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A product thus made is much more serviceable
than if made of one piece, even though the laminated
parts are of the same wood, because the
different strips used will have their fibers overlapping
each other, and thus greatly augment the
strength of the whole.

Generally the alternate strips are of different
materials, black walnut, mahogany, birch, spruce,
and maple being the most largely used, but mahogany
and birch seem to be mostly favored.

LAYING UP A PROPELLER FORM.--The first step
necessary is to prepare thin strips, each, say,
seven feet long, and five inches wide, and three-
eighths of an inch thick. If seven such pieces are
put together, as in Fig. 78, it will make an assemblage
of two and five-eighth inches high.

_Fig. 78. A Laminated Blank._

Bore a hole centrally through the assemblage,
and place therein a pin B. The contact faces of
these strips should be previously well painted
over with hot glue liberally applied. When they
are then placed in position and the pin is in place,
the ends of the separate pieces are offset, one beyond
the other, a half inch, as shown, for instance,
in Fig. 79.

This will provide ends which are eight and a
half inches broad, and thus furnish sufficient
material for the blades. The mass is then subjected
to heavy pressure, and allowed to dry before the
blades are pared down.

_Fig. 79. Arranging the Strips._

MAKING WIDE BLADES.--If a wider blade is desired,
a greater number of steps may be made by
adding the requisite number of strips; or, the
strips may be made thicker. In many propellers,
not to exceed four different strips are thus glued
together. The number is optional with the
maker.

An end view of such an assemblage of strips
is illustrated in Fig. 80. The next step is to lay
off the pitch, the method of obtaining which has
been explained.

_Fig. 80. End view of Blank._

Before starting work the sides, as well as the
ends, should be marked, and care observed to
place a distinctive mark on the front side of the
propeller.

Around the pin B, Fig. 81, make S-shaped
marks C, to indicate where the cuts on the faces
of the blades are to begin. Then on the ends of
the block; scribe the pitch angle, which is indicated
by the diagonal line D, Fig. 80.

_Fig. 81. Marking the Side._

This line is on the rear side of the propeller,
and is perfectly straight. Along the front of this
line is a bowline E, which indicates the front surface
of the propeller blade.

PROPELLER OUTLINE.--While the marks thus
given show the angles, and are designed to indicate
the two faces of the blades, there is still another
important element to be considered, and
that is the final outline of the blades.

_Fig. 82. Outlining._

It is obvious that the outline may be varied
so that the entire width at 1, Fig. 82, may be used,
or it may have an outline, as represented by the
line 2, in this figure, so that the widest part will
be at or near the dotted line 3, say two-thirds of
the distance from the center of the blade.

This is the practice with most of the manufacturers
at the present time, and some of them
claim that this form produces the best results.

FOR HIGHER SPEEDS.--Fig. 83 shows a propeller
cut from a blank, 4" x 6" in cross section, not
laminated.

_Fig. 83. Cut from a 4" x 6" Single Blank._

It should be borne in mind that for high speeds
the blades must be narrow. A propeller seven
feet in diameter with a six foot pitch, turning
950 revolutions per minute, will produce a pull of
350 pounds, if properly made.

Such a propeller can be readily handled by a
forty horse power motor, such as are specially
constructed for flying machine purposes.

INCREASING PROPELLER EFFICIENCY.--Some experiments
have been made lately, which, it is
claimed, largely increase the efficiency of propellers.
The improvement is directed to the outline
shape of the blade.

The typical propeller, such as we have illustrated,
is one with the wide part of the blade at
the extremity. The new type, as suggested, reverses
this, and makes the wide part of the blade
near the hub, so that it gradually tapers down to
a narrow tip.

Such a form of construction is shown in Fig.
84. This outline has some advantages from one
standpoint, namely, that it utilizes that part of
the blade near the hub, to produce a pull, and
does not relegate all the duty to the extreme ends
or tips.

_Fig. 84. A Suggested Form._

To understand this more fully, let us take a
propeller six feet in diameter, and measure the
pull or thrust at the tips, and also at a point half
way between the tip and the hub.

In such a propeller, if the blade is the same
width and pitch at the two points named, the pull
at the tips will be four times greater than at the
intermediate point.



CHAPTER XIV

EXPERIMENTAL GLIDERS AND MODEL AEROPLANES


AN amusing and very instructive pastime is
afforded by constructing and flying gliding machines,
and operating model aeroplanes, the latter
being equipped with their own power.

