CHAPTER XV

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.

=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.

=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.

=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.

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.

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.

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.