Sir Hiram Maxim's experimental biplane. The inventor can be seen standing in front of the aircraft, between the guidance rails. The monster was powered by advanced, light and powerful steam engines. 
In a sense the association of the aircraft and the machinegun began before the first existed. The first automatic machinegun was developed by Sir Hiram Maxim, and it was used in the colonial warfare of the late 19th century. Maxim devoted some of his wealth to the construction of a giant steam-powered biplane, tested in 1894 in Britain. He was wise enough to try to keep the aircraft under control with guiding rails, that allowed it to raise only a few centimeters above the ground. But during a test the rails broke and the aircraft was destroyed.
It is not known whether Maxim envisaged that his guns would be fitted to aircraft. The early aircraft were much smaller than Maxim's 3.5 ton giant, and the weight of the gun and its ammunition was a heavy burden for small aircraft with limited engine power. The development of more powerful aircraft was only a matter of time, however. Rifle-calibre machineguns were not that heavy, and the weight could be reduced by deletion of water-cooling jackets and circuits: On aircraft air cooling was sufficient. The jacket was often retained, but perforated to reduce the weight and improve cooling.
The problem then became the development of a suitable gun mount. We will skip over the problems of observer guns, and only consider forward-firing guns. There was no real problem on aircraft with a pusher engine: The machinegun could be put in the nose. However, such aircraft had a complicated design with tailbooms around the propeller; this was heavy and induced a lot of drag. Therefore, the performance of pusher aircraft was usually inferior to that of the more compact tractor designs. At first this was not a serious problem. At the start of WWI the French even decided to equip their air force entirely with pusher designs, simply because this would make it easier to distinguish friend from foe: The Germans employed mainly tractor aircraft.
Left, the Foster mount on an SE.5a. When this aircraft entered service the Foster mount was obsolescent, but it did have the advantage that it could be used to fire upwards. 
On tractor designs, with the propeller in front, the gun could in principle be fitted outside the propeller circle. On two-seat monoplanes a high gun mount could be installed, and the gunner could stand to fire over the propeller: It was tried on some aircraft, but it was not a good solution. On biplanes the machinegun could be installed on top of the upper wing. Unfortunately the gun was then out of the reach of the pilot, and WWI machineguns were rather unreliable. Pilots needed to clear stoppages frequently, and often carried a small hammer for this purpose. For drum-fed weapons, such as the Lewis, there was also the problem of changing drums: Flying the aircraft with one hand while handling a heavy ammunition drum with the other was a difficult task, and many pilots chose to break off combat and descend to lower altitude before attempting to do this. Finally, the recoil of the gun was enough to disturb the aim. Nevertheless this arrangement was used by some Allied aircraft, using a Lewis gun, because this was more reliable than the Vickers. Later the so-called "Foster mount" was installed on SE.5a fighters. This was a curved rail that allowed the pilot to slide the gun backwards and downwards.
The final alternative, the most practical one but also the technically most complicated one, was to install some form of interruption or synchronisation mechanism, so that the machinegun could fire through the propeller disc. Several such mechanisms had been designed before the war. The Swiss engineer Schneider, who had worked for Nieuport in France and LVG in Germany, patented his design, of which drawings were even published in The Scientific American. In Russia, Poplavko experimented with synchronisation in 1913. In Britain, the Edwards brothers patented another gear, and demonstrated a working model. The French engineer Saulnier also worked on synchronisation, but he discovered that the Hotchkiss machinegun fired too irregularly: It was not suitable for synchronisation. Far too often a round would "hang" and put a bullet in the propeller. As a safeguard, the French had developed wedge-shaped steel deflectors that were fitted to the propeller to protect it, but the results were not encouraging.
Despite all this experimentation before the war, the the French and British air services entered the war with only two machineguns each, and the Germans had none. Therefore, the first shots in air warfare were fired with pistols and rifles, very unsuitable weapons for this kind of combat, but readily available to the crews, who often came from infantry or cavalry units. The RFC (Royal Flying Corps) authorized only the standard service rifle, a far too unwieldy weapon. Apparently the British pilot Lanoe G. Hawker was the only one to win any victories with a carabine.
