As with several other designs, the completion of the working model of this camera occurred after agreements had been reached by the Allies, as to plate size, standard lens cones, and other features, not easily incorporated in it, thus making manufacture inadvisable. The validity of the design for peace-time work is, of course, not affected by this fact.

The deRam Camera.—The only completely automatic plate camera actually produced commercially before the end of hostilities was the French model deRam (Fig. 54). Its plate-changing action has already been described in connection with the American semi-automatic model (Figs. 52, 90 and 91). It differs from the American model in the shutter, which is of the self-capping variety, carried on a rising and falling frame; and in the exposing mechanism. The latter embodies a clutch whose point of attachment to a uniformly rotating disc in the camera is governed through a Bowden wire, whereby the interval between the plate-changing operation and the shutter release is varied. The intervals are indicated by figures on the dial to which the observer's end of the Bowden wire is attached. The source of power for the camera is a constant speed propeller. Complete semi-automatic operation is not possible, as an interval of 1 to 2 seconds elapses between the time a single exposure is called for and its occurrence. No arrangement is provided for hand operation.

It will be noted that while this camera is a true automatic apparatus it does not meet even a majority of the requirements listed above as found desirable by experience. There exists a great opportunity for designing and developing an entirely satisfactory automatic plate camera—provided it is agreed that anything more than semi-automatic operation is ever advisable for plates.

The weight of the glass and the sheaths in the plate camera forms its most serious drawback. This weight must be reckoned at least three quarters of a pound for each 18 × 24 centimeter plate. Consequently, with the use of these large plates, and with the demands for ever increasing numbers of pictures to be taken on long reconnaissance flights, a serious conflict arises between the weight of the photographic equipment and the carrying capacity of the plane. Among plate cameras probably the most economical in weight is the deRam. It carries fifty 18 × 24 centimeter plates, and has a total weight of approximately 100 pounds. An advance to 100 or 200 plates—not feasible in the deRam construction—even if we assume the lightest possible magazines, would bring the weight of camera and plates to 150 or 200 pounds, which would be detrimental to the balance and would seriously infringe on the fuel carrying capacity and ceiling of any ordinary reconnaissance plane.

Early and persistent attention was therefore paid to the possibilities of celluloid film in rolls, as used so widely in hand cameras and in moving picture work. The two great advantages of film would be its practically negligible weight (approximately one-tenth that of plates, not including sheaths) and its small bulk, which would permit the greatest freedom in the development of entirely automatic cameras to make exposures by the hundreds instead of by the dozens. Certain disadvantages were foreseen at the outset: the difficulty of holding the film flat and immune from vibration in the larger sizes; the difficulty of quickly developing and drying large rolls; the question whether as good speed or color sensitiveness could be obtained in sensitive emulsions when flowed on a celluloid base as on glass. Early trials revealed a further problem to solve: how to eliminate the discharge of static electricity occurring at high altitudes, especially when the weather is cold.

As far as camera construction is concerned the chief problems are to hold the film flat, and to eliminate static.

Methods of Holding Film Flat.—Several means have been proposed and used for holding the film flat. Disregarding mere pressure guides at the side, which are suitable only for small area films (up to 4 × 5 inch), the successful means have taken three forms: pressure of a glass plate, pressure of the shutter curtain, and suction. A glass pressure plate can be used in either of two ways; the film may be in continuous contact with it or may be pressed against its surface only at the moment of exposure. The advantage of this first method lies solely in its mechanical simplicity; its disadvantage in the likelihood of scratches or pressure markings on the film. Where a glass plate is used there is always the chance of a dust or dirt film accumulating, or of the condensation of moisture, to impair the quality of the negative. There is, moreover, an inevitable loss of light (about 10%), together with some slight distortion, due to the bending of the marginal oblique rays through the thickness of the glass. In cases where a filter would normally be employed, the loss of light is minimized by using yellow glass for the plate, so that it serves for filter and film holder as well.

