In the present stage of model aeroplane building, rubber strand motors
satisfy every demand. Models have been flown for more than 2,500 feet by
the force of these twisted strands, and doubtless their efficiency will
be still further increased. Such motive power is besides very easily
obtained and applied. Careful tests have shown that more energy may be
stored up in twisted rubber strands than in the same weight of springs
of steel or any other metal.
In gauging the strength of your motor, much depends upon whether your
model is to rise from the ground or be launched from the hand. In the
model tournaments in England, the flying machines are almost invariably
thrown across the starting line, while in America they are required to
rise unaided. It is obviously unfair, therefore, to compare the distance
records of the two countries.
[Illustration: A Metal Motor Anchorage]
It requires a comparatively powerful motor to raise a model from the
ground, whereas a lighter motor would be sufficient to propel it through
the air. Many models, capable of flights of several hundred feet when
thrown will refuse to rise, while, on the other hand, some models which
rise well enough have poor distance qualities.
It should be borne in mind that the length of the motor, speaking
broadly, controls the distance qualities, and its diameter the speed of
the model aeroplane. A long slender motor, capable of from five hundred
to one thousand turns which will revolve the propellers for thirty
seconds or more, should insure a flight of several hundred feet. As you
increase the number of strands of rubber, building up the diameter of
the motor, you cut down the number of turns and therefore its duration,
although you increase its speed.
A motor capable of one thousand turns must be about forty inches in
length and consist of but six, or at most eight, of these strands. A
model which may be driven by this motor, it will be found, must be very
light. A model aeroplane weighing upwards of one pound, on the other
hand, will require motors composed of fourteen strands or more to raise
it from the ground. It is a very simple matter, of course, to add
strands of rubber until your motor develops sufficient energy for the
work it is expected to do.
The length and diameter of your motor, again, has a direct influence on
the height of the flight. Too much power tends to raise the aeroplane
higher than necessary above the ground. Since it requires more energy to
drive a model aeroplane upward than along a horizontal direction, this
is obviously a waste of energy.
If it is desired to fly the model as far as possible, it must be kept
close to the ground. In the case of weight-lifting contests, the problem
of altitude is, of course, entirely different. Overwinding is even worse
than underwinding, since it shortens the life of the motor.
Try out your aeroplane with ten strands on each motor and increase them
later. The motor, as previously explained, is formed by looping the
rubber strands loosely between the hooks, just as zephyr is wound on a
skein. Keep the strands very loose and fasten them to the hooks by tying
with a strand of rubber. In winding, do not turn the propeller after the
rubber has a double row of knots for its entire length. Such a motor
should take up from three hundred to five hundred turns, perhaps more.
Do not keep the elastic wound up too long before starting your flight.
The strain is great and it quickly wears out.
[Illustration: A Metal Motor Anchorage]
The rubber strands should not be allowed to come in contact with any
metal parts of the model. The copper that is often used for wiring is
especially injurious and tends to decompose the rubber. The hooks of
both the propeller and motor anchorage should be covered with a piece of
rubber tubing. This serves a double purpose. With this protection, the
rubber when tightly twisted is in no danger of being cut by the wire or
of taking up the oxides which quickly eat through it.
[Illustration: A notable model possessing unusual stability. Built by
W.S. Howell, Jr.]
[Illustration: Front view of model built by W.S. Howell, Jr.]
It requires an expert to pick out the best quality of rubber. If the
strands be examined under a magnifying glass, it will be found that the
edges of fresh rubber of the best quality are clean-cut, whereas the
cheaper rubber, and that which is worn, has commenced to granulate,
giving the edges a ragged appearance.
A simple test is to stretch the rubber over a ruler. A good rubber, in
first-class condition, will stretch about seven times its length, and on
being released instantly spring back to its original size. The same
rubber should stretch to ten times its length without breaking.
