How machine guns on World War I biplanes never hit the propeller

Was the gun designed to fire through the propeller, or the propeller designed for a machine gun?
biplane machine gun
(National Archives)

There was a lot of new technology brought to the battlefield during World War I. Two of those were used in tandem—and somehow managed to perfectly complement each other.

It was the fighter plane and the machine gun, mounted perfectly for the pilot’s use, without shooting up the propeller that kept the bird aloft. Was it the gun that was designed to fire through the propeller, or the propeller designed to be used with the biplane machine gun?

The answer is yes.

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The system worked because of synchronization gear which kept the gun from firing when the propeller would be hit by the bullet. While airborne, the prop would be spinning five times as fast as the weapon could fire, so there was little margin of error. The problem was solved by the addition of a gear-like disc on the propeller that would only allow the gun to fire in between the blades’ rotation.

Often called an “interrupter” the disc did not actually interrupt the firing of the weapon, it merely allowed it to fire semi-automatically instead of at an even pace. When the prop spun around to a certain position, it would allow the weapon’s firing mechanism to fully cycle and fire a round. Usually, when the round was supposed to be interrupted, the weapon was actually just in the process of cycling.

"The Interrupter" WW1 Machine guns on planes. thumbnail
"The Interrupter" WW1 Machine guns on planes.

So pulling the trigger would essentially connect the weapon to the propeller, and the prop would actually be firing the gun. Letting the trigger go would disconnect the weapon from the propeller.

Later versions, such as the Kauper interrupter used on the Sopwith Camel, allowed for multiple machine guns at different rates of fire.

The interrupter was a welcome change from the early days of combat aviation, where props were sometimes metal plated just in case mechanically uncoordinated rounds hit the propeller, so the bullet would ricochet.

biplane machine gun
Synchronization gear was also needed for later planes, such as the German Me-109 fighter, seen here in World War II.

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Blake Stilwell

Editor-In-Chief, Air Force Veteran

Blake Stilwell is a former Air Force combat cameraman and erstwhile adventurer whose work has been featured on ABC News, HBO Sports, NBC, Military.com, Military Times, Recoil Magazine, Together We Served, the Near East Foundation, and more. He is based in Ohio, but is often found elsewhere.


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