Best synchronizable heavy machine gun of the war?

[taly01]

Tony Williams has written a shortish article on synchronization; it's at his site. CANNON, MACHINE GUNS AND AMMUNITION.

I looks like that is were I got my information from years ago There is some great stuff on his site.

 

[Vincenzo]

I read the article, if i'm in right he missed completely that engine running a different rpm bring also the propeller to run a different rpm so the gun fired a different rpm

 

[bruno_bis]

Dear all,
I'd like to spend my two cents about the impact of the number of blades on the gun synchronization feasibility and effectiveness.

The short version of the story is that, for a given gun, mounted in the same position with respect to the propeller (distance and projection of line of fire on prop disc), the higher the number of blades, the narrower is the engine speed compatibility window for (safely) operating a synchronization system, unless special versions of such gizmo are adopted. Historically speaking, however, these solutions, although proposed, were never adopted in fighters that fought. As explained in the "long version", it can be said that the reason for this is a "missed synchronization" between a real problem and a potential solution occurring when the solution is proposed when still nobody has or sees the problem. This is why gun synchronization and four or five bladed WWII fighters weren't considered a viable or even feasible match.
Another (although minor) impact of a higher number of blades is a slightly greater average loss of rate of fire due to the more severe limitations on the duration of fire enabling time intervals. This loss, however can be compensated by a potentially higher triggering pulse frequency if, with a higher number of blades, it is still possible to stick with the "one triggering pulse per blade" rule that was commonplace in WWII era.
An important remark is that, at the end of the day, what actually matters is the inter-relationship between the propeller and its rpm operating range and the gun/cartridge features and quality.
It is well known the case of the Lewis gun that couldn't be synchronized even with two bladed props. This outcome was the result of both Lewis operating cycle and the existing synchronization concept limitations. These last statements need less straightforward explanations that, therefore, are proposed inside the "long" version of the story, provided in this post.


For people interested in the full story, here we are with the long version. At the price of an overview about some basic aspects of synchronization stuff, hopefully, things should be better clarified.

Synchronization basics

The best way to think about gun-propeller synchronization is to think about a machine-gun as a semi-automatic device that has to be "externally" commanded so to avoid a stream of objects (blades) placed at a close distance and passing the line of fire at a frequency depending on engine speed and number of propeller blades. Semi-automatic means that, once gun is triggered by an "external" command, a cycle is initiated, made of the "firing" phase and other phases and that, once completed, the cycle stops waiting for a new external triggering command.
Another important fact is that, within the basic operation cycle of the gun, once the triggering pulse is provided, the firing phase is not necessarily the first one. Machine gun technical history shows a plenty of such examples. The point is that, while the delay between the trigger command and the actual cartridge ignition is of little importance when the machine gun operates in automatic mode, things are critically different when gun is "forced" to operate in semi-automatic fashion, due to the need of not hitting propeller blades. This fact is, together with cartridge ignition and launch charge features, the most important point to understand how easy or difficult synchronizing an automatic weapon can be and why propeller speed and number of blades matter. Things will be better clarified later on.
If the triggering pulse provided by the synchronization system always arrives at the very moment the gun is (again) ready to fire, then its rate of fire is not affected at all. If the gun is again ready to fire at a time that happens to be in between two consecutive triggering pulses P1 and P2, it has to wait for P2. Thus, worst case is when the gun is again ready to fire immediately after P1 has passed by.
If triggering pulses are generated at fixed positions of the propeller with respect to the gun line of fire, the best and the worst conditions happen when the synchronization pulse frequency is a multiple of the gun "free" rate of fire. That is, when propeller (and thus engine) speed is equal to one of a set of "magical" values corresponding to a pulse frequency equal to 1x, 2x, etc. the gun rate of fire.

Triggering pulse frequency is proportional to engine speed since propeller to engine speed ratio is constant. Thus, higher engine speed means higher triggering pulse frequency. That is, most of pulses are ineffective, since gun is not yet ready to fire, but an "again ready to fire" gun has less time to wait for firing its next round.

