TYPES OF MILITARY AIRPLANES AND THEIR USES
=Modern Airplanes Combining Best Features of Previous Experiments.=—The
modern airplane, of which the Curtiss training machine used at the U.
S. Aviation Schools is typical, is a combination of the best features
referred to above. It is of the biplane type for, as shown by Chanute,
rigid trussing is thus possible, an advantage sufficient to offset the
slight loss of efficiency which exists in the biplane. The landing gear
consists of two wheels provided with shock absorbers; the body is of
the general stream-line type, enclosed from front to back, containing
comfortable seats for the passengers and enclosing the motor and tanks
away from the wind. The motor is at the front where, in an accident,
it will not be on top of the pilot. The warping effect is obtained by
hinging flaps at the wing tips, the same effect being obtained while at
the same time leaving the whole wing structure rigid and strong rather
than flexible and weak, as was the case in the early warping type of
machines.
=Military Airplanes of Today.=—In the modern airplane, therefore,
we see that matters of efficiency, to which the Wrights gave great
attention, have been sacrificed in favor of convenience, particularly
in favor of power and speed. This is the effect of military demands
for airplanes where power, speed, and ability to climb fast are vital
requirements. To escape from or to destroy an enemy, high speed and
ability to climb fast are, of course, prerequisites. Moreover, from the
standpoint of safety in maneuvering it is desirable to have a reserve
of power and speed. Therefore, the design of military machines has
tended in a given direction up to the present.
New considerations have arisen on this account, such as for instance
the question of landing. Fast machines in general make high-speed
landings, and are for that reason dangerous. The original Wright
machines were built to land at such a slow speed that ordinary skids
were sufficient to take the shocks. Nowadays the high-powered airplane
is likely to come to grief in landing more than at any other time.
The question of stability in flight has of recent years been treated
mathematically and experimentally, using of course the fundamental
system of “three axes control” first applied by the Wrights. It has
been found that by properly proportioning the tail surfaces and
properly arranging the wings and center of gravity, any desired degree
of stability may be obtained, such that a machine may be made almost
self-flying or, if preferred, may be made very sensitive.
All of the above features of design have had consideration in the
latest types of military airplanes. Observe the high speed of the
latest speed scouts, where power is concentrated exclusively on speed
and climbing ability and landing speed is dangerously high. We see
the advent of the triplane scout, which is an attempt to secure slow
landing speed combined with high flying speed. We see machines with the
motor and propeller in the rear, or with two motors, one to each side
of the body out in the wings, the object being to avoid interference of
the propeller with the range of gun fire. In short, we see the effect
of many military considerations on the design of the airplane. It will
be interesting at this point to survey what are these military uses of
the airplane.
=Aerial Fighting.=—Fighting in the air is the most spectacular use
to which military airplanes have been put. The first requirements in
a fighting airplane are speed and climbing ability and these must be
obtained at all costs, because speed and climb are weapons of defense
and offense second only in value to the gun itself. The concentration
of motive power for speed and climb requires that as little weight as
possible be used; and therefore the fastest fighters are designed to
carry only one person and are very light and of course very small.
It is usual to have one gun fixed to the body and firing through the
propeller in the case of a tractor, and a second adjustable aim gun
pointing upwards over the top wing. This gives the pilot a chance to
fire a round at the enemy while “sitting on his tail” or following
from behind; and then when diving below the enemy the second gun is
available for shooting overhead. These very high-speed fighters are
difficult to land, due to their speed, and are suitable only for the
highest-trained pilots.
=Directing Artillery Fire.=—The friendly airplane is sent out over
the enemy’s positions, soars above the target, sends back signals
by wireless to the friendly battery regarding the effect of fire;
practically dictating the success of artillery operations.
