Synchronization gear
A synchronization 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 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. However, the first practicalif far from reliablegear 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 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. 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 would therefore obstruct the gun six times every firing cycle of the gun, 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, 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.
True synchronization, though, with a machine gun's rate of fire exactly proportional to the revolutions per minute of a spinning aircraft propeller, would require an impractical level of complexity. A machine gun normally fires a constant number of rounds a minute, 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 the throttle setting and what maneuvers were 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.
It has been pointed out that any mechanism that achieved the feat of firing between the whirling blades of a propeller without striking them could be described as "interrupting" the fire of the gun, and also as "synchronizing", or "timing" its fire to coincide with the revolutions of the propeller.
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.The timing of each impulse had to be adjusted to coincide with a "safe" period, when the blades of the propeller were well out of the way, and this adjustment had to be checked at intervals, especially if the propeller was changed or refitted, as well as after a major engine overhaul. Faults in this adjustment could well result in every bullet fired hitting the propeller, a worse result than if the gun was fired through the propeller with no control at all. The other main type of failure resulted in fewer or no firing impulses, usually due to the generator or linkages either jamming or breaking. This was a common cause of synchronized guns "jamming".
The speed of the propeller, and thus the distance that it travelled between the firing of the gun and the arrival of the bullet at the propeller disc, varied as the rate of engine revolutions changed. Where muzzle velocity was very high, and the guns were sited well forward so that the bullets had a very short distance to reach the disc of the propeller, this difference could be largely ignored. But in the case of relatively low muzzle velocity weapons, or any gun sited well back from the propeller, the question could become critical, and in some cases the pilot had to consult his tachometer, taking care that his engine revolutions were within a "safe" range before firing, otherwise risking speedy destruction of his propeller.
At the gun
The second requirement was for a gun that would reliably fire exactly when required. Not all automatic weapons were equally amenable to synchronization. When it was ready to fire, a synchronized machine-gun needed to have a round in the breech, the breech closed, and the action cocked. Several widely used automatic weapons were triggered from an open bolt, with an unpredictable interval between triggering and firing, and were thus not suitable for synchronization without extensive modification.In practice it was found necessary for the gun to be fired in semi-automatic mode. As the propeller revolved, a series of firing impulses was transmitted to the gun, each of which could trigger it to fire a single shot. The majority of these impulses would catch the gun in the process of ejecting a spent round or loading a fresh one, and would thus have no effect; but as soon as the firing cycle was completed, the gun would be ready to fire as soon as it received the next impulse from the synchronizing gear. The delay between the end of the firing cycle and the arrival of the next firing impulse slowed the rate of fire in comparison with a free-firing machine gun, which fires the moment it is ready to do so; but provided the gear functioned correctly, the gun could fire fairly rapidly between the whirling propeller blades without striking them.
Some other machine-guns, such as the Austrian Schwarzlose and the American Marlin, proved less than perfectly adapted to synchronization, although eventually predictable "single shot" firing was achieved, typically by modifying the trigger mechanism to emulate "closed bolt" firing. Most weapons that were successfully synchronized were based on the original Maxim gun of 1884, a closed bolt weapon operated by barrel recoil. Before these distinctions were fully understood, much time was wasted on attempts to synchronize unsuitable weapons.
Even a closed bolt weapon needed reliable ammunition. If the primer in a cartridge is faulty to the extent of delaying the firing of the gun for a tiny fraction of a second this is of little consequence in the case of a gun in use by infantry on the ground, but in the case of a synchronized "aircraft" gun such a delay can produce a rogue firing, sufficiently "out of time" for it to risk hitting the propeller. A very similar problem could arise where the mass of a special round was different enough to produce a substantial difference in muzzle velocity. This was compounded by the additional risk to the integrity of the propeller due to the nature of the round.
The "trigger motor" could theoretically take two forms. The earliest patent assumed that the synchronization gear would periodically prevent the gun from firing, thus operating as a true, or literal "interrupter". In practice all "real-life" synchronization gears, for which we have reliable technical details, directly fired the gun: operating it as if it were a semi-automatic weapon rather than a completely automatic one.
The linkage between propeller and gun
The third requirement is for a linkage between the "machines" to be synchronized. Many early gears used an intricate and inherently fragile bell crank and push rod linkage that could easily jam or otherwise malfunction, especially when required to work at higher speeds than it had been designed for. There were several alternative methods, including an oscillating rod, a flexible drive, a column of hydraulic fluid, a cable, or an electrical connection.Generally, mechanical systems were inferior to hydraulic or electric ones, but none were ever entirely foolproof, and synchronization gears at best always remained liable to occasional failure. The Luftwaffe ace Adolf Galland in his memoir of the war period The First and the Last describes a serious faulty synchronization incident in 1941.