Pressure of the shutter curtain is utilized in the Duchatellier film camera by furnishing the edges of the curtain aperture with heavy velvet strips, whose light and gentle pressure during the passage of the shutter holds the film against a metal back. In many ways this is the simplest film-holding device; it occasions no loss of light, and needs no mechanical movements or external accessories, such as are called for in the suction devices next described. There is always danger of markings on the film, if the velvet is not of the right thickness and softness, and the operation and speed control of the shutter are necessarily complicated by the additional frictional load.

Suction of the film against a perforated back plate has been found a very successful means of securing flatness. Suction at the moment of exposure may be produced by the action of a bellows, which has been compressed beforehand by the camera-driving mechanism. Continuous suction can be produced either by a continuously driven pump, or by a Venturi tube placed outside the fuselage. The Venturi tube (Fig. 55) consists of a pipe built up of two cones, placed vertex to vertex, to form a constriction. When air is forced through this at high velocity suction is produced in a small diameter tube taken off at the constriction. A suction of two centimeters of mercury, acting through holes about one centimeter equidistant from each other in the back plate, has been found adequate to hold flat a film 18 × 24 centimeters.

One merit of suction applied only at the moment of exposure is that the film-driving mechanism does not have to work against the drag of the suction. Continuous suction, on the other hand, gives a longer opportunity for flattening out kinks in the celluloid, and easily permits movement of the film during the exposure, either for the purpose of permitting a longer exposure or for the purpose of preventing distortion due to the focal-plane shutter. A disadvantage of continuous suction is the production of minute scratches on the celluloid surface as it drags over the suction plate. These are ordinarily too small to cause trouble, but may show up when printing is done in an enlarging camera.

Static discharges are produced by the friction of the celluloid against the pressure back or other surfaces with which it comes into contact. They show in the developed film as branching tree-like streaks (Fig. 56) and in cold dry weather may be numerous enough to ruin a picture. The discharges are noticeably less frequent with film coated on the back with gelatine (“N.C.”), but the extra gelatine surface is extremely undesirable. When handled by developing machines, as large rolls must be, this back gelatine surface becomes scratched and bruised in a serious manner. Plain unbacked film is much to be preferred if the static can be obviated.

To avoid static, it is necessary to provide for the immediate dissipation of all acquired electrical charges. Experiments made by the United States Air Service have shown that nothing is so good as rather rough cloth, thoroughly impregnated with graphite, held in close contact with the celluloid during as great a portion of its travel as possible. In the United States Air Service film camera which uses suction through a perforated back plate, the plate has been covered with thin graphited cloth, and similar cloth sheets are pressed against the film rolls by sheets of spring metal (Fig. 65). In cameras with this equipment no trouble has been experienced with static.

Representative Film Cameras.—The English F type (Williamson). This is one of the earliest cameras designed for film, as is indicated by the nature of the power drive, which presupposes that the camera is to be carried on the outside of the fuselage. Its essential features are shown in Figs. 57 and 58. It consists of a rectangular box with a cone at the front on which is mounted a propeller, intended to be rotated by the wind made by the motion of the plane. This drives, through a governor controlled friction clutch, a train of gears which draws the (5 × 4 inch) film across the focal plane, sets and exposes the shutter at regular intervals.

Above the camera, supported on a tripod, are a compass and altimeter, both recording on a single dial, illuminated from below by the light reflected from a circular white disc painted on top of the camera. An image of the dial is thrown on a corner of the film by a lens, whose shutter is actuated in synchronism with the main focal-plane shutter. No special means are provided for holding the film flat. Special film with perforated edges is used.

The camera was designed for mapping work on the Mesopotamian and other fronts where no maps at all existed.

The Duchatellier camera is essentially a film magazine to fit on the standard French deMaria camera bodies, of the 18 × 24 centimeter size. In its simplest form it embodies a shutter (the regular focal-plane shutter of the camera being removed) and a film-moving mechanism, both actuated by a single motion of the hand. Automatic and semi-automatic operation are accomplished by an auxiliary mechanism to which Bowden wires from the hand lever are attached. The motive power is an air propeller. Variation of speed is obtained by changing the point of contact of a roller on a friction disc, the disc being directly connected to the propeller shaft, the roller to the camera drive shaft.

