Aircraft technical Basics: TM 1-409, Aircraft Armament and Pyrotechnics, 1941: IV. - Aircraft Cameras (Machine Gun)

Section IV - AIRCRAFT CAMERAS (MACHINE GUN)

26. Employment.-a. Aircraft cameras (machine gun) have been designed and are used in the training of pilots and aerial gunners in aerial combat. By means of the gun camera a photographic scoring or record is made of the result of each shot (exposure) and from this the errors can be noted and afterward guarded against, and the skill thus developed in the use of the gun cameras is substantially of the same value as that obtained in the use of a machine gun employing ball ammunition.

b. In the event the Air Corps uses gun cameras for the same purpose as the British, that is, to confirm the destruction of an enemy aircraft or the disabling thereof, the maintenance, bore alinement, adjustments, and care of the cameras are the same as if the materiel is used for training purposes.

c. In either use, that of training or confirmation of hits, the maximum value of the film record is obtained only when the film is properly interpreted and studied. The observations arising should subsequently be used as a guide in making corrections and in otherwise improving the score.

d. Gun cameras must not be disassembled or repaired by other than authorized personnel. All faulty or damaged gun cameras and accessories must be sent to an Air Corps control depot or authorized instrument repair section for repair.

e. It will be noticed while studying the H1 aircraft camera (machine gun) that many references are made to the H2 aircraft camera (machine gun). Basically, the cameras are the same and many of the major assemblies are interchangeable. The greatest difference lies in the construction of the outer cases and in their methods of operation.

27. H2 aircraft camera (machine gun).-a. General deseriprion.-(1) The H2 aircraft camera has been so designed that it will approximate the Ml and M2 caliber .30 machine guns in size, weight, and shape. This feature of its design is of great value in flexible gunnery.

(2) The. H2 aircraft camera may be used as a flexible gun and operated manually, using a spade grip and trigger similar to that used on the M2 caliber .30 machine gun, or it may be used in a fixed installation and operated electrically by means of a solenoid.

(3) The H2 camera takes photographs at the rate of 16 frames (or photographs) a second as long as the trigger is depressed, up to a total of 700 exposures, which is the limiting capacity of the spring motor under low temperature conditions. In order to obtain the full number of 700 exposures, the camera must be loaded with 18 1/2 feet of 16-mm film. Each foot of film will take 40 exposures, and one extra foot must be allowed for the combined lengths to leader and trailer in the magazine. It is a practice when a machine gun is used on a training mission to take about 400 rounds of ammunition. Therefore, in order to simulate more closely the practice with a machine gun, the magazine of a gun camera is usually loaded with 10 feet of film, which is sufficient for 360 exposures. For that reason the standard length of film may be assumed to be 10 feet.

FIGURE 2.-H2 aircraft camera (machine gun) assembly.

(4) Interchangeable lighttight metal magazines are used for holding the film. These magazines, in the number desired, are loaded in an ordinary darkroom by photographic personnel. The camera may be reloaded in the air with fresh magazines provided the camera is accessible.

(5) The reticle embodied in the camera unit includes a full cross, the center of which represents the path of an imaginary bullet (no drift or drop), and a series of concentric circles. These circles rep-resent the proper deflection or lead allowances for speeds of 60, 115, 175, and 230 miles an hour, respectively. The 60 miles an hour ring is a broken line to aid in distinguishing it from the others. These speed deflection rings are computed for an imaginary, average bullet velocity of 2,350 feet a second.

(6) The time registration or picture of the watch occurs at the end of each "burst of fire" and full size frame is used for registering the image of the watch. By counting back from any watch image and measuring 16 exposures a second, the time of any vital hit may be easily determined.

b. Detailed description.-(1) Operating mechanism.-The H2 air-craft camera uses a modified simplex 16-mm motion picture camera unit, which may readily be removed by merely lifting it out of the camera housing. The A4 film magazine fits into this unit. All camera units are interchangeable, the gun camera and camera units being so constructed that there is no change in the alinement of the camera optical axis with regard to the sight when one camera unit is replaced by another.

