Interrupter gear
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Synchronization gear, popularly and incorrectly known as interrupter gear, was a triggering device attached to the machine gun of a tractor-type fighter aircraft so that it would fire only at certain times thereby allowing the gun to fire through the arc of a spinning propeller without the bullets striking the blades. Introduced during the First World War, the gun synchronizer was a significant development in the history of air combat and remained in operational use until the Korean War.
Though their effects were the same, there was a subtle difference between the concept of the interrupter and the synchronizer. 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 synchronization gear had the trigger normally disabled and the synchronizer mechanism would enable the trigger when the propeller was clear. In reality, the technical difficulties associated with reliably halting the firing of a Maxim-type machine gun meant that no working interrupter system was ever developed — all successful implementations used the concept of synchronization.
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Origins
Experimentation with gun synchronization 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 synchronizer in 1913. French aircraft designer Raymond Saulnier built and patented a practical gun synchronizer 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.
Fokker's synchronizer
Popular accounts claim that Dutch aircraft designer Anthony Fokker was then asked to reproduce Saulnier's deflectors and proceeded to invent the synchronization 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 synchronization 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.
Image:Interrupter gear diagram en.png
Fokker's team adapted their system to work with the new Parabellum lMG 14 machine gun fitted to a Fokker A.III unarmed single-seat monoplane (a military version of the Fokker M.5K) usually flown by Leutnant Otto Parschau. This aircraft — the first example of the five M.5K/MG production prototypes 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 a synchronized gun-equipped fighter is believed to have occurred on 1 July 1915 when Lieutenant Kurt Wintgens of Feldflieger Abteilung 67, flying the Fokker M.5K/MG serial number 'E.5/15', 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.
Sole possession of a working synchronizer enabled Germany to dominate the Western Front skies in a period known as the Fokker Scourge. Initially lacking a synchronizer, the Royal Flying Corps relied on pusher aircraft such as the Vickers F.B.5 Gunbus and the Airco D.H.2 which did not have the problem of firing forwards through the propeller. Germany was protective of the synchronizer system, instructing pilots not to venture over enemy territory in case they were forced down and the secret revealed, but by 1916 the Allies had developed various synchronizer mechanisms of their own, usually based cams and links like the Fokker mechanism.
Further development
The first British aircraft to use synchronizer gear was the Sopwith 1½ Strutter which arrived in April 1916 equipped with the Ross synchronizer. Eventually all British aircraft were equipped with the superior Constantinesco synchronization gear (or "CC" gear, invented by Romanian engineer George Constantinesco) which used the impulse on a column of liquid instead of a mechanical system of linkages, though it still relied on a cam to initiate the impulse. The Constantinesco gear remained in use with the Royal Air Force until the Second World War, the Gloster Gladiator being the last British fighter to be equipped with it.
The main problem with the synchronizer 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 Allies did not field an aircraft with synchronized twin guns until the Sopwith Camel and the SPAD S.XIII.
Synchronization gear remained in use 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 fired to the sides of the propeller and their bullet trajectories were "harmonised" to converge at a preset distance. Nevertheless, most air forces, with the notable exception of the RAF, continued to develop fighters with fuselage-mounted guns that required synchronization. The last synchronizer-equipped aircraft to see combat action were the Lavochkin La-11 and the Yakovlev Yak-9 during the Korean War.
References
- Woodman, Harry, "CC Gun Synchronisation Gear", Aeroplane Monthly, September 2005
- Jarrett, Phillip, "The Fokker Eindeckers", Aeroplane Monthly, December 2004de:Unterbrechergetriebe
pl:Zsynchronizowany karabin maszynowy sl:Sinhronizator streljanja zh:射击断续器