The most distinctive features of the Duchatellier camera is its use of the focal-plane shutter to hold the film flat during the exposure. As already explained, this is accomplished by pressure, velvet strips on the shutter edges keeping the film close against the back plate. The return of the shutter curtain to the “set” position is accomplished by locking it to the film by perforating points, so that it is pulled across as the film is wound. This introduces between each pair of pictures a strip of tremendous over exposure, as wide as the curtain opening. A fixed-aperture variable-tension shutter is used. The magazine carries a roll of film long enough for 200 exposures, feeding the long way of the picture. When film needs to be changed in the air, this is done by changing the entire magazine, including its shutter.

The G. E. M. camera (Fig. 59) is a very light self-contained clock-work-drive camera taking 36 pictures six inches square. The film is unrolled from a small-diameter feeding roller on to a large-diameter receiving roller to which the driving mechanism is attached. By this means approximately equal spacing of pictures on the film is assured. The film is held flat by continuous contact with a glass plate, which is made of yellow glass, so that it serves at the same time as a color filter. The lens—of 8 to 12 inch focus—is equipped with a single speed between-the-lens shutter. The operation of the camera is entirely automatic. The interval between pictures is controlled by varying the clock-work speed, through a lever on the outside of the camera box. Protection of the camera from vibration is sought by supporting it on four spring cushions mounted on a solid frame, to which the camera is held by spiral springs attached to its sides.

The Brock Film camera (Fig. 60) is an entirely automatic, very compact self-contained camera, taking one hundred 4 × 5 inch pictures. The motive power is clock-work, regulated in speed by an escapement controlled by a flexible shaft carried to a dial which may be fastened to the instrument board or to some other convenient part of the plane. The lens is 6, 12, or 18 inch focus. The shutter is of the fixed-aperture variable-tension type, of long travel, and with a flap behind the lens for covering during the setting period. None of the special means above described for holding the film flat are provided. A metal plate resting on the back, and a flat metal frame in front with a 4 × 5 inch aperture, are considered sufficient check on the excursions of the small-sized film. A ball bearing double pivoted frame serves to support the camera in a pendulous manner, permitting it to assume a vertical position after tilting. Damping of oscillations and vibration is arranged for by two pneumatic dash pots.

The German film mapping camera, shown in Fig. 61, is distinguished by a number of special features. The size of the pictures, 6 × 24 centimeters, is unusual. It has its advantages, however. Since the short dimension is in the line of flight, the maximum width of field covered by the lens is utilized (Fig. 17). This of course necessitates a larger number of exposures to complete a strip, which is perhaps an added advantage, since the narrower the individual pictures the better the junctions will be, especially if large overlaps are made. This proved to be the case with captured German mosaics. Difficulty is experienced in making overlaps on a turn (Fig. 62), but this is not a vital objection. The shutter has a fixed aperture, narrower at the center than at the ends, to compensate for the falling off in illumination away from the center of the lens. No safety flap is needed because the curtain moves in opposite directions on successive exposures, thereby also compensating for shutter distortion, as has already been discussed. Shutter speed is controlled by varying the tension of the actuating spring.

The camera is driven by an electric motor, connected to a set of gears, whose shifting provides for speed variation. The film is moved by rubber rollers which are cut away for part of the circumference, allowing the film to stand still until they bite again. A yellow glass pressure plate holds the film during the exposure and serves as color filter also (Fig. 63). An electric heater is provided near the shutter, as in all the later German cameras.

United States Air Service automatic film camera—Type K (Figs. 64, 65, 92, 93, 98, 99). This is an entirely automatic camera, manufactured by the Folmer and Schwing Division of the Eastman Kodak Co., taking 100 pictures of 18 × 24 centimeter size at one loading. As with all the American cameras of this size, it uses the standard lens cones of any desired focal length. The camera proper consists of a compact chamber in which the film rollers are carried at each end forward of the focal plane, the shutter lying between. In consequence of this arrangement the vertical depth of the camera is the absolute minimum—short of decreasing the length of the optical path by mirror arrangements—making it possible to suspend the camera diagonally in the American and British planes, for taking oblique pictures.

