controlling the fire of a machine-gun so as not to strike the blade of
the propeller, but it is admitted that the Germans were the first to
make any extensive use of it, introducing it on the Fokker monoplanes,
which caused so much damage on the Western Front in 1915. Shortly
thereafter the Allies adopted similar devices. When the United States
entered the war neither the Ordnance Department nor the Aviation Section
of the Signal Corps had had any experience worthy of the name with
aircraft guns. And if they were ill-informed on the subject of guns,
they were appallingly ignorant on the subject of gun controls. A few
months of study and experiments served to materially increase the War
Department’s knowledge along these lines, however, and by the time the
planes were ready to receive the guns we had adopted a device known
as the Constantinisco control. I should explain, perhaps, that there
are two distinct types of gun control, both of which were in use when
hostilities ceased. One is hydraulic, the other mechanical. The operation
of both types is somewhat similar. In each case a cam mounted on the
shaft of the engine actuates a plunger which in turn operates the rest
of the mechanism. In the mechanical gun control the impulse of the cam
is transmitted to the gun through a series of rods, causing the gun to
fire at the exact moment when there is no propeller-blade in front of
the muzzle. In the hydraulic type the impulse of the cam is transmitted
to the gun through a system of copper tubes containing oil under high
pressure. The hydraulic control, known as the Constantinisco, was adopted
for use on American planes, particularly the De Havilland 4, which
carries two fixed Marlins, each firing at the rate of 650 shots a minute.
By employing the maximum rate of fire, 1,300 shots could be fired in
a minute through the blades of the propeller, which would make 1,600
revolutions in the same space of time—without the blades being struck by
a single bullet.
A machine-gun intended for aerial use must be absolutely reliable in
operation. If a gun jams on the ground there is usually time to overhaul
it or to replace it. Not so in the air. There a jam or a malfunction
is almost certain to prove disastrous, if not fatal, to the gunner,
who is left completely at the mercy of his adversary. An aircraft gun
must also function properly in any position in which it is likely to be
placed by the manœuvres of the plane. Likewise, an intensely high rate
of fire is essential. For groundwork 500 shots per minute is reckoned
as sufficient for the machine-gun, for a higher rate of fire would only
result in several bullets hitting the same man. But a considerably higher
rate of fire—up to 1,000 shots a minute, in fact—is demanded of aircraft
guns, this being necessitated by the great speed at which airplanes
move. The gunner, remember, can train on his target for only a few
seconds, sometimes for only a fraction of a second, at a time, and it is
essential, therefore, that he should have at his command the greatest
possible volume of fire. Do you appreciate that, were an airplane flying
parallel to, say, a high board fence, at a speed of 100 miles an hour,
and shooting at right angles at that fence with a gun firing 880 shots a
minute, _the bullet-marks on the fence would be ten feet apart_?
Single-seater machines carry only fixed guns, which are mounted with
the barrel parallel to the axis of the airplane. These guns, which are
synchronized so as to shoot through the propeller, are put into action by
a trigger on the “joy-stick” of the plane and are aimed by pointing the
entire airplane at the enemy. Flexible guns are used only on two-place
machines, being operated by the observer or gunner. They are carried
on the Universal mount, which permits of the gun being pointed in any
direction. All of the flexible aircraft guns used by the Allies were
based on the principle of the Lewis gun, the invention of a retired
American army officer, Colonel Isaac Lewis. The chief difference between
the ground and aircraft models is that in the latter the cooling radiator
is eliminated, as aircraft guns are never fired continuously for any
length of time.
When the United States entered the war the Vickers was the only type
of fixed gun in use on either English or French planes and was used
on all the planes which General Pershing bought in France. When the
Equipment Division of the Signal Corps faced the machine-gun situation
in September, 1917, it was alarmed to find that the entire production of
Vickers in the United States had already been contracted for to supply
the imperative requirements of the infantry. There was another gun on the
market at this time, however—the Marlin—and toward its development for
aircraft use the officers of the Signal Corps bent all their energies.
