Real LMG 08/15 Firing Rates

[JFM]

Hello,

Looking for info concerning the LMG 08/15 firing rate. I've found several different figures, ranging from 450 rpm to 700 rpm. I've also found information that the interrupter gear significantly reduces the firing rate, but by how much was never stated.

So on an average, say, Albatros D.II, what firing rate could the pilot expect?

Any help appreciated, as well as information toward any good books on the LMG 08/15 (or any WW1 weaponry).

Interupter gear does impact firing rate, of course, but, far more significant is engine rpm. If you are shooting through a propeller, its speed will determine alot of the cyclic rate of the gun.

HTH,

F=MA

[Taz]

JFM- There is an excellent explanation of rate of fire in Flying Guns World War I by Williams and Gustin and it is exactly as F=MA stated, a function of both cyclic MG rate of fire and engine (actually propeller for geared engines) rpm. It also depended on the sophistication of the synchronization gear and what synchronization scheme was used. These could be set up for number of firing pulses per revolution or as a true interrupter system with a cam to stop the firing only when the propeller was in the way. I am definitely not brave enough to quote an exact firing rate for the Albatros since too many variables are involved. The set number of firing pulses per revolution would see rate of fire rise and fall as engine rpm times number of pulses matched or mismatched the rate of fire of the MG and could match cyclic rate of fire at certain propeller rpms. The interrupter gear would give the highest average rate of fire, but would never reach the maximum rate of fire of the MG because cyclic rate of fire would always be reduced by the interrupter.

Hope that was not too confusing, but I would recommend reading the book.

Taz

[Machinbird]

I believe this topic has been covered extensively about a year and a half ago. See Previous Firing Rate Discussion It is a rather long thread. It won't hurt to kick the can around though, because perhaps the discussion will lead in a new direction.

[Dave_Watts]

Hi JFM,

I went through this quite thourghly in the previous thread and this should answer your question. I had an article on this published in Small Arms Review and WW1 Aero.

I've done some thinking on the question of actual "at the prop" rates of fire and have come up with some potential solutions, which I hope to confirm in the near future.

Just of note Dave, in an earlier posting from Dec.6th you stated, "... subsequent LMG08/15s that came into service in 1917...", the LMG 08/15 was put into service in May of 1916.

Some quick production numbers (using observed serial numbers of actual guns) are;

1916: 2310 (+/- 110) produced
1917: 9688 (+/- 152) produced
1918: 11538 (+/- 42) produced, production may be higher

I've been working on the rates of fire for the later Zentralsteuerung system as would have been found on the Fokker Dr.I, D.VI, D.VII, and D.VIII aircraft. With the advent of the Zentralsteuerung system there were a number of firing devices that were fitted to the bottom of the LMGs which at first "fired" the LMG once for every revolution of the device, then twice, and ultimately four times. My tests are working with the later four pulse-firing device and would have been on the above aircraft mentioned. These devices were termed as Abzugsvorrichtung or Sicherheitsabzug.

I'll try to keep this simple as I get bored and lost with equations just as much as the next guy.

I took the gearbox that attaches to the back of the camshaft housing of the D.IIIaü motor and hooked it up to the lathe. First I took a reading of the axis speed of the lathe and found it to be 415 rpm. With the lathe acting as the camshaft driving the gearbox I took the rpm reading of the LMG output drive axis and it was 415 rpm. The tachometer output is the same spindle shaft that goes into the camshaft (or in this case the lathe) so I knew it was turning 415 rpm as well. I attached the tachometer drive and tachometer and observed the tachometer which was reading 830 rpm, which is what we would expect as the camshaft turns at half the speed of the crankshaft. The tachometer has a 1:2 ratio.

In brief, the LMG drive output shaft has the same rpm as the camshaft.

The camshaft rpm is half the speed of the crankshaft rpm.

Now, the firing device that is fitted to the bottom of the LMG has an impeller with four arms with each arm pushing a steel ball. When these balls pass the plunger they depress it and cause the gun to fire. This is where I have to actually conduct the hands-on test of firing the gun with the device via an electric drill to see how many times the gun fires with each depressing of the trigger plunger by the steel balls.

I will try to post a photo of the firing device so you can all get an idea of exactly how this device looks and works.

I believe it only fires the gun once for every "pulse"; one bullet only, not a multi-burst from one pulse of the steel ball.

My next calculations are based on the premise that the LMG only fires once from each depressing of the trigger plunger via the steel balls.

The firing device has four arms with the four steel balls and turns one full revolution for every revolution of the camshaft.

Pulsing firing rate of LMG = 4 X Camshaft rpm.

Pulsing firing rate of LMG = 2 X Propeller rpm.

Note I state "Pulsing firing rate of LMG" instead of "Rate of fire of LMG". This is because there is not a direct correlation of the rate of the firing device and the rate of fire from the LMG. I will elaborate.

The firing rate of the LMG is set on the gun. I have repeatedly observed in photographs set rates of 430 to 440 shots per minute. This is the base information we will work from.

60 seconds divided by 440 shots equals .1363636. One shot is fired every .1363636 of a second, or let's phrase it as every .1363636 of a second the gun fully cycles and is ready to be fired again, as the gun operates from the recoil of the bullet being fired.

This is our base number .1363636 of a second for each cycle of the gun. Let's round .1363636 off to .1364.

So if you follow my thinking, the gun can only be fired every .1364 of a second and no faster at our 440 rate of fire setting.

Now we'll turn our attention to the firing device and its rate of pulsing. We have established it pulses 4 times per revolution and it revolves at the same rpm as the camshaft rpm. The camshaft rpm is half the prop (crankshaft) rpm.

