FINE POINTS OF CONSTRUCTION
A great many experiments have been made to find whether the flat or
curved wings give the best support, and how sharply the curve should
be drawn. The wings of birds are curved slightly upward, and in the
end, after all the experiments, it has been found that this curve is
just the right one. All forms of aëroplanes will fly more swiftly and
steadily if the planes be slightly bowed or flexed. After you have
built your aëroplane with flat wings it will repay you to replace them
with flexed planes, and you will find that the experience in building
models will make this construction very simple.
The lighter and more flexible materials, such as bamboo or cane, are
best for the curved planes. After you have decided upon the dimensions
of the wings cut the pieces for the ends slightly longer than the
width of your planes. These pieces may then be bent by steaming them
over a kettle of boiling water and bending to the desired curve. When
dry they will hold their shape remarkably well. Another plan is to use
a flexible strip and pull the ends together by a strong thread or wire
until the wood is bowed to just the right curve. A corset steel or
whalebone may readily be curved in the same way. It is a common mistake
to curve the plane too sharply, when the resistance offered to the air
will be greater than that with the flat plane.
A plane two or three feet in width cannot be held in shape merely
by curving the end pieces. A series of ribs must be added at equal
distances, each having, of course, exactly the same upward curve. The
ribs may be fastened to the sides of the planes with small brads or
simply with glue or wire. The covering should then be drawn down. A
very smooth covering may be made of rice-paper. Cut the sheets the
proper size and lay them for a few minutes between moistened cloths.
Now stretch the paper carefully over the frame and glue in position.
When dry the paper will contract and leave a smooth, taut surface like
the head of a drum.
Much depends upon the curve of the plane. A wing whose curve is not
a perfect arc of a circle, but which is rounded just back of the
front edge and flattened at the rear, will be found to offer the
least resistance to the air. The best plan is to study the curves in
the aëroplanes or models and imitate them. Different models require
different planes. It is a problem which each young aëronaut must work
out for himself.
[Illustration: A Model Aëroplane Built from the Drawing (Plate C, Chap.
IV).]
The question of rudders or guiding planes is very important. It is
too much to expect of even the best model that it will fly in an
unswerving line. Any simple vertical plane which may be turned from
side to side and held in position will act as a rudder. There is great
difference of opinion as to the proper size and position of these
guiding surfaces. It is argued by some aviators that the rudder should
be placed above the plane, where the air is undisturbed, while others
believe that the partial vacuum created above the wings in flight makes
the propeller ineffective. Still others argue that a rudder placed back
of the planes affords a leverage, and is therefore more effective. Try
a rudder in each position. It is impossible to lay down a law for all
models.
The larger models should be equipped with twin propellers. In building
these the greatest care should be taken to have them exactly the same
size, weight, and pitch. Twin propellers should, as a rule, be placed
at the front of the machine, that is, they should pull and not push the
planes. If by any accident the motor of one should fail, the second
propeller will continue to keep the aëroplane afloat and break its fall
on descending. With the propellers at the stern of the little airship,
the failure of one would cause the plane to pitch downward, and the
remaining propeller would drive it down to possible disaster.
In winding up the two motors, care should be taken to give both the
same number of turns. The aëroplane may be launched by holding a
propeller in either hand and releasing simultaneously. The double
motor insures a steadier as well as a longer flight. Always turn the
propellers in opposite directions. In flying they must spin around
either toward each other or away from each other. If they turn the same
way they will give the model a torque which no rudder could possibly
overcome.
The efficiency of your motor depends more upon its length than its
diameter. In constructing the motor-base, especially for the larger
models, arrange to have the strands of rubber bands extend the entire
length of your aëroplane, and if necessary, project well forward of
the front plane. Such a motor in unwinding will exert a more sustained
force. The shorter strands of greater diameter will unwind much more
quickly and give very short flights.
With a little experience you will soon learn to gauge your motor to the
needs of your air-ship. It is, of course, absolutely necessary that the
force exerted by the motor should be sufficient to keep your aëroplane
in rapid motion, but it is easy to make it too powerful. If it were
possible to attach a “governor” to your motor, this would not matter
so much. But since this is practically out of the question, the motor
itself must be very nicely proportioned to the demand made upon it. You
will soon be able to judge between the steady whir of a good motor, and
the buzz of a propeller which races. There is a distinct note for each.