Flatness of the film is secured by a suction plate covered with graphited cloth and connected with a Venturi tube. The top cover is removed for re-loading. The shutters on the first cameras of this type are of the variable-tension fixed-aperture design, though later ones have the variable-aperture curtain controlled by an idler, as used in the American deRam. An auxiliary curtain shutter serves to cap the true shutter during setting.

The operation of the film driving mechanism is comparatively simple. It consists of a train of gears, driven by a flexible revolving shaft attached to some separate source of power capable of speed variation. The action of the gears is to move the film, set the shutter and then expose it; in the earlier cameras with the film continuously moving. In the first cameras constructed the space between the pictures varies as the film rolls up, due to the increasing diameter of the roll. In later cameras the film roller is disengaged from the gears just before the shutter is tripped, so that the film stands still during the exposure, and is then re-engaged at a new point on a ratchet wheel governed by the diameter of the receiving roll, whereby the pictures are equally spaced. In all the cameras, punch marks made at the time of exposure enable the limits of the picture to be detected in the dark room by touch.

Variable speed is arranged for in any one of several ways. For peace-time uses a turbine attached to the side of the plane is simple and positive, and, provided it is made of sufficient size—which is not the case with the one shown in the Figure—will give adequate speed regulation upon varying the aperture through which the air enters. The Venturi tube may be carried upon the same mount, or a small rotary pump can be attached on the same shaft. Where the high wind resistance of the turbine is an objection the camera is driven electrically, by a motor acting through the intermediary of a variable speed control described in the next chapter (Fig. 68).

The camera weighs complete about forty pounds, and the film rolls about four pounds. The latter can be changed in the air without great difficulty provided the camera is mounted accessibly and so that the top may be opened.

As long as circumstances permit, hand operation still remains the most reliable and satisfactory method of driving a camera. It is always available, can be applied to just the amount desired, and at the time and place needed. For instance, in a magazine of the Gaumont type (Fig. 40), what is needed is the periodic application of a very considerable force rather quickly, and while this can be done quite simply by hand, no mechanism has even been attempted to go through this same operation automatically. Instead, the fundamental design of automatic magazines has been made along other lines calculated to utilize smaller forces more steadily applied.

It must be granted, however, that for war planes, and particularly for single seaters, cameras should be available which are capable of operating semi-automatically or automatically. This necessarily means the employment of artificial power, whose generation, transmission to the camera and control as to speed present a mechanical problem of no small difficulty.

Available Sources of Power.—The sources from which power may be drawn on the plane are four, although the various combinations of these present a large number of alternative approaches to the problem. These sources are:

These may first be considered largely from the descriptive standpoint, leaving questions of performance and efficiency for separate treatment.

Power may be derived directly from the engine through a flexible shaft, similar to that used for the revolution counter. This source of power is inherently the most direct and efficient, since the engine is the seat of all the lifting and driving energy of the plane. There is no loss through transformation into other forms of energy, such as electrical; or by the use of more or less inefficient intermediary apparatus, such as wind propellers. Against the direct drive of the camera from the engine may, however, be urged that the usual distance between engine and camera is too great for reliable mechanical connection, as by flexible shafting. Objections also arise from the standpoint of speed. This cannot be controlled by the camera operator; and varies over too wide a range, as the engine changes from idling to full speed, to fit it for automatic camera operation. The first objection may be met by that combination of methods of power drive which consists in transmitting the power electrically; that is, by letting the engine operate a generator from which cables run to a motor close to the camera. This method, of course, sacrifices efficiency, and it breaks down when the engine speed drops below the speed necessary to generate the requisite voltage. This defect may in turn be met by floating in storage batteries, which brings up the whole question of electrical drive, to be treated presently. While use of the engine for direct drive or for generating electric current has not been adopted in the American service, it is known that some German planes were supplied with electric current in this way.

