Interrupter gearThe interrupter gear was a triggering device attached to a machine gun so that it would fire only at certain times. This allowed machine guns to be mounted directly in front of the pilot of a fighter aircraft, firing through the propeller. The interrupter guaranteed that the gun would only fire when the propeller was not in the way.
The French ace Roland Garros had fitted nose mounted guns to his planes in 1915, simply by placing a metal wedge on the propeller to deflect the bullets. After a particularily bad month in which German aircraft were being shot down with increasing numbers, his plane was forced down in German territory in April 1915 and the mystery was revealed.
Anthony Fokker felt Garros's solution was particularily poor, and set about inventing a better one. His solution was 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 the propeller was horizontal when it pushed on the rod, and the rod in turn pressed the trigger to fire a bullet. The trigger the pilot operated pulled the rod into position over the cam.
The system was immediately fitted to Fokker Fokker E.I design, first flying in July and leading to the Fokker Scourge. The system was soon copied by all air forces.