(2) Counter.-An accurate counter dial is placed on the left side plate far enough away from the gunner's eye position in flexible gunnery that it can be easily read while the gun camera is in the firing position. The side counter can be read from the cockpit when the gun camera is mounted in a fixed mount on top to the center section. Immediately after the loading or reloading of the camera, the exposure counter is set at zero by use of the small knurled knob on the exposure. counter.

(3) Time registering device.-A watch image is recorded automatically after each burst of fire. The watch is a 2-dial 17-jewel Hamilton with separate hour, minute, and second hands, and a stop for synchronizing it with other watches. The lamps used to illuminate the watch receive current from three flashlight batteries located in an easily removable tube that screws into the rear of the gun camera.

(4) Lens.-The H2 aircraft camera is equipped with an F/3.0 35-mm motion-picture lens with an iris diaphragm providing marked settings from F/3.0 to F/16. The lens is mounted in a fixed position on the gun camera frame and is accessible through the door on the left side plate.

c. Installation.-(1) The H2 aircraft camera is designed to be interchangeable with regard to mounts for model 1918M1 or M2 machine guns, caliber .30. When it is used in the flexible mount for one of these machine guns, it should be equipped with handgrips and both front and rear sights.

(2) (a) By removing the handgrips, the H2 aircraft camera can be used in the fixed mount for either of the machine guns just mentioned, provided such mountings do not require that the gun camera be inside the wing or center section structure or under the cowling of the airplane. In other words, the gun camera must. be mounted in the open. This is necessary because the angle of view of the lens is of such width that the opening in the leading edge of the wing or center section or through the cowling would not be sufficient to permit the unobstructed passage of all the rays of light that the camera lens should include. There would be interference which would make it impossible to use the gun camera. By means of a solenoid, the gun camera can be operated remotely.

(b) When installing the gun camera in a fixed mount, the barrel should be removed to reduce vibration of the gun camera.

(c) To attach the solenoid adapter to the camera, the handgrips must be first removed. The trigger arm can then be removed. Attach the solenoid adapter on the rear of the camera by means of four screws making sure that the hook-shaped slide is engaged with the trigger pin.

d. Operation.-(1) General.-The trigger should be operated its full stroke in both directions in order to insure correct operation of the camera unit and time recording mechanism. Unless the trigger is allowed to return all the way to the top of its stroke, no image of the watch will be produced. It is not desirable to return the trigger slowly, and particular care should be taken not to "ride" the trigger by keeping it partially depressed in anticipation of a burst, other-wise the parts allowing focus of the lens on the watch are subject to vibration resulting in wear and poor watch pictures. The trigger should never be depressed when the camera unit is completely run down. Before removing the film from the camera at the end of a flight, depress the trigger for a period of 1,4 second to remove the frame containing the last recorded watch image from the magazine aperture. For instance, when using a 10-foot length of film, an excess of film is provided in each magazine over the 360 scoring shots permitted. Some of this excess should be saved for this last operation of running off a few safety exposures. If not more than 360 shots are taken in the air prior to the last watch image, there will be ample film remaining for these safety exposures. Some weak images of the watch on the film may indicate that batteries need replacement or that one or more lamps are burned out, and not a fogging of film due to light.

(2) To operate.-Regardless of the, type of installation, the first thing to do is to load the camera unit of the gun with a film magazine, check its operation, and then replace the camera unit in the gun. Check the lights and wind and set the watch. Set film consumption indicator at zero. If the installation of the gun camera is flexible, further preparations are unnecessary. When it is desired to operate the camera, it is necessary only to depress the trigger and hold it down as long as it is desired to keep the camera in operation. In case of a fixed installation where the camera is to be operated by a solenoid, the solenoid should be installed, connected, and a check then made to see that it is functioning. The camera is then ready for use.

(3) Winding camera.-Grasp the case firmly and wind the camera unit by turning the winding key in a clockwise direction. Run the camera spring down slightly by releasing the shutter trip pin. The counter gear should be watched and should be allowed to make two or three complete revolutions. This method will eliminate spring breakage due to temperature variations at high altitude operation.

