So the fixed gun was brought down into the fuselage and made to fire through the whirling propeller. At first the aviators took their chances of hitting the propeller blades, and sometimes the blades were armored at the point of fire, being sheathed in steel of a shape calculated to cause the bullets to glance off. This system was not satisfactory. Then, since a single bullet striking an unprotected propeller blade would often shatter it to fragments, attempts were made to wrap the butts of the blades in linen fabric to prevent this splintering, and this protection actually allowed several shots to pierce the propeller without breaking it.
This was the state of affairs on both sides early in 1915. The French Nieuports had their fixed guns literally shooting through the propellers, the bullets perforating the blades, if they did not wreck them. As late as February, 1917, Maj. Bradley, who was by that time a flight commander in the British service, worked a Lewis gun over the Bulgarian lines with the plane propellers protected only by cloth wrappings.
All of this makeshift operation of fixed machine guns was changed by the invention of the synchronizing device. This is an appliance for controlling the fire of the fixed gun so that the bullets miss the blades of the flying propeller and pass on in the infinitesimal spaces of time when the line of fire ahead of the gun is clear of obstruction. The term "synchronizing" is not accurate, since that word implies that the gun fires after each passage of a propeller blade across the trajectory. Such is not the truth. The propeller revolves much more rapidly than the gun fires. The device is also called an "interrupter," another inexact term, since the fire of the gun is not interrupted, but only caused at the proper moments. Technicians prefer the name "gun control" for this mechanism.
Who first invented the synchronizer is a matter of dispute, but all observers agree that the Germans in the Fokker monoplanes of 1915 were the first to use it extensively. Not until some time after this did the allies generally install similar devices. Some have attributed the original invention to the famous French flier, Roland Garros.
Two types of synchronizers were developed, one known as the hydraulic type and the other as the mechanical. In operation they are somewhat similar. In each case there is a cam mounted on the engine shaft so that each impulse of the piston actuates a plunger. The plunger passes on the impulses to the rest of the mechanism. In the mechanical control the impulse is carried through a series of rods to the gun, causing the latter to fire at the proper moments. In the hydraulic control the impulse is transmitted through oil held at a pressure in a system of copper tubes. The hydraulic synchronizer is known as the Constantinisco control, commonly called the "C. C." after the military fashion of using initials. This was the device copied for American planes in the war.
In April, 1917, we knew practically nothing about the use or manufacture of aircraft guns. We had used airplanes at the Mexican border, but not one of them carried a machine gun. The Lewis gun, which is a flexible type of aircraft weapon pointed on a universal pivot by the observer in a two-place plane, was being manufactured by the Savage Arms Corporation for the British Government; but we had never made a gun of the fixed type in this country, nor did we know anything about the construction or manufacture of synchronizers.
One special requirement of the aircraft machine gun is that it must be reliable in the extreme. It is bad enough to have a gun jam on the ground, but in the air it may be fatal, for little can be done there to repair the weapon. A jam leaves the gunner to the mercy of his adversary, so in the production of aircraft armament there must be not only special care in the manufacture of the guns, but the ammunition, too, must be as perfect as human accuracy can make it. The cartridges must be either hand-picked and specially selected from the run of service ammunition, or else manufactured slowly and expressly for the purpose, with minute gauging from start to finish of the process.
Another requirement for the aircraft gun is that it must function perfectly in any position. On the ground a machine gun is fired essentially in a horizontal position, but the airman dives and leaps in his maneuvering and must be able to shoot at any instant.
Aircraft guns are subject to extreme variations of temperature, and so they must be certain to function perfectly in the zero cold of the high altitudes, regardless of the contraction of their metal parts.
Then, too, such guns must be able to fire at a much greater rate than those of the ground service. Five hundred shots per minute is regarded as sufficient for a ground gun, but aircraft guns have been brought up to a rate of fire as high as 950 to 1,000 shots per minute. The Browning aircraft gun, never used by us, but in process of development when the armistice was signed, had been speeded up to 1,300 shots per minute, with all shots synchronized to miss the blades of the propeller.