Though the Marlin was adopted in the face of violent opposition, it
resulted in providing sufficient fixed guns to arm the American planes,
the wisdom of the action being proved by the fact that up to the time of
the Armistice no other fixed guns were ready for delivery. The Marlin
has been adapted to all American-built planes which carry fixed or
synchronized guns, over 37,000 having been produced up to December, 1918.
This gun shoots .30-calibre ammunition at the rate of 600 to 650 shots a
minute and is fed from a belt of the disintegrating metal-link type. In
December, 1917, the first order was placed for Lewis aircraft guns, over
39,000 of them being delivered to the American Air Service within the
following twelvemonth. A notable improvement in the aircraft model of the
Lewis gun was an increase in the depth of the magazine pan, so that each
magazine holds 97 cartridges instead of 47 as previously. The Browning
aircraft machine-gun was just coming into production when the war ended.
This weapon embodies the best features of every known machine-gun and
would probably have replaced all other types in use. It is a belt-fed gun
of the recoil type—both the Marlin and Lewis are gas-operated—is as near
fool-proof as a machine-gun can be made, and has the amazing rate of fire
of 950 shots a minute. Of it the inventor is said to have remarked: “If
it had four more parts it could play a tune; if it had seven more parts
it could talk.”
The ammunition for fixed aircraft guns, such as the Marlin and Browning,
is carried in belts containing a maximum of 500 rounds. In the earlier
days of the war these belts were of woven web, but it was found that
taking care of them, when empty, in the limited space of the fuselage,
was always a source of annoyance and not infrequently a source of danger
to the aviator. To remedy this a belt was designed and furnished to
the American Expeditionary Forces which consisted of small metallic
links held together by the cartridges themselves. As the gun fires, the
links drop apart, chutes being provided so that they fall clear of the
airplane. Another minor though interesting feature of aircraft armament
is the small electric heater which is now provided for the purpose of
keeping the gun warm and thus preventing the oil from congealing in high
altitudes.
Efforts to make the bursts of fire from aircraft guns of maximum
effectiveness have led to the development of three distinct types of
ammunition—tracer, armor-piercing, and incendiary. The tracer type of
ammunition was developed to assist the gunner in correcting his aim, and
is equally useful by night or day, as the course of the bullet can be
traced by a trail of white smoke in the daytime and by a bright spark at
night. Armor-piercing ammunition has a projectile consisting of a hard
steel core with a soft nickel casing. The object of this ammunition, as
its name implies, is to pierce any of the metallic parts of an enemy
plane, particularly the gasoline-tanks or the engine, the soft nickel
casing acting as a lubricant and preventing the steel core from glancing
off. Incendiary ammunition is loaded with yellow phosphorus. When the
cartridge is fired the rifling in the barrel of the machine-gun opens a
small hole in the case of the projectile, thus permitting the phosphorus
to come in contact with the air, whereupon it immediately ignites and
sets fire to any inflammable part of a plane which it may hit. It is
customary to load the belts or pans of aircraft machine-guns with these
three types of special ammunition in a certain sequence, depending upon
the notions of the pilot himself. A sequence commonly used was, first,
the tracer cartridge, which assisted the gunner in correcting his aim;
next, two or three armor-piercing cartridges, in the hope that they would
pierce the enemy’s gasoline-tank or damage his engine; and then one or
two incendiary cartridges, which if the gasoline-tank was pierced would
ignite the leaking gasoline and set fire to the machine. This sequence
was continued throughout the loading of the belt or pan.
Another branch of sky warfare which was being rapidly developed was
aerial bombing. Though bombs of a sort were used by Italian aviators
against the Arabs during the Libyan campaign, and by American soldiers
of fortune serving with the Villista forces in northern Mexico, these
attempts were so amateurish and ineffective as to merit no serious
consideration. It may be said that the first bombs dropped from an
aircraft in the history of warfare were those loosed from the German
Zeppelin which raided Antwerp in August, 1914. I speak with a certain
personal knowledge of my subject, for the first bomb dropped on the night
in question exploded less than a hundred yards from the window in which
I was sitting, demolishing a house and killing three persons.
