Interrupter gear

The interrupter gear, also known as synchronisation gear, was a triggering device attached to a fighter aircraft's machine gun so that it would fire only at certain times. This allowed machine guns to be mounted directly in front of the pilot, firing through the propeller. Introduced during the First World War, the interrupter or synchroniser guaranteed that the gun would only fire when the propeller was not in the way and thus avoided damaging it.

Though their effects were the same, there was a subtle difference between the mechanisms of the interrupter and the synchroniser. A machine gun fitted with interrupter gear had the trigger normally enabled and the interrupter mechanism would disable the trigger when a propeller blade was in the way. A machine gun fitted with synchronisation gear had the trigger normally disabled and the synchroniser mechanism would enable the trigger when the propeller was clear.

Experimentation with gun synchronisation had been underway in France and Germany before the First World War but the engineers involved received little support or encouragement from the military who disregarded the need for armed aircraft, believing them solely useful for reconnaissance. Swiss engineer Franz Schneider, working for LVG, designed and patented a synchroniser in 1913. French aircraft designer Raymond Saulnier built and patented a practical gun synchroniser in April 1914, having borrowed a machine gun from the army for testing. No design was developed to the point of being operational in the field, one significant problem being the inconsistency of ammunition propellant resulting in hang fire rounds.

Saulnier pursued a simpler method using armoured propeller blades. In December 1914, French pilot Roland Garros approached Saulnier to arrange for this device to be installed on his aeroplane but it was not until March 1915 that he took to the air with a forward-firing Hotchkiss 8mm (.303 in) machine gun mounted on his Morane-Saulnier Type L. In addition to the armoured blades, Garros's mechanic, Jules Hue, attached deflector wedges to the blades. While this reduced the chance of a dangerous ricochet, the wedges severely diminished the propeller's performance.

On 18 April, 1915, having shot down three German aircraft, Garros' plane was forced down in German territory. Before he could burn his aircraft, he was captured and the gun and propeller were sent for evaluation by the Inspektion der Fliegertruppen (Idflieg) at Döberitz near Berlin.

Popular accounts claim that Dutch aircraft designer Anthony Fokker was then asked to reproduce Saulnier's deflectors and proceeded to invent the interrupter system in a matter of days — according to some accounts, Fokker was given the problem on a Tuesday evening and presented a working system on Friday. However, Fokker's team, including engineer Heinrich Lübbe, had been working on an interrupter mechanism since late 1914, probably based on Schneider's patent. Indeed in 1916 LVG and Schneider sued Fokker for patent infringement — the battle continued until 1933 and though the courts repeatedly found in Schneider's favour, Fokker refused to acknowledge the rulings.

Fokker's team adapted their system to work with the new Parabellum lMG 14 machine gun fitted to a Fokker A.III single-seat monoplane (a military version of the Fokker M.5K). This aircraft — the prototype for the Fokker E.I — was demonstrated on 19–20 May, 1915 and shipped to the Western Front on 30 May, 1915.

The solution used a cam attached to the propeller shaft that pressed on a long rod running to the trigger of the guns. The cam was set such that the propeller was horizontal when it pushed on the rod, and the rod in turn pressed the trigger to fire a bullet. The trigger operated by the pilot pulled the rod into position over the cam.

The first victory using an interrupter-equipped fighter is believed to have occurred on 1 July, 1915 when Leutenant Kurt Wintgens of Feldflieger Abteilung 67, flying a modified M.5K, forced down a French Morane-Saulnier Type L east of Lunéville however the plane landed in French territory and the victory could not be confirmed. The first confirmed victory went to Max Immelmann flying a Fokker E.I on 1 August, 1915, forcing down a Royal Flying Corps B.E.2c. This marked the beginning of the Fokker Scourge.

The first British aircraft to use interrupter gear was the Sopwith 1½ Strutter which arrived in April 1916. As the technology developed, both sides adopted the synchronisation mechanism as standard though the S.E.5 continued to use Constantinesco interrupter gear.

The main problem with the interrupter or synchroniser was the reduction in the rate of fire of the machine gun. A pilot would usually only have the target in his sights for a fleeting moment so a concentration of bullets was vital for achieving a kill. The obvious solution was to increase the number of guns. The final version of the Fokker Eindecker, the Fokker E.IV, came with two Spandau machine guns and this became the standard armament for all the German D-type scouts starting with the Albatros D.I. Fokker experimented with mounting three machine guns on the E.IV but the extra weight rendered the aircraft virtually unflyable. The British did not field an aircraft with synchronised twin guns until the Sopwith Camel and the French SPAD S.XIII.

The interrupter gear remained widely used until the jet engine replaced propeller-driven fighters. As aircraft gunsight technology improved, however, the importance of locating the guns in front of the pilot diminished, and many fighters of World War II had wing-mounted guns instead that simply fired to the sides of the propeller.de:Unterbrechergetriebe pl:Zsynchronizowany karabin maszynowy sl:Sinhronizator streljanja