Abroad this work has been very successful as
a means of interesting boys, and, indeed, men
who have taken up the science of aviation are
giving this sport serious thought and study.

When a machine of small dimensions is made
the boy wonders why a large machine does not
bear the same relation in weight as a small machine.
This is one of the first lessons to learn.

THE RELATION OF MODELS TO FLYING MACHINES.
--A model aeroplane, say two feet in length, which
has, we will assume, 50 square inches of supporting
surface, seems to be a very rigid structure,
in proportion to its weight. It may be dropped
from a considerable height without injuring it,
since the weight is only between two and three
ounces.

An aeroplane twenty times the length of this
model, however strongly it may be made, if
dropped the same distance, would be crushed, and
probably broken into fragments.

If the large machine is twenty times the dimensions
of the small one, it would be forty feet in
length, and, proportionally, would have only
seven square feet of sustaining surface. But an
operative machine of that size, to be at all rigid,
would require more than twenty times the material
in weight to be equal in strength.

It would weigh about 800 pounds, that is, 4800
times the weight of the model, and instead of
having twenty times the plane surface would require
one thousand times the spread.

It is this peculiarity between models and the
actual flyers that for years made the question of
flying a problem which, on the basis of pure calculation
alone, seemed to offer a negative; and
many scientific men declared that practical flying
was an impossibility.

LESSONS FROM MODELS.--Men, and boys, too,
can learn a useful lesson from the model aeroplanes
in other directions, however, and the principal
thing is the one of stability.

When everything is considered the form or
shape of a flying model will serve to make a large
flyer. The manner of balancing one will be a
good criterion for the other in practice, and
experimenting with these small devices is, therefore,
most instructive.

The difference between gliders and model aeroplanes
is, that gliders must be made much lighter
because they are designed to be projected through
the air by a kick of some kind.

FLYING MODEL AEROPLANES.--Model aeroplanes
contain their own power and propellers which,
while they may run for a few seconds only, serve
the purpose of indicating how the propeller will
act, and in what respect the sustaining surfaces
are efficient and properly arranged.

It is not our purpose to give a treatise on this
subject but to confine this chapter to an exposition
of a few of the gliders and model forms which
are found to be most efficient for experimental
work.

AN EFFICIENT GLIDER.--Probably the simplest
and most efficient glider, and one which can be
made in a few moments, is to make a copy of the
deltoid kite, previously referred to.

This is merely a triangularly-shaped piece of
paper, or stiff cardboard A, Fig. 84, creased in
the middle, along the dotted line B, the side wings
C, C, being bent up so as to form, what are called
diedral angles. This may be shot through the
air by a flick of the finger, with the pointed end
foremost, when used as a glider.

_Fig. 85. Deltoid Glider._

THE DELTOID FORMATION.--This same form may
be advantageously used as a model aeroplane, but
in that case the broad end should be foremost.

_Fig. 86. The Deltoid Racer._

Fig. 86 shows the deltoid glider, or aeroplane,
with three cross braces, A, B, C, in the two forward
braces of which are journaled the propeller
shaft D, so that the propeller E is at the broad
end of the glider.

A short stem F through the rear brace C, provided
with a crank, has its inner end connected
with the rear end of the shaft D by a rubber band
G, by which the propeller is driven.

A tail may be attached to the rear end, or at
the apex of the planes, so it can be set for the
purpose of directing the angle of flight, but it will
be found that this form has remarkable stability
in flight, and will move forwardly in a straight
line, always making a graceful downward movement
when the power is exhausted.

It seems to be a form which has equal stabilizing
powers whether at slow or at high speeds,
thus differing essentially from many forms which
require a certain speed in order to get the best
results.

RACING MODELS.--Here and in England many
racing models have been made, generally of the
A-shaped type, which will be explained hereinafter.
Such models are also strong, and able to
withstand the torsional strain required by the
rubber which is used for exerting the power.

It is unfortunate that there is not some type of
cheap motor which is light, and adapted to run
for several minutes, which would be of great value
in work of this kind, but in the absence of such
mechanism rubber bands are found to be most
serviceable, giving better results than springs or
bows, since the latter are both too heavy to be
available, in proportion to the amount of power
developed.

Unlike the large aeroplanes, the supporting
surfaces, in the models, are at the rear end of
the frames, the pointed ends being in front.