Coming next to the wind motors, these possess one very great merit: they utilize a motive power that is always present as long as the plane is in motion through the air. On the other hand, the process of using the main propeller of the plane to pull another smaller propeller through the air appears a roundabout way to utilize the driving power of the airplane engine. Yet on the whole it is probable that some form of propeller or wind turbine is the simplest and most convenient device we have for the operation of airplane auxiliaries. As long as the amount of power required is small, such inefficiency as is inherent in its use is offset by its convenience and reliability. An advantage of the propeller is that its speed is almost directly proportional to that of the plane through the air, a desirable feature in automatic cameras provided the proportionality is under control. Yet it is just in this matter of varying the speed at will that the propeller presents difficulties, to be met only by additional mechanisms for gearing down or governing. Propellers have the practical disadvantage that they present an easily bent or broken projection to the body of the plane (Figs. 83 and 84). The strength of small propellers for operating auxiliaries is never so much in question with reference to their resistance to whirling and thrust of air as it is to their ability to withstand the inevitable knocks and careless handling that will fall to their lot. The propeller bracket is just what the pilot is looking for to scrape the mud off his boots before climbing in.

The wind turbine has the advantage over the propeller that its speed can be varied rather simply by exposing more or less of its face to the wind. A turbine fitted with an adjustable aperture for admitting the wind is shown in Fig. 64, in connection with the type K automatic film camera. The turbine has the advantage of being compact and lying close against the body of the plane. In the form figured, altogether too much head resistance is offered—just as much for low as for high speeds—but with proper design this need not be the case. It is, moreover, quite too small to give the needed speed regulation, as it only begins to operate near its full opening.

Spring motors have the very real advantage that by their use the camera can be made entirely self contained. The simplest application of the spring motor would be to the semi-automatic camera, where no close regulation of speed is required. In such a camera the operation of exposing the shutter would release the spring, which would then change the plate or film and re-set the shutter, repeating this operation as long as the spring retained sufficient tension. Small film hand-cameras of this type, using self-setting between-the-lens shutters, have been designed, though not for aerial work. The possibilities of using springs as motive power in semi-automatic cameras have not apparently been seriously considered.

When a spring motor is used for automatic camera operation it at once becomes necessary to add to the motor an elaborate clock mechanism for controlling and regulating its speed of action. Springs are much better fitted for giving power by quick release of their tension than by slow release, and the necessary clock mechanisms for their regulation become very heavy, as well as complicated and delicate, when they are made large enough to do any real work. For their repair they require the services of clock makers rather than the usual more available kind of mechanic.

Coming next to electric motors, we meet with a source of power of very great flexibility both in its derivation and in its application. If a source of electric current is already provided for heating and lighting as it is on the fully equipped military plane, and if it has sufficient capacity to handle the camera, its use is rather clearly indicated, irrespective of how efficiently or by what method it is produced. Especially is this the case, from the standpoint of economy and simplicity, if a propeller-driven generator is the source of current, and the alternative power drive is an additional propeller for the camera. If, on the other hand, the camera must have its own source of electric power, the advantages and disadvantages must be closely scrutinized. In this case either a generator must be provided, or resort be made to storage batteries, or a combination of the two.

Ruling out a special propeller-driven generator, we are left with either the generator driven from the engine or the storage battery. Inasmuch as storage batteries are practically indispensable with generators, in order to maintain the voltage constant at all speeds, it is on the whole advisable to rely upon batteries alone. An advantage of their use is that the power plant is entirely within the plane: All projections such as propellers are avoided. Another merit is that the power is drawn upon only as needed. Against storage batteries is their weight, the need for frequent charging, and their loss of efficiency at low temperatures—a loss so serious with those of the Edison form as to preclude their use.