This explains why the basic underlying dependence of synchronized rate of fire on engine speed has a "tooth saw" shape and why the sudden "vertical" drops of rate of fire happens and have a progressively reduced depth as engine speed increases and, finally, why, at higher engine speeds, the toll synchronization system takes on rate of fire is lighter.
If K is the ratio between the triggering pulse frequency and the gun "free" rate of fire, 1/(K+1) is the maximum relative loss of rate of fire of the "vertical" drop. For K=1 (pulse frequency = free rate of fire) the maximum loss can be 50%. For K = 2, the maximum loss is about 33%. The max loss is 25% when pulse frequency is three times the free rate of fire, and so on. Max here means that it can happen only if triggering pulse duration is as short as zero. A limit "ideal" case. A straightforward "take" of this up& down game is that WWII fighters whose engines had narrow combat engine speed ranges (thanks to the constant engine speed concept) very seldom were so poorly matched with guns, so to force them to fire in correspondence with the bad areas of synchronization performance. Designers generally managed to look for nice areas, rather.
Pulse duration can be very short or not but it is finite. Generally speaking, as far as the pulse duration doesn't create the risk of hitting prop blades, increasing the duration is beneficial since it helps to reduce the loss of rate of fire in the critical speed regions. Going into deeper details imply some math and I'm going to skip this stuff.

So, the higher the frequency of triggering pulses the better. Isn't it? Unfortunately, not. Better said, there is an upper limit to the maximum number of triggering pulses per propeller turn. And this limit stems from a set of parameters including, among others, the number of propeller blades.

And, here we are with blades!

Once fire is enabled, time of the "bullet to propeller disc" must be compatible with the time interval between two consecutive propeller blades. A gun must fire in-between two consecutive blades, after all. Time to propeller disc depends on gun operating cycle/cartridge types and performances and, of course, on the gun distance from the propeller. This time can be worked out by using nominal data: the list and the duration of serial operations gun mechanism has to complete before actually igniting the cartridge primer, the combustion speed of the launching charge, the length of the barrel and the average bullet speed inside it and, finally, the muzzle speed of the bullet and the distance between gun muzzle and propeller disc. On top of this, safety margins must be considered in order to cope with geometric factors (the impact section of the blade at gun line of fire level) and uncertainties that, to keep it simple, are mainly due to gun and cartridge performance dispersion with respect to their nominal features (a matter of quality of production process and care of ammunition storage but also a matter of influence of temperature, that is altitude and heating due to automatic fire, on the gun parts).

As to the time interval between two consecutive blades, it is easy to see that it is inversely proportional to propeller (and therefore engine) speed and the Number of Blades (NoB) since the angle between two consecutive blades is 360°/NoB.

Finally, synchronization gears used during WWI and WWII were based on a constant offset angle concept, that is: each triggering pulse was generated at a constant angle between a reference (triggering) blade and the gun line of fire.

By the way, many WWII with a three bladed prop had synchronization mechanisms that provided three pulses per propeller turn. That is: one pulse per blade. That is: each pulse had its own reference blade. As to WWI, beside cases of two pulses per prop turn, examples exist of just one pulse per propeller turn or things like "x" pulses every "y" propeller turns, being x and y integers and x not greater than y. At that time, the large majority of props had just two blades and the rare birds with a four bladed propeller didn't need synchronization (prop behind the pilot) or never got a synchronized gun.
The constant offset angle concept is the simplest way to set up a synchronization system. This explains why it was universally adopted.
As far as the amount of this angle, three options are possible: zero, positive (anticipated) and negative (delayed) offset.
A zero offset angle means that the firing impulse occurs when the blade axis is aligned to the line of fire. An anticipated (delayed) offset means that the firing impulse occurs before (after) the reference blade has reached the line of fire.
The "zero" and the "anticipated" options, while providing more room for firing without hitting the blade that is consecutive to the reference one, imply that gun cannot be triggered below a minimum engine speed. Otherwise, the risk occurs of hitting the reference blade.

Anyway, depending on the chosen option, inter-related limitations arise on the minimum and maximum propeller (and therefore engine) speeds compatible with gun fire synchronization. Firing the gun at a speed that is lower than the minimum or higher than the maximum permissible regime, implies the risk of hitting the blades (the triggering blade or the successive blade) rather than the enemy.
This explains warning labels in the cockpit or on the fuselage, indicating min and max engine speed for safely operating the whole stuff.
Pilots had to carefully manage and check engine speed, especially before constant speed propeller control became a widespread solution as in WWII fighters.
Beside other factors, these limits had to cope with both too quickly (for zero or anticipated offset angles) and too slowly exploding cartridges (for any option), that is, cartridges quality.