Many people seem to be under the impression that bomb-dropping is about
as simple as dropping a brick out of an upper-story window onto the head
of a man beneath. This is not so. As a matter of fact, it is extremely
difficult to drop a bomb from an airplane so that it will hit a desired
target, for, owing to the speed at which the plane travels, the bomb when
released does not drop to the ground vertically, but falls in a parabolic
curve, something like that described by a man who jumps from a street-car
when it is in motion. For this reason the bomb must be released some
moments before the airplane is directly over the target, the ability of
an aviator to determine the exact moment to pull his release mechanism
being acquired only through long experience. Bomb-sights have recently
been perfected, however, which have largely eliminated this element of
chance. These sights have numerical scales mathematically calculated,
so that when adjusted for height, air-speed as shown by the air-speed
indicator, and calculated speed of the wind with or against the airplane,
two sighting points are moved into such a position that if the bomb is
dropped when the desired target comes in line with them, it will reach
its objective—provided, of course, the aviator has made his calculations
and set his sights correctly. All this sounds rather complicated, I
know, and it _is_ complicated, but if the pilot uses the sight correctly
his chances of hitting his target are enormously increased. All bombing
planes are fitted with quick-release mechanisms, which hold the bombs
firmly in a vertical or horizontal position, according to the type and
size carried. On the smaller bombing planes, such as the De Havilland
4, the release mechanisms are placed underneath the fuselage or the
lower wings, but on the large types, such as the Handley-Page, the bombs
are carried inside the fuselage. By a quick jerk of a lever the pilot
releases his bomb precisely as a hangman, by jerking a lever, drops the
trap on which the condemned man stands. And the consequences are usually
much the same in both cases.
[Illustration: BOMBING PRACTICE.
An illustration of how the enemy’s lines of communication can be
destroyed by bombs dropped from airplanes.
_Photograph by Signal Corps, U. S. A._]
[Illustration: EGGS OF DEATH.
Attaching dummy bombs to the rack of a bombing plane.
_Photograph by Signal Corps, U. S. A._]
[Illustration: PIGEONS HAVE BEEN REPEATEDLY USED WITH SUCCESS FROM BOTH
AIRPLANES AND BALLOONS.]
[Illustration: THE EYE IN THE SKY; AN AIRPLANE CAMERA IN OPERATION.
During the offensive in the Argonne the American Photographic Sections
made 100,000 aerophotographs of battle lines in four days.
_Photograph by U. S. Air Service._]
There were three distinct types of bombs—demolition, fragmentation,
and incendiary—in use by the American Air Service when the war ended.
American demolition bombs are made in 50, 100, 250, 500, and 1,000 pound
weights, the 100 and 250 pound sizes being used chiefly. These bombs
consist of a light steel casing filled with TNT or other high explosive
and a detonator separated from the explosive by a safety-pin. When the
bomb is released from the airplane the safety-pin is automatically pulled
out, permitting the detonator to slide down into such a position that the
bomb will explode the instant it strikes the ground. These demolition
bombs are primarily designed for use against buildings, fortifications,
and other heavy structures where a high-explosive charge is desired. Had
the war continued long enough to have permitted of our aviators letting
loose a few 1,000-pound bombs on some of the trans-Rhine strongholds,
the Germans would have learned what the San Francisco earthquake was
like. Fragmentation bombs are considerably smaller, the size most
frequently used weighing twenty pounds. They have a thicker case than the
demolition bombs and are constructed so as to explode a few inches above
the ground. These bombs are for use against troops in trenches or in the
open and depend upon the scattering of the fragments for their effect.
Incendiary bombs weigh about fifty pounds and contain charges of oil
emulsion, thermite, and metallic sodium, which burn for several minutes
with the intense heat of a plumber’s blow-lamp. They are used against
ammunition-depots, storehouses, and other structures of inflammable
construction, the purpose of the metallic sodium being to discourage
the efforts of any one who attempts to put out the fire, as it explodes
violently when water is poured on it.