=Reconnaissance.=—The friendly airplanes go out, usually in squads for
the sake of protection, and observe by means of photographs or vision
size of enemy troops, batteries, trenches, lines of communication,
etc.; report the situation to headquarters as a source of daily
photographic record of the operations of the enemy, to such an extent
that any change of the enemy’s position can be analyzed. Of course the
value of reconnaissance is lessened when the enemy disguises his gun
emplacements, etc. In reconnaissance machines it is important to have
two persons, one to steer and the other to scan the countryside. The
reconnaissance machine is therefore a two-place type which may or may
not have armament. It need not be so fast, especially when convoyed by
fighting speed scouts. The two-place machines are frequently used for
fighting, in which case the pilot will have a gun fixed to the body and
shooting through the propeller, and the passenger, especially in German
machines, will also have a gun mounted in the turret so that it may be
shot in a variety of directions by the passenger.
=Bomb Dropping.=—This maneuver requires squad flights to be of great
value. The fundamental characteristic of a bombing airplane is its
ability to carry great weight. Such machines are of comparatively
large size and not particularly fast. Weight carrying is of course
incompatible with speed and climbing ability and therefore the bombing
machine must be a compromise if it is to have any reasonable speed.
It may be said that airplanes compare very unfavorably with dirigible
balloons for bomb raids because the latter are able to carry several
tons of bombs as against the airplane’s quarter of a ton.
=Locating Submarines.=—For coast patrol or submarine spotting, the
airplane is an important factor, for from an airplane it is possible
to see for a considerable depth into the water, and to locate hostile
submarines.
=Training Student Aviators.=—The training machine on which prospective
aviators secure their flying instruction may be considered as a
type in which great speed and power is not essential, but in which
reliability and ease of control is desirable. The typical military
training airplane in this country is a single-motor tractor of
moderate horsepower (about 100) having of course the seats in tandem
and furnished with dual control so that operation may be from either
pilot’s or passenger’s seat. The dual-control system of training which
prevails in this country differs from the French method of starting
the pupil out alone to try his wings; it enables the pilot to keep
a constant eye upon the pupil’s control manipulations and to correct
them instantly whenever they are in error before any damage is done.
A possible improvement in the dual-control training machine will be
the substitution of side by side seats for tandem seats. At present,
communication is difficult due to the great noise of the motor; but
with the adoption of side by side seats such as is used in naval
training schools, the pilot and pupil will be able to communicate to
better advantage.
[Illustration: FIG. 7.—U. S. training airplane, dual control (Curtiss
JN-4).
Speed 43 to 72 mi. per hr.; climbing ability 300 ft. per min.; 90 h.p.;
weight fully loaded 1,890 lbs.]
=Types of Airplanes.=—To suit the foregoing purposes flying machines
exist in seven distinct different shapes at the present time, namely:
monoplanes, biplanes, triplanes, single-motor tractors, single-motor
pushers, double-motor machines and marine airplanes. The last four
types may be either monoplanes, biplanes or triplanes. In order to
understand the adoption of one or the other type for military use, it
is well to run over the characteristics of the seven types mentioned.
=Monoplanes.=—The simplest form of airplane is the monoplane which
is fashioned after the manner of a bird (see Fig. 34). There are two
things to say in favor of the monoplane: first, that the passengers
have an unobstructed view forward and range of gun fire upward because
there is no wing above them; second, the aerodynamic efficiency of
the monoplane is superior to any other type. But when the bird design
is applied to a man-carrying apparatus, it becomes impracticable
to construct spars to take the place of the bird’s wing bones; and
therefore to give the wings proper strength it becomes necessary to
truss them with numerous tension wires stretching from the running gear
out to various portions of the wings. There are also wires running
from a vertical mast above the body to a point on the top part of the
wing; these wires, while they give the wing no added strength during
a flight, are necessary in order that the shock of landing shall not
break the wings off sharp at the shoulder. It is characteristic of
monoplane construction that from a point below the body and also from
a point above the body a number of heavy wires run outward to various
points on the wings; and it may be said that the strength to be secured
from this construction is not all that could be desired.
