From the beginnings of practical flight, possible military uses for aircraft were considered, although not all writers came to positive conclusions on the subject. By 1913,
military exercises in Britain, Germany, and France had confirmed the likely usefulness of aircraft for reconnaissance and surveillance, and this was seen by a few forward looking officers as implying the need to deter or destroy the enemy's reconnaissance machines. Thus aerial combat was by no means entirely unanticipated, and the machine gun was from the first seen as the most likely weapon to be used. What was not generally agreed on was the superiority, at least for an attacking aircraft, of
fixed forward-firing guns, aimed by pointing the aircraft at its target, rather than flexible weapons, aimed by a gunner other than the pilot. As late as 1916, pilots of the
DH.2 pusher fighter had problems convincing their senior officers that the forward-firing armament of their aircraft was more effective if it was fixed to fire forward rather than being flexible. On the other hand, August Euler had patented the idea of a fixed gun as early as 1910 – long before
tractor aircraft became the norm, illustrating his patent with a diagram of a machine gun-armed
pusher. The linkage between the propeller and the gun is achieved with a spinning drive shaft, rather than a reciprocating rod. The impulses needed to operate the trigger, or in this case to prevent the trigger from operating, are produced by a cam wheel with two lobes at 180° apart situated at the gun itself since firing is to be interrupted by both blades of the propeller. No attempt was made (so far as is known) to build or test an actual operating gear based on this patent, which attracted little or no official interest at the time.
The Raymond Saulnier patent (1914) Unlike the Schneider patent design, Saulnier's device was actually built, and may be considered the first practical synchronization gear to be tested. For the first time, the cam producing the to-and-fro movement conveying firing impulses to the gun is situated at the engine (driven in this case by the same spindle that operated the oil pump and the tachometer) and the impulses themselves are transmitted by a reciprocating rod rather than Schneider's rotating shaft. The idea of literally "interrupting" the firing of the gun gives way (probably as the result of experience) to the principle of pulling the trigger for each successive shot, like the action of a semi-automatic weapon. It has been pointed out that this was a practical design that should have worked, but it did not. Apart from possible inconsistencies in the ammunition supplied, the real problem was that the gun used to trial the gear, a gas-operated
Hotchkiss 8 mm (.323 in) machine gun borrowed from the French army, was fundamentally unsuitable for "semi-automatic" firing. Following initial unsuccessful tests, the gun had to be returned, and the experiments ceased. was the expedient of firing straight through the propeller arc and "hoping for the best". A high proportion of bullets would in the normal course pass the propeller without striking the blades, and each blade might typically take several hits before there was much danger of its failing, especially if it were bound with tape to prevent splintering (see diagram below, and illustration to the left). Garros himself and his personal mechanic Jules Hue are sometimes credited with testing and perfecting the "deflectors". This crude system worked after a fashion, although the wedges diminished the propeller's efficiency, and the not inconsiderable force of the impact of bullets on the deflector blades must have put undesirable stress on the engine's crankshaft.
Anthony Fokker was able to persuade Idflieg to arrange the loan of a Parabellum machine gun and ammunition so that
his device could be tested, and for these items to be transported forthwith to the
Fokker Flugzeugwerke GmbH at
Schwerin (although probably
not in his railway compartment or "under his arm", as he claimed after the war). The story of his conception, development and installation of the Fokker synchronization device in a period of 48 hours (first found in an authorised biography of Fokker written in 1929) is not now believed to be factual. Another possible explanation is that Garros's Morane, partly destroyed by fire as it was, had sufficient traces of the original synchronization gear remaining for Fokker to have guessed how it worked. For various reasons this also seems unlikely, and the current historical consensus points to a synchronization device having been in development by Fokker's team (including engineer
Heinrich Lübbe) prior to the capture of Garros's machine. but ''Saulnier's''. Like the Saulnier patent, Fokker's gear was designed to actively fire the gun rather than interrupt it, and, like the later Vickers-Challenger gear developed for the RFC, it followed Saulnier in taking its primary mechanical drive from the oil pump of a rotary engine. The "transmission" between the motor and the gun was by a version of Saulnier's reciprocating push-rod. The main difference was that instead of the push rod passing directly from the engine to the gun itself, which would have required a tunnel through the firewall and fuel tank (as shown in the Saulnier patent drawings), it was driven by a shaft joining the oil pump to a small cam at the top of the fuselage. This eventually proved unsatisfactory, as the oil pump's mechanical drive spindle was insufficiently robust to take the extra load. This prototype was demonstrated to IdFlieg by Fokker in person on 19–20 May 1915 at the
Döberitz proving ground near Berlin.