(4) Loading camera with film magazine.-In the case will be found the camera unit equipped with a small lifting ring in the center. Grasp this ring and lift out the camera unit, being careful not to strike the trip pin on the side of the case or any of the projecting parts. At the front of the case will be seen the shutter aperture; at the left side the winding key which folds flat against the case; and near the top of the nearest edge, the magazine release button. Press this button and pull open the door. Insert the magazine in the camera unit through this door opening. It is impossible to insert the magazine in any other than the correct manner. The magazine rear spring should then project slightly to the rear of the case of the camera unit. Push the rear of the magazine to make certain that it is properly seated in the compartment. By means of the shutter trip pin, trip the camera for about 5 to 8 exposures. If it fails to run, remove and replace the magazine and then endeavor to trip it again. This is important as the magazine sometimes fails to engage. This check insures that the camera is functioning.

(5) Replacing camera unit.-Turn the camera unit over and hold it by the lifting ring. It should be inserted in the gun case with the shutter aperture forward. Push it down until it strikes the bottom, making sure that it is all the way in and forward as far as it will go. During this operation, care should be taken to avoid striking the trip pin on the side of the case, as this will set the camera unit in operation.

(6) Setting exposure counter.-The exposure counter is located on the left side of the gun camera case. After the camera unit has been placed in position correctly, set the exposure counter to zero.

(7) Time recording feature.-The watches of the cameras intended for mock combat should now be synchronized to the hour, minute, and second in order to determine later which contestant first secured the vital hits. The watch is contained in the upper front chamber and may be reached by pushing the door release button located on the lower left of the barrel flange on the front surface of the case. The second hands of several watches may be synchronized by stopping them all at the same numeral and then releasing simultaneously so that they start running together. The hour and minute hands can now be adjusted by means of the crown reset in the usual manner. While the watch is out, any necessary data for identifying the flight or pilot may be written in lead pencil on the celluloid card near the watch face, taking care to keep the writing near the edge of the watch. Any pencil eraser will remove previous pencil marks. These data will be photographed with the watch.

With the watch door open, depress the trigger to the bottom and release it and observe whether all the lamps can be made to light. Do not keep them on as the load is considerable on the small battery and will shorten its useful life. When replacing the lamps, always be sure that 2.5 volt miniature lamps are supplied, since any other size will be unsatisfactory. Although the lamps are overloaded, they should have in regular use a life of at least 10 rolls of film. After the watch and lamps have been replaced, close the watch compartment door and be sure that the spring catch of this door is securely seated so as to prevent the opening of the door. This spring catch must be depressed slightly in order to allow the door to close.

(8) Diaphragm setting.-There is one important adjustment to be made when using the gun camera and that involves the setting of the diaphragm. The following is a table of adjustment:

(a) F/4.5 for the 3 hours preceding or after noon if the sun is shining.

(b) F/3.0 for very early or very late hours when the sun is shining, or near noon during overcast weather.

For those who wish to compute their own diaphragm setting, the shutter speed of the camera is 1/160th of a second when the gun camera speed is at 16 frames a second, which is the normal speed and the one at which the camera is set when it leaves the factory. The filter is used for the purpose of improving the pictures, especially those taken toward the light on hazy days, and has a factor of approximately 2½ times normal exposure.

(9) Trigger.-If the trigger seems to stick slightly while being released from the operating position, depress it slightly and it will come back at once.

e. Accessories.-(1) Indicator, gun camera alining.-(a) The gun camera alining indicator (fig. 3) for the H2 aircraft camera consists of a casting, accurately machined so that when it is placed in the position ordinarily occupied by the motion picture camera unit, the cross engraved on the indicator prism is in the same position that the center of the reticle in the camera unit ordinarily occupies.

(b) The gun camera alining indicator comes in a sturdy mahogany carrying case especially fitted to it, and the indicator should always be kept in this case when not in use in order to avoid accidental damage.