=Biplanes.=—The biplane is an improvement over the monoplane from the
latter standpoint; in the biplane there are two parallel surfaces
separated by vertical sticks or struts, thus forming parallelograms
which are susceptible of being trussed by means of tension-wire
diagonals in a manner familiar and well understood in case of bridges.
It is possible to build up biplane wings of great rigidity and strength
by this system, much more easily than in case of monoplanes. However,
the biplane type is from the standpoint of efficiency inferior to the
monoplane. This is due to the fact that the vacuum above the bottom
wing which is so necessary for high duty is somewhat interfered with
by the upper wing; thus while in a biplane the upper wing operates
about as efficiently as it would operate in a monoplane, yet the lower
wing has its efficiency materially reduced and the resulting overall
efficiency of a biplane compared area for area with the monoplane
is about 85 per cent. as great. However, recent developments of the
airplane have more or less put efficiency in the background and as
a result today the biplane is more popular than the monoplane. In
addition to the greater strength of biplane wings their span may be
less than the monoplane for the same supporting area. This makes them
less unwieldy. Moreover, for certain reasons a biplane machine of high
speed may be landed at a lower speed than equivalent monoplanes.
=Triplanes.=—What is true of the biplane is more true in almost every
item of the triplane, that is, it is comparatively strong, compact, and
of low landing speed, but of reduced efficiency.
[Illustration: FIG. 8.—U. S. speed scout triplane, single seater.
(Curtiss Model S3), 55 to 115 mi. per hr.; climbing ability 900 ft. per
min.; 100 h.p.; weight fully loaded 1,320 lbs.]
=Single-Motor Tractors=.—The single-motor tractor received its name
simply because the propeller is in front and draws the machine forward;
but this location of the propeller necessitates a distinct type of
airplane, wherein the power plant is located at the very nose of the
machine. The tractor type has the pilot and passenger located in or
to the rear of the wings in order that their weight may balance the
weight of the motor. This means that the view and range of fire of
the passengers is obstructed in a forward direction by the wings, and
in machines such as the U. S. training machine, the passenger, who is
practically in the center of the wings, can not look directly upward
nor directly downward. Moreover, as concerns gun fire, the propeller
of a tractor obstructs the range straight ahead. In the tractor the
tail is supported at the rear and on the same body which contains the
motor and passengers; this body constitutes a stream-line housing for
the machinery, seats, etc., and therefore has low wind resistance.
The tractor is a very shipshape design, compact and simple and is
at present the prevailing type on the European war front. However,
it has disadvantages which are only overcome in other types. One of
these disadvantages is of course the obstruction to range of gun fire.
The present practice in fighting airplanes is simply to shoot the
gun straight through the circle of rotation of the propeller on the
assumption that most of the bullets will get through and that those
which hit the shank of the propeller blade will be deflected by proper
armoring. An attempt is made to insure that all the shots will get
through by connecting the gun mechanism mechanically to the motor shaft
in such a way that bullets will be discharged only at the instant
when their path is unobstructed by a propeller blade. This practice
is possible of course only in guns which are fixed immovably to the
airplane.
[Illustration: FIG. 9.—Fuselage diagram, Curtiss “R4” reconnaissance
biplane.
Speed 48 to 90 mi. per hr.; climbing ability 400 ft. per min.; 200
h.p.; weight fully loaded 3,245 lbs.]
[Illustration: FIG. 10.—An American pusher biplane design.
Crew in front, motor and propeller in the rear, tail support on
outriggers.]
=Single-motor Pusher Airplanes.=—The pusher type has popularity because
the propeller and motor rotate to the rear of the passenger, who takes
his place in the very front of the body and has an open range of vision
and gun fire downward, upward and sideways. Another point in favor of
the pusher is that the oil and fumes of the motor do not blow into his
face as in the case of the tractor. The disadvantage of the pusher is
that the motor, being located behind the pilot, will be on top of him
in the case of a fall. Another disadvantage is that the body can not
be given its shipshape stream-line form because to do so will interfere
with the rotation of the propeller. Therefore, the body is abruptly
terminated just to the rear of the wings and it is just long enough to
hold the passenger and the motor, the propeller sticking out behind.