What should be clear now is that, beside the fact that a max allowable engine speed limit always existed, improving this limit was possible only at the cost of increasing the minimum engine speed compatible with synch fire and, finally, the higher the number of blades the lower the max engine speed compatible with synch fire and the narrower the engine speed region compatible with the synchronized fire. even if in this narrower window, the number of pulses per propeller turn can be higher (if the one pulse per blade rule holds).
Keeping the time to propeller disk low can help of course. By way of example, "closed bolt" guns with electrically primed cartridges of a gun that is close to the propeller disk and fires bullet with very high muzzle velocities is, under this respect, the "ideal" combination since it minimizes the time to propeller disc of the bullet. But, for the same "boundary conditions", more blades mean more problems, to the point that, moving from a three bladed to a four or five bladed propeller may turn synchronization into a poor or an even impossible option.
Thus it should be clear now why given a number of propeller blades, there could be guns that cannot be synchronized and vice versa, so that this game is a matter of mutual compatibility.

The only way to overcome problems with high (four and even five or more) propeller blades is to get rid of the constant offset angle concept. An example of this improvement is to change the offset angle as a function of engine speed. Something similar to what has been always done with spark ignition phase variation in internal combustion gasoline engines and, more recently, with the smart control of engine valves.

For the sake of completeness it is useful reporting that, actually, devices implementing this concept were proposed to air forces. The basic idea was to enhance centrifugal force to adjust (anticipate) the firing pulse as engine speed increases. Of course, a sufficient cartridge quality still remains a prerequisite. Without keeping the cartridge "ignition and explosion" features within a "narrow enough" min-max intervals, no trick can work.
Some of such devices were proposed well in advance with respect to the WWII outbreak. I dare say too much in advance to be considered worth of being taken into consideration. Simply stated, they solved an issue that wasn't yet considered a major problem by decision makers that had to do with two or three bladed propellers, well known, proven in use and simpler synchronization gears and, as an alternative, simply putting the gun line of fire outside the propeller disk. Later on, when these gears, if existing and "proven in use", could have solved an actual and urgent problem, they weren't available. Just concepts or prototypes. Too risky for being pursued. Engine and combat aircraft development programs are and were based on concepts and philosophies that create long lasting legacies. Missing the right opportunity window can make incorporating some even potentially smart solutions simply impossible. Exceptions are possible under very special (e.g. emergencies) circumstances only.

Therefore, complexity and, very likely, perceived reliability of such "smart" variants, together with the "sudden" advent of jets can be considered the main show stoppers of the above mentioned solutions that could have coped with the shortcomings implied by four-five bladed propellers.

Even if the main effect of the number of propeller blades is on the minimum and maximum value of the synchronization operating rpm interval, another impact exists on the average loss of rate of fire: the lower the number of blades, the better the average synchronized rate of fire.
This in most cases is a minor effect. Nevertheless, it exists and can be explained: the higher the angle between two consecutive blades, the wider the fire enabling pulse can be without risking a blade damage. In other words, the "enable" time intervals for firing a "ready to fire" gun are longer lasting. Of course, their frequency increases proportionally to the engine speed. A wider firing/triggering angle reduces the sudden "saw tooth shaped" drop of rate of fire occurring as soon as the propeller speed become higher than a multiple of the unsynchronized "unconstrained" rate of fire of the gun, thus reducing the average loss of rate of fire due to the synch gear. Therefore, since the angle between two consecutive blades is 90° for a four bladed propeller whereas it is 120° for a three bladed propeller, the enabling pulse width of a four bladed propeller must be narrower and, as a consequence, the average loss of rate of fire is higher.

 

[P-39 Expert]

Dear all,
I'd like to spend my two cents about the impact of the number of blades on the gun synchronization feasibility and effectiveness.