_Fig. 87. A-Shaped Racing Glider._

Fig. 87 shows the general design of the A-
shaped gliding plane or aeroplane. This is composed
of main frame pieces A, A, running fore
and aft, joined at their rear ends by a cross bar
B, the ends of which project out slightly beyond
their juncture with the side bars A, A. These
projecting ends have holes drilled therein to receive
the shafts a, a, of the propeller D, D.

A main plane E is mounted transversely across
this frame at its rear end, while at its forward
end is a small plane, called the elevator. The
pointed end of the frame has on each side a turnbuckle
G, for the purpose of winding up the shaft,
and thus twisting the propeller, although this is
usually dispensed with, and the propeller itself
is turned to give sufficient twist to the rubber for
this purpose.

THE POWER FOR MODEL AEROPLANES.--One end
of the rubber is attached to the hook of the shaft
C, and the other end to the hook or to the turnbuckle
G, if it should be so equipped.

The rubbers are twisted in opposite directions,
to correspond with the twist of the propeller
blades, and when the propellers are permitted to
turn, their grip on the air will cause the model to
shoot forwardly, until the rubbers are untwisted,
when the machine will gradually glide to the
ground.

MAKING THE PROPELLER.--These should have
the pitch uniform on both ends, and a simple
little device can be made to hold the twisted blade
after it has been steamed and bent. Birch and
holly are good woods for the blades. The strips
should be made thin and then boiled, or, what is
better still, should be placed in a deep pan, and
held on a grid above the water, so they will be
thoroughly steamed.

They are then taken out and bent by hand, or
secured between a form specially prepared for
the purpose. The device shown in Fig. 88 shows
a base board which has in the center a pair of
parallel pins A, A, slightly separated from each
other.

_Fig. 88. Making the Propeller._

At each end of the base board is a pair of holes
C, D, drilled in at an angle, the angles being the
pitch desired for the ends of the propeller. In
one of these holes a pin E is placed, so the pins
at the opposite ends project in different directions,
and the tips of the propeller are held
against the ends of these pins, while the middle
of the propeller is held between the parallel pins
A, A.

The two holes, at the two angles at the ends of
the board, are for the purpose of making right
and left hand propellers, as it is desirable to use
two propellers with the A-shaped model. Two
propellers with the deltoid model are not so necessary.

After the twist is made and the blade properly
secured in position it should be allowed to thoroughly
dry, and afterwards, if it is coated with
shellac, will not untwist, as it is the changing
character of the atmosphere which usually causes
the twisted strips to change their positions.
Shellac prevents the moist atmosphere from affecting
them.

MATERIAL FOR PROPELLERS.--Very light propellers
can also be made of thin, annealed aluminum
sheets, and the pins in that case will serve as
guides to enable you to get the desired pitch.
Fiber board may also be used, but this is more
difficult to handle.

Another good material is celluloid sheets,
which, when cut into proper strips, is dipped in
hot water, for bending purposes, and it readily
retains its shape when cooled.

RUBBER--Suitable rubber for the strips are
readily obtainable in the market. Experiment
will soon show what size and lengths are best
adapted for the particular type of propellers
which you succeed in making.

PROPELLER SHAPE AND SIZE.--A good proportion
of propeller is shown in Fig. 89. This also
shows the form and manner of connecting the
shaft. The latter A has a hook B on one end to
which the rubber may be attached, and its other
end is flattened, as at C, and secured to the blade
by two-pointed brads D, clinched on the other
side.

_Fig. 89. Shape and Size._

The collar E is soldered on the shaft, and in
practice the shaft is placed through the bearing
hole at the end of the frame before the hook is
bent.

SUPPORTING SURFACES.--The supporting surfaces
may be made perfectly flat, although in this
particular it would be well to observe the rules
with respect to the camber of large machines.



CHAPTER XV

THE AEROPLANE IN THE GREAT WAR


DURING the civil war the Federal forces used
captive balloons for the purpose of discovering
the positions of the enemy. They were of great
service at that time, although they were stationed
far within the lines to prevent hostile guns from
reaching them.

BALLOON OBSERVATIONS.--Necessarily, observations
from balloons were and are imperfect. It
was found to be very unsatisfactory during the
Russian-Japanese war, because the angle of vision
is very low, and, furthermore, at such distances the
movements, or even the location of troops is not
observable, except under the most favorable conditions.

