THE AIRPLANE CONSIDERED AS A CAMERA PLATFORM
An essential part of the equipment of either the aerial photographer or
the designer of aerial photographic apparatus is a working knowledge of
the principles and construction of the airplane, and considerable actual
experience in the air. Conditions and requirements in the flying plane
are far different from those of the shop bench or photographic studio.
As a preliminary to undertaking any work on airplane instruments a good
text-book on the principles of flight should be studied. Such general
ideas as are necessary for understanding the purely photographic
problems are, however, outlined in the next paragraphs.
[Illustration:
FIG. 1.—The elements of the plane.]
=Construction of the Airplane.=—The modern airplane (Fig. 1) consists of
one or more _planes_, much longer across than in the direction of flight
(_aspect ratio_). These are inclined slightly upward toward the
direction of travel, and their rapid motion through the air, due to the
pull of the _propeller_ driven by the _motor_, causes them to rise from
the earth, carrying the _fuselage_ or body of the airplane. In the
fuselage are carried the pilot, observer, and any other load. Wheels
below the fuselage forming the _under-carriage_ or _landing gear_ serve
to support the body when running along the ground in taking off or
landing. The pilot, sitting in one of the _cockpits_, has in front of
him the _controls_, by means of which the motion of the plane is guided
(Figs. 2 and 3). These consist of the engine controls—the _contacts_ for
the ignition, the _throttle_, the oil and gasoline supply, air pressure,
etc., and the steering controls—the _rudder bar_, the _stick_ and the
_stabilizer control_. The rudder bar, operated by the feet, controls
both the _rudder_ of the plane, which turns the plane to right or left
in the air, and the _tail skid_, for steering on the ground. The _stick_
is a vertical column in front of the pilot which, when pushed forward or
back, depresses or raises the _elevator_ and makes the machine dive or
climb. Thrown to either side it operates the _ailerons_ or wing tips,
which cause the plane to roll about its fore and aft axis. The
stabilizer control is usually a wheel at the side of the cockpit, whose
turning varies the angle of incidence of the small stabilizing plane in
front of the elevator, to correct the balance of the plane under
different conditions of loading.
[Illustration:
FIG. 2.—Forward cockpit of DeHaviland 4, showing instrument board.]
[Illustration:
FIG. 3.—Rear cockpit of DeHaviland 4, showing rear “stick” and rudder
bar.]
The _fuselage_ consists usually of a light hollow framework of spruce or
ash, divided into a series of bays or compartments by upright members,
connecting the _longerons_, which are the four corner members, running
fore and aft, of the plane. Diagonally across the sides and faces of
these bays are stretched taut piano wires, and the whole structure is
covered with canvas or linen fabric. Cross-wires and fabric are omitted
from the top of one or more bays to permit their being used as cockpits
for pilot and observer. In later designs of planes the wire and fabric
construction has been superseded by ply-wood veneer, thereby
strengthening the fuselage so that many of the diagonal bracing wires on
the inside are dispensed with. This greatly increases the accessibility
of the spaces in which cameras and other apparatus must be carried.
[Illustration:
FIG. 4.—Biplane in flight.]
The fuselage differs greatly in cross-section shape and in roominess
according to the type of engine. In the majority of English and American
planes, with their vertical cylinder or V type engines, the fuselage is
narrow and rectangular in cross-section. In many French planes, radial
or rotary engines are used and the fuselage is correspondingly almost
circular, and so is much more spacious than the English and American
planes of similar power. The shape and size of the plane body has an
important bearing on the question of camera installation.
[Illustration:
FIG. 5.—A single-seater.]
=Types of Planes.=—The most common type of plane is the _biplane_ (Fig.
4), with its two planes, connected by struts and wires, set not directly
over each other, but _staggered_, usually with the upper plane leading.
_Monoplanes_ were in favor in the early days of aviation, and
_triplanes_ have been used to some extent. According to the position of
the propeller planes are classified as _tractors_ or _pushers_, tractors
being at present the more common form. Planes are further classified as
_single-seaters_ (Fig. 5), _two-seaters_, and _three-seaters_. These
motor and passenger methods of classification are now proving inadequate
with the rapid development of planes carrying two, three, and even more
motors, divided between pusher and tractor operation, and carrying
increasingly large numbers of passengers. Aside from structure, planes
may be further classified according to their uses, as _scout_, _combat_,
_reconnaissance_, _bombing_, etc. Planes equipped with floats or
pontoons for alighting on the water are called _seaplanes_ (Fig. 182),
and those in which the fuselage is boat-shaped, to permit of floating
directly on the water, are _flying boats_ (Fig. 183).