The short version of the story is that, for a given gun, mounted in the same position with respect to the propeller (distance and projection of line of fire on prop disc), the higher the number of blades, the narrower is the engine speed compatibility window for (safely) operating a synchronization system, unless special versions of such gizmo are adopted. Historically speaking, however, these solutions, although proposed, were never adopted in fighters that fought. As explained in the "long version", it can be said that the reason for this is a "missed synchronization" between a real problem and a potential solution occurring when the solution is proposed when still nobody has or sees the problem. This is why gun synchronization and four or five bladed WWII fighters weren't considered a viable or even feasible match.
Another (although minor) impact of a higher number of blades is a slightly greater average loss of rate of fire due to the more severe limitations on the duration of fire enabling time intervals. This loss, however can be compensated by a potentially higher triggering pulse frequency if, with a higher number of blades, it is still possible to stick with the "one triggering pulse per blade" rule that was commonplace in WWII era.
An important remark is that, at the end of the day, what actually matters is the inter-relationship between the propeller and its rpm operating range and the gun/cartridge features and quality.
It is well known the case of the Lewis gun that couldn't be synchronized even with two bladed props. This outcome was the result of both Lewis operating cycle and the existing synchronization concept limitations. These last statements need less straightforward explanations that, therefore, are proposed inside the "long" version of the story, provided in this post.


For people interested in the full story, here we are with the long version. At the price of an overview about some basic aspects of synchronization stuff, hopefully, things should be better clarified.

Synchronization basics

The best way to think about gun-propeller synchronization is to think about a machine-gun as a semi-automatic device that has to be "externally" commanded so to avoid a stream of objects (blades) placed at a close distance and passing the line of fire at a frequency depending on engine speed and number of propeller blades. Semi-automatic means that, once gun is triggered by an "external" command, a cycle is initiated, made of the "firing" phase and other phases and that, once completed, the cycle stops waiting for a new external triggering command.
Another important fact is that, within the basic operation cycle of the gun, once the triggering pulse is provided, the firing phase is not necessarily the first one. Machine gun technical history shows a plenty of such examples. The point is that, while the delay between the trigger command and the actual cartridge ignition is of little importance when the machine gun operates in automatic mode, things are critically different when gun is "forced" to operate in semi-automatic fashion, due to the need of not hitting propeller blades. This fact is, together with cartridge ignition and launch charge features, the most important point to understand how easy or difficult synchronizing an automatic weapon can be and why propeller speed and number of blades matter. Things will be better clarified later on.
If the triggering pulse provided by the synchronization system always arrives at the very moment the gun is (again) ready to fire, then its rate of fire is not affected at all. If the gun is again ready to fire at a time that happens to be in between two consecutive triggering pulses P1 and P2, it has to wait for P2. Thus, worst case is when the gun is again ready to fire immediately after P1 has passed by.
If triggering pulses are generated at fixed positions of the propeller with respect to the gun line of fire, the best and the worst conditions happen when the synchronization pulse frequency is a multiple of the gun "free" rate of fire. That is, when propeller (and thus engine) speed is equal to one of a set of "magical" values corresponding to a pulse frequency equal to 1x, 2x, etc. the gun rate of fire.

Triggering pulse frequency is proportional to engine speed since propeller to engine speed ratio is constant. Thus, higher engine speed means higher triggering pulse frequency. That is, most of pulses are ineffective, since gun is not yet ready to fire, but an "again ready to fire" gun has less time to wait for firing its next round.

This explains why the basic underlying dependence of synchronized rate of fire on engine speed has a "tooth saw" shape and why the sudden "vertical" drops of rate of fire happens and have a progressively reduced depth as engine speed increases and, finally, why, at higher engine speeds, the toll synchronization system takes on rate of fire is lighter.
If K is the ratio between the triggering pulse frequency and the gun "free" rate of fire, 1/(K+1) is the maximum relative loss of rate of fire of the "vertical" drop. For K=1 (pulse frequency = free rate of fire) the maximum loss can be 50%. For K = 2, the maximum loss is about 33%. The max loss is 25% when pulse frequency is three times the free rate of fire, and so on. Max here means that it can happen only if triggering pulse duration is as short as zero. A limit "ideal" case. A straightforward "take" of this up& down game is that WWII fighters whose engines had narrow combat engine speed ranges (thanks to the constant engine speed concept) very seldom were so poorly matched with guns, so to force them to fire in correspondence with the bad areas of synchronization performance. Designers generally managed to look for nice areas, rather.
Pulse duration can be very short or not but it is finite. Generally speaking, as far as the pulse duration doesn't create the risk of hitting prop blades, increasing the duration is beneficial since it helps to reduce the loss of rate of fire in the critical speed regions. Going into deeper details imply some math and I'm going to skip this stuff.

