WW1 Aircraft Synchronization Gear Demonstrated in Super Slow Motion

At one point in the video I had a go at counting the approximate number of shots per revolution. I would hazard a guess at 6 to 7 rounds per prop revolution which would work out to around 1200 rounds per minute based on their stated prop rpm of 200. Sounds a bit high.

It certainly did, yes. As with everything in the air..it will never kill you, until it does 🙂

Most often it was badly adjusted gear or, on the later electrical types, I believe, sometimes “shorts”. My knowledge of the gear is really WWI based, so I can’t really comment on Spain. But in the majority of cases, failure will be failure to fire 14-18, rather than lost prop. During WWI structural failures, engine fires and engine failures were so common place that they do not even rate special mention. It was perfectly feasible to set a rotary on fire just by blipping for a bit too long whilst landing. The danger of shooting your prop off was small in comparison and provided you got shut down straight away, relatively harmless…just one more forced landing. The Camel, as an example, killed as many RFC pilots in training and active service accidents as it did enemy in action!
I think that could also be the rational behind lack of ground testing… I mean airmen fell into the props they were swinging so often, it also hardly gets a mention other than as a casualty. Pilots drank rum and milk before flying to ward off the effects of castor oil injested from the exhaust in rotaries, A different attitude, at least in the First War.

I have several CitARs that discuss it from memory but it is a long time since I have spooled up that microfilm machine. I will have a look at a couple of references on the weekend because I am almost certain it happened to both Max Immelmann and Boelke at least once each.

Buried in 27 years of OTF and C&C are several stories about sync gear, CC gear and plans etc. If anybody has a record of the issue numbers, by chance, I’ll be happy to pull them out.

Another advantage of firing, not interrupting, is that failure of the gear generally has the effect of stopping the guns hitting the propellor anyway. Failure of an interrupter gear has the opposite, unfortunate effect!!

But it did happen; with interrupter-gear and, presumably, synchronising-gear too?

I remember reading about an incident from the Spanish Civil War where a biplane fighter shot-off its own propeller. I cannot remember any details, actually it is the illustration I remember more than anything, but it was a biplane-versus-biplane engagement. Thinking about it, the cause was blamed on incorrect adjustment of the synchronising-gear by ground-crew who weren’t familiar with the aircraft that had been supplied by a foreign power (Italy possibly?) but it begs the question: why didn’t they try the guns on the ground (or even test-fire them in the air)?

G’day Archer,

600 rounds per minute cyclic was considered the practical rate of fire of a Vickers in aircraft installations. I’ve fired an M60 but can’t remember what the cyclic rate is.

In reality, no sensible pilot fired more than a few rounds at a time, due to the high likelihood of jamb.

Variations in ammo quality and primitive feed systems meant that the best pilots often insisted on loading their own belts!!

With a blade passing the muzzle 2400 times every minute there is no way that a Vickers machine-gun could keep-up with a cam-operated synchronising-gear. Several blades must pass the muzzle, the ‘trigger’ being pulled by the synchronising-gear for each one, while the machine-gun is in the middle of its cycle (and therefore unable to fire).

That makes a lot more sense than the 200 RPM demonstration in the video (which seems to fire after every passing blade); at anything over 600 ‘passing blades’ a minute a Vickers machine-gun will effectively revert to its maximum cyclic rate of fire.

Precisely. Effectively the weapon becomes a semi automatic, being fired each time the action is ready AND a hydraulic pulse, mechanical pull or electromagnetic contact is made. That gives the maximum practical rate of 600 rounds per minute at any point the rpm are over 300. I say “over” not “at least” because there is some “wastage” of opportunity when a blade passes at a point that the gun action isn’t ready. Not a problem, as during combat it is extremely unlikely that a 1200rpm engine will be much throttled. Usually, then, the practical rate achieved is close to 600… Something north of 550.

It is interesting that two guns are really not any more complicated to manage than one if you fire the guns with the engine. Another advantage of firing, not interrupting, is that failure of the gear generally has the effect of stopping the guns hitting the propellor anyway. Failure of an interrupter gear has the opposite, unfortunate effect!!

CC Gear is an absolutely marvellous bit of engineering and very interesting to see. Being hydraulic, it was extremely reliable .

They did say at one point that it was a modified M60 they were using as the correct type of weapon wasn’t available. Perhaps they slowed down the prop to get the correct ratio between prop rpm and firing rate, but that is a guess on my side.

Edit: I see that a typical Clerget rotary delivers maximum power at 1250 rpm, so that is not too far from the 2400 blades per minute mentioned above. How do the firing rates of a Vickers and an M60 compare?

With a blade passing the muzzle 2400 times every minute there is no way that a Vickers machine-gun could keep-up with a cam-operated synchronising-gear. Several blades must pass the muzzle, the ‘trigger’ being pulled by the synchronising-gear for each one, while the machine-gun is in the middle of its cycle (and therefore unable to fire).

That makes a lot more sense than the 200 RPM demonstration in the video (which seems to fire after every passing blade); at anything over 600 ‘passing blades’ a minute a Vickers machine-gun will effectively revert to its maximum cyclic rate of fire.

In what way incorrect ???? The device interrupted the firing of the weapon.

Actually, anything but.

The truest “interupter” system was Roland Garros’ and even in that the wedges interupted the FLIGHT of the bullet not the firing.

In the other systems a cam FIRED the gun. The reason is quite simple. It is easier to FIRE the gun with a cam than to STOP it firing with one.
Each minute there was a blade passing the gun muzzle 2400 times. With a gun having a typical rate of 600 rounds per minute, there was ample opportunity to trigger the weapon rather than “blocking” it from firing, which could only ever REDUCE the firing rate and risk misfeeds etc.

Interupter systems were tried, inefficient, complicated and liable to jamming..at least in WW1 technology levels.

Cams with mechanical triggers, hydraulic impulses (like the British CC Gear) and even solenoid fired electro systems.

Yes, didn’t they say 120 RPM at one point in the video; makes you wonder why it wasn’t hit more often when they just let the machine-gun fire continuously.

Edit: actually 200 RPM in the video.

Is it just me or did the rotational speed of the propellor appear to be rather low (no I do not mean in the slo-mo clips)?

Are there two distinct systems then?

The mock-up in the video seemed to be set up so that the rotation of the propeller shaft fired the gun; one round (and only one round) triggered to fire just before each blade passed. In this way the machine-gun is behaving as a semi-automatic rifle; the rate of fire absolutely proportional to (with two blades) twice the RPM of the engine. (Incidentally at 500 RPM (low?) with two blades that would give 1000 rounds-per-minute; which is pretty good for a Vickers machine-gun at the best of times!)

Isn’t there another system where the machine-gun fires at its normal (higher) cyclic rate and the firing is actually ‘interrupted’ by the passing of each propeller blade? In this way the machine-gun is behaving as a true machine-gun; the rate of fire being dictated by the gun alone.

I’m pretty sure I’ve seen a diagram of how this second system works; effectively there is a second ‘sear’ in the trigger mechanism of the machine-gun. Only when both sears are activated, one by the pilot, one by the interrupter-gear, can the machine-gun fire. If the pilot holds his trigger down the gun fires continually except when it is interrupted by the passing of a propeller blade.

Great video, thanks!

(And SO nice to see it called Sync gear rather than incorrect “interupter gear” usually used!

In what way incorrect ???? The device interrupted the firing of the weapon.

The notice on the cowl of a friend’s recently completed Sopwith Pup replica.

Great video, thanks!

(And SO nice to see it called Sync gear rather than incorrect “interupter gear” usually used!