(c) The gun camera alining indicator is supplied to make a definite check on alinement of fixed guns with telescope or cowl sights and for the purpose of alining gun sights or checking them when damaged and repaired. When in use, it is put in the gun camera in place of the camera unit.

(2) A4 film magazine.-The A4 film magazine (fig. 3) is a self-contained lightweight magazine for 16-mm film. It is constructed throughout of corrosion-resistant materials and equipped with a lighttight metal container for storage or transportation purposes. The magazine has a capacity of 25 feet of 16-mm negative film which provides for 1,000 exposures or shots. A brief examination of the magazine will make its construction obvious.

f. Lubrication.-A drop or two of aircraft instrument lubricating oil (Spec. No. 3562) may be placed on the retard pallet at overhaul periods to prevent the pallet from rusting. By all means avoid an excess and remove all superfluous oil with a lintless cloth as the rapid action of the mechanism may cause the oil to spatter on the lens and prisms. No other lubrication of the gun camera is recommended or is necessary, except to the oiling of the exterior surfaces to preserve the finish as described in the next paragraph.

g. Inspection and maintenance.-In addition to the inspection made in accordance with the instructions contained on Air Corps Form No. 45, and recorded and submitted on that form, the following inspection should be, made of gun cameras at least once in every 10 days while the cameras are in continuous use :

(1) Watch.-The watch (fig. 3) will he checked, and if found to be inaccurate, will be sent to the instrument repair shop at the Air Corps control depot for necessary adjustment or repair.

FIGURE 3.-H2 aircraft camera (machine gun) open, with accessories.

(2) Lamps.-The lamps are located on a. removable panel in the watch compartment. When one or more lamps have burned out, it is desirable to insert a complete new set of four lamps to provide a safeguard against poor watch illumination caused by lamp failure during a mock combat. Although the lamps are overloaded, they should have a. life of at least 10 rolls of film. Always be sure that 2.5 volt miniature lamps are used. Any other size is unsatisfactory.

(3) Batteries.-The batteries for supplying the lamps used to illuminate the watch are located in a tube (fig. 3) screwed into the rear of the gun camera and locked in place by a knurled eccentric safety lock. Fresh batteries should always be used and if the watch image is seen to be faint on the film, the batteries should be replaced. The lights may be tested by depressing the trigger and observing their operation through the top door or through the watch door.

(4) Speed regulation.-In special circumstances, some advantage may be obtained by slowing the camera mechanism to 12 frames a second by turning the rosette head screw on the forward upper right side of the camera unit as indicated by the marking on that unit. This decreases the shutter speed to 1/120 of a second and this change should be noted on the data card to allow correct time scoring of the hits.

(5) Alining.-In order to insure correct alinement of the gun camera sights or to aline the gun camera, when used as a fixed or flexible gun, the camera should be alined as explained in paragraph 30.

(6) Cleaning film slide and reticle glass of camera unit.-A small soft camel's-hair brush is provided to clean the film slide and the back surface of the, reticle, glass of the camera unit.. This should be done from time to time to prevent the scratching of the film. The front surface of the reticle glass may be dusted off with an air jet while the camera unit is running. The prisms should also be dusted off from time to time. The brush should be entirely free from oil and must be washed occasionally with soapy, hot water and dried before using. When not actually in use, the camera. should be kept in its case.

(7) Exterior of gun camera.-It is desirable that the exterior of the gun camera be oiled once a month with any good grade of machine oil to preserve the finish. All excess oil should be removed with a dry cloth. Avoid getting oil on the filter, or scratching its surface in any way.

(8) The alining indicator.-The alining indicator for the H2 aircraft camera is properly adjusted at the factory, and with reasonable care no further adjustments should be necessary. The prisms of the indicator should be cleaned occasionally with a clean, dry cloth or soft lens tissue. Other than this, no servicing or adjustments are required.

FIGURE 4. H1 aircraft camera (machine gun), assembled.