=The Plane in the Air.=—The first flight of the photographic observer or
of the instrument expert who is to work upon airplane instruments is
very profitably made as a “joy ride,” to familiarize him with conditions
in the air. His experience will be somewhat as follows:
The plane is brought out of the hangar, carefully gone over by the
mechanics, and the engine “warmed up.” The pilot minutely inspects all
parts of the “ship,” then climbs up into the front cockpit. He wears
helmet and goggles, and if the weather is cold or if he expects to fly
high, a heavy wool-lined coat or suit, with thick gloves and moccasins,
or an electrically heated suit. The passenger, likewise attired, climbs
into the rear cockpit and straps himself into the seat. He finds himself
sitting rather low down, with the sides of the cockpit nearly on a level
with his eyes. To either side of his knees and feet are taut wires
leading from the controls to the elevator, stabilizer, tail skid and
rudder. If the machine is dual control, the stick is between his knees,
the rudder bar before his feet. None of these must he let his body
touch, so in the ordinary two-seater his quarters are badly cramped.
At the word “contact” the mechanics swing the propeller, and, sometimes
only after several trials, the motor starts, with a roar and a rush of
wind in the passenger's face. After a moment's slow running it is
speeded up, the intermittent roar becomes a continuous note, the plane
shakes and strains, while the mechanics hold down the tail to prevent a
premature take-off. When the engine is properly warmed up it is
throttled to a low speed, the chocks under the wheels are removed, the
mechanics hold one end of the lower wing so that the plane swings around
toward the field. It then “taxis” out to a favorable position facing
into the wind with a clear stretch of field before it. After a careful
look around to see that no other planes are landing, taking off, or in
the air near by, the pilot opens out the engine, the roar increases its
pitch, the plane moves slowly along the ground, gathers speed and rises
smoothly into the air. Near the ground the air is apt to be “bumpy,” the
plane may drop or rise abruptly, or tilt to either side. The pilot
instantly corrects these deviations, and the plane continues to climb
until steadier air is reached.
At first the passenger's chief impressions are apt to be the deafening
noise of the motor, the heavy vibration, the terrific wind in his face.
If he raises his hand above the edge of the cockpit he realizes the
magnitude of wind resistance at the speed of the plane, and hence the
importance of the _stream-line section_ of all struts and projecting
parts.
When he reaches the desired altitude the pilot levels off the plane and
ceases to climb. Now his task is to maintain the plane on an even keel
by means of the controls, correcting as soon as he notes it, any
tendency to “pitch,” to “roll” or to “yaw” off the course. The resultant
path is one which approximates to level straight flying to a degree
conditioned by the steadiness of the air and the skill of the pilot. If
he is not skilful or quick in his reactions he may keep the plane on its
level course only by alternately climbing and gliding, by flying with
first one wing down and then the other, by pointing to the right and
then to the left. The skilled photographic pilot will hold a plane level
in both directions to within a few degrees, but he will do this easily
only if the plane is properly balanced. If the load on the plane is such
as to move the center of gravity too far forward with respect to the
_center of lift_ the plane will be _nose-heavy_, if the load is too far
back it will be _tail-heavy_. Either of these conditions can be
corrected, at some cost in efficiency, by changing the inclination of
the stabilizer. When the plane reaches high altitudes in rare air, where
it can go no further, it is said to have reached its _ceiling_. It here
travels level only by pointing its wings upward in the climbing
position, so that the fuselage is no longer parallel to the direction of
flight. An understanding of these peculiarities of the plane in flight
is of prime importance in photographic map making, where the camera
should be accurately vertical at all times.
The direction and velocity of the wind must be carefully considered by
the pilot in making any predetermined course or objective. The progress
of the plane due to the pull of the propeller is primarily with
reference to the air. If this is in motion the plane's _ground speed_
and direction will be altered accordingly. In flying with or against the
wind the ground speed is the sum or difference, respectively, of the
plane's _air speed_ (determined by an air speed indicator) and the speed
of the wind. If the predetermined course lies more or less across the
wind the plane must be pointed into the wind, in which case its travel,
with respect to the earth, is not in the line of its fore and aft axis.
The effect of “crabbing,” as it is called, on photographic calculations
is discussed later (Figs. 136 and 138).
When the plane has reached the end of its straight course and starts to
turn, its level position is for the moment entirely given up in the
operation of _banking_ (Fig. 6). Just as the tracks on the curve of a
railroad are raised on the outer side to oppose the tendency of the
train to slip outward, so the plane must be tilted, by means of the
ailerons, toward the inside of the turn. A point to be clearly kept in
mind about a bank is that if correctly made a plumb line inside the
fuselage will continue to hang vertical _with respect to the floor of
the plane, and not with respect to the earth_, for the force acting on
it is the combination of gravity and the acceleration outward due to the
turn. Only some form of gyroscopically controlled pointer, keeping its
direction in space, will indicate the inclination of the plane with
respect to the true vertical. If the banking is insufficient the plane
will _side slip_ outward or _skid_; if too great, it will side slip
inward.
[Illustration:
FIG. 6.—Banking.]