When once the source of electrical energy is decided upon, its method of application needs to be considered. Here we meet at once the peculiar merit of electrical energy, namely, the ease and convenience with which it may be transmitted. All we need is a pair of wires, led to any part of the plane by any convenient route and connected by simple binding posts. It may with equal ease be turned on or off by merely making or breaking a contact with a switch. For operating semi-automatic cameras this feature may be utilized in the interest of economy, if the power is automatically turned off as soon as the plate-changing operation is finished. Exceptionally reliable make and break contacts are necessary to insure the success of this latter scheme.

Two methods of transforming the supply of electrical energy into mechanical motion are available. The first is by the use of a solenoid and plunger. This is a device practically restricted to semi-automatic cameras, in which the operation consists of a straight to-and-fro motion, initiated at the will of the operator. It has been used little if at all. The second motion is the continuous rotary one secured by the use of an electric motor. This motion is the most practical one for the continuous operation of any mechanism, but on the other hand requires that the imposed load be reasonably uniform at all times through the cycle of operations. Assuming that the camera mechanism is of this character, the motor may be attached directly to the camera, or if it must be so large as to cause danger by vibration, it may be connected through a flexible shaft. This use of an electric motor is very practical for semi-automatic cameras such as the “L” or the American deRam, in planes supplied with a suitable source of current.

When it comes to entirely automatic cameras, where uniform and regulatable speed is required, as in making overlapping pictures for mapping, the electrical drive is not so convenient. The shunt-wound motor runs at nearly constant speed, while the series-wound motor in which the speed can be regulated by the interposition of resistance, has nothing like a sufficient range of variation for the purpose (at least five to one is imperative) before it fails to carry the load. Hence we must either incorporate in the camera some mechanism for varying the interval between exposures while the speed of the motor remains constant, or introduce an auxiliary device to effect the required transformation in speed. If we do use an auxiliary device the train of apparatus, consisting of battery (or generator), motor, speed control and camera, is altogether too long; it is apt to cause annoying delays in connecting up in an emergency, and it offers an excessive number of chances for break-down.

Performance and Efficiency Data.—The first step in deciding upon methods of power drive, and indeed in deciding whether power drive is feasible at all, is to assemble definite data as to the power required to drive representative cameras. Approximate figures for some of the cameras described in previous chapters are:

These requirements—not exceeding ⅒ horse power—are insignificant in comparison with the total of 100 to 400 horse power available for all purposes from the plane's engine.

Propeller characteristics. Data on the performance of small propellers are somewhat meagre. However, the results of the rather extensive researches on large ones, suitable for driving planes, may be applied, with proper reservations, to give a fair guide to the study of the application of small propellers for driving plane auxiliaries.

The first factor to be considered is the thrust or head resistance offered by a propeller to motion through the air. This varies as the square of the velocity, as the density of the medium, and as the area of the body projected normally to the wind, the formula being

where T = thrust, d = density, a = area, V = velocity. Data on the L camera propeller are shown in Fig. 66, where its thrust both when free and when loaded with the camera is given, as well as that of a solid disc of the same diameter as the propeller. For this propeller, which is double-bladed, and six inches in diameter, cda = .000275 with the load on. The total thrust amounts to only about three pounds when the plane velocity is 100 miles per hour. The head resistance of the whole plane is a matter of hundreds of pounds, so that the propeller resistance is quite negligible.

The next factor is the speed of revolution of the propeller, expressed in revolutions per minute. This varies with the design—the number of blades, their area, and pitch. For a given design the speed of revolution is directly proportional to the speed of motion through the air, and to the density of the air. Representative data for the L camera propeller are shown in Fig. 67. It will be noted that the speed goes up to 8000 for 120 miles per hour air speed. This illustrates the necessity for great strength to withstand centrifugal force. Propellers should be constructed of tough material, and subjected to whirling tests up to speeds considerably in excess of any the plane will attain in any maneuver. At low speeds the linear relationship fails, as a critical velocity is reached—about 3500 r. p. m. for this propeller—where it refuses to turn.