Leutnant Otto Parschau was test flying this aircraft by 30 May 1915. The five production prototypes (factory designated
M.5K/MG and serialed E.1/15 – E.5/15) were undergoing military trials shortly thereafter. These were all armed with the Parabellum gun, synchronized with the first version of the Fokker gear. This prototype gear had such a short life that a redesign was necessary, producing the second, more familiar, production form of the gear. The gear used in the production
Eindecker fighters replaced the oil pump's mechanical driveshaft-based system with a large cam wheel, almost a light flywheel, driven directly from the
spinning rotary engine's crankcase. The push rod now took its reciprocating motion directly from a "follower" on this cam wheel. At the same time the machine gun used was also changedan
lMG 08 machine gun, the so-called "Spandau", replacing the Parabellum used with the prototype gear. At this time the Parabellum was still in very short supply, and all available examples were required as observers' guns, the lighter and handier weapon being far superior in this role. The first victory using a synchronized gun-equipped fighter is now believed to have occurred on 1 July 1915 when
Leutnant Kurt Wintgens of
Feldflieger Abteilung 6b, flying the Parabellum-armed Fokker M.5K/MG aircraft "E.5/15", forced down a French
Morane-Saulnier Type L east of
Lunéville. Exclusive possession of a working gun synchronizer enabled a period of German air superiority on the
Western Front known as the
Fokker Scourge. The German high command 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 synchronizers were already available in quantity. By this time, the Fokker
Stangensteuerung gear, which had worked reasonably well for synchronizing a single gun, firing at a modest cyclic rate through a two-bladed propeller driven by a rotary engine, was becoming obsolete.
Stangensteuerung gears for "stationary",
i.e., in-line engines, worked from a small cam immediately behind the propeller (see illustration). This produced a basic dilemma: A short, fairly robust push rod meant that the machine gun had to be mounted well forward, putting the breech of the gun out of the pilot's reach for clearing jams. If the gun was mounted in the ideal position, within easy reach of the pilot, a much longer push rod was required, which tended to bend and break. The other problem was that the
Stangensteuerung never worked well with more than one gun.
Two (or even three) guns, mounted side by side
and firing simultaneously, would have produced a wide spread of fire that would have been impossible to match with the "safe zone" between the whirling propeller blades. Fokker's initial answer to this was the fitting of extra "followers" to the ''Stangensteuerung's'' large cam wheel, to (theoretically) produce the "ripple" salvo necessary to ensure that the guns were aimed at the same point on the propeller disc. This proved a disastrously unstable arrangement in the case of three guns, and was rather less than satisfactory, even for two. Most of the early Fokker and Halberstadt biplane fighters were limited to a single gun for this reason. In fact, the builders of the new Albatros twin-gunned stationary-engine fighters of late 1916 had to introduce their own synchronization gear, known as the
Hedtke gear or
Hedtkesteuerung, and it was evident that Fokker were going to have to come up with something radically new. The firing button for the gun simply engaged a clutch at the engine which set the flexible drive (and thus the trigger motor) in motion. In some ways this brought the new gear closer to the
original Schneider patent (q.v.). A major advantage was that the adjustment (to set where on the propeller's disc each bullet was to impact) was now in the gun itself. This meant that each gun was adjusted separately, an important feature, since twin synchronized guns were not set to be fired in strict unison, but when they were pointing at the same point on the propeller disc. Each gun could be fired independently, since it had its own flexible drive, linked to the engine camshaft by a junction box, and having its own clutch. This provision of a quite separate set of components for each gun also meant that a failure in the gear for one gun did not impinge on the other. This gear was available in numbers by mid 1917, in time for installation on the
Fokker Dr.I triplane and all later German fighters. In fact it became the standard synchronizer for the Luftstreitkräfte for the remainder of the war, although experiments to find an even more reliable gear continued. The aircraft crashed on its way to the front and nothing more was heard of it, or its synchronization gear, although it was presumably based on Schneider's own patent. The system was specifically intended to overcome the problems that had arisen in applying the Fokker
Stangensteuerung gear to in-line engines and twin gun installations, and was a variation of the rigid push-rod system, driven from the rear of the crankshaft of the
Mercedes D.III engine. The
Albatros D.V used a new gear, designed by
Werkmeister Semmler: (the
Albatros-Semmler Steuerung). It was basically an improved version of the Hedtke gear.