As part of the “joy ride” the pilot may do a few “stunts,” such as a
“stall,” a “loop,” a “tail spin,” or an “Immelman.” From the
photographic standpoint these are of interest in so far as they bear on
the question of holding the camera in place in the plane. The thing to
be noted here is that (particularly in the loop), if these maneuvers are
properly performed, there is little tendency toward relative motion
between plane and apparatus. In a perfect loop it would, for instance,
be unnecessary, due to the centrifugal force outward, for the observer
to strap himself in. It is, however, =un=wise to place implicit
confidence in the perfection of the pilot's aerial gymnastics. No
apparatus should be left entirely free, although, for the reason given,
comparatively light fastenings are usually sufficient.
When nearing the landing field the pilot will throttle down the engine
and commence to glide. If he is at a considerable altitude he may come
down a large part of the distance in a rapid spiral. As the earth is
approached the air pressure increases rapidly, and the passenger, if
correctly instructed, will open his mouth and swallow frequently to
equalize the air pressure on his ear drums. Just before the ground is
reached the plane is leveled off, it loses speed, and, if the landing is
perfect, touches and runs along the ground without bouncing or bumping.
Frequently, however, the impact of the tail is sufficiently hard to
cause it to bump badly, with a consequent considerable danger to
apparatus of any weight or delicacy. This is especially apt to occur in
hastily chosen and poorly leveled fields such as must often be utilized
in war or in cross-country flying.
=Appearance of the Earth from the Plane.=—The view from the ordinary
two-seater is greatly restricted by the engine in front and by the
planes to either side and below (Figs. 7, 8, and 9). By craning his neck
over the side, or by looking down through an opening in the floor, the
passenger has an opportunity to learn the general appearance of the
subject he is later to devote his attention to photographing. Perhaps
the most striking impression he receives will be that of the _flatness_
of the earth, both in the sense of absence of relief and in the sense of
absence of extremes of light and shade. The absence of relief is due to
the fact that at ordinary flying heights the elevations of natural
objects are too small for the natural separation of the eyes to give any
stereoscopic effect. The absence of extremes of light and shade is in
part due to the fact that the natural surfaces of earth, grass and
forest present no great range of brightness; in part to the small
relative areas of the parts in shadow; in considerable part to the layer
of atmospheric haze which lies as an illuminated veil between the
observer and the earth at altitudes of 2000 meters and over (Figs. 10
and 11). Due to the combination of these factors the earth below
presents the appearance of a delicate pastel.
As the gaze is directed away from the territory directly below, the
thickness of atmosphere to be pierced rapidly increases, until toward
the horizon (which lies level with the observer here as on the ground)
all detail is apt to be obliterated to such an extent that only on very
clear days can the horizon itself be definitely found or be
distinguished from low lying haze or clouds (Fig. 4).
[Illustration:
FIG. 7.—The view ahead.]
[Illustration:
FIG. 8.—The view astern.]
=Airplane Instruments.=—Mounted on boards in front of the pilot and
observer are various instruments to indicate the performance of engine
and plane (Fig. 2). Those of interest to the photographic observer are
the _compass_, the _altimeter_, the _air speed indicator_, the
_inclinometers_.
[Illustration:
FIG. 9.—The view between the wings.]
[Illustration:
FIG. 10.—Appearance of the earth from a low altitude—3000 feet or
less.]
The _compass_ is usually a special airplane compass, with its “card”
immersed in a damping liquid. Like most of the direction indicating
instruments on a plane its indications are only of significance when the
plane is pursuing a steady course. On turns or rapid changes of
direction of any sort perturbations prevent accurate reading.
The _altimeter_ is of the common aneroid barometer type. On American
instruments it is usually graduated to read in 100-foot steps. While
somewhat sluggish, it is quite satisfactory for all ordinary
determinations of altitude in photographic work. Were primary map making
to be undertaken, where the scale was determinable only from the
altitude and focal length of the lens, the ordinary altimeter is hardly
accurate enough.
[Illustration:
FIG. 11.—Appearance of the earth from a high altitude—10,000 feet or
more.]
The _air speed indicator_ consists of a combination of Venturi and Pitot
tubes, producing a difference of pressure when in motion through the air
which is measured on a scale calibrated in air speed. This instrument is
important for determining, in combination with wind speed, the _ground
speed_ of the plane, on the basis of which is calculated the interval
between exposures to secure overlapping photographs. Its accuracy is
well above that necessary for the purpose.
_Inclinometers_ for showing the lateral and fore and aft angle of the
plane with the horizontal, are occasionally used, and have also been
incorporated in cameras. The important point to remember about these
instruments is that they are controlled not alone by gravity but as well
by the acceleration of the plane in any direction. They consequently
indicate correctly only when the plane is flying straight. On a bank the
lateral indicator continues to indicate “vertical” if the bank is
properly calculated for the turn.
II
THE AIRPLANE CAMERA