There is a great difference of opinion among the most successful model
builders as to the best shape of rubber strands. Some prefer the flat,
band rubber, while others are obtaining satisfactory results with rubber
cut in square strands. The strand used by the English model builders is
seldom more than one-sixteenth of an inch square, while in America
one-eighth of an inch strand is commonly used.
Experiments have been made with a single strand of rubber one-fourth of
an inch square, but the results have not been satisfactory. One theory
is that the corners of the square rubber tend to cut into one another
and quickly wear out, and that a perfectly round strand would be the
more efficient. At present there are no such strands on the market. It
is argued by some that the square strand in twisting must be turned on
itself further than the flat strand, and is therefore placed under an
unnecessary strain. After all, the advantage of one form over another is
fractional, and an extra strand added to the motor will balance any
possible defects.
Figures have been prepared giving the exact relation of the size of
rubber to the number of turns, although such statistics are elastic. The
problem may be worked out with your own motor. Differences of
temperature will be quickly noted. The rules prepared by V. E. Johnson,
M.A., an English authority on aviation, are as follows: The motive power
is doubled by increasing the number of rubber strands one-half; by
doubling the number of strands, the motive power is increased more than
two times; and the tripling of the strands increases the motive power
seven times. As regards the number of turns the same authority states
that the doubling of the number of strands diminishes the number of
turns by one-third to one-half.
[Illustration: A Metal Skid]
It is also found that each strand will have doubled knots of 310 turns;
four strands, 440; sixteen strands at 200; and eight at 210. This is
working with strands one-sixteenth of an inch square. As a rule, rubber
should not be turned after the second row of knots is formed. And by the
way, you will find that the rubber, after being tightly twisted, tends
to stick together, and should be carefully separated after a flight so
that the air can reach all surfaces.
According to the experiments made by Mr. Johnson, one pound of rubber
may be made to store up 320 foot pounds of energy, while one pound of
steel, in the form of springs, will only store up 65 pounds.
In the early model aeroplanes much valuable energy was lost through
friction. There has been a marked improvement in the construction of the
propellers, axles, and bearings. Friction has been reduced to
practically nothing. It is possible, of course, to drive a propeller
with the shaft turning in a hole drilled at random through a stick, with
a glass bead for a washer. It is very important, however, that the axle
should turn exactly at right angles, and to hold it in position requires
careful adjustment. To meet the demands of model aeroplane builders,
several shaft mechanisms have been prepared, even to a very complete
arrangement of miniature ball bearings.
The model builder who cannot avail himself of these parts can,
nevertheless, imitate their action with reasonable fidelity. The axle
attached to the propeller should be heavy enough to resist bending in
ordinary wear and tear. A bicycle spoke is just the thing. When you have
decided upon this axle, procure a piece of metal tubing in which the
axle will turn freely, without binding or rattling about. The tubing
should then be passed through the frame supporting the propeller exactly
at right angles, and extend out at either side about half an inch. To
fasten it securely in position, glue and if necessary drive small
wedges,—a match or toothpick,—about it.
Several metal washers should be strung on the axle between the upper
edge of the shaft and the propeller. These may be punched from a sheet
of metal. A section of this tube may also be inserted part way in the
propeller, and washers introduced where they meet. The second tube will
insure smooth action.
The projecting tube will serve also to remove the propeller far enough
from the frame to prevent its striking. By freely oiling these parts,
the propeller may be made to turn very freely.
[Illustration: An ingenious adjustment of ailerons]
[Illustration: Tuning up the model for a flight.]
In bending the axle into a hook for holding the rubber strands of the
motor, care must be taken to keep the ends of the strands on a line with
the axle. After turning the wire into a hook, bend back the shank to the
proper angle. It will be seen that if the motor carries the axle about
from side to side, the friction will be considerably increased. Over
this hook, slip a piece of rubber tubing before attaching the strands of
rubber, since the metal is likely to cut and wear the motor. It will be
found a good plan to tie the strands together just below the hook to
keep them from slipping off. And, by the way, do not keep your motor
wound up any longer than you can help before a flight, since the strain
on the rubber in this position is very great.
