A synchronization gear (also known as a gun synchronizer or interrupter gear) was a device enabling a single-engine tractor configuration aircraft to fire its forward-firing armament through the arc of its spinning propeller without bullets striking the blades. This allowed the aircraft, rather than the gun, to be aimed at the target.
There were many practical problems, mostly arising from the inherently imprecise nature of an automatic gun's firing, the great (and varying) velocity of the blades of a spinning propeller, and the very high speed at which any gear synchronizing the two had to operate. In practice, all known gears worked on the principle of actively triggering each shot, in the manner of a semi-automatic weapon.
Design and experimentation with gun synchronization had been underway in France and Germany in 1913–1914, following the ideas of August Euler, who seems to have been the first to suggest mounting a fixed armament firing in the direction of flight (in 1910). However, the first practical—if far from reliable—gear to enter operational service was that fitted to the Fokker Eindecker fighters, which entered squadron service with the German Air Service in mid-1915. The success of the Eindecker led to numerous gun synchronization devices, culminating in the reasonably reliable hydraulic British Constantinesco gear of 1917. By the end of the First World War, German engineers were well on the way to perfecting a gear using an electrical rather than a mechanical or hydraulic link between the engine and the gun, with the gun being triggered by a solenoid rather than by a mechanical "trigger motor".
From 1918 to the mid-1930s the standard armament for a fighter aircraft remained two synchronized rifle-calibre machine guns, firing forward through the arc of the propeller. During the late 1930s, however, the main role of the fighter was increasingly seen as the destruction of large, all-metal bombers, for which this armament was inadequately light. Since it was impractical to try to fit more than one or two extra guns in the limited space available in the front of a single-engine aircraft's fuselage, this led to an increasing proportion of the armament being mounted in the wings, firing outside the arc of the propeller. The conclusive redundancy of synchronization gears did not, however, finally come until the introduction of jet propulsion and the absence of a propeller for guns to be synchronized with.
Nomenclature
A mechanism to enable an automatic weapon to fire between the blades of a whirling propeller is usually called an interrupter or synchronizer gear. Both these terms are more or less misleading, at least insofar as explaining what happens when the gear functions.
The term "interrupter" implies that the gear pauses, or "interrupts" the fire of the gun at the point where one of the blades of the propeller passes in front of its muzzle. The difficulty is that even the relatively slowly revolving propellers of First World War aircraft typically turned twice or even three times for each shot a contemporary machine gun could fire. A two-bladed propeller would therefore obstruct the gun six times every firing cycle of the gun, a four-bladed one twelve times. Another way of putting this is that an "interrupted" gun would have been "blocked" more than forty times every second, while it was firing at a rate in the region of seven rounds per second. Unsurprisingly, the designers of so-called interrupter gears found this too problematic to be seriously attempted, as the gaps between "interruptions" would have been too short to allow the gun to fire at all.
And yet, "synchronization", in the usual sense of the word, between the rate of fire of a machine gun (firing as such, in a fully automatic manner) and the revolutions per minute of a spinning aircraft propeller is also a conceptual impossibility. A machine gun normally fires a constant number of rounds a minute, and while this may be boosted by, for instance, strengthening and increasing the tension on a return spring, or redirecting the gasses produced by each firing, it cannot be varied at will while the gun is operating. On the other hand, the propeller of an aircraft, especially before the advent of the constant-speed propeller, turned at widely differing rates of revolution per minute, depending on the throttle setting, and whether the aircraft was climbing, flying level, or diving. Even if it had been feasible to pick a particular point on an aircraft engine's tachometer at which a machine gun's cyclic rate would permit it to fire through the propeller arc, this would be very limiting.