So, the higher the frequency of triggering pulses the better. Isn't it? Unfortunately, not. Better said, there is an upper limit to the maximum number of triggering pulses per propeller turn. And this limit stems from a set of parameters including, among others, the number of propeller blades.

And, here we are with blades!

Once fire is enabled, time of the "bullet to propeller disc" must be compatible with the time interval between two consecutive propeller blades. A gun must fire in-between two consecutive blades, after all. Time to propeller disc depends on gun operating cycle/cartridge types and performances and, of course, on the gun distance from the propeller. This time can be worked out by using nominal data: the list and the duration of serial operations gun mechanism has to complete before actually igniting the cartridge primer, the combustion speed of the launching charge, the length of the barrel and the average bullet speed inside it and, finally, the muzzle speed of the bullet and the distance between gun muzzle and propeller disc. On top of this, safety margins must be considered in order to cope with geometric factors (the impact section of the blade at gun line of fire level) and uncertainties that, to keep it simple, are mainly due to gun and cartridge performance dispersion with respect to their nominal features (a matter of quality of production process and care of ammunition storage but also a matter of influence of temperature, that is altitude and heating due to automatic fire, on the gun parts).

As to the time interval between two consecutive blades, it is easy to see that it is inversely proportional to propeller (and therefore engine) speed and the Number of Blades (NoB) since the angle between two consecutive blades is 360°/NoB.

Finally, synchronization gears used during WWI and WWII were based on a constant offset angle concept, that is: each triggering pulse was generated at a constant angle between a reference (triggering) blade and the gun line of fire.

By the way, many WWII with a three bladed prop had synchronization mechanisms that provided three pulses per propeller turn. That is: one pulse per blade. That is: each pulse had its own reference blade. As to WWI, beside cases of two pulses per prop turn, examples exist of just one pulse per propeller turn or things like "x" pulses every "y" propeller turns, being x and y integers and x not greater than y. At that time, the large majority of props had just two blades and the rare birds with a four bladed propeller didn't need synchronization (prop behind the pilot) or never got a synchronized gun.
The constant offset angle concept is the simplest way to set up a synchronization system. This explains why it was universally adopted.
As far as the amount of this angle, three options are possible: zero, positive (anticipated) and negative (delayed) offset.
A zero offset angle means that the firing impulse occurs when the blade axis is aligned to the line of fire. An anticipated (delayed) offset means that the firing impulse occurs before (after) the reference blade has reached the line of fire.
The "zero" and the "anticipated" options, while providing more room for firing without hitting the blade that is consecutive to the reference one, imply that gun cannot be triggered below a minimum engine speed. Otherwise, the risk occurs of hitting the reference blade.

Anyway, depending on the chosen option, inter-related limitations arise on the minimum and maximum propeller (and therefore engine) speeds compatible with gun fire synchronization. Firing the gun at a speed that is lower than the minimum or higher than the maximum permissible regime, implies the risk of hitting the blades (the triggering blade or the successive blade) rather than the enemy.
This explains warning labels in the cockpit or on the fuselage, indicating min and max engine speed for safely operating the whole stuff.
Pilots had to carefully manage and check engine speed, especially before constant speed propeller control became a widespread solution as in WWII fighters.
Beside other factors, these limits had to cope with both too quickly (for zero or anticipated offset angles) and too slowly exploding cartridges (for any option), that is, cartridges quality.

What should be clear now is that, beside the fact that a max allowable engine speed limit always existed, improving this limit was possible only at the cost of increasing the minimum engine speed compatible with synch fire and, finally, the higher the number of blades the lower the max engine speed compatible with synch fire and the narrower the engine speed region compatible with the synchronized fire. even if in this narrower window, the number of pulses per propeller turn can be higher (if the one pulse per blade rule holds).
Keeping the time to propeller disk low can help of course. By way of example, "closed bolt" guns with electrically primed cartridges of a gun that is close to the propeller disk and fires bullet with very high muzzle velocities is, under this respect, the "ideal" combination since it minimizes the time to propeller disc of the bullet. But, for the same "boundary conditions", more blades mean more problems, to the point that, moving from a three bladed to a four or five bladed propeller may turn synchronization into a poor or an even impossible option.
Thus it should be clear now why given a number of propeller blades, there could be guns that cannot be synchronized and vice versa, so that this game is a matter of mutual compatibility.