A Vickers F.B.5 captured by the Germans. 
The pusher design was the first to be applied. Already in September 1912 Vickers had tested a biplane with a machinegun. By 1913, Vickers had developed the EFB.2, perhaps the world's first purpose-designed fighter aircraft. The EFB.2 was a two-seat pusher biplane, so that the gunner in front had a free field of fire. The installation of the Vickers machine gun, in a fairing in the tip of the nose, was entirely unpractical. By June 1914 a more practical gun mount had been developed, but it was July 1915 before the first Vickers FB.5, nicknamed "Gun Bus", appeared in France! Unfortunately the Vickers was underpowered and slow, and the machinegun initially so unreliable that many gunners took a rifle with them anyway.
A similar installation was made on some French Voisin biplanes, with Hotchkiss machineguns that were operated by the observer. The pilot sat in the front seat, and the observer fired the gun over his head. On 5 October 1914 a Voisin piloted by Joseph Frantz and with Louis Quenault as observer shot down a German Albatross biplane. It was the first victory in the air. By February 1915 the French had installed about 50 machineguns on their aircraft: Not very much, but enough to force the German aircraft to a hasty retreat whenever they appeared over the battlefield.
A fixed machinegun on a single-seater would be even more effective. The famous pilot Roland Garros dispensed with the synchronizing gear altogether, and installed a machinegun and deflectors on his Morane-Saulnier L, a parasol monoplane. His mechanic worked hard to improve the design of the deflectors. Starting on 1 April 1915, he shot down five German aircraft in seventeen days, before he and his aircraft came down behind the lines and were captured by the Germans. His success was so convincing that the Morane L and N, equipped with fixed guns and deflectors, soon equipped three French squadrons and some RFC units.
Fokker E.III. 
In Germany, the aircraft manufacturer Anthony Fokker was asked to deliver a synchronisation mechanism. Engineers Heinrich Luebbe and Fritz Huber were already developing a suitable mechanism, and after a few days Fokker could demonstrate the prototype. (The Fokker mechanism infringed Schneider's patents, and the legal battle would continue until Schneider's death.) The result of this experiment was the Fokker E.I, a modified M.5K monoplane with an Oberursel rotary engine, and a single fixed forward-firing machinegun. The first Fokker Es were delivered in the summer of 1915, and came in the hands of pilots such as Boelcke and Immelmann. The Fokker E.I, and the improved E.II and E.III, did not have a very good performance, but their armament was very effective. One or two fixed forward-firing machineguns became the standard armament for the fighters of World War I and the interbellum, usually installed on top of the front fuselage.
The Fokker became famous, but already before it entered service the Allies had put in production superior designs: The Airco DH.2 pusher, and the Nieuport 11 with the machinegun on the upper wing. Hence the Fokker Scourge was relatively brief. Far more deadly were the Albatross biplane fighters, with two synchronized machineguns. They took a heavy toll in "bloody April" of 1917, and set the standard pattern for the rest of the war and the interbellum.
The Sopwith F.1 biplane fighter received the name Camel because of the hump that covered the two Vickers guns. The fairing was often partially cut away to make it possible to clear stoppages in the air. Note the loading handles in the cockpit. 
There was a wide array of different synchronisation mechanisms. The early ones were not very reliable, and accidents were common. The most successful was the C.C.-gear, named after its inventors Constantinesco and Colley. This operated with an hydraulical instead of a mechanical link, and that gave much more flexibility. The C.C.-gear could be used with any engine and any machinegun.
A refinement that was useful for air combat was the development of incendiary ammunition, but this was not without danger: The earliest German incendiary ammunition was so unreliable that it could seriously damage the aircraft that fired it. The quality of ammunition was a problem throughout the war, and a wise fighter pilot personally checked every round that was loaded in his guns. Rounds with slightly irregular dimensions were likely to jam a gun.
