Editing Synchronization gear (section)

=== At the propeller ===
[[File:Severed propeller blade.jpg|thumb|Propeller of an Albatros C.III. One blade severed by a faulty or badly adjusted synchronization gear]]

First, a method of determining the position of the propeller at a given instant was required. Typically, a [[Cam (mechanism)|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.<ref name=Volker2p78/> 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.
[[File:Synchronised gun - catastrophic failure.svg|thumb|upright|Synchronised gun firing badly "out of sync". All or most rounds strike one blade of propeller, quickly destroying it]]
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 (for example, a cam wheel slipping a millimetre or two, or a pushrod flexing)<ref group="Note">The normal expansion and contraction due to changing temperature was quite enough, especially for longer rods.</ref> 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,<ref name=Volker4p60>Volker 1992, pt. 4, p. 60</ref> 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.<ref group="Note">This phenomenon was particularly marked in Austro-Hungarian fighters armed with the [[Schwarzlose MG M.07/12|Schwarzlose gun]]: which had a low muzzle velocity and very marginal suitability for synchronization.</ref>