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In WWII there was a plane called the Focke-Wulf Fw 190. It was designed by an engineer called Kurt Tank.

How is it possible that the Fw 190 fires through the propeller from 4 different places as you can see in the picture below? There are two machine guns right in front of the pilot, and one at either side on the wing, close to the body of the plane.

enter image description here

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    $\begingroup$ There were early planes where they simply didn't bother with synchronization, and just "hoped for the best" (en.wikipedia.org/wiki/…) $\endgroup$ – abelenky Sep 1 '16 at 21:37
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The Internet is incredible. I found a PDF of the manual for the armament of the Fw-190 online here. In it, the components are described sufficiently to be sure that synchronization in the FW-190 was achieved electrically.

General arrangement of the armament in the Fw-190

General arrangement of the armament in the Fw-190 (picture source). The pipes running to the wing cannons feed hot air from the engine to the ammunition box to keep the temperature there above -35°.

Detail view of the Sicherungs- und Verteilerkasten SVK-2 for the MG151 in the wing root.

Detail view of the Sicherungs- und Verteilerkasten SVK-2 (circuit breaker and distribution box) for the MG 151/20 in the wing root (picture source). Clearly, the weapon was electrically controlled.

The Wikipedia page on gun synchronization already mentions that with firing rates above 400 rounds/minute, mechanical synchronization became unreliable, and at the end of WW I, the first electrical gun synchronization by Siemens was used in LVG attack planes, and Aviatik employed their own system. The much higher firing rate of WW II guns required electrical synchronization, and only some Russian fighters continued to use mechanical synchronization into and even beyond WW II.

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    $\begingroup$ Very good and definitely specific to the question! I hope that the asker will consider accepting this answer since this is actually using different technology than the early mechanical implementation. $\endgroup$ – user2943160 Sep 3 '16 at 20:28
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Note: See Peter Kämpf's answer for specifically how the Fw 190 Achieved this Electronically

This answer is specific to how this was achieved via mechanical means in earlier aircraft and not specific to the Fw 190.

It's called a "Synchronization Gear" and it allows the gun to fire only when the blade is not in front of the barrel.

enter image description here Source: Kaiserliches Patentampt (Imperial German patent Office) / Franz Schneider - Patentschrift No. 276396, 1913 First published in Flugsport" (1914) and in many places since, Public Domain,

[From Wikipedia] Interestingly, the linkage between the propeller and the gun is achieved with a spinning drive shaft rather than a reciprocating rod. The impulses needed to operate the trigger, or in this case to prevent the trigger from operating, were produced by a cam wheel with two lobes at 180° apart situated at the gun itself, since the gun is interrupted by both blades of the propeller.

These have been around since before 1920, long before the aircraft you mention in your original post.

The one on the Fokker (not Focke-Wulff, but still German) differs a little bit from the patent image above:

enter image description here Source: By Gsl - Version with English text of Image:Interrupter_gear_diagram.png, Public Domain

But not significantly, the basic idea is that the trigger mechanism follows a cam lobe which is tied to the propeller shaft. When the cam lobe spins, the trigger is allowed to fire the gun only when the propeller isn't "in the way".

See Wikipedia Article on the Synchronization Gear for more information.

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    $\begingroup$ Only the inner ones would have to be synchronized, and it would just mean another rod on the cam wheel to feed the other gun. The cam wheel on the FW190 probably had multiple lobes for the 3 bladed propeller. $\endgroup$ – Ron Beyer Sep 1 '16 at 15:30
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    $\begingroup$ Worth noting that this is essentially the same type of synchronization mechanism that does things like lifting engine valves and firing spark plugs at the right time. $\endgroup$ – J... Sep 1 '16 at 16:44
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    $\begingroup$ Fokker (not Focke-Wulff, but still German) uhm, Fokker was Dutch? $\endgroup$ – Federico Sep 1 '16 at 21:33
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    $\begingroup$ I have to look it up, but I am almost sure the Fw-190 used electric synchronization. The mechanical way was first, but Focke-Wulff was far ahead of all others on the way to the all-electric aircraft. Just look at the cockpit and count the number of circuit breakers in a Fw-190. $\endgroup$ – Peter Kämpf Sep 2 '16 at 6:46
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    $\begingroup$ @JourneymanGeek Anthony Fokker was Dutch. He designed and built his first aircraft in Germany and moved his business to the Netherlands after WW1. $\endgroup$ – DeltaLima Sep 2 '16 at 9:27
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Since WWI, there was a device called a Synchronisation Gear which staggered the firing of the gun so that it would not hit the propeller of the plane.

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I can't seem to locate the reference but the synchronizing gear was not at first used with the forward firing gun. The inventor of the forward firing gun had steel plates mounted to his propellers to deflect the occasional round that might hit the prop. This was satisfactory right up until he got his prop shot off. That's when the synchronizing systems were sought out.

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    $\begingroup$ Before they put deflector plates on they just fired the gun and "hoped" it missed the prop. Usually they would wrap the prop in a fiber tape that helped keep the prop together when inevitably hit by the bullets. Deflector plates came after "hope for the best". $\endgroup$ – Ron Beyer Sep 2 '16 at 2:43

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