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 triggered by an electro-mechanical solenoid.
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. In 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 inadequate. Since it was impractical to fit more than two guns in the limited space available in the front of a single-engine aircraft's fuselage, guns began to be mounted in the wings instead, firing outside the arc of the propeller so not requiring synchronising. Synchronizing became unnecessary on all aircraft with the introduction of propellerless jet propulsion.