Examination 1. Rate of fire at 200 rpm.

Gun set at 440 shots per minute and motor/prop at 200 rpm.

To determine pulse rate of firing device we take 200 rpm divide it by two to get the camshaft/LMG drive speed which equals 100. We then mutiply the 100 by the 4 pulses a revolution for the firing device and we get 400 pulses per minute. To get the pulse per second rate we divide 60 seconds by the 400 pulses and get a rate of .15 of a second per pulse.

Firing device rate of pulsing of .15 LMG cyclic rate of .1364
.15 FIRE
.1364
.15 FIRE
.1364
.15 FIRE

With the firing device pulsing at a rate of .15 of a second and the gun cycling at .1364 of a second the gun will fire every .15 of a second.
.15 x 400 = 60 seconds.
Rate of fire at 200 rpm is 400 rounds per minute.

Examination 2. Rate of fire at 220 rpm.

Gun set at 440 shots per minute and motor/prop at 220 rpm.

To determine pulse rate of firing device we take 220 rpm divide it by two to get the camshaft/LMG drive speed which equals 110. We then mutiply the 110 by the 4 pulses a revolution for the firing device and we get 440 pulses per minute. To get the pulse per second rate we divide 60 seconds by the 440 pulses and get a rate of .1364 of a second per pulse.

Firing device rate of pulsing of .1364 LMG cyclic rate of .1364
.1364 FIRE
.1364 FIRE
.1364 FIRE
.1364 FIRE
.1364 FIRE

Here we have the full rate of fire of 440 x .1364 = 60 seconds.

Examination 3. Rate of fire at 240 rpm.

Gun set at 440 shots per minute and motor/prop at 240 rpm.

240 divided two equals 120. 120 x 4 pulses a revolution for the firing device equals 480 pulses per minute. To get the pulse per second rate we divide 60 seconds by the 480 pulses and get a rate of .125 of a second per pulse.

Firing device rate of pulsing of .125 LMG cyclic rate of .1364
.125 FIRE
.125
.1364
.250 FIRE
.125
.1364
.250 FIRE

As you see here the mismatching of the pulsing and the reload cycle rates slow down the rate of fire of the gun to every .25 of a second. .25 x 240 = 60 seconds.
Rate of fire at 240 rpm is 240 rounds per minute.

I've crunched the numbers and have arrived at the following findings, (given the gun is set at a rate of fire of 440 rounds per minute).

For motor rpms from 1 to 220 the rate of fire is double the motor rpm.

For motor rpms from 221 to 440 the rate of fire is the same as the motor rpm.

For motor rpms from 441 to 659 the rate of fire is 2/3 (.66) of the motor rpm.

For motor rpms from 660 to 880 the rate of fire is 440.

For motor rpms from 881 to 1099 the rate of fire is .4 of the motor rpm.

For motor rpms from 1100 to 1319 the rate of fire is 1/3 (.33) of the motor rpm.

For motor rpms from 1320 to 1450 (sugg. max rpms) the rate of fire is .286 of the motor rpm.

This would look more logical on a graph chart. The point is the rate of fire for the LMG goes up and down according to the motor rpm and not at a constant rate. You can have the full rate of fire of 440 at 220 rpm and then at 221 the rate drops to 221. Note that the gun remains constant at 440 for motor rpms of 660 to 880. You have a drop again and then the rate is pretty constant from 1100 to 1319.

This is given that the gun and everything else is working perfectly. Many other factors could slow this rate down, but this should be a good base to work from. Again this is based on one shot per pulse of the firing ball.

I know the findings sound strange and hard to believe, but it appears these will be rates of fire. I will conduct my test with the drill motor attached to the firing device as soon as the weather breaks. I have about 8000 rounds of ammo so no problem there! A couple of twirls of the firing device and I'll have my answers.

If anyone sees where I've fumbled up on my calculations please speak up. The findings do seem to be out of kilter.

Best of wishes,
Dave W.

P.S. I know the motor is an engine, but Daimler calls it a motor in all their publications and were talking about a Mercedes. Flugmotor not Flugengine. If we're talking British or American, then by all means engine!

[JFM]

Fantastic, guys! Tanks for the linkage, book suggestion and all the above info. I look forward to hearing about the upcoming experiments.

Going to go absorb all this information! Thanks for helping me learn.

JFM

[TonyWilliams]

Thanks for all your work on pinning down the Maxim's system, Dave.

Now what I'd like to see is the same sort of data on the Vickers, with different synchroniser systems and with or without the Hazelton adaptor!

Tony Williams: Military gun and ammunition website and discussion forum

[Taz]

JFM- Looks like you got the real experts working on this problem. At least I hope I quoted Tony's book to him accurately. Dave is obviously very serious about this issue. Would love to see one of his demos. Never got to fire my
F-111's gun, only dropped 3-4000 bombs from it. Our gun was not synchronized, for obvious reasons.

Taz

[Volker_Nemsch]

""" Dave Watts: P.S. I know the motor is an engine, but Daimler calls it a motor in all their publications and were talking about a Mercedes. Flugmotor not Flugengine. If we're talking British or American, then by all means engine! """

Dave is absolutely right, in the German language there is no difference between the electric motor (Elektromotor) and the combustion engine (Verbrennungsmotor) - in both cases the word "Motor" is used.

That is one of the few problems German pupils are facing while learning English ...

[TonyWilliams]

Interesting point - in British English the distinction between 'motor' and 'engine' isn't that clear.

An electric motor would never be called an engine, but an IC engine might be called a motor; in fact, the original 'long form' of the term 'car' was 'motor car' and we still refer to car drivers as 'motorists'.

TW