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The term covers two related technologies: the first, more accurately referred to as synchronization gear, or a gun synchronizer, is attached to the armament of a tractor-type craft so that it can 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 aerial warfare and remained in operational use until the Korean War, after which the universal adoption of propeller-less jet aircraft rendered such gears unnecessary.
The other, true interrupter gear stops the firing of the machine gun when some part of the aircraft is 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 gun turret mounts on bomber aircraft.
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, essentially with the rotating parts of the engine (the crankshaft and other parts connected to it with an inline engine, the crankcase with a rotary engine) firing the gun. In reality, the technical difficulties associated with reliably halting, or co-ordinating, 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. August Euler applied for German Patent No. DRP 248.601 for a fixed forward mounting on an airplane for a machine gun in 1910.
However, 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 1 April 1915, flying for MS26, Garros shot down his first German aircraft, killing both the crew. 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.
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, all the way to the time of the Third Reich in 1933.
The only known date that is certain in the development of Fokker's pioneering Stangensteuerung system is on May 16, 1915, when an officer of the Bavarian armed forces, working with Idflieg, alerted his province of Germany that "firing trials of an interesting nature, from a light monoplane, were to take place on the 19th of 20th of May". The trials were meant to take place at the famous Döberitz proving ground near Berlin, but no record of any sort remains to definitively state what happened there.
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 for almost the entire first year of hostilities in World War I by Leutnant Otto Parschau, marked with the IdFlieg number A.16/15. This aircraft, with factory serial number (or Werknummer) 216, for the Fokker E.I — was demonstrated on 19–20 May 1915 and shipped to the Western Front on 30 May 1915. The five M.5K/MG production prototypes, with Fokker factory serial numbers 191 through 194, and s/n 198, received the IdFlieg military serial numbers E.1/15 through E.5/15, followed Parschau's A.16/15 aircraft into military trials very shortly thereafter, with Leutnant Parschau himself receiving E.1/15 as the replacement for his then worn-out No. 216 aircraft, which was returned to the Fokker factory for further trials with the lMG 08 "Spandau" air-cooled machine gun.
The new gear 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 when the propeller was horizontal 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, essentially allowing the engine's own rotation to fire the gun.
The first victory using a synchronized gun-equipped fighter, based on late 20th century research of surviving German and French early World War I aviation records, is strongly believed to have occurred on 1 July 1915 when Leutnant Kurt Wintgens of Feldflieger Abteilung 6b, flying the Fokker M.5K/MG aircraft that bore IdFlieg's 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, as with Wintgens' second engagement only three days later, against a second Morane Parasol. The first "confirmed" victory, in the official German records of that time, would finally go to Wintgens on July 15, 1915, after he was re-assigned to Feldflieger Abteilung 48 near Mühlhausen im Elsaß by downing a third Morane Parasol.
Sole possession of a working gun 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 in which the propeller was behind the pilot, and therefore out of the way of forward firing guns. 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
In December 1915, the Vickers-Challenger interrupter gear was put into production for the Royal Flying Corps and in a few weeks a similar order for the Scarff-Dibovski gear was placed for the RNAS. Neither was a copy of the Fokker gear, as is sometimes reported - work on both was advanced before a captured Fokker was available for technical analysis. The first British aircraft to use these gears was the Sopwith 1½ Strutter which arrived in April 1916, although some other service types were retrofitted with synchronised guns about this time, including the Nieuport 12 and the Bristol Scout. Later British mechanical gears included the Ross, and Sopwith-Kauper gears. The main problem with all 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 Constantinesco synchronization gear (or "CC" gear, invented by Romanian engineer George Constantinesco) which used sonic 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. In addition, it was hard for the Germans to copy captured examples because it appeared to them to be a hydraulic system, whereas the actual principle of operation was based on Constantinescu's "theory of sonics", which remained secret until the end of the war. 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 initially experimented with mounting three machine guns on the E.IV, but the extra weight rendered the aircraft virtually unflyable, and production E.IVs were fitted with two guns. The Allies did not field a production aircraft with twin synchronized guns until the Sopwith Camel and the SPAD S.XIII came into service in mid-1917 although a few (five to 10) Sopwith Triplanes had been fitted with twin guns prior to the arrival of the Camel.
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 twin engine biplane bomber that served with the RAF, and the almost simultaneous introduction of the much more advanced Martin B-10 all-metal monoplane bomber 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 usefulness of synchronization gears naturally disappeared altogether when jet engines eliminated the propeller, at least in fighter aircraft; but gun synchronization, even in single piston engined aircraft, had already been in decline for the decade prior to this.
The increased speeds of the new monoplanes of the late 1930s meant that the time available to deliver a sufficient weight of fire to bring down an enemy was greatly reduced, and two guns were no longer enough. Since it was never practicable to mount more than two synchronized guns in the forward fuselage, any additional guns had to be mounted in the wings. 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 almost as steady a mounting as the fuselage. The retention of fuselage mounted guns, with the additional weight of their synchronization gear (which slowed their rate of fire, albeit only slightly, and still occasionally failed, resulting in damage to propellers) became increasingly unattractive as the extra firepower offered by two machine guns came to represent a decreasing percentage of a fighter's total armament.
Against these arguments, "centralised" guns still rewarded the true marksman, regardless of advances in gunsight technology: their range was limited only by ballistics, as they did not need the "harmonisation" necessary to concentrate the fire of wing mounted guns. These considerations resulted in a reluctance (especially in Germany, Russia, and Japan) to abandon fuselage-mounted guns altogether—in fact, even the old (and highly problematic) idea of mounting a cannon to fire through the centre of the propeller hub for geared-propshaft, inline-engined fighters, pioneered by the World War I-era SPAD S.XII's 37mm calibre moteur-canon, was revived to this same end, notably in the mass-produced Messerschmitt Bf 109 in its Motorkanone mount.
 Further reading
- 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
- Grosz, Peter M., Windsock Datafile No. 91, Fokker E.I/II, Albatros Publications, Ltd. 2002. ISBN 1-902207-46-7
- Guttman, Jon, et al. Pusher Aces of World War 1. Osprey Pub Co, 2009. ISBN 1-84603-417-5, ISBN 978-1-84603-417-6.