As motors have increased in power and distance qualities, the process of
winding up has become a serious problem. The simple method of turning
the propeller by hand takes too long, and a slip with a powerful motor
may give one an ugly cut. An ordinary machine drill will do the work
much more quickly. Some drills are geared so that a single turn of the
wheel will give you ten revolutions of the propeller.
To arrange your motor for winding with a drill, run the axle through the
propellers and turn in the form of a closed hook. A small hook should
then be fixed to the end of the drill, which may be inserted in this
loop. Some boys use a double hook on the propeller, detach the strands
of rubber, wind them up, and then attach them to the propeller.
A very simple and ingenious method of winding up has been adopted in the
remarkable model constructed by Mr. Mungokee. The motor is anchored by
running the wire holding the strands through the supporting stick of the
base, and bending the end into a hook which, as the rubber pulls, is
held securely in a second hole at the side.
[Illustration: Showing Construction And Mounting Of Propeller And Axle.]
To wind up, it is only necessary to draw out this hook, attach it to the
winding drill and turn. When wound up, the pull of the motor will
obviously hold the end of the hook firmly in the hole, making it
impossible for it to turn. This does away with extra hooks and the
trouble of hooking up the motor when once it has been wound.
The life of your rubber motor may be lengthened by careful winding. As
long as you wind up by turning the propeller by hand, it is safe to turn
it as fast as you can, since the process is slow at best. In case the
turning is done with a machine drill or some similar geared wheel, there
is danger at some points of winding too fast.
It is safe to wind as quickly as you can until the first row of knots
has formed in the rubber strands, but at the moment the double strands
begin to appear the winding should proceed very slowly. You will find
that if you wind very quickly the double knots will appear in tight
groups or bunches, and that only after considerable winding do these
begin to spread out evenly. This puts the rubber under a severe and
unnecessary strain and shortens its life.
The simplest way of locking the propellers when once wound up is to
thrust a piece of cane or reed through the hooks. The twist of the motor
will hold it tightly in position, so that you can carry your model
about, even shake it vigorously without danger of dislodging it. If you
have twin propellers, use a strip long enough to pass through both
hooks. Remove the strip just before starting. Be careful, of course,
that your axles have not been thrown out of plumb.
It will be found very convenient to equip your model with a single clasp
for holding the propellers after they have been wound up, which may be
easily released. It is awkward to keep them from slipping. An effective
break may be made by attaching two strips of reed or cane, such as you
use for skids, to either side of the motor base, so that the free ends
will pass between the propeller blades and the frame, thus locking them
fast.
These bands should spring outward and be held in position by rubber
bands running from one to the other. To release the propellers, simply
pinch the two free ends together, and the propellers will be freed at
the same instant. Do not keep your motor wound up a moment longer than
you can help. It is very trying to the rubber to be held in this
tightly-twisted position.
In mounting your propeller, it is well to make the support for the
bearing of the propeller axle as long as possible. An axle turning in a
shaft one inch in length will meet with much less resistance than in a
half-inch shaft, and with a good motor an inch-and-a-half shaft is still
better. The rear stick of your motor base, which often holds the
propeller axle, is usually made as thin as possible and rarely gives you
more than a half-inch support.
It is a good plan to lengthen the shaft by attaching a block of wood to
the frame and passing the axle through it. Cut from a strip one-half an
inch square a piece one inch in length, or whatever seems necessary.
This may be mortised slightly into the stick and glued at right angles.
[Illustration: An excellent monoplane capable of long flights.]
[Illustration: Long-distance model built by Percy Pierce.]
Now drill a hole through the stick, with the grain, and the stick of
your motor base and pass the tube holding the propeller shaft through
both. To make this look shipshape, round off the edges. A great
advantage of this stick is that it enables you to mount the propeller as
far as you like from the frame, thus preventing it from striking.