Note that rate of fire would be strongly reduced when a gun was synchronized.
|Name||Ammunition||Rate of Fire||Muzzle velocity||Weight|
|Hotchkiss Mle 1909||8 x 50R||600 rpm||725 m/sec||12.3 kg|
|Lewis Mk.I||7.7 x 56R||550 rpm||745 m/sec||11.4 kg|
|Lewis Mk.II||7.7 x 56R||700 rpm||745 m/sec||7.7 kg|
|Vickers Mk.I||7.7 x 56R||850 rpm||745 m/sec||13.0 kg|
|LMG.08/15 Spandau||7.92 x 57||450 rpm||5.9 kg|
|MG 14 Parabellum||7.92 x 57||700 rpm||890 m/sec||4.3 kg|
|Becker||20 x 70RB||325 rpm||30 kg|
|Schwarzlose 07/16||8 x 56R||570 rpm||625 m/sec|
|Browning .30||7.62 x 63||490 rpm||7.2 kg|
|Madsen Mdl 1902||7.62 x 54||425 rpm||4.1 kg|
|Fiat Revelli 1914||6.5 x 52||450 rpm||7.7 kg|
|Marlin 1917||7.7 x 63||640 rpm||4.5 kg|
The Vickers was the standard Allied machinegun in fixed installations, typically two guns directly in front of the pilot. It was recoil-operated, but aircraft versions used the muzzle blast to speed up the mechanism. It was belt-fed, initially by a fabric belt, but this was changed to a disintegrating belt of metal links. The Vickers remained the standard armament of British biplane fighters until the late 1930s. The Mk.II version was characterised by a cooling jacket of smaller diameter, for installation within the fuselage. During WWI, an 11mm version was hastily developed, mainly for use against balloons and Zeppelins: It could carry a larger incendiary load. The 11mm version was not very successful.
The gas-operated Lewis was designed in the USA. It was an improved version of the gun designed by McClean, and in June 1912 it became the first machinegun fired in the air. However, the US Army rejected it, because it had adopted the Benet-Mercie (a weapon that was unsuitable for aircraft use). The Lewis then entered production in Belgium and Britain. It was the standard flexible gun of the Allies. Occasionally it was found in fixed installations, especially on pusher aircraft such as the DH.2 and FE.8, and on top of the wing of the Nieuport 11 and the SE.5a, but it was not suitable for synchronisation. It was fed with 47-round (Mk.I) or 97-round (Mk.II and Mk.III) drums. Even the fixed installation retained a pistol grip with a conventional trigger. In later aircraft designs the pilot had a Bowden cable to pull this trigger, but the pilot of a Nieuport 11 had to reach up to fire his gun.
The French Hotchkiss was a good gun, put it was fed by clips and too hard to reload in the air. It was quickly replaced by the Vickers and Lewis.
The LMG.08/15 was better known as the Spandau, after the place where it was manufactured. This German development of the Maxim machinegun was the standard fixed armament of their fighters. It was fed with a fabric belt.
The MG 14, also known as the Parabellum, was an improved, lightened version mostly used as flexible weapon. The name Parabellum was the codename of the DWM (Deutsche Munitions und Waffenfabriken). Apparently the Parabellum was less reliable than the Lewis, for German aircraft readily used captured Lewis guns.
The Becker 20mm cannon was the precursor of the Oerlikon cannon. This weapon was installed on some German bombers, not on fighters.
Austrian fighters used the Schwarzlose, a machinegun with blowback operation. It was slow-firing and short-ranged. For this gun, ammunition with case lengths of 50mm or 52mm was also in use. In addition, it was often installed in a bulky fairing on the upper wing of biplanes, were the pilot could not reach it to clear a jam.
The Marlin was used by the USAAS and the USN from 1917 to 1921, a fairly short career. This gas-operated weapon was reliable and fast-firing, but was soon replaced by the Browning machinegun.
Next: Gun tables
|Introduction||What preceded||Gun Tables||Ammunition|
|WWII Fighters||Analysis||Firing Up||Big Guns|
|Fighter Armour||Bomber's Defense||Postscript||Korean War Fighters|
|Fighters Table||Fighters Charts||Ballistics|