PRACTICAL CAMERA MOUNTINGS
=General Considerations.=—Camera mountings as used during the war were
far from being developed on the basis of scientific study or test. At
first the need for special supporting apparatus was not realized, and
the suspensions later in use were largely field-made affairs, often
dependent on adjustments made according to individual taste. Through
lack of accurate methods of test and of conclusive evidence on the
subject, it was quite common to find extremists who, on the one hand,
denied the efficacy of suspensions in general, and on the other ardently
supported crazily conceived supporting arrangements which accurate
comparative test show to be even worse than useless.
In the French service, despite numerous types of suspension available,
the very general practice was to lift the camera from its support and
hold it between the knees. Or else the hand was pressed on the top of
the camera during exposure, more reliance being placed on the damping
qualities of the body than on any of the rubber or spring mechanisms.
As is clearly shown by the experimental data described in the last
chapter, a correctly designed supporting device, carrying the camera
accurately in the plane of its center of gravity, accomplishes
practically perfect elimination of vibrational troubles. So important is
the use of a mount and so important is it that the mount should be
correctly dimensioned and adjusted for the camera, that an entirely
different attitude should be adopted from the prevalent one which
focuses attention on the camera and regards the mounting as a mere
auxiliary to be left more or less to chance. _The mounting should be
considered an integral part of the camera._ The man in the field should
receive camera and mount together, leaving as his only problem the
attachment of the complete camera—and—mount unit to the plane. This may
be arranged, by proper designing, to be a simple matter of rigid bolting
or strapping, requiring ingenuity perhaps but not the scientific
knowledge which is required for mounting design.
[Illustration:
FIG. 81.—“L” camera mounted outside the fuselage. Observer using
exposure plunger, pilot using Bowden wire release.]
=Outboard Mountings.=—In the English service the camera was first
attached to the plane outside the fuselage by a rigid frame, to which
the camera was strapped or bolted (Fig. 81). Obvious objections exist to
placing the camera in this position, such as the resistance of the wind
and the difficulty of changing magazines. However, in the earlier
English planes with their fuselages of small cross section no other
accessible place for the camera was to be found. Vibrational
disturbances with the rigid outboard mounting are quite serious, as is
so clearly indicated by the trace shown in Fig. 78. Extremely short
exposures are alone possible, and a very large proportion of the
pictures are apt to be indistinct.
=Floor Mountings.=—A step in advance of the outboard mounting is to
support the camera snout in a padded conical frame on the floor of the
plane (Fig. 82). This mounting avoids the objection on the ground of
wind resistance that holds with the outboard, and has possibilities of
being worked out as an entirely satisfactory support. Yet to be
satisfactory, the point of support must lie in the plane of the center
of gravity of the camera, and the camera must be of a type that
preserves its center of gravity unchanged in position as the plates are
exposed. Unless these conditions are fully met the floor mounting gives
results little better than does the outboard.
=Cradles or Trays.=—Floor space in the cockpit being unavailable in the
battle-plane, due to duplicate controls, bomb sights, etc., the English
service was driven to the practice of carrying the camera in the
compartment or bay behind the observer. Here it was attached either to
the structural uprights or longerons, or to special uprights and
cross-pieces built into the plane to serve photographic ends. As an
intermediary between the camera and the supporting cross-pieces there
was introduced the camera _tray_ or _cradle_. This is essentially a
frame carrying sponge rubber pads into which the camera is more or less
deeply bedded. Figs. 83 and 84 show an American L camera cradle based on
the design of the English L camera tray. Thick sponge rubber pads
support the two ends of the camera top plate, and additional pads are
provided to hold the nose of the camera. Careful tests show this cradle
to be superior to the outboard mounting, but still leave much to be
desired. Its performance is better with the nose of the camera left
free.
[Illustration:
FIG. 82.—“L” camera in floor mounting.]
[Illustration:
FIG. 83.—“L” camera and cradle mount in skeleton DeHaviland 4
fuselage, side view.]
=Tennis-ball Mounting.=—A very simple mount used by the French consists
of a frame enclosing the nose of the camera, and carrying four tennis
balls, on which the whole weight rests (Fig. 40). If the center of
support is in the plane of the center of gravity and if the four balls
are of uniform age and elasticity, this form of support is good. As
provided by the camera manufacturer, the tennis ball frame fits much too
far down on the camera. Another application of the tennis ball idea was
frequently made in the French service, in which the balls were close up
under the shutter housing (Fig. 85). Additional support was, however,
given to the camera nose by flexible rubber bands, the success of the
whole being largely a matter of the adjustment of the tension on the
bands.
[Illustration:
FIG. 84.—“L” camera and cradle mount in skeleton DeHaviland 4
fuselage, front view.]
=Parallel Motion Devices.=—A form of suspension favored by the French
consists of parallel bell cranks, rigidly linked together and held up by
springs. Mountings of this sort are illustrated in Figs. 86, 87, 88 and
96. The guiding principle is that any sort of shock will be transformed
into a straight up-and-down or side-wise motion of the camera, which is
harmless. The mounting as adapted by the English surrounds the camera
body, making the plane of support somewhere near the center of gravity.
