Bullets Versus Propellers, or Why Synchronizer Gears Were So Important in World War I – The SlowMo Guys

In World War I, the Germans developed a secret technology that helped them dominate the skies during 1915 and early 1916. The tech? A device that synchronized the firing of a machine gun with the rotation of an aircraft’s propeller, allowing accurate low-mounted forward-firing weapons on warplanes for the first time.

Synchronization gears, also called “synchronizers” or “interruptor gears”, are fairly simple devices that essentially connect a disconnector mechanism – such as the those used in semiautomatic weapons to prevent continuous fire – to a cam system that activates the disconnector at the right time to prevent a blade strike. The SlowMo Guys took to the Nevada desert to show us a synchronizer in action, under the eye of a high speed camera, of course:

Gav and Dan have created a pretty interesting rig: An M60, with some aesthetic enhancements to make it look more like a period-correct Vickers machine gun, coupled to a propeller via a synchronization gear (I am guessing an electrical unit, based on what appears to be an electric motor on top of the gun – historical WWI-era synchronization devices were purely mechanical). This allows them to show how the ignition of the cartridges in the machine gun is timed correctly, such that the bullets do not impact the propeller blades.

Of course, what happens without a synchronization device? Gav and Dan obliged us there, too, by simply disabling their device. The results, even at a sedate 200 rotations per minute for the propeller, were what you’d expect:


The hole Dan (off camera) is pointing to was made by two bullets striking the same location!

Nathaniel F

Nathaniel is a history enthusiast and firearms hobbyist whose primary interest lies in military small arms technological developments beginning with the smokeless powder era. In addition to contributing to The Firearm Blog, he runs 196,800 Revolutions Per Minute, a blog devoted to modern small arms design and theory. He is also the author of the original web serial Heartblood, which is being updated and edited regularly. He can be reached via email at nathaniel.f@staff.thefirearmblog.com.


  • Alex Waits

    Is that a strike industries “cookie cutter” comp on that?

    • Scott Laidler

      It is. A great muzzle brake and even better seeing it in super slow mo

  • iksnilol

    HMmm… if the rounds impact at the same spot for given RPM just make a hole there and ignore it 🙂

    • Avery

      You forgetting friction from the air coming from the front and being moved by the propeller. Best case is the air wafts through the holes, loosing blade efficiency and requiring the engine to work harder, while the worse case scenario is the force of the air and/or engine torque overcoming the compromised structural integrity of the blade and snapping off.

    • Martin M

      Speed/power was controlled through RPM. Since your RPM would constantly be changing, so would the point of impact.

      • iksnilol

        Yeah, but that’s what gears are for. To help keep it at a constant RPM. I’d guess that concept would be foreign to you since there’s few manual cars in the US.

        • Tony Williams

          Constant-speed propellers didn’t feature until WW2. The rate of fire of a synchronised gun with a variable speed prop fluctuated quite considerably. From “Flying Guns: WW1”:

          “The effect of synchronisation on the rate of fire can best be explained by describing a simple system like that introduced by Fokker, in which one firing signal was sent to the gun for each rotation of the propeller. If the gun was capable of firing at 500 rounds per minute, then for propeller speeds of up to 500 revolutions per minute the RoF would be the same as the propeller rpm. However, as soon as the propeller exceeded 500 rpm, the gun mechanism could no longer keep up and could then only fire on every other rotation, so the RoF would drop to 250 rpm. It would then accelerate again with increasing propeller speed but at half the rate, so when the propeller was spinning at 1,000 rpm, the gun would be back to firing at 500 rpm again. Once more, propeller revs faster than this would cause the RoF to drop, but this time only to two-thirds of the full RoF, as it would fire on every third rotation, so it would be achieving 330 rpm. As the propeller continued to accelerate to 1,500 rpm, the gun would be back up to 500 rpm again, and so on. Any quoted figure for synchronised rates of fire could therefore only be an average.”

        • Martin M

          Are you talking about gearing the firing mechanism? WWI aircraft had their props connected directly to the crankshaft, or in the case of rotary engines like the Clerget, to the engine case. No gearing. No transmission. The props were fixed (not variable), so the only way to increase power (thrust) was to increase RPM.

          • iksnilol

            I refuse to believe that. You trying to tell me they flew in first gear all the time? Then they’d be slow af

          • Martin M

            Yeah. No gears. All Throttle. Like a push lawnmower. Like a string trimmer. Like an electric fan. Simple aircraft made of bits of wood and fabric. Even air combat began by people literally throwing bricks at each other.

            Even in WWII, aircraft gearing was a single gear modifying the ratio between crank and prop or connecting two power-plants. Even with variable pitch propellers thrust was ultimately achieved by increasing RPM.

          • iksnilol

            Whaat, you can’t throw bricks out of an airplane. Explosive decompression would get you then.

