WORLD WAR I

The word "fighter" was first used to describe a two-seater aircraft with sufficient lift to carry a machine gun and its operator as well as the pilot. The first such "fighters" belonged to the "gunbus" series of experimental gun carriers of the British Vickers company which culminated in the Vickers F.B.5 Gunbus of 1914. The main drawback of this type of aircraft was its lack of speed. It was quickly realized that an aircraft intended to destroy its kind in the air needed at least to be fast enough to catch its quarry.
Fortunately another type of military aircraft already existed, which was to form the basis for an effective "fighter" in the modern sense of the word. It was based on the small fast aircraft developed before the war for such air races as the Gordon Bennett Cup and Schneider Trophy. The military scout airplane was not expected to be able to carry serious armament, but rather to rely on its speed to be able to reach the location it was required to "scout" or reconnoiter and then return quickly to report – while at the same time making itself a difficult target for anti-aircraft artillery or enemy gun-carrying aircraft. British scout aircraft in this sense included the Sopwith Tabloid and Bristol Scout; French equivalents included the light, fast Morane-Saulnier N.
In practice, soon after the actual commencement of the war, the pilots of small scout aircraft began to arm themselves with pistols, carbines, grenades, and an assortment of improvised weapons with which to attack enemy aircraft. It was inevitable that sooner or later means of effectively arming "scouts" would be devised. One method was to build a "pusher" scout such as the Airco DH.2, with the propeller mounted behind the pilot. The main drawback was that the high drag of a pusher type's tail structure meant that it was bound to be slower than an otherwise similar "tractor" aircraft. The other approach was to mount the machine gun armament on a tractor-type airplane in a manner that enabled the gun to fire outside the arc of the propeller.

Only two configuration options were practical initially for tractor aircraft. One involved having a second crew member added behind the pilot to aim and fire a swivel-mounted machine gun at enemy airplanes. However, this limited the area of coverage chiefly to the rear hemisphere, and the inability to effectively coordinate the pilot's maneuvering with the gunner's aiming, which reduced the accuracy and efficacy of the gunnery. This option was chiefly employed as a defensive measure on two seater reconnaissance aircraft from 1915 on. The alternative configuration mounted a gun on the upper wing to fire over the propeller arc. While more effective for offensive combat, since the pilot could move and aim the guns as a unit, this placement made determining the proper aim point more difficult. Furthermore, this location made it nearly impossible for a pilot to maneuver his aircraft and have access to the gun's breech – a very important consideration, given the tendency of early machine guns to jam – hence this was a stopgap solution. Nevertheless, a machine gun firing over the propeller arc did have some advantages, and was to remain in service from 1915 (Nieuport 11) until 1918 (Royal Aircraft Factory S.E.5). The British Foster mounting was specifically designed for this kind of application.

The need to arm a tractor scout with a forward-firing gun whose bullets passed through the propeller arc was evident even before the outbreak of war, and its approach motivated inventors in both France and Germany to devise a practical synchronization gear that could time the firing of the individual rounds to when the propeller was not in the way. Franz Schneider, a Swiss engineer, had patented such a device in Germany in 1913, but his original work was not followed up. French aircraft designer Raymond Saulnier patented a practical device in April 1914, but trials were unsuccessful because of the propensity of the machine gun employed to hang fire due to unreliable ammunition.
In December 1914, French aviator Roland Garros asked Saulnier to install his synchronization gear on Garros' Morane-Saulnier Type L. Unfortunately the gas-operated Hotchkiss machine gun had a firing cycle which caused the bullet to leave the weapon too late to effectively and consistently synchronize the gunfire with a spinning propeller. Because of this, the propeller blades were armored, and Garros' mechanic, Jules Hue, fitted metal wedges to the blades to protect the pilot from ricochets. Garros' modified monoplane was first flown in March 1915 and he began combat operations soon thereafter. Firing 8 mm (.323 in) solid copper bullets, Garros scored three victories in three weeks before he himself was shot down on 18 April and his airplane – along with its synchronization gear and propeller – was captured by the Germans.

The actual aircraft used by Wintgens in his pioneering aerial engagement, his Fokker M.5K/MG with IdFlieg military serial number "E.5/15", as it appeared at the time of the engagement.

However, the synchronization gear (called the Stangensteuerung in German, for "pushrod control system") devised by the engineers of Anthony Fokker's firm was the first gear to attract official sponsorship, and this would make the pioneering Fokker Eindecker monoplane a feared name over the Western Front, despite its being an adaptation of an obsolete pre-war French Morane-Saulnier racing airplane, with a mediocre performance and poor flight characteristics. The first victory for the Eindecker came on 1 July 1915, when Leutnant Kurt Wintgens, flying with the Feldflieger Abteilung 6 unit on the Western Front, forced down a Morane-Saulnier Type L two-seat "parasol" monoplane just east of Luneville. Wintgens' aircraft, one of the five Fokker M.5K/MG production prototype examples of the Eindecker, was armed with a synchronized, air-cooled aviation version of the Parabellum MG14 machine gun, which did not require armored propellers. In some respects, this was the first "true" fighter victory of military aviation history.

The success of the Eindecker kicked off a competitive cycle of improvement among the combatants, building ever more capable single-seat fighters. The Albatros D.I of late 1916, designed by Robert Thelen, set the classic pattern followed by almost all such aircraft for about twenty years. Like the D.I, they were biplanes (only very occasionally monoplanes or triplanes). The strong box structure of the biplane wing allowed for a rigid wing that afforded accurate lateral control, which was essential for fighter-type maneuvers. They had a single crew member, who flew the aircraft and also operated its armament. They were armed with two Maxim-type machine guns – which had proven much easier to synchronize than other types – firing through the propeller arc. The gun breeches were typically right in front of the pilot's face. This had obvious implications in case of accidents, but enabled jams (to which Maxim-type machine guns always remained liable) to be cleared in flight and made aiming much easier.

 The use of metal in fighter aircraft was pioneered in World War I by Germany, as Anthony Fokker used chrome-molybdenum steel tubing (a close chemical cousin to stainless steel) for the fuselage structure of all his fighter designs, and the innovative German engineer Hugo Junkers developed two all-metal, single-seat fighter monoplane designs with cantilever wings: the strictly experimental Junkers J 2 private-venture aircraft, made with steel, and some forty examples of the Junkers D.I, made with corrugated duralumin, all based on his experience in creating the pioneering Junkers J 1 all-metal airframe technology demonstration aircraft of late 1915.

As collective combat experience grew, the more successful pilots such as Oswald Boelcke, Max Immelmann, and Edward Mannock developed innovative tactical formations and maneuvers to enhance their air units' combat effectiveness and accelerate the learning – and increase the expected lifespan – of newer pilots reaching the front lines.
Allied and – until 1918 – German pilots of World War I were not equipped with parachutes, so most cases of an aircraft catching fire, or structurally breaking up in flight were fatal. Parachutes were well-developed by 1918, and were adopted by the German flying services during the course of that year (the famous "Red Baron" was wearing one when he was killed), but the allied command continued to oppose their use, on various grounds.