The tail surfaces are then attached to the airplane by means of long
outriggers springing from the wing beams at points sufficiently far
from the propeller axis so as not to interfere with the propeller.
[Illustration: FIG. 11.—U. S. army battle plane.
Two 100 h.p. motors; speed 85 mi. per hr.]
=Double-motor Machines.=—In order to combine the advantages of the
tractor and pusher types and eliminate their disadvantages, the
double-motor machines have been developed. In these there is no
machinery whatever in the body either in front or back, and the
passengers may take seats at the extreme front as is desirable. The
body then tapers off to the rear in stream-line form and supports the
tail surfaces. The power plants are in duplicate and one is located
to each side of the body out on the wings. It is customary to enclose
each of these two motors in a casing so that the whole power plant
presents a more or less stream-line shape to the wind, the propellers
projecting from the front or rear of these stream-line shapes. It
may be said that in the double-motor airplane it makes very little
difference whether the propeller is in front or behind so that while a
“twin-motor” machine may be more accurately specified as a “twin-motor
pusher” or a “twin-motor tractor,” it is usually sufficient indication
of a machine’s characteristics to call it a twin-motor machine.
By adopting this twin-motor form we bring in new disadvantages. One of
these is due to the fact that the heavy motors are now located some
distance from the center of gravity of the machine. This requires
stronger supporting members between the motor and the body. It also
makes the lateral control comparatively logy for now the heavy masses
are far from the center of gravity, resisting the pilot’s efforts to
use the lateral control. The second disadvantage in the twin-motor
type results from possible stoppage of either motor. In this case, of
course, the propelling force is some distance off center and is also
reduced to one-half its value requiring energetic exercise of the
control wheel to maintain equilibrium. It is reported, however, that
twin machines can continue to fly and even climb with only one motor
running. In this country the twin-motor type has not developed as was
hoped at first, and on the European firing lines it is not so numerous
as the single-motor tractor type.
=Marine Airplanes=.—The possibility of mechanical flight having once
been established and wheels having been applied to the airplane so that
it could start from and land on the ground, the logical next step was
to substitute some form of boat for the wheels so that flights could be
made over the water.
Experiments were made in France by M. Fabre in this direction and in
this country by G. H. Curtiss. The latter, in his flight down the
Hudson from Albany to New York, equipped his airplane with a light
float to provide against forced landing in the river. Pursuing this
general idea he made some experiments under the auspices of Alexander
Graham Bell’s Aerial Experiment Association, in which a canoe was
substituted for the wheels, and in which an attempt was made to start
from the surface of the water. Success did not come at first and this
plan gave no satisfaction. Curtiss next turned his attention to the
hydroplane type of boat and made a series of experiments at San Diego.
The hydroplane appeared to be much better adapted to his purpose than
the canoe had been, and he was able to obtain success.
[Illustration: FIG. 12.—Thomas Type H. S. seaplane. Double pontoons.
Speed 47 to 82 mi. per hr.; climbing ability 270 ft. per min.; 135
h.p.; weight, fully loaded 2,600 lbs.]
[Illustration: FIG. 13.—Curtiss Model F flying boat.
Speed 45 to 65 mi. per hr.; climbing ability 150 ft. per min.; 90 h.p.;
weight fully loaded 2,100 lbs.]
=The Hydro-airplane (or “Seaplane”).=—From analogy to the airplane one
might at first imagine that a suitable hydroplane would have a wide
span and fore and aft length; but such proportion would give a very
poor stability on the water, and would require auxiliary hydroplanes
in the same way that an airplane requires auxiliary guiding surfaces.
