how did propellor warplanes shoot their machine guns through the propellor?
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There were several “fixes” to the problem tried in WW1: Here they are in the order they were implemented:
1 – Avoid the problem entirely by building a plane with a pusher propeller (engine in the rear). Con: These pusher planes had poor performance.
2 – Put the gun up high on the top wing so it can shoot forward just above the propeller. Con: Pilot can’t sight the gun since it’s not in front of his eyes. Also it’s hard to change the ammo drum up there.
3 – Go ahead and shoot the gun through the propeller anyway, but affix armored metal wedges to the propeller blades at the spot the bullets will be hitting. Most bullets miss the blades and go straight through, but the ones that don’t get slapped aside by the metal wedge. Con: The engine rattles like mad whenever the propeller wedge slaps a bullet aside. The wear from this meant engines had to be replaced after just a handful of flights. Also, bullets that happen to hit the wedge straight-on would ricochet back toward the pilot, which isn’t ideal.
4 – Create an “interrupter” mechanism driven by the engine’s rotation. The interrupter is a mechanical part linked to the engine’s shaft that will interfere with the trigger mechanism of the machine gun when the engine’s drive shaft has rotated into certain positions. The interrupter stops the trigger from working during the split second in which the bullet would hit the propeller if it was fired right then. Then the interrupter rotates out of the way and the machine gun trigger will work again, until the engine rotates around again to that spot and the interrupter halts the mechanism. Thus let’s say you graph the machine gun’s timing of bullets and it would normally be like this:
|
|—bullet—bullet—bullet—bullet—bullet—bullet—bullet—> time
|
With the interrupter mechanism, it might end up looking more like this:
|
|—bullet—bullet————bullet—bullet————bullet—> time
| ^ ^
| Interrupter says “no”. Interrupter says “no”.
5 – Instead of having the engine *suppress* the firing of the gun at certain times, have the engine *be* the trigger that fires the gun. In other words, design the gun to use the cycling motion of the engine *as* its triggering mechanism. Then, just like, say, an engine piston moving into the right position causes a the spark plug to spark, you have the gun fired By the engine moving into the right position where it triggers the gun. Thus instead of interrupting the normal flow of the machine gun, the machine gun is fired totally in sync with the engine’s movements. A gun might be arranged to fire a bullet at, say, at 3 or 4 specific points of the engine’s rotation. When you use this method, the pilot’s trigger isn’t so much firing the machine gun, as it is just connecting the machine gun to the engine mechanism that fires it.
In the beginning, planes were used as observers and artillery spotters, not shooting at each other. Generals decided that having the other side do that was bad, so planes were equipped with guns to shoot down other side’s observers.
During WW1, it was common for planes to have a gun mounted on the upper wing. This was done to put the gun above the propeller arc. Another way (soon abandoned) was to use a pusher configuration. Propeller behind the pilot doesn’t get in the way of the guns. Some planes had cowl-mounted guns, and tan the risk of shooting off the propellor. One French pilot had big slabs of metal fitted to the propeller to deflect any bullets that hit it.
Anthony Fokker came up with the idea for interruptor gear. Many guns have an automatic safety that keeps them from firing if the breech is not closed and locked. Interruptor gear is an added automatic safety that keeps the gun from firing if a propeller blade will be in the way.
A final technique, that can only be used if the plane uses reduction gear on the engine (engine turns faster than propeller to allow more power without getting the bad stuff that happens if the propeller turns too fast) and can only work for one gun, is to have the gun mounted on the propeller centreline and fire through the hub. Examples of this include the BF-109 and the P-39.
Imagine a WW2 dogfight between a P-51D and a BF-109. The guns on the P-51D are mounted as a cluster of 3 in each wing, outside the propeller arc. The BF-109 has two cowl-mounted machine guns fitted with interruptor gear and a cannon firing through the hub of the propeller. 3 techniques, all of which were viable.
There’s a good description of the interruptor gear above so I’ll leave that off. Before the interruptor gear was figured out there were several “pusher” style fighters that had the propeller in the rear so the gun could shoot forward clearly. They also put angled metal plates on the propeller at the level where bullets would hit on conventional propeller designs.
By WWII, especially the later years, most fighters had their guns in the wings. That had its own challenge: convergence. You wanted your fire lines to converge so you were putting all your bullets at one point to do the most damage. When your guns were on the fuselage and firing forward that was automatic. But with guns in the wings your bullets came from a good way off to the side. So the guns had to be angled inward just slightly so that all of the bullets would converge at a point a certain distance ahead of the fighter. Then you would need to do your best to engage a target at that distance so you would have the most firepower possible hitting it at once.
Very carefully, they used a mechanism that synchronized the machineguns rate of fire with the revolutions of the helix so it could shoot through, without destroying it
I feel like [this awesome video](https://youtu.be/ysB-SH19WRQ) from TheSlomoGuys needs some love from this question. It shows it really well in slow motion!
I’d really recommend checking out the Sloe Mo Guys video on it:
It’s a mechanical gear configuration that is synchronized to engage the gun when the propeller is out of the way of the barrel. It was a very clever solution and a VERY reliable one.
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