[Illustration: Showing An Excellent Way Of Fastening The Propellers To
The Framework.]
In mounting the propellers above or below the frame, you will need
similar supports. The blocks should measure half an inch in width by one
and a half inches square and should be cut with the grain of the wood
running lengthwise. The hole for the propeller shaft is drilled near the
top, and the block is securely fastened to the frame.
It will be found a good plan to mortise the frame slightly in order to
make the joint perfectly rigid, even in the face of a bad smash-up. Many
boys merely glue the stick in position and bind it securely to the motor
base with fine strong thread, and paint it with glue or shellac to hold
it in position. These blocks may be fastened either above or below the
frame or at the sides.
In mounting the propellers, bear in mind that a position above the
planes tends to drive the aeroplane downward, while the thrust exerted
below tends to throw the aeroplane upward. The builders of model
aeroplanes differ as much as to the best position of the propellers as
the designers of man-carrying machines. Excellent models have been built
with the propellers in either position. It is obviously impossible to
lay down a rule for all models, since the properties of the planes vary
so widely.
A very simple and efficient support for the propeller shaft may be made
of metal to correspond to the motor anchorage. Procure a sheet of heavy
tin—a piece of sheet aluminum is still better—one-half inch in width and
three inches in length. Now mark off a one-half inch, one inch, two
inches, and two and one-half inches, and bend over the ends at right
angles, as shown in the accompanying drawing.
This support may be nailed or screwed rigidly to the end of the motor
base, and a hole for the shaft of the propeller drilled through the two
uprights. The propeller must be mounted so that the blades will, of
course, be free of the base. The size of the support may be altered to
suit the frame. In case you are using a heavy propeller, say an inch
blade, the metal must be heavy enough to resist the pull of the
propeller.
The forward ends of the motors may be held by a cross piece cut from a
sheet of aluminum six inches in width and two inches in depth, which is
bound rigidly to the end of the motor base with shoemaker’s thread.
Aluminum suitable for this purpose costs about fifteen cents a square
foot, and is soft enough to be cut with heavy shears.
An ingenious motor anchorage of metal construction has been hit upon by
the builders of model aeroplanes in France. We are all familiar with the
difficulty of raising the hook, holding the rubber bands, high enough
above the main frame, or fusilage, to be perfectly free. Instead of
attaching a wooden block, the French boys bend a piece of tin, or some
such metal, very simply into a support for the hook.
You will need a sheet of metal heavy enough to withstand the full force
of the motor when wound up. The tin used in cans, as a rule, is not
heavy enough. For each support you will need a rectangle of tin or metal
measuring three by one and one-half inches. Parallel to the longer base,
draw a line one-quarter of an inch above. From the center, erect a long
rectangle one-quarter of an inch wide, extending to the opposite side.
Now connect the ends of the line above the base with the points at which
the upright rectangle intersects the top line, round off the edges
neatly and cut away this triangle. Four holes should be cut or punched
in the tin, as indicated in the drawing.
Now bend the tin on the two upright lines until the two sides are
parallel. This support is fitted to the end of a motor base and secured
by driving nails through the three holes at the base covering the wood.
The end of the hook which holds the rubber strands of the motor should
be passed through the opening at the end, bent over and fastened into
position with a drop or two of solder. Such a support adds practically
nothing to the weight of the frame, and obviously anchors the motor
rigidly.
[Illustration: Model built by Rutledge Barry, winner of spectacular
flight contest.]
[Illustration: A model by Percy Pierce, winner of the indoor
long-distance record.]
The efficiency of a rubber-strand motor may be considerably increased by
careful adjustment. If the strands first be wound rather loosely, as a
rope is formed, and strung between the propeller hook and the motor
anchorage, you will find that about thirty per cent. more rubber may be
added without increasing the length and that a five to ten per cent.
increase in the number of effective turns may be gained as well. By
increasing the amount of rubber, you will, of course, add accordingly to
the power of the motor. It is safe to say that the efficiency of your
motor is increased upwards of twenty-five per cent. by this adjustment.
