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The early two-seater aircraft purchased by the Royal Flying Corps (RFC) relied on the observer to fire at the enemy. The observer used a revolver, rifle or an hand grenade. In September 1914, the RFC began fitting Lewis machine-guns that were angled to fire forwards clear of the propeller. However, it was not until October 1914 that the first aircraft was shot down by a machine-gun. This success went to a French observer-gunner who used a Hotchkiss machine-gun to bring down a German Aviatik C-I reconnaissance aircraft near Rheims.
Like most pilots, Louis Strange found that his observer had a great deal of difficulty using the machine-gun against enemy aircraft. He therefore devised a safety strap system in his Avro 504 so that it was possible for his gunner to "stand up and fire all round over the top of the plane and behind". Strange's gunner had considerable success against German planes and the Royal Flying Corps decided to fit this safety harness to all their aircraft.
From the beginning of the war attempts were made to find ways of firing the machine-gun through the propeller. A French manufacturer, Raymond Saulnier, developed a system that he hoped would enable the pilot to fire the gun only when the propeller was out of line with the target. Early versions did not work properly and the pilots found they were destroying their propellers rather than enemy aircraft with their machine-gun fire.
In the early months of 1915, the French pilot, Roland Garros, added deflector plates to the blades of the propeller of his Morane-Saulnier. These small wedges of toughened steel diverted the passage of those bullets which struck the blades. He was now able to use a forward-firing machine-gun and on 1st April 1915, he went out searching for his first victim. Garros approached an German Albatros B II reconnaissance aircraft. The German pilot was surprised when Garros approached him head-on. The accepted air fighting strategy at the time was to take 'pot-shots' with a revolver or rifle. Instead Garros shot down the Albatros through his whirling propeller.
In the next two weeks Roland Garros shot down four more enemy aircraft. However, the success was short-lived because on 18th April, a rifleman defending Courtrai railway station, managed to fracture the petrol pipe of the aircraft that Garros was flying. Garros was forced to land behind the German front-line and before he could set-fire to his machine it was captured by the Germans. It was immediately sent to Anthony Fokker, a Dutch designer who was producing aircraft at his factory in Germany.
Since 1914 Fokker had been working on a new single-seater fighter plane. Fokker was convinced that it was vitally important to develop a system where the pilot could fire a machine-gun while flying the plane. His solution to this problem was to have a forward-firing machine-gun synchronized with the propeller.
After inspecting the deflector blades on the Morane-Saulnier Fokker and his designers decided to take it one stage further by developing an interrupter mechanism. A cam was attached to the crankshaft of the engine in line with each propeller blade, when the blade reached a position in which it might be struck by bullets from the machine-gun, the relevant cam actuated a pushrod which, by means of a series of linkages, stopped the gun from firing. When the blade was clear, the linkages retracted, allowing the gun to fire.
This synchronized machine-gun was fitted to the new Fokker E aircraft. The EI and two other models, the EII and EIII, began arriving on the Western Front during the summer of 1915. The synchronized machine-guns gave the Germans a considerable advantage over the Allied pilots. German aces such as Max Immelmann and Oswald Boelcke became national heroes as the number of victories over their opponents grew.
(1) Sergeant Chaney was an observer-gunner before the invention of the synchronized machine-gun.
The disadvantage was that I had to hold the machine-gun during the whole of the flight as there was no means of clamping it. This was extremely awkward as I was hampered by the presence of the bombs in my arms.
(2) Wing Commander Maurice Baring described the invention of the interrupter gear in his memoirs published in 1920.
The problem of how to fire through the propeller was engaging everyone's attention at this time. The question was solved for the moment by having a deflector on the propeller, of which the bullet ricocheted, when it would, without a deflector, have hit the propeller. This system was invented by Garros, the French pilot, and copied by the Germans. They then adopted a gun which fired through the propeller, by virtue of an interrupter gear, a system which was definitely proposed by the Royal Aircraft Factory before the war, although it did not then get as far as the drawing stage. Our synchronizing gear first came into existence in 1916.
(3) In October 1915, British fighter-pilots began reporting that the Fokker planes were using interrupter gear.
21st October 1915: Sgt. Courtney and Sgt. Thornton, when dropping hand grenades and flechettes on La Bassee were attacked by a Fokker monoplane with deflector propeller. Sgt. Thornton was hit in the hand and had his face grazed. When getting out his rifle he was hit in the other hand.