page_title: Synchronization gear
section_title: At the propeller
hierarchical_section_title: Synchronization gear / Definitions / Basic problems / At the propeller
caption_attribution_description: English: Propeller of an Albatros C.III - one blade severed by faulty synchronisation gear
context_page_description: A synchronization gear, or a gun synchronizer, sometimes rather less accurately called an interrupter, is attached to the armament of a single-engine tractor-configuration aircraft so it can fire through the arc of its spinning propeller without bullets striking the blades. The idea presupposes a fixed armament directed by aiming the aircraft in which it is fitted at the target, rather than aiming the gun independently.
  There are 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 has to operate.
  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 practical—if far from reliable—gear to enter operational service was that fitted to the Eindecker monoplane fighters, which entered squadron service with the German Air Service in mid-1915.
context_section_description: Firstly, 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 (or, say, a cam wheel slipping a millimetre or two, or a pushrod flexing) 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 involved a break in the stream of firing impulses, usually due to the generator or linkages either jamming or breaking (or disintegrating). This simply meant the gun no longer fired, and 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.
caption: Propeller of an Albatros C.III. One blade severed by a faulty or badly adjusted synchronization gear
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