The credit of this ingenious arrangement is due to Mr. W. Howell, Jr.
It will be well to experiment with a short-strand motor, using a single
strand of rubber for the test. Let us assume that your motor is twelve
inches in length, thus making a double strand twenty-four inches long.
First knot this, string it between the two hooks and turn it, counting
the revolutions until the first row of double knots begins to appear.
Note the number of turns.
Now untie the strand and, holding one end, twist it until the lines of
the edges make a continuous loose spiral throughout its entire length.
The easiest way of twisting them is to lay them on a flat surface and
rub with the palm of the hand.
Now bring the two ends together and let the strands twist and wriggle
until they come to rest. Fasten the ends and measure the double twisted
strand. You will find that it measures less than ten inches.
To prepare a strand for a twelve-inch motor, you must therefore begin
with a piece of rubber fully thirty inches in length. It is clear,
therefore, that the new plan enables you to gain considerably more
rubber length for length. Now string your twisted rubber on the hooks of
your motor so that to wind up you must turn against the twisted strand.
You will find that a number of turns are required before the rubber
strands are untwisted and lie parallel, which is pure gain. Count the
number of turns up to the time the first line of the double knots
appears, and you will find that it is about five per cent. greater than
in the case of the single strands.
The power exerted by your motor is meanwhile increased in direct
proportion to the amount of rubber added. Still another advantage of
this adjustment lies in the fact that such a motor will unwind to the
first turn. In preparing a multiple-strand rubber, each strand must, of
course, be twisted in the same direction and exactly the same number of
times before being installed.
The builders of model aeroplanes may profit from the experience of the
automobile tire manufacturers in studying the properties of the rubber
used for motors. Rubber is at best comparatively short-lived. It suffers
a surface deterioration on being exposed to the air, which in time
affects the entire mass.
This process of decay goes on fastest in very warm weather and in very
bright sunlight. It is believed that sea air and the rarified air of the
mountains are bad for rubber. On the other hand, a very low temperature,
as you may perhaps have discovered, robs the rubber of much of its
elasticity.
It will pay you to take some trouble to protect the rubber strands as
far as possible. Lay them away in a can or jar in some cool, dry and
dark place when they are not in use. Some boys cover the rubber with
powdered chalk. When the surface of the rubber begins to granulate, its
best days are over. Rubber is originally white in color, while the
refining process gives it the familiar gray tone.
The Para rubber is generally considered the best of the many kinds now
on the market. As a rule, any oil or grease tends to decay the rubber,
as any motorist can tell you. This is particularly unfortunate for the
aviator, since the efficiency of the rubber motor is increased by
slightly lubricating the strands. Many boys prefer to chance injuring
their motors in order to gain the advantage of the oiled strands.
The strands thus prepared slide smoothly on one another and do not grip
each other even when tightly wound. The simplest preparation for
greasing the strands is a solution of ordinary soap and water. A few
drops poured over the strands will suffice. When your motor unwinds, be
careful to keep your face out of range, since a few drops might be flung
into your eyes. Several preparations for lubricating the motors have
been placed upon the market.
The direction flights may be controlled very easily by winding the
motor. If you care to fly your model in circles or spirals, the simplest
plan for influencing its direction is to give different power to your
propellers. It often happens that a model must be in a restricted place,
perhaps a straight-away flight is out of the question.
The model may be deflected to the right or left by the use of vertical
propellers, but they require delicate adjustment, and a gust of wind may
destroy their effect. By winding up one double the number of turns of
the other, a circular flight is assured. To gauge the diameter of the
circle merely alter the relation of the number of turns. You will soon
find that you can control the diameter of the circle with remarkable
accuracy.
[Illustration: A Motor Anchorage]