CHAPTER XIV
THEORY AND EXPERIMENTAL STUDY OF METHODS OF CAMERA SUSPENSION

General Theory.—In addition to the limitation of exposure set by the ground speed of the plane another limitation is set by the vibration of the camera. This may be caused either by the motor, or by the elastic reactions of the plane members to the strains of flight. Unlike the movement of the image due to the simple motion of the plane, movements due to vibration may be eliminated by proper anti-vibrational mounting of the camera.

The effect of vibration may show as an indistinctness of the whole image—this is its only effect with a between-the-lens shutter—or as a band or bands of indistinctness parallel to the curtain opening (Fig. 76). These are due to shocks or short period vibrations during the passage of the focal-plane shutter.

The obvious remedy for vibration troubles is to mount the camera on some elastic, heavily damping support, like sponge rubber or metal springs. Such a mounting should, however, be designed on sound principles derived from a proper analysis of the nature and effect of the possible motions of the camera. Otherwise, the vibrational disturbances may be increased rather than diminished by the camera mount. Such an analysis, based merely on general mechanical principles, shows that all motions of the camera are resolvable into six. These are three translational motions, namely, two at right angles in one plane such as the horizontal, and one in the plane at right angles to this (vertical); and three rotational motions, one about each of the above directions of translational motion as an axis (Fig. 77).

Brief consideration will show that only the latter—the rotational motions—are of any importance when the small displacements due to vibration are in question. To illustrate the negligible effect of vibrations which merely move the camera parallel to itself in any direction it is only necessary to imagine the camera moved parallel to the ground through a large distance, such as 10 centimeters. Now 10 centimeters motion of the camera at 3000 meters elevation means, with a 25 centimeter camera lens,

motion on the plate, which would be only a tenth the distance separable by a good lens. If we reduce this motion to the small fraction of a centimeter which vibration would actually produce, it is evident that such vibration is of absolutely no importance. Similarly, if we imagine the camera, under the same conditions, moved vertically with reference to the ground by ten centimeters, the scale of the picture would merely be changed by 1
12000 or by 1
1000 centimeter on a 12 centimeter plate, again quite negligible.

When we consider motions of rotation, however, the case is quite different. If the camera is mounted so that the effect of any vibration is to rotate it around a horizontal axis, this is exactly equivalent to rotating the beam of light from the lens so that it sweeps across the plate. Thus a millimeter displacement of the lens of the camera with the plate remaining fixed gives approximately a millimeter motion of the image. Consequently, a rotation producing only a fraction of a millimeter's relative motion of lens and plate during the period the curtain aperture is over a given point would cause fatal blurring—and the visible vibration of plane longerons and cross members is easily of half millimeter amplitude or more. Reduced to angular units it is easily shown that a rotation of one degree per second—which is quite slow as plane oscillations go—is beyond the limits of toleration. Translational motions of large amplitude may be allowed, but the mounting of the camera must not permit these translations to be at all different for different parts of the camera.

The proper way to eliminate vibrational effects is to devise a mounting that will transmit only the translational shocks or that will transform the rotational ones into translations. Platforms pivoted and cross-linked so as to be free to move only parallel to themselves (described in the next chapter) represent one attempt to reach this result. Quite the simplest and most scientific form of mounting to achieve this end is to support the camera solely in the plane of the center of gravity. The principle here involved is easily grasped if we note that when we jar a camera supported above or below its center of gravity, the effect is to start the camera vibrating with the center of gravity oscillating pendulum-like about the point of support. The closer the center of gravity to the center of support, the smaller the moment of the body about the latter point.

Experimental Study of Methods of Camera Support.—Conclusive evidence as to the merits of any system of camera mounting can be obtained only under conditions that eliminate the effect of other variables which may be equally efficacious in diminishing the effects of vibration, but which have only limited application. Very brief exposures—1
500 second and less—will, for instance, result in good pictures with almost any condition of vibration. Hence a sharp picture offers no proof of the merits of a camera mounting unless it is known that the exposure was no shorter than the limit set by the ground speed of the plane. In fact it may be said that the chief object of studying methods of camera suspension is to increase the allowable exposure to a maximum, thus lengthening the working hours and multiplying the useful working days for aerial photography.

The most satisfactory method of test yet developed is to fly over a light or a group of lights on the ground with the camera shutter open. In the first use of this method, which originated in the English Service, such flights were made at night, but later it was found that good results could be got by placing the lights in a forest and making the tests when the sun was fairly low. One of the group of lights must be periodically interrupted, at a known rate, to furnish the time intervals.

