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.
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.
CHAPTER XVI
THE INSTALLATION OF CAMERAS AND MOUNTINGS IN PLANES
Conditions to Be Met.—The characteristic difficulty in installing the airplane camera is that there is no place for it. After the gasoline supply, the armament, the wireless, the oxygen tank, the bombs, and other necessities are taken care of there is neither space available nor weight allowable. Where space may be found it will be inaccessible, or accessible only through a maze of tension and control wires; or it will be in a position where any weight will endanger the balance of the plane. Plane design has in fact been more or less of a conflict between the aeronautical engineer, who is designing the airplane primarily as a machine to fly, and the armament and instrument men, who look upon it as a platform for their apparatus. Lack of appreciation of the extreme importance of aerial photography resulted, during a large part of the war, in the camera installation being neglected until the plane was supposedly entirely designed, and even in production. At that stage the installation could be but a makeshift. Only in the later stages of the war, when plane design became a matter of coöperation between all concerned, were fairly convenient and satisfactory arrangements made for the camera. Always, however, the rapid succession of new plane designs, with various shapes of fuselage and details of structure, made camera installation in the war plane a matter calling for the greatest ingenuity.