Electrical gears Post First World War German fighters were fitted with electrical synchronizers. In such a gear, a contact or set of contacts, either on the propeller shaft itself, or some other part of the drive train revolving at the same number of revolutions per minute, generates a series of electrical pulses, which are transmitted to a solenoid driven trigger motor at the gun. Unlike the French and Italians, who were eventually able to acquire supplies of Vickers guns, the Austrians were unable to obtain sufficient quantities of "Spandaus" from their German allies and were forced to use the Schwarzlose in an application for which it was not really suited. Although the problem of synchronizing the Schwarzlose was eventually partially solved, it was not until late 1916 that gears were available. Even then, at high engine revolutions Austrian synchronizer gears tended to behave very erratically. Austrian fighters were fitted with large
tachometers to ensure that a pilot could check that his "revs" were within the required range before firing his guns, and propeller blades were fitted with an electrical warning system that alerted a pilot if his propeller was being hit. There were never enough gears available, due to a chronic shortage of precision tools; so that production fighters, even the excellent Austrian versions of the
Albatros D.III, often had to be sent to the front in an unarmed state, for squadron armourers to fit such guns and gears as could be scrounged, salvaged or improvised. Rather than standardising on a single system, different Austrian manufacturers produced their own gears. The research of Harry Woodman (1989) identified the following types:
Zahnrad-Steuerung (cogwheel-control) Drive was from the camshaft operating rods of the
Austro-Daimler engine via a wormgear. The early Schwarzlose gun had a synchronized rate of 360 rounds per minute with this gear – this was later boosted to 380 rounds with the MG16 model.
Bernatzik-Steuerung Drive was taken from the rocking arm of an exhaust valve, a lever fixed to the valve housing transmitting impulses to the gun through a rod. Designed by
Leutnant Otto Bernatzik, it was geared down to deliver a firing impulse every second revolution of the propeller, and fired at about 380 to 400 rounds per gun. As with other gears synchronizing the Schwarzlose gun, firing became erratic at high engine speeds.
United Kingdom British gun synchronization got off to a quick but rather shaky start. The early mechanical synchronization gears turned out to be inefficient and unreliable and full standardisation on the very satisfactory hydraulic "C.C." gear was not accomplished until November 1917. Synchronized guns seem to have been rather unpopular with British fighter pilots well into 1917 and the over-wing Lewis gun, on its
Foster mounting, remained the weapon for Nieuports in British service, being also initially considered as the main weapon of the
S.E.5. Significantly, early problems with the C.C. gear were considered one of the
less pressing matters for No. 56 squadron in March 1917, busy getting their new S.E.5 fighters combat worthy before they went to France, since they had the over-wing Lewis to fall back on!
Ball actually had his Vickers gun removed altogether for a while, to save weight.
The Vickers-Challenger gear The first British synchronizer gear was built by the manufacturer of the machine-gun for which it was designed: it went into production in December 1915.