          • Martin M

            Men were men in those days. Men were men.

          • iksnilol

            So were women and children, but strangely men weren’t gay for having sex with these women that were also men.

            T’was a different time 🙂

          • Martin M

            Plus they were able to sport wicked cool beards in the absence if rolled up skinny jeans.

          • Madcap_Magician

            It was WW1, they WERE slow af.

          • iksnilol

            But fuel efficency? Weight transfer?

            How’d they even engage in fierce dorifto battles without a clutch!?

          • Tony Williams

            There’s possible confusion here, because some WW1 planes did have geared propellers, in that the engine revs were higher than the prop revs (with the rest, the engine and prop revs were the same). However, the gearing could not be changed.

    • Tassiebush

      Hmm it’s almost like you’d need to synchronise the propeller and gun to achieve that…;)

    • Peter Balzer

      Yeah, and best have it fire continuously, too. Remember that a non-mechanic item (aka The. Pilot.) fires the gun at different points in time. That and the fact that RPMs are not the same at all times make your logic somewhat flawed.

    • Tassiebush

      Perhaps Iksniltroll would be a more appropriate handle

      • iksnilol

        Shhh, that’d make it too easy. I’ve had several comments back and forth with people thinking I was serious, it was hilarious 😛

        • Tassiebush

          Oh man I empathise with them and consider you a rogue! I get drawn in easy by such stuff. I earnestly attempt to clear the air only to discover I’m the naive victim of a villainous masquerade! 🙂

          • iksnilol

            Trust me, so do I. You get pulled in and then you’re just like “whaaa… oh wait, nobody is actually that stupid” … well, except for that one time when somebody was actually that stupid. That was a scary one.

            First time I’ve been described as rogue tho, nice, those are the ones that are good with women, right ?

          • Tassiebush
          • iksnilol

            Minus the nefarious mustache (tho I am working on it) that’s basically me to a T. 😮

  • Darhar M.

    I had read about this many years ago but to see the German tech slowed down
    to be able to see the Interrupter working in tandem with the prop and Vickers Gun
    is excellent.

    Thank you for posting this.

    • Herr Wolf

      The Bloody Red Baron has done it again!

      • Madcap_Magician

        Ten, twenty, thirty, forty, fifty or more…

  • pbla4024

    1) Low mounted forward firing machine gun was first allowed by deflectors by Garros
    2) German interruptor was constructed by Dutch guy
    3) WW1 synchronization devices were not purely mechanical, British used hydraulic one.

    • Martin M

      I always thought the deflectors were a crazy idea.

      • retfed

        I remember reading that Louis Bleriot, a French pilot who was the first to fly across the English Channel, actually shot himself down with the deflectors in the early part of the war. Apparently they took so many hits the cumulative impacts either loosened or cracked the propeller, That’s when the French decided to spend the extra money for the interruptors.

    • Tony Williams

      All true except that, to be really picky, the British CC gear (Constantinescu-Colley) was hydrosonic, not hydraulic: the liquid column in the pipes was not moved to and fro to fire the gun, it was the carrier for a sound signal which triggered each shot.

    • Renato H. M. de Oliveira

      1 yes, but the deflector was only a paliative, as the propeller would break in short order anyway. Low mounted MGs weren’t really useful before sync, unless you used a pusher prop or a twin-engined plane
      2 yes, but the first practical use was with the Fokker Eindekker. Before the E, most MGs were top-wing mounted, which severely limited their use in dogfights. Recall that inventing and making practical use are different things, so much so that the 1st successful “assault rifle” was the StG-44, not the Fedorov and not the Ribeyrolles
      3 yes, but most were purely mechanical – German ones used in large scale were purely mechanical

    • roguetechie

      Constantinescou or something along those lines made the really really nice one that worked the best.

  • Tony Williams

    Strictly speaking the practical synchronisers were not “interrupter gears” (although they were called that at the time). A synchronised MG was semi-auto only, each shot being fired by the synchro gear as soon as the gun was ready to fire and the blades were out of the way, for as long as the pilot kept pressing the button.

  • Herr Wolf

    If the Germans haven’t already thought of it then it’s not worth inventing

    • roguetechie

      Except the good one wasn’t invented by the Germans lol

  • Some of you may enjoy this vintage article on aircraft machineguns from the Jan/Feb 1921 issue of Army Ordnance magazine.


  • MisterTheory

    In what world does a WW1 aircraft engine turn 200 rpm?

  • uisconfruzed

    That’s neat.
    In Canada alone do their engines spin WITH the prop?!?

  • J. gM

    The caption in French was a nice touch. Merci beaucoup !

  • Nils Ollum

    Is that Gavin from Achievement Hunters?

  • PCS

    One thing tried before the interupter was to put metal on the back side area of the prop that would get hit by the bullet. it helped but was not a solution.