(9) Filter.-A minus blue filter of optical flat glass is always used with the H2 aircraft camera. It is provided with screw threads for attaching to the front of the camera body. The filter should be cleaned occasionally with a clean, dry cloth or soft lens tissue.

(10) Solenoid.-The gun camera may be operated by means of an H2 solenoid and adapter. Its operation may be checked by applying a 12-volt power supply.

28. H1 aircraft camera (machine gun).-a. General description.-The H1 aircraft gun camera (fig. 4) is a fixed-type gun camera, designed to be attached to the upper side of the airplane top wing in front of the pilot, or to be faired into a structural part of the aircraft. It is streamlined to give the least amount of wind resistance.

(1) The camera is attached to the airplane by means of gun mounts of the type used for Ml and M2 caliber .30 aircraft machine guns.

(2) The camera is tripped electrically by a solenoid, which is connected through the operator's switch to a 12-volt power supply.

(3) Photographs are taken at the rate of 16 a second as long as the trigger is depressed, up to a total of 700 exposures, which is the limiting capacity of the spring-driven camera unit. In order to obtain the full number of 700 exposures, the camera must be loaded with 18 1/2 feet of 16-mm film.

(4) The time registration or picture of the watch occurs at the end of each "burst of fire," and a full size frame is used for registering the image of the watch. By counting back from any watch image and measuring 16 exposures a second, the time of any vital hit may be easily determined.

(5) An accurate exposure counter dial (fig. 5) is located at the rear of the camera near the top. The numerals can be read by the operator while the camera is running.

(6) The H1 gun camera has a removable camera unit (fig. 5) which contains the film magazine and shutter. This camera unit is interchangeable with other camera units and may be used in any H1 gun camera. The film magazine can be removed from the camera unit in daytime since it is lighttight.

(7) The reticle system of the H1 aircraft camera (machine gun) is identical with that. of the H2 aircraft camera (machine gun), as explained in paragraph 27a(5).

(8) The watch unit (fig. 5) of the. H1 aircraft camera (machine gun) is identical with that of the H2 aircraft camera (machine gun), as explained in paragraph 27b (3).

b. Installation.-(1) If the camera is to be mounted within the leading edge of the airplane wing, a check should be made when bore-sighting the camera to see that there is a sufficient angle of opening in front of the camera lens opening (fig. 4) so that light rays are not obstructed, the angle should be at least. 7° from the axis of the lens, or an included angle of 14°. Appropriate cable connections must then be made from the solenoid and lights to the power supply, and to the operator's switch.

FIGURE 5.-H1 aircraft camera (machine gun) partially assembled, with acessories

(2) After mounting, the camera must be boresighted before being used. This is done as follows :

(a) An instrument, known as the boresight (fig. 5), is used for checking sight alinement and for completing the fixed installation of the gun camera.

(b) Remove the front fairing assembly and then the subframe assembly. Remove the camera unit from the subframe assembly and replace it with the boresight assembly. Return the subframe to the main frame but do not reassemble the front fairing. Move the prism adapter arm as far forward as possible and hold it in this position while boresighting. Focus the microscope tube of the bore-sight so that the cross lines on the prism are sharply defined. The central portion of the image formed by the gun camera lens will also be sharply defined if the boresight is seated properly and is located securely against the subframe. The cross lines which can be seen superimposed on the image accurately represent the point where the imaginary bullet (without drift or drop) would strike, as indicated by the center of the reticle in the camera unit. If the reticle center and the pattern do not line up, the gun camera must be adjusted in its mount until they do. After the alinement is made, the camera mount should be fastened securely. The boresight should then be removed from the camera and placed in its carrying case to avoid accidental damage.

(c) If the camera is to be used following the boresighting, the camera unit should be reassembled to the subframe before securing the latter to the main frame. The front fairing should then be assembled. To complete the installation, the cable connections from the camera to the power supply and operator's switch should be made. If the gun camera is not to be used, the dummy nose cap assembly (fig. 5) should be secured in place of the front fairing and subframe assemblies.

c. Inspection.-(1) Inspections will be accomplished as prescribed by Air Corps Form No. 45.