The only way to overcome problems with high (four and even five or more) propeller blades is to get rid of the constant offset angle concept. An example of this improvement is to change the offset angle as a function of engine speed. Something similar to what has been always done with spark ignition phase variation in internal combustion gasoline engines and, more recently, with the smart control of engine valves.

For the sake of completeness it is useful reporting that, actually, devices implementing this concept were proposed to air forces. The basic idea was to enhance centrifugal force to adjust (anticipate) the firing pulse as engine speed increases. Of course, a sufficient cartridge quality still remains a prerequisite. Without keeping the cartridge "ignition and explosion" features within a "narrow enough" min-max intervals, no trick can work.
Some of such devices were proposed well in advance with respect to the WWII outbreak. I dare say too much in advance to be considered worth of being taken into consideration. Simply stated, they solved an issue that wasn't yet considered a major problem by decision makers that had to do with two or three bladed propellers, well known, proven in use and simpler synchronization gears and, as an alternative, simply putting the gun line of fire outside the propeller disk. Later on, when these gears, if existing and "proven in use", could have solved an actual and urgent problem, they weren't available. Just concepts or prototypes. Too risky for being pursued. Engine and combat aircraft development programs are and were based on concepts and philosophies that create long lasting legacies. Missing the right opportunity window can make incorporating some even potentially smart solutions simply impossible. Exceptions are possible under very special (e.g. emergencies) circumstances only.

Therefore, complexity and, very likely, perceived reliability of such "smart" variants, together with the "sudden" advent of jets can be considered the main show stoppers of the above mentioned solutions that could have coped with the shortcomings implied by four-five bladed propellers.

Even if the main effect of the number of propeller blades is on the minimum and maximum value of the synchronization operating rpm interval, another impact exists on the average loss of rate of fire: the lower the number of blades, the better the average synchronized rate of fire.
This in most cases is a minor effect. Nevertheless, it exists and can be explained: the higher the angle between two consecutive blades, the wider the fire enabling pulse can be without risking a blade damage. In other words, the "enable" time intervals for firing a "ready to fire" gun are longer lasting. Of course, their frequency increases proportionally to the engine speed. A wider firing/triggering angle reduces the sudden "saw tooth shaped" drop of rate of fire occurring as soon as the propeller speed become higher than a multiple of the unsynchronized "unconstrained" rate of fire of the gun, thus reducing the average loss of rate of fire due to the synch gear. Therefore, since the angle between two consecutive blades is 90° for a four bladed propeller whereas it is 120° for a three bladed propeller, the enabling pulse width of a four bladed propeller must be narrower and, as a consequence, the average loss of rate of fire is higher.

So, the rate of fire for the 4 bladed P-39 was lower than for the three bladed P-39?

 

[bruno_bis]

Rather than a matter of rate of fire, it's mainly a matter of engine rpm interval in which firing through the propeller disc is allowed. Main limitation is on the upper limit of this interval.

 

[bruno_bis]

Angle between two consecutive blades is smaller so, given a constant time from the trigger pulse to the moment the bullet reaches the propeller disc, the engine speed at which two consecutive blades cross the line of fire is lower.

 

[bruno_bis]

If designer wants the same engine max speed, trigger pulse must be anticipated but then the lower limit of engine rpm allowable interval must be raised.

 

[P-39 Expert]

Rather than a matter of rate of fire, it's mainly a matter of engine rpm interval in which firing through the propeller disc is allowed. Main limitation is on the upper limit of this interval.

It was a yes or no question.

 

[bruno_bis]

For a "yes or not" answer, some data should be known. If, moving from three to four blades, the constant speed typical engine regimes remain unchanged, synchronized rate of fire variations are, for the worst o for the best, quite similar.
For a better answer the information should be known about the actual number of triggering pulses per propeller turn in the two cases of interest. Assuming that the "one pulse per blade" solution was adopted in the four blade case too, rate of fire could be even slighty better. Unfortunately, without better info, this is just a guess. Again, the main expected effect is a narrower engine speed range useful for operating synchronization.