George Challenger, the designer, was at the time an engineer at Vickers. In principle it closely resembled the first form of the Fokker gear, although this was not because it was a copy (as is sometimes reported) it was not until April 1916 that a captured Fokker was available for technical analysis. The fact is that both gears were based closely on the Saulnier patent. The first version was driven by a reduction gear attached to a rotary engine oil pump spindle as in Saulnier's design and a small impulse-generating cam was mounted externally on the port side of the forward fuselage where it was readily accessible for adjustment. Unfortunately, when the gear was fitted to types such as the
Bristol Scout and the
Sopwith 1½ Strutter, which had rotary engines and their forward-firing machine gun in front of the cockpit, the long push rod linking the gear to the gun had to be mounted at an awkward angle, in which it was liable to twisting and deformation as well as expansion and contraction due to temperature changes. For this reason the
B.E.12, the
R.E.8 and Vickers' own
FB 19 mounted their forward-firing machine guns on the port side of the fuselage so that a relatively short version of the push rod could be linked directly to the gun. This worked reasonably well although the "awkward" position of the gun, which precluded direct sighting, was initially much criticised. It proved less of a problem than was at first supposed once it was realized that it was the aircraft that was aimed rather than the gun itself. The last aircraft type to be fitted with the Vickers-Challenger gear, the R.E.8, retained the port-side position of the gun even after most were retrofitted with the C.C. gear from mid 1917.
The Scarff-Dibovski gear Lieutenant Victor Dibovski, an officer of the
Imperial Russian Navy, while serving as a member of a mission to England to observe and report on British aircraft production methods, suggested a synchronization gear of his own design. According to Russian sources, this gear had already been tested in Russia, with mixed results, although it is possible that the earlier Dibovski gear was actually a deflector system rather than a true synchronizer. In any case, Warrant Officer F. W. Scarff worked with Dibovski to develop and realize the gear, which worked on the familiar cam and rider principle, the connection to the gun being by the usual push rod and a rather complicated series of levers. It was
geared in order to slow the rate that firing impulses were delivered to the gun (and hence improve reliability, although not the rate of fire). The gear was ordered for the RNAS and followed the Vickers-Challenger gear into production by a matter of weeks. It was more adaptable to rotary engines than the Vickers-Challenger, but apart from early Sopwith 1½ Strutters built to RNAS orders in 1916, and possibly some early
Sopwith Pups, no actual applications seem to have been recorded.
Ross and other "miscellaneous" gears The Ross gear was an interim, field-built gear designed in 1916 specifically to replace the unsuitable Vickers-Challenger gears in the 1½ Strutters of the
RFC's No.70 Squadron. Officially it was designed by Captain Ross of No.70, although it has been suggested that a flight-sergeant working under Captain Ross was largely responsible. The gear was apparently used only on 1½ Strutters, but
No. 45 squadron used at least some examples of the gear, as well as No. 70. It was replaced by the Sopwith-Kauper gear when that gear became available.
Norman Macmillan, writing some years after the event, claimed that the Ross gear had a very slow rate of fire, but that it left the original trigger intact, so that it was possible "in a really tight corner" to "fire the gun direct without the gear, and get the normal rate of fire of the ground gun". Macmillan claimed that propellers with up to twenty hits nonetheless got their aircraft home. Some aspects of this information are hard to reconcile with the way a synchronized gun actually worked, and may well be a matter of Macmillan's memory playing tricks. Only
Sopwiths' gear (next section) was to go into production.