(2) Before removing the film from the gun camera at the end of a flight, the camera should be run for a period of half a second to remove the film frame containing the last recorded watch image from the magazine aperture.

(3) At the end of a flight, when the aircraft is to be stored over-night, the front fairing and the camera subframe with the camera unit should be removed and replaced by the dummy nose cap in order to protect the main frame assembly.

d. A4 film magazine.-See H2 aircraft camera (machine gun).

e. Pointers on reloading film magazines.-(1) If a magazine jams, set it aside and do not use it again until it has been repaired and fully tested with dummy film.

(2) Jams are mainly caused by one of the following reasons:

(a) Bent covers or containers. Remedy-Straighten and test.

(b) Sticky or frozen take-up clutches. Remedy-Disassemble by pushing out inner core while turning counterclockwise with respect to outer sleeve, while facing recessed end. Clean thoroughly and replace. Oil with a trace of aircraft instrument lubricating oil (Spec. No. 3562).

(c) Film place improperly on the supply core. This may be felt by touching the finger to the edge of the roll before loading. If a raised edge is felt above the end surface of the supply core, the film will jam against the magazine cover. Remedy-Work the film endwise between both thumb and forefinger until the edges of the film are smoothly flat with the surface of the core. If this cannot be done, reject the film.

f. Operation.-(1) Winding camera unit-See H2 aircraft camera (machine gun).

(2) Loading camera unit with film magazine.-Remove the camera unit from the camera subframe by pressing on the locking hinge at its lower edge near the locking hinge pin. Wind exactly the same as the H2 camera unit.

(3) Replacing camera unit on the camera subframe.-(a) Place the camera unit on the subframe. The camera unit shutter aperture should seat over the lens aperture boss on the subframe. The camera unit should also seat on and against the finished pads on the subframe.

(b) Push on lock spring hinge at its upper edge until it snaps in place. Make sure that the camera unit is positioned properly and that the locking hinge is secure. During this operation, care should be taken to avoid striking the shutter trip pin on the side of the camera unit and setting the camera unit in motion.

(4) Setting exposure counter.-The exposure counter is located at the rear of the camera near the top of the main frame fairing where it can be seen by the pilot. After inserting the camera sub-frame into the main frame and latching it in place, reset the counter. This may be done by turning the counter knob at the rear of the main frame fairing. There are two dials and both should be set at zero. The inner circle of numerals indicates hundreds of exposures, and the outer circle of numerals indicates single exposures.

(5) Diaphragm setting.-See H2 aircraft. camera (machine gun).

(6) Setting of watches.-See H2 aircraft camera (machine gun).

g. Interpretation of film for failures.-(1) If one burst follows another without a watch image or blank space between, it indicates that the solenoid adjustment would not allow the mechanism to return to the single exposure position.

(2) A blank frame between bursts indicates that the lamps did not give sufficient illumination on the watch or failed entirely due to burned out bulbs, "dead" battery, broken connections, or bad switch contact.

(3) A film which is partially or entirely blackened outside the row of perforations shows that it was light-struck during loading operations.

(4) A blank film that has run through the camera, usually indicates that the lens diaphragm was accidentally closed too far.

(5) A film showing blurred reticle lines indicates that the camera unit shutter is out of synchronism with the film spacing mechanism.

(6) A thin image on the film indicates underexposure. Open up the lens diaphragm to the next larger stop. A thin watch image indicates poor illumination. The watch holder, lamp holder, and compartment are painted white to use the light efficiently. If these get dirty, clean and repaint with white paint when necessary.

h. Lubrication.-Lubrication of the H1 gun camera is not recommended or necessary, except at overhaul.

i. Inspection and maintenance.-(1) Watch-See H2 aircraft camera (machine gun).

(2) Lamps.-See H2 aircraft camera (machine gun).

(3) Optical system.-See H2 aircraft camera (machine gun).