The motor is, at present, the great problem of the model aëroplane. The
rubber bands are, at best, only a make-shift. It is practically out of
the question to get a flight of more than fifteen seconds in this way,
so that the distance is limited to a little more than two hundred feet.
It is doubtless only a question of time before a much more efficient
form of motor will be invented. Very probably, some amateur aviator
will be the first to apply a new means of propulsion, which would be an
important achievement indeed.
The simplest form of motor after the rubber bands would seem to be some
form of metal spring which could be wound up. Long before the days
of automobiles, as we now know them, wagons were built with motors
of springs, and some surprising runs were obtained. The spring lends
itself to many forms of construction, and is not expensive. It will
be necessary to control its action in some way, however, to prevent
it from racing and running down in almost no time, like the too heavy
rubber motors. It might be found interesting to experiment with the
spring to be found in the ordinary roller-shade. The weight of these
springs is not too great to be carried by a good aëroplane model,
which, of course, is a great factor in their favor.
[Illustration: Detail of Rudder and Propeller of Model Built from
Drawing (Plate C).]
A number of experiments have been made in France to equip aëroplane
models with compressed-air motors. The compressed air is carried in
a hollow tube in much the same position as the rubber bands. Many
believe that the motor problem, for the toy aëroplane will be solved in
this way. A number of interesting models have also been equipped with
clock-work motors. A small movement, such as may be borrowed from some
mechanical toys, will run for a minute or more. What glorious flights
would be possible if our models could be kept aloft--say five times as
long as at present. When you feel that you thoroughly understand your
model, borrow the clock work from some old toy and make the experiment.
It is possible to buy motors for model aëroplanes. The smallest of
these develops one half horsepower, weighs seven pounds and will run
for fifteen minutes.
The best covering for the wings still remains largely an open question.
Although your model will make successful flights with almost any kind
of covering, you will find that its stability will be increased and
the flight lengthened by a little attention to this detail. According
to the Wright Brothers, the most successful covering is the one which
offers the greatest resistance to the air. The pressure of the air
upward under the planes tends to force its way through the meshes of
even the finest cloth. The addition of a coat of varnish will prevent
this leakage. A light parchment will also be found effective. It will
be well to experiment with a variety of coverings.
A very light, serviceable frame may be made for your motor-base by
using hollow shafts or sticks. Procure a very thin, light wood, such as
is used for veneering, and after cutting it carefully into strips, glue
them together to form a hollow shaft about an inch square. Although the
shell may be only one sixteenth of an inch thick, the frame will be
found strong enough for all practical purposes. A hollow frame of this
kind will save several ounces of weight.
The builder of aëroplane models will find a good friend in aluminium.
It is strong enough for all purposes of the model air-ship and, even
when used freely, adds almost nothing to the weight. The metal costs
ninety cents a pound, but it is so light that, at this rate, it will
be found a very cheap material. Comparatively thick pieces may be used
for braces or for angles, thus making the frame absolutely rigid, while
adding but a fraction of an ounce to the weight. The metal, being
comparatively soft, is easily worked, and simple castings may be made
at little expense.
Many builders of aëroplanes waste time and ingenuity quite
unnecessarily in constructing sets of wheels for carrying their models.
The time would be better employed in looking to your planes. The
amount of friction saved by attaching wheels, even good ones, to your
model, is after all very trifling. Should the wheels jam or stick,
which is likely to be the case with such small models, they are worse
than skids, and besides, add appreciably to the weight. A light skid
is better than a clumsy wheel. If your model fails to rise from the
ground, the fault is not at all likely to be in the skids, but in the
thrust or lifting-surface.
An excellent plan for guiding the flights is to add square frames of
soft lead wire to the front or cutting-edge of your front planes. Bend
a piece of wire to form three sides of a square, each two or three
inches long, and fasten the loose ends to the plane. By bending these
up or down, the center of gravity may be altered at a touch. If your
model goes askew, you may bend one of these up and the other down,
until you get the desired balance.
In actual practice, the soaring- or floating-planes seem to add greater
stability to the model and effect to a marked degree the length of the
flight. It is difficult to tell exactly why. The planes in passing may
create an eddy in the air, a following wave, as it were, which tends to
retard the flight, while the floating-plane smoothes this out. In any
event, here is an experiment well worth trying.