The Synchronized Machine Gun.

Hi Pipe,

I'm interested in some technical aspects related to WWI Aircrafts synchronizers.

What I'm looking for, are two basic data: 1) How many firing pulses per propeller turn and 2) the amplitude of the "percussion" angle", the different devices featured. Of course, the easiest to find out should be the first one, while the second parameter is something that had to/could be adjusted within a certain range, depending on the actal application and, usually, is far more difficult to retrieve. Anyway, while the meaning of the first number is "sel explainining, the secondo parameter described how large the firing angle (refferred to the propeller disc) linked to the single firing pulse was. That is, if the percussion angle was, say 20°, the shot enabled by the firing pulse (provided the weapon was again ready to fire) could be fired within this angular interval.
As to the number of firing pulses, since the synchronizer could be driven by the propeller (direclty or, i.e., via a couple of gears) or by some other device belonging/driven/linked to the engine (camshaft or something like),the number of pulses associated to the synchronizer could be refferred to the propeller turns or to the engine turns or to something different (i.e,its own dirving shaft), depending on the actual device operating and connecting mode. Anyway, by knowing the actual ratio between the propeller turns and the turns of the device driving the synchronizer, it is (was) possible to get anyway how many firing pulses per propeller turn the device was able to generate. In the same way, it is possible to get the number of fiiring pulses per engine turn.

Both parameters were fudamental in onder to get the actual synchronized rate of fire as a function of the engine speed, to set up the device with respect to the prop blades and to state the engine speed range for a safe shooting. Of course, all this stuff, in combination with the knowledge about weapon/cartridge(s) techincal and ballistic features and the actual mounting of the weapon with respect to the propeller "disc".

But coming back to historically relevant devices, one of the most interesting device (at least for me) is the Alkan-Hamy since, as far as I know, this was the device employed on Ni17 and (maybe) on the Hanriot HD1. It would be nice to get at least the info about the firing pulses per prop turn since, if I'm not wrong, the cam(s) driving the device was (were) placed on the rotary engine (i.e. the propeller shaft) so that their number should give immediately the info I'm looking for. What would actually be interersting is to know if the magci number was just 1 firning pulse per prop turn or just the double of it or, maybe, early version featured 1 and later versions had two cams rahter than just one.

Have you the info I'm looking for about this device, in particular, and or to some other of the many synch gears pioneering the era of firing through the propeller disc? Or at least, could tell me where to search for?
Up to now I wasn't able to find anything "sure" about this device, in sipte of my long lasting search on books and/or internet sites.

Thnaks in advance (to you and or to everybody could give me useful data on this subject)

Dear Bruno:

The PIPE Here again...happy holidays, by the way, as a BIIG sporting event in my neck of the woods takes place on New Year's Day, as mentioned at 2010 Bridgestone NHL Winter Classic - Flyers v. Bruins , with my favorite sports team taking part against a huuge rival of theirs from Philly...and it's one that's FAR better than any of the college sports taking place that day, IMHO...

...I'd thought I'd start this reply by stating, where machine guns HAVE been described as "VERY deadly single-stroke engines", as a type of "internal combustion engine", getting the internal combustion engine that spins the prop to "synch-up" with the single-stroke "fatality engine" that is mounted on the fighter, to get it to do ITS main job, is the whole raison d'etre for the gun synchronizer's existence in the first place.

Right now, I've got my copy of WW I AERO issue no. 138 right in front of me as I'm tyiping this text, and from section 2.5, entitled "Description of a Typical Synchronizer" from part 2 of Mr. Volker's article, I quote Mr. Volker, with my additions for clarification in [brackets] within the text, as I always do in such a situation:

"In a Maxim-type machine gun, such as the 7.92mm calibre lMG 08 [machine gun], with each firing of a round the lock assembly inside the gun traveled rearward, carrying the trigger mechanism, including the trigger arm and the firing pin with it. It then returns forward to complete the cycle of operations. Therefore, the pulsating output-end of the synchronization could act on the trigger arm on the lock assembly only when it had returned to the firing position; that is, when it was held against the breech face of the gun barrel with the firing pin cocked. The rate of fire generated and controlled by the synchronizer and the gun [it controlled] in this manner varied with propeller speed, [which draws out a sawtooth curve when graphed with the horizontal axis showing prop speed, and the vertical axis showing the rate of synchronized fire]. Since a [machine] gun operating in conjunction with a synchronizer was capable of producing single shots only, in response to impulses generated by the synchronizer, it was actually made to function as a semi-automatic gun, also known as a self-loader, instead of a fully-automatic machine gun".