In certain of the French suspensions employing this principle the whole
camera is hung below the bell cranks (Fig. 86), and then the nose is
restrained by heavy rubber bands. The net result is largely a matter of
adjustment.
[Illustration:
FIG. 85.—Tennis ball suspension, assisted by elastic bands attached to
nose of camera.]
[Illustration:
FIG. 86.—French spring and bell crank suspension.]
[Illustration:
FIG. 87.—U. S. hand-operated 18 × 24 centimeter plate camera on bell
crank mount with rotating turret.]
[Illustration:
FIG. 88.—Same camera in plate changing position.]
[Illustration:
FIG. 89.—Tests of camera mountings: (_a_) deRam camera on
bell-crank-and-spring mount, below the center of gravity; (_b_)
same, at center of gravity; (_c_) type “K” film camera on universal
mounting (Fig. 88).]
Tests on the English design made in the United States Air Service appear
to show that the chief virtue of the mounting lies in the approximation
of the point of support to the center of gravity in the English cameras.
A deRam camera supported by its cone, so that its center of gravity was
considerably above the center of support gave rather poor results (Fig.
89_a_), but when the bell cranks were attached near the center of
gravity, highly successful results were obtained (Fig. 89_b_). The
French deRam camera as ordered for the American Expeditionary Force was
fitted with a bell crank supported in this position.
Figures 90 and 91 show a bell crank mounting furnished with a rotating
turret. This was designed to facilitate the changing of magazines in the
English B M camera, which is swung around through 90 degrees from the
exposing position to bring the magazine near the observer. The camera
shown in the mounting is the American hand-operated model (type M), in
which there is the same necessity for turning in order to manipulate the
bag magazine easily. The camera is shown in both exposing and plate
changing positions. An important detail of these mounts is a _safety
catch_, which must be fastened before the plane lands, in order to
prevent the shocks of landing from producing oscillations sufficient to
throw the camera out of the mount.
=Center of Gravity Rubber Pad Supports.=—Given a camera whose center of
gravity does not change during operation, a simple and entirely adequate
anti-vibration support is furnished by a ring of sponge rubber in the
plane of the center of gravity. But if provision has to be made for
oblique views or for adjusting the camera to the vertical, something
more elaborate is necessary.
[Illustration:
FIG. 90.—U. S. model deRam camera on anti-vibration mounting
adjustable for the angle of incidence of the plane.]
[Illustration:
FIG. 91.—U. S. deRam camera and mount installed in photographic
DeHaviland 4 (Fig. 100). Viewed from above the observer's cockpit.]
[Illustration:
FIG. 92.—U. S. type “K” film camera on universal mounting, vertical
position.]
[Illustration:
FIG. 93.—U. S. type “K” film camera on universal mounting, oblique
position.]
Mountings for the American deRam and for the Air Service film camera,
embodying the results of complete study of the anti-vibration problem,
are shown in Figs. 90, 92 and 93. Trusses carrying the cameras on pivots
rest on four pads of sponge rubber which are mounted on frames correctly
spaced ready for attachment to the cross-pieces of the airplane camera
supports. In the deRam (Fig. 90) the pivots, attached to the camera
body, permit it to be leveled fore and aft, to compensate for the
inclined position of the fuselage assumed at high altitudes or in some
conditions of loading. This will sometimes amount to as much as 11 or 12
degrees, which is very serious, since one degree causes (with an angular
field of 20 degrees) about one per cent. difference of scale at the two
sides of the plate. The film camera mounting carries the camera in a
conical ring, and is pivoted not only for vertical adjustment, but for
the taking of obliques as well (Fig. 93). These mounts transmit
practically no vibration.
[Illustration:
FIG. 94.—Tests on two types of camera mount: (_a_) Support at bottom
of camera; (_b_) support above center of gravity.]
A caution must be noted with regard to center of gravity mountings. Any
change in the camera, in particular the substitution of a short for a
long lens cone, must be made so as to cause no alteration of the
relative positions of the center of support and the center of gravity.
Either the short cone must be weighted, or additional supporting pivots
must be provided in the plane of the new center of gravity.
=The Italian and G. E. M. Mountings.=—These mounts (Figs. 49 and 59) are
similar in that the protection from vibration is furnished by an elastic
support at the bottom of the camera. Tests show that these two cameras
give very similar results, of the unsatisfactory sort to be expected
from this kind of mounting in view of the lessons of the last chapter on
the proper point of support. Fig. 94, _a_, shows a trace given by the
Italian mount. The permissible exposure, on the criterion adopted, is
very short with either mount, about 1/200 second.
=The Brock Suspension.=—This consists of a pair of frames into which the
camera is fitted by ball bearing pivots, so that it is free to move in
any direction (Fig. 60). In order to permit gravity to control the
direction of the camera, the point of support is made considerably (ten
inches) above the center of gravity. Air dash pots are provided for
damping the swings. As already explained, the pendular method of support
is in basic contradiction to the requirements for vibration elimination.
Tests of the Brock suspension, shown in Fig. 94, _b_, indicate it to be
of low efficiency in damping out the short period vibrations which are
responsible for poor definition.