Comparatively few persons realize, I suppose, that fireworks almost
identical with those we used to set off on the Fourth of July in the good
old days before the safe-and-sane laws went into effect are utilized
in aerial warfare and form a valuable and often vital asset for the
aviator. Most of these aerial pyrotechnics resemble in their effects
the colored lights and the Roman candles of our childhood and are used
for signalling from the airplane to the ground and vice versa, or from
one plane to others in the air at the same time. For this purpose every
active service airplane carries one or more signalling pistols, depending
upon the number of the crew. These rather formidable-appearing weapons,
which look not unlike the big-barrelled affairs the pirates were wont
to carry in their scarlet sashes, are similar to the Very pistols used
in the trenches; their ammunition consists of cartridges very similar
to shotgun shells, but larger, containing stars of various colors, like
those in Roman candles, and the necessary powder charge to eject the
stars. Three colors, red, green, and white, are furnished, the color of
the star being indicated on the base of the cartridge, which is also
serrated in such a manner that the aviator can tell the color by touch
when flying at night. By different combinations of these colors an almost
endless variety of signals can be conveyed. One of the strangest and most
fascinating night sights on the Western Front was to see these countless
stars, scarlet, yellow, emerald, shot from invisible airplanes, drifting
across the purple velvet of the sky. The stars are clearly visible in
the daytime and were used for many purposes, such as indicating the
position of enemy troops, the presence of hostile aircraft, requests for
assistance from other planes, and as a means of transmitting orders from
the leader of a squadron to other machines in formation. At night the
signalling pistol is of exceptional value in aiding the aviator to effect
a safe landing. When approaching his home-field the pilot fires a light
of a prearranged color, and if answered by a light of a proper color from
the ground, he knows that the field is clear of obstructions and other
machines and safe to land on. Pilots have also used their signalling
pistols for firing into their gasoline-tanks and thus setting fire to
their machines when forced to land in enemy territory. There are also a
few cases on record of the pilot being able to hold enemy soldiers at bay
with his signalling pistol long enough to prevent them from extinguishing
the fire.
Night-flying is one of the most hazardous duties of the aviator,
the chief danger being in the difficulty of making a safe landing.
Night-landing fields are, as a rule, well illuminated by flood-lights,
but near the front this was not always advisable or safe, and, owing to
the difficulty of judging the distance of the machine above the ground in
the darkness, accidents were by no means uncommon. In order to minimize
this danger there was developed the “wing-tip flare,” which consists of a
small cylinder of magnesium material in a metallic holder, one of which
is fitted under each lower wing of the plane. The flares are ignited by
an electric current and are controlled by push-buttons, one for each
flare, in the pilot’s cockpit. In making a night-landing, when the pilot
judges the plane to be but a few feet above the ground, he presses one
of the buttons. The flare instantly ignites and for about fifty seconds
burns with a light of approximately 20,000 candle-power, which, reflected
on the ground by the under surface of the wing, enables the pilot to
judge his distance and effect his landing without trouble.
The requirements of night-bombing have led to the development of a
new and very interesting form of pyrotechnic known as the “airplane
flare.” This flare, which weighs thirty-five pounds, is contained in a
cylindrical case of sheet-iron about four feet long and five inches in
diameter. The flare consists of an illuminating charge, capable of giving
32,000 candle-power for approximately ten minutes, which is attached to a
silk parachute twenty feet in diameter. The cylinder is attached to the
airplane by a light release mechanism similar to those used for holding
bombs. On the end of the cylinder is a small pinwheel, which, revolved by
the rush of air as the released cylinder hurtles downward, ignites the
illuminating charge and at the same time detonates a small black-powder
charge sufficient to eject the flare and its tightly rolled parachute
from the case. The parachute immediately opens and the burning flare
descends very slowly, illuminating a large area of territory underneath
almost as brightly as though it were day. These flares were used
particularly for night-bombing raids, the pilots thus being enabled to
illuminate the objectives so that they could accurately drop their bombs.