The Sopwith-Kauper gear s (1917) The first mechanical synchronization gears fitted to early Sopwith fighters were so unsatisfactory that in mid 1916 Sopwiths had an improved gear designed by their foreman of works
Harry Kauper, a friend and colleague of fellow Australian
Harry Hawker. This gear was specifically intended to overcome the faults of earlier gears. Patents connected with the extensively modified Mk.II and Mk.III versions were applied for in January and June 1917. Mechanical efficiency was improved by reversing the action of the push rod. The firing impulse was generated at a low point of the cam instead of at the lobe of the cam as in Saulnier's patent. Thus the force on the rod was exerted by tension rather than compression, (or in less technical language, the trigger motor worked by being "pulled" rather than "pushed") which enabled the rod to be lighter, minimising its inertia so that it could operate faster (at least in early versions of the gear, each revolution of the cam wheel produced two firing impulses instead of one). A single firing lever engaged the gear and fired the gun in one action, rather than the gear having to be "turned on" and then fired, as with some earlier gears. 2,750 examples of the Sopwith-Kauper gear were installed in service aircraft: as well as being the standard gear for the Sopwith Pup and
Triplane it was fitted to many early
Camels, and replaced earlier gears in 1½ Strutters and other Sopwith types. However, by November 1917, in spite of several modifications, it was becoming evident that even the Sopwith-Kauper gear suffered from the inherent limitations of mechanical gears. Camel squadrons, in particular, reported that propellers were frequently being "shot through", the gears having a tendency to "run away". Wear and tear, as well as the increased rate of fire of the Vickers gun and higher engine speeds were responsible for this decline in performance and reliability. By this time the teething problems of the hydraulic C.C. gear had been overcome and it was made standard for all British aircraft, including Sopwiths. In May 1916, he prepared the first drawing and an experimental model of what became known as the Constantinesco Fire Control Gear or the "C.C. (Constantinesco-Colley) Gear". The first provisional patent application for the Gear was submitted on 14 July 1916 (No. 512). At first, the meticulous Constantinesco was dissatisfied with the odd slightly deviant hit on his test disc. It was found that carefully inspecting the ammunition cured this fault (common, of course, to all such gears); with good quality rounds, the performance of the gear pleased even its creator.
A. M. Low who commanded the
Royal Flying Corps secret Experimental Works at Feltham was involved in the testing. The system was perfected by Constantinesco in collaboration with the
Fleet Street printer and engineer Walter Haddon at the Haddon Engineering Works in Honeypot Lane, Alperton. The first working C.C. gear was air-tested in a B.E.2c in August 1916. The new gear had several advantages over all mechanical gears: the rate of fire was greatly improved, the synchronization was much more accurate, and above all it was readily adaptable to any type of engine and airframe, instead of needing a specially designed impulse generator for each type of engine and special linkages for each type of aircraft. In the long run (provided it was properly maintained and adjusted) it also proved far more durable and less prone to failure.
No. 55 Squadron's DH.4s arrived in France on 6 March 1917 fitted with the new gear, It was late in 1917 before a version of the gear that could operate twin guns became available, so that the first Sopwith Camels had to be fitted with the Sopwith-Kauper gear instead. From November 1917 the gear finally became standard; being fitted to all new British aircraft with synchronized guns from that date up to the
Gloster Gladiator of 1937. Over 6,000 gears were fitted to machines of the Royal Flying Corps and the Royal Naval Air Service between March and December 1917. Twenty thousand more "Constantinesco-Colley" gun synchronization systems were fitted to British military aircraft between January and October 1918, during the period when the
Royal Air Force was formed from the two earlier services on April 1, 1918. A total of 50,000 gears were manufactured during the twenty years it was standard equipment.
The Betteridge gear The C.C. gear was not the only hydraulic gear to be proposed; in 1917 Air Mechanic A.R. Betteridge of
No.1 Squadron Australian Flying Corps built and tested a gear of his own design while serving with his unit in Palestine. No official interest was expressed in this device; possibly the C.C. gear was already in prospect. The illustration seems very likely to be of the test rig for this gear.
France The French
Aviation Militaire was fortunate in that they were able to standardise on two reasonably satisfactory synchronization gears – one adapted for rotary engines, and the other for "stationary" (in-line) ones – almost from the beginning.