Some characteristic “trails” obtained by this method of test are shown in Fig. 78. The first trail is that given by a camera rigidly fastened to the fuselage. The second and third show hand camera trails, made by an inexperienced and by an experienced observer, respectively. They show by comparison with the other figures that the human body is an excellent block to vibration, but in unskilled hands a poor check to rapid erratic (probably rotational) motions of the camera. The fourth is the trail given by a camera supported by gimbals bedded in sponge rubber accurately in the plane of the camera's center of gravity. Other trails are shown in the next chapter in connection with the description of practical camera mountings. Clearly the best suspension is that giving the smallest amplitude of displacement during the interval of time covered by an average exposure. It is, in fact, possible to determine from these trails the permissible exposure for any assumed permissible blurring of the image. The rigid mounting trail indicates very bad conditions, calling for literally instantaneous exposures. The center of gravity trail, at the other extreme, shows practically no limitation of exposure in so far as vibration is concerned, thus bearing out the theoretical conditions above discussed. An interesting conclusion from these experiments is that a rapidly running motor gives less harmful vibration than a slow one, although in the war it was a common practice to throttle the motor before exposing. As might be expected, the greater the number of cylinders, the shorter the period and the smaller the amplitude of the vibration.

Pendular Camera Supports.—The design of the camera support may be approached from a different standpoint, namely, with the aim of carrying the camera so that it will tend to hang always vertical. In mapping this is of fundamental importance. It is, indeed, a question whether aerial mapping will ever be worthy of ranking as a precision method unless the camera can be mounted so that its pictures are taken in the horizontal, undistorted position.

The simplest way to hold the camera vertical is to mount it on gimbals, with its center of gravity below the point of support. When so mounted the camera swings as a pendulum. Delicacy of response to variation of level is obtained by leaving a considerable distance between the center of gravity and the center of support. Oscillation about the vertical position is to be prevented by some system of dash pots or other damping. A suspension of this kind is furnished with the Brock film camera (Fig. 60).

It will be seen at once that the relation of center of gravity to center of support called for here is in direct contradiction to the requirements for eliminating vibration. Either one requirement or the other must be sacrificed, or else a compromise made in which neither delicate response to inclination of the plane nor fully satisfactory freedom from vibration is attained. This is a very serious objection to the pendular support. But the really vital objection to the pendular support is that it performs its function only very partially. It is entirely satisfactory only under conditions of steady flying, as in a uniform climb or glide, with the plane tail or nose heavy, or in flying with one wing down. As soon as we introduce any acceleration, as in making a turn, the camera follows the plane and not the earth.

It is true that mapping photography is done from a plane flying as level as possible, and that except under bad air conditions it holds its course with very little turning, if handled by a skilled pilot. Nevertheless, a surprisingly small deviation from straight flying causes quite serious variations from the vertical. It is of interest to calculate how large may be the horizontal accelerations that accompany swervings from a straight course which one might think insignificant. For instance, consider the horizontal acceleration due to a turn having a radius of a kilometer when the plane is moving at 100 kilometers per hour. If a is the acceleration, v the velocity of the plane, and r the radius, we have from elementary dynamics that

Substituting the values chosen, we have—

The acceleration of gravity being 9.80 meters
sec² we have that the ratio of the horizontal acceleration to the vertical is

This is the tangent of the angle of deviation from the vertical, from which the angle turns out to be about 4½ degrees, a very considerable error, rapidly multiplied as the speed of the plane is increased. It is, indeed, open to question whether the average deviations from the vertical are not apt to be less with the camera rigidly fixed to the plane, if guided by a skilled pilot who will hold the ship level at the expense of “skidding” the slight turns he must make to hold his direction.

Gyroscopic Mountings.—The ideal support for the aerial camera will undoubtedly be one embodying gyroscopic control of the camera's direction. By proper utilization of the principles of the gyroscope it is to be expected that not only can the camera be maintained vertical, but it may be supported anti-vibrationally as well. At the present time the problem of gyroscopic control is in the experimental stage, so that only the elements of the problem and the possible modes of solution can be laid out.

