Machine gun synchronizer in slow motion - GIF

[neilbourjaily]

Why didn't they just mount the guns above the propellers? I've always found that intriguing. I know you probably needed them in the pilot's line of sight so he could aim properly, but surely they could have mounted guns on the wings above the props and worked out a system to help the pilot aim them at his quarry...

Then again, it's not like no one thought of that, so it must not have worked out.

https://www.uh.edu/engines/epi1369.htm

The British were first to mount forward-firing guns on the upper wing -- shooting over the propeller. But that made aiming hard, and it put the guns out of the pilot's easy reach when they jammed. The French took the next step. They put metal deflectors on the propeller so the pilot could fire straight through the blades, with a bullet glancing off now and then. That worked until crankshafts deformed under the hammering of their own pilots' bullets.

Enter now Holland's Anthony Fokker. The year before WW-I began, Fokker was only 23 and building airplanes. Germany contracted with him to build ten airplanes, and he went to work. War broke out months later, and Fokker was suddenly Germany's man-of-the-hour. By 1915 his monoplane, the Eindecker, was doing frontline scout work. Then the Germans brought him a captured French plane with metal plates on the propeller. Could he do that with the Eindecker? Fokker tells what happened next, in his autobiography:

They handed him the plane late on a Tuesday afternoon. Fokker said, "Wait a minute!" The way around the problem is to let the propeller fire the gun. The propeller turns at 1200 rpm, and the gun fires 600 times a minute. Put a cam on the shaft and let it fire the gun every other turn. Then no bullet will ever hit the prop. Fokker came back with a synchronized machine gun that Friday.