So "there you have it" from Mr. Volker himself...generally only ONE bullet got fired from a Fokker E.I's single Spandau gun from a single complete rotation of the propeller and its "umlaufmotor" ("rotary engine" im Deutsch).

I suppose a synchronizer COULD be driven off any rotating component of the engine that was essentially operating at the same exact speed of the prop, or at a proper "whole number integral" speed in relation to the prop (like an inline engine's camshaft, rotating at 1/2 the speed of the crankshaft). With a rotary engine-powered aircraft like the Eindecker, the rear end of its umlaufmotor's spinning crankcase is where the synchronizer's trigger cam would have been mounted.

And, if using the original Stangensteuerung system format from the Eindecker, one cam per machine gun would most likely be needed for twin-gun installations. The Albatros firm came up with their own system, initially, for their D-series fighters, as one example of a non-Fokker synchronizer showing up at the Front, with the Allies' Vickers-Challenger system being a first step for the eventual winning side, as the Constantinesco unit became the most-widely used synchronizer during WW I for the Allies, if my mind serves me properly.

You CAN get reprints of the Hank Volker three-part article from WW I AERO, just by contacting them though their page for it at World War 1 Aeroplanes, Inc.-Contact Us ...I'd strongly suggest you get a copy of the entire three-part article for yourself, so then you can understand the whole issue of gun synchronization quite a bit better.

The Osprey and Windsock Datafiles are also good references to use alongside the article by Mr. Volker...so those might be a good idea to get, to look at the WHOLE picture of how the gun synchronizer began to evolve almost a century ago.

Hope you can get copies of that complete article from WW I AERO...

Yours Sincerely,

The PIPE!

Dear The Pipe,
thanks a lot for quickly anwering my post . I've appreciated your analogy between the internal combustion engine and a rifle or a machine gun! The ICE too is a semi-automatc system once the injection and ignition system are properly synchronized wth piston etc stuff.
For sure I'll follow your hint to get in touch with people you suggested. Getting the papers on the subject is anyway a good idea especially if I will find there data I'm looking for.
But, in order to make more clear and possibly more fruitful my search, please, allow me to explain that the "principia" behind the synchronization are clear to me (at least so they seem to me) and, indeed, once I get the two parameters I spoke about in my previous post, it would be easy to produce Rate of Fire vs engine speed graphs for any aircraft (engine+prop)/weapon/cartridge set. Clearly, once some few other basic info about the trio are known.
The "sawthoot" aspect of such graphs is the basic "ideal" "universal" form and, for an ideal enabiling (possibly firing) pulse, it is also the "exact" form in any engine (propeller) speed range, provided the enabling angle (the percussion angle) tends to zero. In this case, the linear growth and the sudden down aspect, repeated whenever the frequency of enabling pulses is equal to or is an exact multiple of the weapon "natural" (unsynchronized) rate of fire is quite straightforward to get. The graph shows a vertical drop from the "full" natural rate of fire to a local minimum that increases as the order of multiplicity increases according to the N/(N+1) law, where N is the order of multiplicity between the frequency of enabling pulse and the gun natural rate of fire. Engine speeds (critical speeds) corresponding to these "situations" are easily calculated once some basic parameters are known.I.e: the number of firing pulses per prop turn and the ratio between prop and engine speed (but this is not the only way to get the same results). Same reasoning for the link between Rate of fire and prop speed.
Things are a little bit more puzzling if the amplitude (in terms of angle and, consequently,of time) of the enabling pulse is different from zero, as usually happened in actual systems. Basically the sawthoot aspect is still there but in the area of the crritical speeds, the drop of the rate fo fire will be no more vertical. Its behavior will be more complicated. The reason why I'd like to get info about percussion angle too (i.e. to produce more accurate graphs) should be more clear now and I really hope to get a better insight into the "raw" data" of the synchronization systems of WWI aircrafts so to complete my quantitative understanding about their behavior.
Finally let me explain why I'm so interested in Alkan-Hamy gear: Ni 17 and Hanriot HD1 equipped a lot of squadriglie of the Italian Air Force during the WWI and Italy is my Country
Thanks again!

Intresting read guys, I have always wondered how these guns work. Thanks for sharing.