The gyroscope consists essentially of a heavy ring or disc rotating at a high speed on an axis free to point in any direction (Fig. 79). If mounted so that the axes of the supporting gimbals pass through the center of gravity of the rotating disc, the result is a neutral gyroscope. Its characteristic is that its axis maintains its direction fixed, but this fixity is with respect to space and not with respect to the gravitational vertical. Consequently, as the earth revolves the inclination of the gyroscopic axis changes with respect to the earth. In latitude 45° this change is approximately a degree in five minutes. Furthermore, the action of friction in the supports, which can never be entirely eliminated, also acts to slowly alter the direction of the gyroscopic axis. Therefore, unless some erector is applied even the gyroscope will not perform the task required of it.

Before discussing possible forms of erectors it may be noted in general, first, that these must depend upon gravity; second, that such being the case, they must respond to the resultant of gravity and any acceleration, that is, to the apparent or pseudo-gravity. As already seen, this pseudo-gravity, during a turn, is exactly what limits the usefulness of the pendular support, and necessitates recourse to the gyroscope. The problem thus becomes one of making an erector-gyroscope combination which will respond to true gravity and not to pseudo-gravity.

In general this problem would be insoluble, since there is no difference in the nature of the acceleration of gravity and that due to centrifugal force. A way out is offered, however, by the fact that true gravity acts continuously and at a small angle to the axis of the gyro, while the components which cause the pseudo-gravity are of short duration, liable to rapid changes of direction, and, on a turn, act at a large angle. What we require, therefore, is an erector which will respond slowly but surely to the average acceleration, which is downward, but too sluggishly to be affected by the shorter period accelerations due to turns or rolls. Slowness of response is a matter of the erecting forces being small and of the mass and angular velocity of the gyro disc being large. The success of the compromise called for depends on the relative times taken for the gyroscope to tilt seriously from the true vertical, due to the causes above mentioned, and for the average turn or roll. Fortunately the former is a matter of minutes, the latter of seconds or at the worst of fractions of a minute. More than this, since the roll or turn is apt to be of much greater angle than any normal deviation of the gyroscopic axis from the vertical in the same time, we are offered the possibility of some device for filtering out the deviations which alone are to effect the erector. For instance, by shunting the restoring force whenever it is called upon to act through more than a predetermined small angle.

As to the method of erecting the gyroscope, its characteristic property must be kept in mind. This is that the axis does not tilt under an applied force in the direction it would if the gyro were not rotating, but around an axis at right angles to that of the applied couple. Thus in Fig. 79, if a weight is attached as shown, the disc does not incline downward toward the weight, around the axis Y, Y´, but precesses about the vertical axis Z, Z´. Some means is therefore needed to translate the pull which any gravitational control, such as a freely swinging pendulum, would give, into a pull with at least a component at a finite angle to this.

In the Gray stabilizer several metal balls are slowly rotated in a tray above the center of gravity of the gyroscope. Specially shaped grooves or compartments limit the freedom of motion of these balls so that when the gyro is inclined the balls travel at different distances from the center on the ascending and descending sides. By this scheme a couple is produced about the axis through the center and the low point of the disc, which tilts the apparatus to the gravitational vertical. In an alternative form the balls are carried past the low point by their momentum and are prevented from returning by the walls of the containing compartment, which have meanwhile been advanced by the rotation of the erector as a whole. The net result is to shift the center of gravity of the system of balls in the proper direction to erect the gyro. The rectifying action is purposely made quite slow so that the displacements of the balls due to pseudo-gravity will be averaged out.

In a design due to Lucian, small pendulums work through electric contacts to actuate solenoids which in turn move small weights in the appropriate directions to give the desired tilt. Response is made fairly quick and delicate, and pseudo-gravity, due to turns and rolls, is rendered inoperative by the contacts breaking whenever the pendulums swing more than three or four degrees. This can only happen if they move too quickly for the erecting forces to act, reliance being here placed on the characteristic differences of action in respect to time of real and pseudo-gravitational forces.

Besides the neutral gyroscope as just considered there is the pendular or top type, in which the center of gravity is not in the plane of the supports. In general this type depends on a couple resulting from the gravitational pull and the inevitable friction of the supports to slowly tilt the axis to the gravitational vertical. This type is slower to respond than the designs in which a definite couple in the proper direction is provided and it reaches the true vertical only through a circuitous path.

Three methods of controlling a camera by a gyroscope are suggested. One is to fasten the gyroscope rigidly to the camera and mount the whole system on gimbals. A second is to mount both camera and gyro side by side on gimbals, linking the two so that the camera is moved parallel to the gyro (Fig. 80). A third method is to utilize the gyro to make electric contacts to operate motors which in turn move the camera.