So Curtiss, with his customary eye for simplicity and convenience,
adopted a type of hydroplane which had the general proportions of
an ordinary boat, _i.e._, was long and narrow, thus obviating the
necessity of auxiliary hydroplanes at the tail of the machine. To
prevent the machine’s tipping over sidewise, “wing pontoons” were
attached at the lower wing tips to prevent capsizing.
[Illustration: FIG. 14.—Building a flying boat hull.
Note wing stumps and hydroplane fins.]
[Illustration: FIG. 15.—Method of hoisting a marine airplane aboard
ship.]
=The Flying Boat.=—In the early hydro-airplane, which was thus
developed, the motor and pilot were above in the usual position in the
wings, while the hydroplane itself was a considerable distance below
the wings. Thus there was a good deal of head resistance. Curtiss set
about reducing this head resistance as far as possible and tried to
incorporate the pilot’s seat with the hydroplane pontoon. The outcome
of his endeavor was that he developed a boat with a tapering stern.
The pilot, gasoline tanks, etc., are located inside of the hull; the
tapering stern provides a backbone to which the tail surfaces can be
readily attached; the wings fixed to the sides of the hull in a manner
analogous to the wing fastenings of the modern military airplane; and
the motor alone remains exposed to the wind. This is the flying boat;
its action on the water is analogous to the action of the hydroplane
for the bottom of this boat hull is made in hydroplane form; indeed, in
the latest types of flying boat, the hydroplane area is increased by
extending it to right and left of the boat hull. The flying boat is an
ingenious combination, wherein the characteristics of the hydroplane
are combined with the seaworthiness of the ordinary boat, and at the
same time wind resistance is reduced to a minimum.
The hydro-airplane remains in use, however, being preferable to the
flying boat for certain purposes, and often is termed seaplane.
=Future of the Airplane.=—In order to be commercially successful and
have a commercial future after the war, the following weak points in
airplane design must be rectified.
1. _Motor._—Airplane motors are imperfect and unreliable at present and
there must be considerable progress before this type of motor which is
very light and delicate can be considered as reliable or can be made in
large enough quantities to cut down the cost.
2. _Landing._—The necessity of landing at considerable speed, say 40
to 50 miles per hour, requires a wide flat space, such as is not easy
to find, and if the present type of airplane is to become commercially
numerous, a large number of landing fields must be developed all over
the country.
3. _Danger._—The airplane is by no means so dangerous as the public has
been led to think from the exploits of the daredevil circus performers
of the past 10 years; with careful manipulation it will make trips day
after day without any damage. However, it is not a foolproof machine
and there remains an element of danger on this account, which it is
hoped will one day be eliminated.
=Future Uses of the Airplane.=—Future uses of the airplane are many
after the war is over. The postal service of several governments are
considering this means of mail delivery; the sports use as in the
past will continue to flourish; express carrying may be expected
in inaccessible countries where railroads and roads do not give
access and where high-speed delivery by countless airplanes would
aid materially in the development of newly opened countries. For
airplane transportation will require no expensive right-of-way,
rubber-tire renewals, etc. Minor uses of airplanes are on such duties
as forest-fire patrol, working at life-saving stations, etc.
=American Airplane Industries.=—The magnitude of the airplane industry
in this country is great, although not so great as in Europe. Leading
business men have invested in this industry with the firm belief that
it will become a profitable one, irrespective of war. We see a number
of leading bankers and also automobile manufacturers in various parts
of the country putting their money into this new industry. Now that
a great demand has sprung up on our side of the water for airplanes,
we will expect to see this industry increase more rapidly still. The
only result can be, from all the interest and importance attached
to aviation, that after the war is over, large commercial uses will
develop which will offer employment to those who go into the work at
this time for military reasons. No one can predict exactly what turn
the situation will take, but there is every indication that aviation
has graduated from the primary class of experimental work and is to
be considered now as an industry along with the automobile business,
motor-boat business, etc.