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 Romanian 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.
Nomenclature A mechanism that achieves the feat of firing between whirling blades of a propeller without striking them could be described as "interrupting" the fire of the gun (to the extent that it no longer actually works as an automatic weapon at all), and also as "synchronizing", or "timing", its fire to coincide with the revolutions of the propeller. 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. Even the relatively slowly revolving propellers of First World War aircraft, however, typically turned twice or even thrice for each shot a contemporary machine gun could fire. A two-bladed propeller could therefore obstruct the gun up to six times every firing cycle, a four-bladed one twelve times. A gun set up this way would be interrupted more than forty times per second, while firing at only around seven rounds per second. Unsurprisingly, designers of so-called interrupter gears found this too problematic to be seriously attempted, as the gaps between the interruptions would have been too short to allow the gun to fire at all. True synchronization, though, with a machine gun's rate of fire exactly proportional to the rotational speed of an aircraft propeller, would require an impractical level of complexity. A machine gun normally fires at a fixed rate, and, while this may be changed by modifying the gun, it cannot be varied at will while the gun is operating. The rate of rotation of an aircraft propeller, meanwhile, especially before the advent of the constant-speed propeller, could vary widely depending on throttle setting and maneuvers being performed. 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.
Components A typical synchronizing gear had three basic components.
At the propeller
First, a method of determining the position of the propeller at a given instant was required. Typically, a cam, driven either directly from the propeller shaft itself, or from some part of the drive train revolving at the same speed as the propeller, generated a series of impulses at the same rate as the propeller's revolutions. There were exceptions to this. Some gears placed the cam within the gun trigger mechanism itself, and the firing impulses were sometimes timed to occur at every two or three revolutions of the propeller, or, especially in the case of hydraulic or electric gears, at the rate of two or more for each revolution. The diagrams in this section assume, for simplicity's sake, one impulse for one revolution, so that each synchronized round is "aimed" at a single spot on the propeller disc.
… excerpt ends here. Continue reading the full article.