The Alkan-Hamy gear The first French synchronizer was developed by
Sergeant-Mécanicien Robert Alkan and
Ingénieur du Génie maritime Hamy. It was based closely on the definitive Fokker
Stangensteuerung gear: the main difference being that the push rod was installed within the Vickers gun, using a redundant steam tube in the cooling jacket. This mitigated a major drawback of other push rod gears in that the rod, being supported for its whole length, was much less liable to distortion or breakage. Vickers guns modified to take this gear can be distinguished by the housing for the push rod's spring, projecting from the front of the gun like a second barrel. This gear was first installed and air-tested in a
Nieuport 12, on 2 May 1916, and other pre-production gears were fitted to contemporary Morane-Saulnier and Nieuport fighters. The Alkan-Hamy gear was standardised as the
Système de Synchronisation pour Vickers Type I (moteurs rotatifs), becoming available in numbers in time for the arrival of the
Nieuport 17 at the front in mid 1916, as the standard gear for forward-firing guns of rotary-engine French aircraft. The
Nieuport 28 used a different gear – now known only through American documentation, where it is described as the "Nieuport Synchronizing gear" or the "Gnome gear". A spinning drive shaft, driven by the rotating crankcase of the Nieuport's 160 CV
Gnome 9N Monosoupape rotary engine, drove two separately adjustable trigger motors – each imparting firing impulses to its gun by means of its own short rod. Photographic evidence suggests that an earlier version of this gear, controlling a single gun, might have been fitted to the
Nieuport 23 and the
Hanriot HD.1.
The Birkigt gear The
SPAD S.VII was designed around Marc Birkigt's
Hispano-Suiza engine, and when the new fighter entered service in September 1916 it came armed with a single Vickers gun synchronized with a new gear provided by Birkigt for use with his engine. Unlike most other mechanical gears, the "SPAD gear" as it was often called, did without a pushrod altogether: the firing impulses being transmitted to the gun
torsionally by a moving
oscillating shaft, which rotated through about a quarter of a revolution, alternately clockwise and anticlockwise. This oscillation was more mechanically efficient than the reciprocating motion of a push rod, permitting higher speeds. Officially known as the
Système de Synchronisation pour Vickers Type II (moteurs fixes) the Birkigt gear was later adapted to control two guns, and remained in use in French service up to the time of the Second World War.
Russia No Russian synchronization gears went into production before the
1917 Revolution – although experiments by Victor Dibovski in 1915 contributed to the later British Scarff-Dibovski gear (described above), and another naval officer, G.I. Lavrov, also designed a gear that was fitted to the unsuccessful
Sikorsky S-16. French and British designs licence-built in Russia used the Alkan-Hamy or Birkigt gears. but in the meantime the engineer
Adolph L. Nelson at the Airplane Engineering Department at McCook Field had developed a new, mechanical gear especially adapted to the Marlin, officially known as the
Nelson single shot synchronizer. In place of the push rod common to many mechanical gears, or the "pull rod" of the Sopwith-Kauper, the Nelson gear used a cable held in tension for the transmission of firing impulses to the gun. Another approach, common to
Germany, the
Soviet Union, and
Japan, while recognising the necessity to increase armament, preferred a system that included synchronized weapons. Centralised guns had the real advantage that their range was limited only by ballistics, as they did not need the
gun harmonisation necessary to concentrate the fire of wing-mounted guns. They were seen as rewarding the true marksman, as they involved less dependence on gun sight technology. Mounting guns in the fuselage also concentrated mass at the centre of gravity, thus improving the fighter's roll ability. More consistent ammunition manufacture, and improved synchronization gear systems made the whole concept more efficient and effective, whilst facilitating its application to weapons of increased calibre such as
autocannon; moreover the
constant-speed propellers that quickly became standard equipment on WW II fighters meant that the ratio between the propeller speed and the rate of fire of the guns varied less erratically. The swan-song of synchronization belongs to the last reciprocating engine Soviet fighters, which largely made do with slow firing synchronized cannon throughout the
World War II period and after. The very last synchronizer-equipped aircraft to see combat action were the
Lavochkin La-11 and the
Yakovlev Yak-9 during the
Korean War. ==Popular culture==