A MORE accurate control may be gained over a strand motor by using gear
wheels. Both their speed and duration may be increased indefinitely. The
gear will restrain a powerful motor from "racing," acting much the same
as the governor of an ordinary engine. Still another advantage is the
steadiness they insure to a machine in flight by cutting down the
vibration.
The geared wheels and the frames for mounting them will be found to add
very little weight, and they make possible a saving of rubber in the
motor which renders such equipment no heavier than the ordinary
direct-drive motor. Gears may be installed upon any ordinary motor base.
Here is a fascinating field for experiment.
The great advantage of the geared motor lies, of course, in the fact
that it enables you to divide up your rubber motor into smaller groups
of strands. As you increase the power by adding more rubber, you of
course cut down the number of the turns it will take. A motor forty
inches in length, comprising six strands of rubber, one-eighth of an
inch square, may be twisted to about one thousand turns. Double the
number of strands and you will find that you cannot get more than five
hundred turns with safety. Double the diameter of your motor one more
and it cannot be twisted more than to two hundred times.
Now it is clear that if the motor consists of two groups of six strands
each, and the axles be geared together, it may be wound up with one
thousand turns while the power exerted will be that of the twelve
strands. A motor with three groups of strands in turn will give you one
thousand turns with the propelling force of the combined number, or
eighteen strands, and so on.
As a general rule, it may be laid down, that by dividing your motor you
double the number of revolutions. The heavier the motor the greater is
the vibration in unwinding, and as you have doubtless discovered, a
model which vibrates in flight offers a greatly increased resistance to
the air. The geared motor cuts the vibration in half, or a third, as the
case may be. Incidentally, this renders your motor practically
noiseless.
The resistance offered by a set of smoothly-running geared wheels is
slight, and may be compensated by adding more strands to the motor. In
mounting them use the same care as used in the shafts of ordinary
motors. By mounting them on a simple metal frame the friction may be cut
down still further. This mechanism, moreover, is so slight that you can
afford to select substantial material for the purpose. The clock wheels
used in most of our American clocks are too thin for the purpose and
easily slip apart.
A wheel one-sixteenth of an inch thick will add little weight to your
model and will run much more smoothly. They may be bought very cheaply
from clockmakers. The ingenious miniature ball bearings constructed for
model aeroplanes are excellent for mounting gears, but they are not
essential by any means, and the ordinary arrangement of washers used for
motor axles will be delicate enough for gear adjustments. Complete sets
of gear wheels mounted on metal frames, ready to be attached to the
motor bases, may be purchased from the supply houses, but they are so
simple that almost any bright boy can construct them for himself.
In experimenting with gears it will be well to begin with but two sets
of strands with gears of the same number of teeth. Several models
equipped in this way have been flown with success in America. The best
flights are those made by a model built by Mr. Frank Schoeber of New
York, a winner in several competitions. Mr. Schoeber’s model is driven
by two motors of three strands each. He uses wheels of the same size and
number of teeth. The support on which the gear wheels are mounted must
be perfectly rigid, since any play will result in a serious loss of
power through increased friction.
Compressed-air motors have not yet been developed to a form practical
for use in small model aeroplanes.
[Illustration: An aeroplane of simple construction that flies remarkably
well, built by R. S. Barnaby]
A number of inventors are at work on this problem and high hopes are
entertained that a practical form of compressed-air engine will soon be
invented. An unlimited supply of compressed air may of course be carried
without adding to the weight of the model. It costs nothing again to get
a supply. Anyone with a bicycle pump can get up enough energy for his
model as easily as he can inflate a bicycle tire. The best
compressed-air engines at present weigh from one-third to one-half as
much as the model they drive, and will run only from ten to thirty
seconds.
Spring motors hold out a promise for the model aeroplane builder. A
number of fascinating experiments have been made to utilize the power of
a coil spring. One young aviator mounted the spring of an ordinary shade
roller upon a model aeroplane. The spring was connected up for a direct
drive, a single propeller being used. It was found that such a spring
could turn fifty times, storing up a relatively large amount of energy.
Once released, however, it unwound with startling rapidity. The
unwinding process was so rapid that the propeller spun through the air
without taking hold or exerting any appreciable driving power.
A propeller of very high pitch was employed, but with the same results.
Evidently a spring of this nature is not practical. Experiments have
been made in Germany, however, with coil springs better suited for model
aeroplanes, with satisfactory results. Since trolley cars have been
driven for considerable distances by energy wound up in coil springs,
the model aeroplane should be able to take advantage of this elasticity.
Doubtless another year will find model aeroplanes flying by energy
stored in this way.
The Petrol motors are now offered for sale in sizes suitable for model
aeroplanes by several firms. A motor developing one-half of one
horsepower weighs but little. The principal difficulty of these little
power plants is that they set up a more or less violent vibration which
racks the entire machine and renders a stable flight difficult. To set
up one of these engines and adjust it to so delicate a task requires the
highest engineering skill. Unless one has had such experience it will be
well to avoid such experiments for the present at least. As a rule the
engines are used to equip scale models which are not intended to fly.
The beautiful model illustrated on the cover of this volume built by Mr.
Karl H. De Leon is equipped with a rotary Knome engine especially
imported.