The fact that the speed of the propeller depends on the density of the air has an interesting corollary, which is that a propeller adequate at low altitudes will fail at high ones. The density of the air varies with altitude according to the following figures:

If we take the r. p. m. at 90 miles per hour at sea level as 6000, then at the above altitudes the speeds will be 4300, 3500, and 3000, respectively. The last figure is below that for which this size of propeller stalls with its normal load, as noted in the last paragraph. Consequently, if flying is to be done at these altitudes a larger propeller must be carried, which will still deliver enough power at the lower density.

The next factor to be considered is the power furnished by the propeller. As a representative figure may be quoted the performance of the L propeller. This gives 27 watts at 3600 revolutions per minute (56 miles per hour). From this figure the performance of other propellers may be deduced from the basic laws, which are: that the power varies as the density of the medium and as the cube of the velocity (assuming constant efficiency). Since the power delivered by the six inch diameter L propeller is already adequate at 60 miles per hour, the necessary dimension to function satisfactorily at 100 miles per hour would need to be only a little more than three inches, except for the desirability of a safety factor for high altitudes and low air densities.

The efficiency of the propeller is defined by the relation—

The denominator of this fraction is the thrust times the velocity, for which the curves of Fig. 66 supply us data for the L propeller. Using the figures 3600 r. p. m., 56 miles per hour, and 27 watts, we find the efficiency to be about 50 per cent. This increases with the velocity, with a possible upper limit of 70 to 80 per cent. Since the main propeller of the plane is not over 80 per cent. efficient we have at most an efficiency of 64 per cent. in using a propeller drive, as compared with taking the power directly off the engine.

In considering the use of spring and clock-work motors we meet at once with the problem of comparing the effect on the performance of a plane of a carried weight, as against a head resistance. The efficiency of a spring motor is measured in terms of its weight, that of a propeller in terms of its head resistance. The general answer to this question is given by the relation that a pound of dead weight is equivalent to ⅕ pound head resistance.

In order to apply this relation to the study of spring motors for driving cameras, data are necessary on the power delivery per pound weight of such mechanisms. Such data are not easily accessible, largely because clock-work has not generally been seriously considered as a motive power for large apparatus. To arrive at an approximate figure we may take the fact that in an 8 × 10 inch film camera designed by one of the manufacturers who have utilized clock-work, the motor weighed 30 pounds. This is equivalent to six pounds head resistance. Now the type K, 18 × 24 centimeter film camera is operated, even with the addition of a friction drive speed control, by means of the L camera propeller. As shown in Fig. 66, at 100 miles per hour the head resistance of this propeller is still less than three pounds. Consequently, it appears that from the efficiency standpoint the clock mechanism is quite outclassed by the wind propeller.

Coming next to the electric motors, the L camera and the K are both operated satisfactorily with a 1
20 horse power motor, weighing 6 pounds. For the deRam a ⅒ horse power motor has been adopted.

Taking up efficiency considerations, we have, if the current is supplied by a generator from the engine, a transformation factor of 70 to 80 per cent. from mechanical to electrical energy and a similar factor in using a motor for the camera. When batteries are employed the matter of weight versus head resistance again arises. The batteries found most satisfactory for operating the K and deRam cameras are of the six-cell 12 volt lead type. Their capacity is 40 ampère hours at three ampères or 36 at five ampères—more than is necessary for a single reconnaissance, but a practical figure when economy of charging and replacement are considered. The weight of this unit is 27 pounds. To this must be added the weight of the motor—6 lbs.—making a total of 33 pounds, equivalent to a head resistance of nearly 7 pounds. This is more than twice the propeller head resistance invoked to do the same work.

These considerations of efficiency have been gone into because they are usual in studying any engineering problem and because of the insistent demand from the plane designer that every ounce of weight and head resistance be saved. Actually, as already stated, the load imposed by any method of power drive is trivial in comparison with the whole load of the plane. There is, however, an important reservation to be made, which applies against clock-work and batteries: This is, that while the equivalent head resistance of any camera motive power carried as dead weight is small, its effect on balance may not be so. While the use of a propeller need not disturb the plane's balance, the weight of the camera alone, without any driving apparatus, is already seriously objected to on this score. The merely mechanical superiority of the propeller as a source of motive power is on the whole rather marked.