Balloon observation during the progress of a
battle is absolutely useless, because the smoke
from the firing line is, necessarily, between the
balloon and the enemy, so that the aerial scout
has no opportunity to make any observations, even
in detached portions of the fighting zone, which
are of any value to the commanders.

CHANGED CONDITIONS OF WARFARE.--Since our
great war, conditions pertaining to guns have been
revolutionized. Now the ranges are so great that
captive balloons would have to be located far in
the rear, and at such a great distance from the
firing line that even the best field glasses would
be useless.

The science of war has also evolved another
condition. Soldiers are no longer exposed during
artillery attacks. Uniforms are made to imitate
natural objects. The khaki suits were designed
to imitate the yellow veldts of South Africa;
the gray-green garments of the German
forces are designed to simulate the green fields
of the north.

THE EFFORT TO CONCEAL COMBATANTS.--The
French have discarded the historic red trousers,
and the elimination of lace, white gloves, and
other telltale insignias of the officers, have been
dispensed with by special orders.

In the great European war armies have burrowed
in the earth along battle lines hundreds of
miles in length; made covered trenches; prepared
artificial groves to conceal batteries, and in many
ingenious ways endeavored to make the battlefield
an imitation field of nature.

SMOKELESS POWDER.--While smokeless powder
has been utilized to still further hide a fighting
force, it has, in a measure, uncovered itself, as
the battlefield is not now, as in olden times, overspread
with masses of rolling smoke.

Nevertheless, over every battlefield there is a
haze which can be penetrated only from above,
hence the possibilities of utilizing the aeroplane
in war became the most important study with all
nations, as soon as flying became an accomplished
fact.

INVENTIONS TO ATTACK AERIAL CRAFT.--Before
any nation had the opportunity to make an actual
test on the battlefield, inventors were at work to
devise a means whereby an aerial foe could be
met. In a measure the aerial gun has been successful,
but months of war has shown that the
aeroplane is one of the strongest arms of the
service in actual warfare.

It was assumed prior to the European war that
the chief function of the aeroplane would be the
dropping of bombs,--that is for service in attacking
a foe. Actual practice has not justified
this theory. In some places the appearance of
the aeroplane has caused terror, but it has been
found the great value is its scouting advantages.

FUNCTION OF THE AEROPLANE IN WAR.--While
bomb throwing may in the future be perfected,
it is not at all an easy problem for an aviator to
do work which is commensurate with the risk
involved. The range is generally too great; the
necessity of swift movement in the machine too
speedy to assure accuracy, and to attack a foe at
haphazard points can never be effectual. Even
the slowly-moving gas fields, like the Zeppelin,
cannot deliver bombs with any degree of precision
or accuracy.

BOMB-THROWING TESTS.--It is interesting, however,
to understand how an aviator knows where
or when to drop the bomb from a swiftly-moving
machine. Several things must be taken into consideration,
such as the height of the machine from
the earth; its speed, and the parabolic curve that
the bomb will take on its flight to the earth.

When an object is released from a moving machine
it will follow the machine from which it is
dropped, gradually receding from it, as it descends,
so that the machine is actually beyond
the place where the bomb strikes the earth, due
to the retarding motion of the atmosphere against
the missile.

The diagram Fig. 90 will aid the boy in grasping
the situation. A is the airship; B the path
of its flight; a the course of the bomb after it
leaves the airship; and D the earth. The question
is how to determine the proper movement
when to release the bomb.

METHOD FOR DETERMINING MOVEMENT OF A
BOMB.--Lieut. Scott, U. S. A., of the Coast Survey
Artillery, suggested a method for determining
these questions. It was necessary to ascertain,
first, the altitude and speed. While the barometer
is used to determine altitudes, it is
obvious that speed is a matter much more difficult
to ascertain, owing to the wind movements,
which in all cases make it difficult for a flier to
determine, even with instruments which have
been devised for the purpose.

_Fig. 90. Course of a Bomb._

Instead, therefore, of relying on the barometer,
the ship is equipped with a telescope which may
be instantly set at an angle of 45 degrees, or vertically.

Thus, Fig 91 shows a ship A, on which is
mounted a telescope B, at an angle of 45 degrees.
The observer first notes the object along the line
of 45 degrees, and starts the time of this observation
by a stop watch.

