Interrupter gear

Interrupter gear

Interrupter gear is a term that covers two related technologies.

The first is the synchronization gear, which is often incorrectly referred to as "interrupter gear"; this is a triggering device attached to the machine gun armament of a tractor-type fighter aircraft so that it would fire only at certain times. This allows 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 when the widespread adoption of jet aircraft rendered them obsolete.

A true interrupter gear stopped the firing of the machine gun when some part of the aircraft was in the way. For much of the early history of the fighter aircraft this was limited to the propeller. This would change with the introduction of turret mounted armament firstly to the bomber aircraft and briefly to the fighter.

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.


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 Luftverkehrs Gesellschaft, 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 8 mm (.323 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 diminished the propeller's efficiency. 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

This initiated a concentrated phase of consideration of the interrupter gear concept. The Dutch aircraft designer Anthony Fokker was heavily involved in this process but the story of his conception, development and installation of a synchronisation device in a period of 48 hours (first found in an authorised biography of Fokker written in 1929) has been shown to be not factual. The available evidence points to a synchronisation device having been in development by Fokker's team including engineer Heinrich Lübbe, and probably based on Schneider's patent, for perhaps six months prior to the capture of Garros' machine. [van Wyngarden, G. Early German Aces of World War 1. Oxford: Osprey Publishing, 2006. ISBN 1-84176-997-5]

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.Fact|date=July 2008

Fokker's team adapted their system to work with the new Parabellum MG14 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 the basic principles involved were already common knowledge, and by the middle of 1916 several Allied synchronizer gears were already available in quantity.

Further development

The first British aircraft to use a synchronizer gear was the Sopwith 1½ Strutter which arrived in April 1916 equipped with the Ross synchronizer, although some other service types were retrofitted with synchronised guns about this time, including the Nieuport 12 and the Bristol Scout. The main problem with early mechanical synchronizers was that the rate of fire of the machine gun was dependent on the engine revolutions, and was slowed, especially when the engine was throttled back. The mechanical linkages were also very liable to failure, resulting in the unfortunate pilot shooting away his own propeller.

Eventually all British aircraft were equipped with the superior hydraulic Constantinesco synchronization gear (or "CC" gear, invented by Romanian engineer George Constantinesco) which used impulses transmitted by a column of liquid instead of a mechanical system of linkages. This was not only inherently more reliable, but delivered firing impulses at a much higher rate, so that a synchronised gun now fired at more or less the same rate as a normal machine gun, regardless of engine revolutions. The gear could also be easily fitted to any type of aircraft instead of having to have type-specific linkages designed. 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.

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 twin synchronized guns until the Sopwith Camel and the SPAD S.XIII came into service (mid 1917).


With the introduction of the "high speed" bomber came a need to protect the gunner from the elements and to give protection but retain the wide firing arcs and so the power driven multi-gun turret evolved. One of the first instances was the single nose mounted turret of the Boulton Paul Overstrand that served with the RAF, and the almost simultaneous introduction of the much more advanced Martin B-10 with the US Army Air Corps.

For maximum efficiency the bomber turret needed to be able to rotate in all directions and cover as wide a range of elevation as possible - this meant that there would be some combinations of elevation and direction where the turret was aiming at some part of the aircraft itself. To prevent the guns firing an electrical system was used. The guns were fired by solenoids and by introducing a break in the electrical power to the guns that coincided with the forbidden arcs of fire the aircraft would be safe from its own guns. The Boulton Paul design used a brass drum and brush contacts that corresponded to the direction of the turret and angle of the guns. Where the brass was removed and replaced with insulating material the electrical circuit would be broken and the guns prevented from firing.

The End of Synchronization

The need for synchronization gears finally disappeared altogether when jets replaced propeller-driven fighters - but their use, even in piston engined aircraft, had already declined - especially after 1940. It was no longer practicable to mount guns with their firing mechanisms within the cockpit, which eliminated one of the reasons for having guns mounted in the fuselage in the first place. The importance of locating the guns in front of the pilot also diminished, as aircraft gunsight technology improved and the requirement for heavier armament increased. Cantilever monoplane wings provided much more space than the fuselage to mount armament - and being much more rigid than the old cable braced wings they provided a much steadier mounting. The result was that many fighters of the World War II era had all their guns mounted in the wings outside the propeller disc - their direction of fire "harmonised" to converge at a preset distance. Nevertheless, German and Soviet fighter types in particular were slow to abandon fuselage-mounted guns, usually in addition to wing mounted guns.

The last synchronizer-equipped aircraft to see combat action were the Lavochkin La-11 and the Yakovlev Yak-9 during the Korean War.


*Woodman, Harry, "CC Gun Synchronisation Gear", "Aeroplane Monthly", September 2005
*Jarrett, Phillip, "The Fokker Eindeckers", "Aeroplane Monthly", December 2004
*"The Electro-Hydraulic Turret", "The Aeroplane" No: 1654, February 1943

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