired.
The word _elastic_, in physics, is the name given to the tendency which
a body exerts, when distorted, to return to its original shape. Rubber
possesses more elasticity than any other material known, it being
possible to stretch a piece of rubber cord to eight or nine times its
original length without fracture. Rubber also possesses the added
requisite of lightness and will store up more energy than any form of
steel spring.
*The Simplest Form of Motor* is a single skein of elastic stretched
between two hooks, one fixed and the other to which the propeller is
attached, free to rotate. In some cases it is a decided advantage to
divide the motor into one or more parts. One phase of this question has
already been considered. The others will be discussed in the following
chapter.
*The type of Elastic* which gives at once the longest life and the
greatest power is the _square_ rubber, preferably about 3/32 x 3/32
inches, and not the flat strip. When examined under the microscope the
edges of the square rubber show to be cleaner and sharper and not so
ragged as those of the flat strips. To be of any value for use in a
model aeroplane, the rubber should be absolutely pure and fresh.
There are certain precautions which if observed will add greatly to the
power and efficiency of a rubber band motor.
Always remove the elastic from the machine when the flights are over for
the time being. Rubber spoils very quickly when kept under tension. It
also deteriorates if warm, so keep in a cool place. Strong sunlight
causes rubber to harden and lose its elasticity, due to the presence of
the sulphur used in vulcanizing. If talcum powder or finely powdered
soapstone is rubbed on the bands from time to time it will prevent them
from sticking together. The strands will then run and slip more easily
upon each other, making it possible to store up a greater number of
propeller revolutions.
In spite of the use of talcum powder, however, when a skein of rubber is
twisted very tightly, the strands stick together, causing it to soon
break up.
This nuisance may be somewhat alleviated if the strands are lubricated
with _pure redistilled_ glycerine free from grease, etc. Such a
precaution will not always greatly lengthen the life of the rubber, but
will increase the number of turns which it is possible to give the skein
(and this is a very important advantage in model contests). Due to its
sticky nature, however, the glycerine will cause the rubber to gather
dust and particles of dirt which, if allowed to grind into the rubber,
would soon weaken it. The skein should therefore be washed from time to
time in warm soda and water and fresh glycerine applied. By all means,
avoid all oils or substances of a greasy nature, such as lubricants.
They quickly soften and rot the rubber.
*The Amount of Elastic* required for a model will vary considerably for
propellers of the same pitch and diameter. There is always a tendency to
use too much rather than too little and this fault should be carefully
guarded against. In nine cases out of ten it is the cause of the
unsatisfactory behavior of a model.
The motor should always be "stranded," that is, made up of a skein of
bands. It is then possible to secure a larger number of turns than if a
single strip were used.
Always start a new machine with a small number of strands and gradually
add to the number until the proper amount of power is obtained. The
distance between the propeller and the fixed hook should always be as
great as possible so as to secure the maximum number of turns.
*Doubling the Number of Elastic Strands* increases the power of the
motor but cuts down the number of turns which it is possible to give the
propeller. That is to say, a certain skein composed of six strands of
rubber will take perhaps two hundred and twenty-five turns while a
twelve strand skein of he same sized rubber strands strands is only
capable of less than half or about one hundred turns before it is wound
tight.
Doubling the number of strands and at the same time keeping them the
same length increases the torque more than three times but diminishes
the number of turns from one-half to one-third.
*Doubling the length* of the strands does not materially reduce the
torque for the first hundred turns. After two hundred turns have been
reached, the torque is only about one-half as great as it would be in
case the length were not doubled.
Doubling the length of the strands doubles the number of turns it is
possible to give the skein. It is easy to see from this why it is always
advisable to make the _motor as long as possible_ and to compose it of
the fewest number of strands if long flights are desirable.
By using several separate skeins geared together so as to apply their
energy to one screw, it is possible to obtain a greatly increased number
of turns. The weight of the gearing is very small and hardly a factor,
considering the advantages derived therefrom. Since the skeins revolve
in opposite directions the frame of the machine is relieved of the
harmful twisting effect so often present in a single skein.
The gears should be of steel accurately cut and of no larger diameter
than is necessary to separate the rubber skeins the requisite distances
so that they will not rub.
Holes may be bored in the gears to lighten them. The gears are easily
and conveniently cut out of steel pinion wire.