(4) Removal of solenoid.-If while the camera is installed it is necessary to remove the solenoid, it should be done as follows : Without disturbing the alinement of the camera, remove the cable connector plugs and then the camera main fairing. Unscrew the stud and loosen the clamp screw. Unscrew the solenoid from its bracket.

(5) Film slide and reticle glass of camera unit.-See H2 aircraft camera (machine gun).

(6) Storage.-Except when installed on an airplane, the H1 gun camera and its accessories should be kept in the carrying cases provided and away from dust and moisture. Storage or transportation in other than these especially designed cases should not be undertaken. The H1 gun camera and its accessories are high quality, precision instruments, and are to be handled and stored as such.

29. Boresighting.-a. Method of boresighting H2 gun. camera-(1) Place gun camera in level, stable position.

(2) Install A3 post sight and B8 ring sight. The post sight is merely placed in position. The ring sight is tightened in position.

(3) Measure vertical distance from center of lens to center of inner ring sight.

(4) Construct pattern using measurement found in paragraph 29a(3).

(5) Place pattern at convenient range, not less than 20 feet.

(6) With alining indicator, aline cross on reticle with lower cross on pattern.

(7) By means of the eccentric front sight base, adjust sights until they coincide with the upper cross of pattern.

(8) Check and safety installation.

b. Method of boresighting H1 gun cameras.-(1) Place airplane in flying position.

(2) Harmonize line of sight and line of flight.

(3) Measure vertical and horizontal distance from line of sight to center of lens.

(4) Construct pattern, using above measurements.

(5) Place pattern at convenient distance, not less than 50 feet away.

(6) Move airplane or pattern until line of sight coincides with corresponding cross on pattern.

(7) By means of the boresight, aline cross on reticle with corresponding cross on pattern. This is accomplished by the adjustable mount.

(8) Check and safety installation.

NOTE.-The routine operations outlined above have been found satisfactory under normaI conditions. They should not be considered mandatory.

30. Film interpretation.-a. General.-Without the proper interpretation of gun camera film after exposure and development, the training derived from mock aerial combat will be valueless. It must he remembered that aerial machine gunnery presents a much different. problem than ground gunnery, particularly in range estimation.

b. Range finding.-In order to calculate the range from which a gun camera film was exposed, it is necessary to know

(I) The focal length of the camera lens. (The focal length of the lens on both H2 and H1 aircraft cameras is fixed at 1.378 inches.)

(2) The dimensions of the aircraft, included in the picture. (These may be found in Air Corps Technical Orders on the particular type aircraft, otherwise the airplane must be measured.)

(3) The size of the image of the aircraft on the developed film. (Found by measuring the image on the film.)

c. Example.

Focal length-1.378 inches.

Length of aircraft-30 feet.

Size of image-1/8 inch.

Using the following formula :

It will be found

496.080x 8/1=3968.64 inches or (after dividing by 36) 110.24 yards.

It should be noted that applying the formula, the length of the air-plane, 30 feet, was converted to 360 inches by multiplying by 12 so that all factors will be in the same unit of measurement.

d. Leading a target.-(1) It must be remembered that in ascertaining whether a hit was made it is necessary to take into consideration the extent to which the target was "led."

(2) A projectile in moving from a weapon to a given point takes a certain length of time to reach the given point. For that reason concentric circles, which allow for leads up to and including 230 miles an hour, have been added to the reticle system. This reticle system allows for an average (imaginary) bullet velocity of 2,350 feet a second.

(3) In using the reticle system to check on lead it, will be necessary to know the average cruising speed of the aircraft participating in the mock combat.

(4) After finding the average cruising speed of the aircraft, locate its image on the film and then form an estimate as to whether a hit was made.

Example: If an airplane is flying at the average cruising speed of 175 miles an hour, from right to left in front of a gun camera on a fixed platform, and examination of the film shows that the image of the plane is superimposed on the third circle, to the right of the cross line intersection and on the horizontal line, the conclusion that should be drawn is that of a hit. The "lead" circles are correct for "leads" indicated for all ranges (fig. 6).

FIGURE 6.-Diagram interpreting "lead."