On several occasions, when raiding airplanes were met by heavy fire from
the enemy’s antiaircraft batteries, it was found that the light from
these flares was so dazzling as to make it impossible for the gunners to
take accurate aim. So wide is the radius illuminated by these flares, and
so intense their light, that it has been found possible by their aid to
obtain aero photographs of excellent detail even on the darkest nights.
I can personally vouch for the amazing brilliancy of these flares, for I
saw one dropped by the Germans during one of their air-raids on Paris in
the summer of 1918. It apparently landed on the Pont Alexandre III or in
the Seine, yet both banks of the river, the façades of the Grand and the
Petit Palais, and the Champ Elysées for several blocks in both directions
were almost as bright as though illuminated by a midday sun. Standing
alone in the Cours de la Reine, I had the feeling that the Kaiser’s
eye was on me and that, having discovered me, he intended to drop upon
me one of his steel visiting-cards. The brilliancy and unexpectedness
of the glare reminded me of boyhood days in the Thousand Islands, when
the captain of the _Island Wanderer_, making his nightly excursions
amid the clustered, cottage-dotted isles, took keen delight in suddenly
turning the beam of his powerful search-light upon some affectionate pair
love-making on the shore.
* * * * *
It has been said that the airplane is the eye of the army, and it is
equally true that the camera is the eye of the airplane. Nothing more
strikingly emphasizes the enormous importance attached to pictures taken
from the air, showing the progress of the operations, than the fact that,
during the offensive in the Argonne, the American photographic sections
made _one hundred thousand aero photographs of the battle-lines in four
days_.
As aerial photography was an entirely new military subject at the
outbreak of the war in 1914, there were no precedents to act as guides,
nor was there any special apparatus in existence. Consequently, the
entire art of aerial photography was developed and brought to its present
state of perfection by the Allies under the incentive of military
necessity and after the war had begun. As trench warfare made aerial
photography not only important but vital to the success of any proposed
operations, the changes and improvements in the apparatus employed
came with incredible rapidity, practices employed one week becoming
obsolete the next. By April, 1917, the British Air Service alone had
issued approximately 280,000 prints, and this number was equalled, if
not surpassed, by the French _Section Photographique_. At the beginning
of the war it was possible to fly at low altitudes and secure reasonably
satisfactory pictures with such cameras, plates, and lenses as were then
available. But as antiaircraft artillery was developed, the planes were
forced to climb higher to keep out of their range, and owing to the
necessity for longer-focus lenses, special plates, and color filters to
overcome the haze existing between the camera and the earth, photography
at these high altitudes became increasingly difficult.
When the United States entered the war the British, French, and Italians
were using plates exclusively and we followed their lead, it not being
until some months later that we turned to films. At this time the British
were using 4 × 5 plates, and cameras equipped with lenses of from 8 to
12 inch focus. Instead of making contact prints from these negatives,
enlargements 6½ × 8½ were made on glossy paper, it being claimed that
this process gave greater control in printing. Whether the British system
really had all the advantages claimed for it is open to question, but in
any event we adopted it and followed it through the first nine months
of the war. The great masters of photography in Rochester were by no
means content to let another nation set the pace for the United States,
however, and in January, 1918, a concern in that city completed a very
remarkable aero camera, radically different from anything which had been
seen in Europe up to that time, which was promptly adopted by the War
Department. This camera, which took an 18-cm. by 24-cm. picture, had a
focal length of 20 inches, held a roll of film on which 100 successive
exposures could be made, and weighed only 35 pounds. Its most novel
feature was the “vacuum back,” consisting of a perforated sheet which
extended across the top of the chamber and over the face of which the
film passed. A slight air-suction, produced by a Venturi tube placed
where it would catch the rush of air past the plane, served to hold the
film absolutely flat—for the slightest curvature of its surface would
play havoc with the perspective of a picture taken from a height, say, of
10,000 feet. This ingenious instrument was driven by an electric motor
which changed the film and automatically set the shutter, the observer
having only to start the machinery going and regulate its speed according
to the rate of travel of the airplane in order to obtain a series of
pictures forming a continuous photograph of the territory over which the
machine was passing.