aeroplanes-第26节
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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。
RUBBERSuitable 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 foi