 

[P-39 Expert]

The two gun synchronizers (one for each gun) were on the back of the remote engine reduction gear in the nose. Each one sent an electrical impulse to their respective gun firing that gun when the propeller blades were in proper position. Shouldn't make any difference whether it was a three bladed propeller or a four bladed propeller, that point was the same. Just a slightly more narrow window for the gun to fire with the four blade prop than the three blade prop.

 

[bruno_bis]

If the triggering pulse width remains unchanged no other effect. Otherwise, as I previously wrote, if, for safety margins reason, the pulse witdh is reduced, higher loss of rate of fire at the "sawtooth places" can occur. But this matters only if engine is running at these unfavourable speeds. That is what I wrote in the long version part of my initial post. So I think that everything is agreed now.

 

[bruno_bis]

Dear all,
with respect to P39 could you kindly provide me with the following data?:
1) Rate of fire for the M2 gun that can be consider the right one between 750 and 850 Rounds per minute (800?)
2) Some (three?) engine speeds of your interest (maybe in the 2000 - 3000 rpm range)
3) Reduction gear ratios for both 3-bladed and 4-bladed versions of P39 (ratio between propeller speed and engine speed)

Thanks in advance

 

[P-39 Expert]

1. I always assumed 600rpm since the P-39 was in production in 1941 and these would have been early M2s.
2. I always assumed the guns would be fired with the engine at combat speed 3000rpm.
3. Reduction gear ratio was 1.8 for early Allison-35 models, 2.0 for mid models with the Allison -63 and 2.23 for the Allison-85 models in the N/Q. The Q had both three and four blade props.

My figures used 3000rpm with 2.23 reduction gear or 1345 propeller rpm.
1345 propeller rpm divided by 600rpm for each M2 gave 2.2 propeller rpm per round fired. Can't fire a partial round per propeller rpm so I used 3 propeller rpm per round fired, or 447rpm per gun (1345 propeller rpm divided by 3).
447rpm divided by normal firing rate of 600rpm means each gun lost about 25% to synchronization.
Meaning the two M2 machine guns were worth about 1.5 machine guns after synchronization. Both of them were worth one super M2 machine gun.
Just my estimate, look forward to yours.
Thanks.

 

[Shortround6]

I am not sure the US continued production of the 600rpm M2 aircraft gun after they changed to the 750-850 rpm version. The US was big on standardization.
The 750-850 rpm was a range that production guns would fire over, not different models.
Depending on fit of the parts, smoothness of the bearing surfaces and the strength of the springs (and recoil buffer) two guns just one serial number apart could, in theory, fire at opposite ends of the rate of fire range. A good armourer could swap parts or polish surfaces to help even things out. But the usual goal was smooth running guns, not peak rate of fire.

 

[P-39 Expert]

I am not sure the US continued production of the 600rpm M2 aircraft gun after they changed to the 750-850 rpm version. The US was big on standardization.
The 750-850 rpm was a range that production guns would fire over, not different models.
Depending on fit of the parts, smoothness of the bearing surfaces and the strength of the springs (and recoil buffer) two guns just one serial number apart could, in theory, fire at opposite ends of the rate of fire range. A good armourer could swap parts or polish surfaces to help even things out. But the usual goal was smooth running guns, not peak rate of fire.

Great. The more rpm the better.
3000rpm divided by 2.23 reduction gear = 1345 propeller rpm.
1345 prop rpm divided by 750rpm = 1.79 propeller rpm per round fired. Can't fire a partial round per propeller rpm so use 2 propeller rpm per round fired.
1345 prop rpm divided by 2 = 672rpm. Divided by normal uninterrupted 750rpm = 90% of uninterrupted rate of fire. Personally I don't know if this was possible, but in theory...

 

[bruno_bis]

Well, I'going to use:
3 triggering pulses per propeller turn for 3-bladed version of P39 Q
4 triggering pulses per propeller turn for 4-bladed version of P39 Q

and, for both versions:

800 Rounds per minute for the M2 gun
Propeller speed/Engine speed = 1/2,23
Engine speed range 2000-3000 rpm

Thanks and see you later

 

[bruno_bis]

 

[bruno_bis]

Just for clarity
SROF synchronized rate of fire
PpT (triggering) Pulses per (propeller) Turn
RG reduction gear (prop speed/enginespeed)
Rpm Rounds per minutes

 

[bruno_bis]

 

[bruno_bis]