Control of Camera Speed.—In the semi-automatic camera the only control required on the speed of the operating motor is at the upper and lower limits. It must not go so fast as to anticipate the completion of any steps in the cycle of camera operation, such as the fall of plates or pawls into position, which would jam the camera. On the other hand, it must not be so slow that pictures cannot be obtained with the requisite overlap for maps or stereoscopic views. In the American deRam camera the cycle of operations cannot safely be put through in less than four seconds, a short enough interval for most purposes. It is also highly desirable in the semi-automatic camera to have the motive power capable of stopping completely. This saves wear and tear on both motor and camera mechanism.

In the automatic camera an extreme range of speed is called for by the several problems of mapping, oblique photography, and the making of stereoscopic views. For mapping alone, the shortest likely interval may be taken as that required for work at approximately 1000 meters altitude, for a plane speed of 150 kilometers per hour, which demands an interval of six seconds with a ten inch lens on a 4 × 5 inch plate. For vertical stereos at the same altitude and speed this interval is divided by three, and low oblique stereos need even quicker operation. Hence a range of from 1 to 30 pictures per minute should be provided for. This requirement is difficult to meet with any simple mechanism.

From the standpoint of simplicity in speed regulation the wind turbine of adequate vane surface has much to recommend it. It is only necessary to present more or less of its vane area to the wind in order to secure a considerable range of speed. The method of doing this by a shutter interposed in front is uneconomical, but it is probable that the design can be so altered that more or less of the turbine is exposed beyond the side of the plane, possibly by varying the angle, to secure the same result without introducing useless head resistance. A serious practical objection to the turbine lies in the large vane surface necessary to give adequate power combined with proper speed variation. In the automatic film camera (Type K) this area should be as much as 40 to 50 square inches.

The wind propeller does not lend itself at all well to speed variation. It cannot be partially covered from the air stream, as can the turbine, because of the resulting strain on its mount. A possible form of variable speed propeller, one which, however, has not yet been practically developed, is a propeller with controllable variable pitch. If this could be made mechanically sound it would be well-suited for camera operation. That such a propeller could be worked out is indicated by the good performance of a constant speed propeller developed for radio generators and used on the French deRam camera (Fig. 54). Parenthetically, it maybe questioned whether a constant speed propeller is really desirable with an airplane camera. What is required is not exposures at a definite time interval—although most of the data are in that form—but exposures at definite intervals with respect to the motion of the plane, which practically means with reference to its air speed. Rather than build a camera calculated to give exposures at intervals of so many seconds when it is attached to a constant speed propeller, we would do better to use a propeller which responds to the speed of the plane, in conjunction with some form of tachometer to show the rate at which exposures are being made. This in turn should be coördinated with the indications of a proper camera-field indicating sight.

One solution of the problem of speed control with a propeller of practically fixed speed, is to use a governor and slip clutch as in the English Type F film camera (Fig. 57). Here the propeller shaft and the camera driving axle are connected by two friction discs. That on the camera mechanism is forced against the other by a spiral spring, whose tension is controlled by a ball governor. If the camera speed becomes too high the governor reduces the tension on the spiral spring and the discs slip over each other. The point where this slipping occurs is determined by the position of the governor as a whole, and this is controlled by a lever on top of the camera.

Another speed control device, perhaps more positive but certainly more complicated and wasteful of power, consists of a large flat disc, driven by the propeller or electric motor, and from which the camera is driven by a shaft from a smaller friction disc which may be pressed against any point from the center to the periphery of the larger disc. The speed range attainable in this way is limited only by the size of the large disc. An application of this idea is shown in the speed control (Fig. 68), designed for the American Type K camera when operated on an electric motor or on a simple propeller. The same idea is utilized in the Duchatellier film camera, in connection with the constant speed propeller already described.