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Aviation History
1943
1943 - 0377.PDF
FEBRUARY IITH, 1943 FLIGHT AircraftGun Control Systems Pneumatic Operation Light and Simple : The Need for Selector Units: Pioneer Work by the French By H. G. COJMWAY EARLY machine guns fitted tofighting aircraft were manuallycontrolled or fired by the rotat- ing airscrew. In some cases simple mechanical remote controls were used to operate a gun in the nose of the aircraft out of reach of the pilot or gunner. The introduction of the multi-gun — or cannon aircraft — brought a new problem: the firing con- trol of the guns- relatively far away in the wings. Certain machine guns such as the Browning used on modern British fighters require merely the pulling of the trigger once loaded up, a simple pneumatic jack on the gun being used. A single button on the control column admits air to all jacks, thus firing all guns simultaneously. In the event of a cartridge jamming or misfiring, the gun cannot be reloaded in the air and remains out of action. Operational Requirements In the case, however, of the mixed gun or cannon fighter, the control sys- tem cannot be so simple. The machine guns are usually focused, say, 300 yards ahead of the aircraft, and the paths of the bullets being divergent beyond this point, th% effectiveness of the fire is lost beyond about 400 yards. This focusing effect gives the equiva- lent of " depth of focus'' in photo- graphy. The cannon, however, can fire in parallel or be focused at a much greater range. If all guns and cannon are fired at once by a single button, considerable wastage of ammunition may occur. It is useless, for instance, to fire six machine guns at the same time as four cannon when attacking tanks, small ships or heavily armoured bombers. It may be equally wasteful to shoot up reconnaissance aircraft, troops in open country, gliders, etc., with cannon when machine guns would do, and the cannon ammunition needed for more serious work. Consequently, some means of selecting the guns to be fired is required. The most obvious method on an air- craft such as the Beaufighter, with a mixture of machine guns and cannon, The gun trigger onthe spade grip of an eight-gun Spitfire.All guns fire on pressing the button,there being no pro- vision for selectinggroups of guns. would be to provide two trigger but- tons on the control column such that either one or both could be pressed and elementary selection of the guns or cannon effected. This system is, in fact, used. However, this may not be enough in certain cases. The Beau- fighter is now fitted wilh a selector allowing any one of its four cannon to be fired at will, thus allowing the maximum use of the relatively limited amount of ammunition, and avoiding wastage. The Fw 19b, which presumably represents the latest German viewpoint on the problem, is equipped with a number of mixed guns—two machine guns, two 20 mm. Oerlikon cannon, and two 20 mm. Mauser cannon, firing through the airscrew. A selector is provided allowing either all guns, or the two Oerlikon cannon, or the machine guns and synchronised Mausers to be fired. Future Armament The four-cannon Spitfire and Hurri- cane, however, have no selectors, be- ing primarily defensive weapons, and unlikely to carry out excursions call- ing for varying types of fire. The whole policy in their armament is to be able to produce the maximum weight of fire at once. It is reasonable to suppose that in future designs of fighter, at any rate on the two-motor long-range type, armament will be even heavier. Pos- sible equipment in the not too distant future would be six machine guns, four 20 mm. and two 40 mm. cannon. The first two would be for general work as at present, and the two heavy shell guns would be effective for tank or flak-ship " busting." In an aircraft equipped in this manner the need for selection is even more obvious. Control Systems As mentioned above, the existing British system for firing 8 or 12 machine guns is a simple button- controlled pneumatic one. An air bottle at a pressure of 200-300 lb./sq. in. provides the air supply. The 20 mm. cannon such as the Hispano is generally provided with three controls : (1) Loading or cocking control, pulling back the bolt to admit a shell; (2) fire control, pulling the equivalent of a trigger ; and (3) safety control, locking the gun trigger mechanism by a safety catch. Other and larger cannon will require similar controls. These functions can be car- ried out by any of the usual remote control systems, but special systems have been developed with particular efficiency. Mechanical controls would be heavy and require considerable effort to operate several guns at once. Electrical controls are used in U.S.A. and in Germany (e.g., on the Fw 190), but the solenoids are heavy and bulky. They have one notable advantage in that they are not directly susceptible to freezing at high altitude. Selection is relatively simple with a multiple switch block. Hydraulic controls have been tried, but they have not been successful. It is a relatively easy matter to apply hydraulic pressure rapidly, but very difficult to remove the pressure and oil from an operating jack equally rapidly. Pneumatic controls have been used for a number of years and, in spite of early fears that trouble would be experienced due to moisture freezing at high altitudes, have proved ex- tremely reliable. In order to over- come the supposed difficulty of freezing, which occurs when the air is expanded down an operating pipe and
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