The telescope is then turned so it is vertical,
as at C, and the observer watches through the
telescope until the machine passes directly over
the object, when the watch is stopped, to indicate
the time between the two observations.

_Fig. 91. Determining Altitude and Speed._

The height of the machine along the line D is
thus equal to the line E from B to C, and the time
of the flight from B to a being thus known, as
well as the height of the machine, the observer
consults specially-prepared tables which show
just what kind of a curve the bomb will make at
that height and speed.

All that is necessary now is to set the sighter
of the telescope at the angle given in the tables,
and when the object to be hit appears at the sight,
the bomb is dropped.

THE GREAT EXTENT OF MODERN BATTLE LINES.--
The great war brought into the field such stupendous
masses of men that the battle lines have
extended over an unbroken front of over 200
miles.

In the battle of Waterloo, about 140,000 men
were engaged on both sides, and the battle front
was less than six miles. There were, thus massed,
along the front, over 20,000 men every mile of
the way, or 10,000 on each side.

In the conflict between the Allies and the Germans
it is estimated that there were less than
7500 along each mile. It was predicted in the
earlier stages of the war that it would be an easy
matter for either side to suddenly mass such an
overwhelming force at one point as to enable the
attacking party to go through the opposing force
like a wedge.

Such tactics were often employed by Napoleon
and other great masters of war; but in every effort
where it has been attempted in the present
conflict, it was foiled.

The opposing force was ready to meet the attack
with equal or superior numbers. The eye
of the army, the aeroplane, detected the movements
in every instance.

THE AEROPLANE DETECTING THE MOVEMENTS OF
ARMIES.--In the early stages of the war, when
the Germans drove the left of the French army
towards Paris, the world expected an investment
of that city. Suddenly, and for no apparent
reason, the German right was forced back and
commenced to retreat.

It was not known until weeks afterwards that
the French had assembled a large army to the
west and northwest of Paris, ready to take the
Germans in flank the moment an attempt should
be made to encircle the Paris forts.

The German aviators, flying over Paris, discovered
the hidden army, and it is well they did
so, for it is certain if they had surrounded the
outlying forts, it would have been an easy matter
for the concealed forces to destroy their communications,
and probably have forced the surrender
of a large part of the besiegers.

The aeroplane in warfare, therefore, has constantly
noted every disposition of troops, located
the positions and judged the destination of convoys;
the battery emplacements; and the direction
in which large forces have been moved from
one part of the line to the other, thus keeping the
commanders so well informed that few surprises
were possible.

THE EFFECTIVE HEIGHT FOR SCOUTING.--It has
been shown that aeroplane scouting is not effective
at high altitudes. It is not difficult for aviators
to reach and maintain altitudes of five thousand
feet and over, but at that elevation it is impossible
to distinguish anything but the movement
of large forces.

SIZES OF OBJECTS AT GREAT DISTANCES.--At a
distance of one mile an automobile, twenty feet
in length, is about as large as a piece of pencil
one inch long, viewed at a distance of thirty-five
feet. A company of one hundred men, which in
marching order, say four abreast, occupies a space
of eight by one hundred feet, looks to the aviator
about as large as an object one inch in length, four
and a half feet from the eye.

The march of such a body of men, viewed at
that distance, is so small as almost to be imperceptible
to the eye of an observer at rest. How
much more difficult it is to distinguish a movement
if the observer is in a rapidly-moving machine.

For these reasons observations must be made
at altitudes of less than a mile, and the hazard
of these enterprises is, therefore, very great,
since the successful scout must bring himself
within range of specially designed guns, which
are effective at a range of 3000 yards or more,
knowing that his only hope of safety lies in the
chance that the rapidly-moving machine will avoid
the rain of bullets that try to seek him out.

SOME DARING FEATS IN WAR.--It would be impossible
to recount the many remarkable aerial
fights which have taken place in the great war.
Some of them seem to be unreal, so startling are
the tales that have been told. We may well imagine
the bravery that will nerve men to fight
thousands of feet above the earth.

One of the most thrilling combats took place
between a Russian aeroplane and a Zeppelin, over
Russian Poland, at the time of the first German
invasion. The Zeppelin was soaring over the
Russian position, at an altitude of about a mile.
A Russian aviator ascended and after circling
about, so as to gain a position higher than the
airship, darted down, and crashed into the great
gas field.

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