Considerable weight and space are required for a gyroscope capable of stabilizing a camera. The rotating disc should be about half the weight of the camera, and with its mounting may be expected to double the room required for the camera alone. Motive power for maintaining the gyro in continuous rotation may be supplied by an air blast, or the gyro may be made up as an induction motor—the latter necessitating an alternating current supply.

In view of the space and weight limitations in a plane it is a question still to be decided whether it is more economical to stabilize the camera or to stabilize an inclinometer and photograph its indications simultaneously with the release of the shutter which takes the aerial picture.

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. 89a), but when the bell cranks were attached near the center of gravity, highly successful results were obtained (Fig. 89b). 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.

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.

The problem was met in part by constructing both cameras and mountings in sections, to be laboriously wormed in through inadequate apertures, in part by later structural changes in the planes, such as the substitution of veneer rings or frames for the tension wires. In certain cases the rear cockpit controls were omitted, thereby freeing accessible and often adequate space for the larger cameras. Rear controls were never used in the German planes, so that their standard practice was to carry the camera forward of the observer. This, together with the general restriction to the 13 × 18 centimeter size plate, made the installation problem less difficult in the German aircraft than in the Allied.

Practical Solutions.—An important feature of camera installation has already been mentioned, but may well be repeated for emphasis. The camera and its anti-vibration mounting should always be considered as a unit, and should be so designed that simple bolts or straps will suffice to fasten it in its place in the plane. Even should the spacing of the structural parts of the plane not correspond to that anticipated by the mounting design, the ingenuity of the man in the field may be depended upon to make the necessary alterations or additions to the plane. The design of the camera suspension itself cannot, however, be left to uneducated ingenuity.

Assuming the camera and mounting supplied, the next step—a very difficult one—is to insure uniformity in the structures to be built into the planes for the purpose of supporting the camera mountings. With this uniformity must, however, be combined the greatest possible flexibility to provide for various designs of cameras.

In the English service the standard camera installation consists of wooden uprights with cross bars athwart the plane, adjustable as to height (Fig. 95). A distance between the cross bars of 13¼ inches has been standardized, and all camera cradles and mountings are notched or otherwise spaced to fit this dimension. The installation adopted in the American planes is similar, but with a distance of 16 inches between cross bars. These uprights and cross bars are ordinarily situated in the bay behind the observer, but can be placed in any available space. Fig. 83 shows, in a model bay, the arrangement of uprights and cross bars in the American DH 4, with the L camera in place in its cradle. It is just possible to introduce camera and cradle separately from the observer's cockpit through the tension wires, and, by uncomfortable reaching, magazines may be changed.

A step in advance is made when the top tension wires and superstructure are replaced by a rigid frame with an opening large enough to admit the entire camera and mounting. When this is done considerably larger cameras may be accommodated in the same sized bay, as shown in Fig. 96. A further advance, from the standpoint of accessibility and convenience of installation, follows when the tension wires between observer's and camera bay are replaced by a ply-wood ring, as shown in Fig. 97. Here the only serious limitations are those due to the vertical height of the camera, and of course its weight.

Openings for the lens to point through are simply provided in the fabric covered aircraft, by cutting through the canvas and stiffening the edge of the hole by wire. Tension wires are often in the way. They may either be disregarded, since they merely cut off a little light, or replaced in part by metal rings, as shown in Fig. 96. In veneer covered fuselages the hole must of course go through the wood. This may be undesirable, since the veneer is depended on to furnish structural strength, a point which further emphasizes the importance of the photographic requirements being thoroughly considered while the plane is being designed.

Single seater or scout planes do not lend themselves to the insertion of such standardized uprights and cross-pieces, because of their small size and the common utilization of all space inside the fuselage for gasoline tanks and control wires. Some French scouts, whose fuselages are very wide, due to the rotary engines, have been fitted with compartments for contemplated automatic film cameras. The most commonly used camera in the single seater was, however, the Italian 24-plate single-motion apparatus (Fig. 49). This camera and its carrying tray occupy very little lateral space and have in actual practice been carried beneath the seat or pushed up through an opening in the bottom of the fuselage under the gasoline tank. Whatever criticism may be made of the adequacy of the mounting, it must be said that the camera, as used, is perhaps the most eminently practical of all developed in the war, as its use on scouts testifies.

Fig. 96.—U. S. type “K” film camera on bell-crank mount, in camera bay of DeHaviland 4. Veneer frame at top of bay in place of usual cross-wires.