P-39 unsynchronized gun

[daveT]

While investigating a P-39Q crash site, it was discovered that the propeller had bullet holes in it and the plane shot itself down with its own .50 cal. The Plane was on a practice straffing run at the time. I have read that U.S. airmen complained about the P-39 slow synchronized rate of fire for the browning .50 Cal (12.7 mm) averaging around 300 r/m.
Questions: Anyone ever heard of a ww2 P-39 .50 cal. synchronization problem?
What is the synchronized rate of fire for the .50 cal in the P-39Q?
Anyone heard of a P-39 shooting itself down?

Looking for info about synchronization issues with nose-mounted .50 cal machine guns during WW2. Any issues, examples, maintenance problems, and accidents. I have discovered an example of P-39 shooting itself down. I'm looking for more examples.

 

[FLYBOYJ]

While investigating a P-39Q crash site, it was discovered that the propeller had bullet holes in it and the plane shot itself down with its own .50 cal. The Plane was on a practice straffing run at the time. I have read that U.S. airmen complained about the P-39 slow synchronized rate of fire for the browning .50 Cal (12.7 mm) averaging around 300 r/m.
Questions: Anyone ever heard of a ww2 P-39 .50 cal. synchronization problem?
What is the synchronized rate of fire for the .50 cal in the P-39Q?
Anyone heard of a P-39 shooting itself down?

Hi Dave -

A few things - do you have an accident report that specified that this aircraft shot itself down? Just because the synchronization failed and a round went through the prop doesn't mean immediate failure. I read at least one occasion where synchronization failed on a fighter and a hole was shot through the propeller and the plane continued to fly (and was not immediately repaired!) This was mentioned in the book "Some Still Live" by Frank Tinker, the aircraft was Soviet I-15. Now if a major portion of the prop was shot away or if the prop strike resulted in the prop gearbox to fail, then absolutely.

I read somewhere about some synchronization issues with the P-39's nose .50s, but it was isolated and due to poor maintenance.

 

[Wurger]

Also , if it was the synchronization issue the damages should be at the same level and quite limited area. If the bullet holes are not concentrated these could be caused by another factor rather.

 

[daveT]

Hi Dave -

A few things - do you have an accident report that specified that this aircraft shot itself down? Just because the synchronization failed and a round went through the prop doesn't mean immediate failure. I read at least one occasion where synchronization failed on a fighter and a hole was shot through the propeller and the plane continued to fly (and was not immediately repaired!) This was mentioned in the book "Some Still Live" by Frank Tinker, the aircraft was Soviet I-15. Now if a major portion of the prop was shot away or if the prop strike resulted in the prop gearbox to fail, then absolutely.

I read somewhere about some synchronization issues with the P-39's nose .50s, but it was isolated and due to poor maintenance.

Yes, I do have the accident report. The P-39 was underwater until recently. It is in process of recovery now. They recovered the propeller and that is when the bullet holes in the prop were discovered. The official accident report only mentions that parts/debris were witnessed coming off the front of the plane before it dived into the lake. the official cause was listed as mechanical failure unknown cause.

"I read somewhere about some synchronization issues with the P-39's nose .50s, but it was isolated and due to poor maintenance." I badly need this reference!

More on the accident and its historical importance: P-39Q, Serial Number: 42-21226, Pilot: Moody, Frank H, crashed into lake Huron. Moody was among the Tuskegee Airmen, America's first black military pilots.

The state of Michigan is doing a full recovery of the wreck. Pictures and story available upon request or I can post here.

 

[FLYBOYJ]

Yes, I do have the accident report. The P-39 was underwater until recently. It is in process of recovery now. They recovered the propeller and that is when the bullet holes in the prop were discovered. The official accident report only mentions that parts/debris were witnessed coming off the front of the plane before it dived into the lake. the official cause was listed as mechanical failure unknown cause.

"I read somewhere about some synchronization issues with the P-39's nose .50s, but it was isolated and due to poor maintenance." I badly need this reference!

More on the accident and its historical importance: P-39Q, Serial Number: 42-21226, Pilot: Moody, Frank H, crashed into lake Huron. Moody was among the Tuskegee Airmen, America's first black military pilots.

The state of Michigan is doing a full recovery of the wreck. Pictures and story available upon request or I can post here.

I'll try to look around for it, as you probably know we have a huge tread on the P-39, some good information, some gibberish, there might be mention about this happening

 

[daveT]

Picture of recovered P-39 propeller with a bullet hole in it. One propeller was completely shot off, one still attached to hub with bullet holes in it and this one.

 

[daveT]

Also , if it was the synchronization issue the damages should be at the same level and quite limited area. If the bullet holes are not concentrated these could be caused by another factor rather.

The bullet holes are level with the guns at about 9 inches above the hub and in a limited area. see next reply including a picture.

 

[Wurger]

So it looks like it is the synchronizer issue.

 

[daveT]

What is the synchronized rate of fire for the cowl .50 Cal. Machine Guns? I have read 300 rounds per minute but unsure.
Did the reduction gear dictate a 300 r /m rate each no matter if the cowl gun was a Browning or anything else (faster UBS 12.7mm etc...)?
How fast was the rate of fire for cowl mounted Browning .50 Cal. MGs if geared to the Allison engine mounted in the nose like the early P-40 or early P-51 for example? I have read 5 rounds per second but unsure.

 

[daveT]

any comments on this statement: Propeller blades sweep past the gun muzzle at a much faster rate than any gun can fire. For example, a three-bladed propeller rotating at a typical 1,200 rpm, a blade will pass the muzzle 3,600 times per minute. Gun synchronization was therefore about choosing the correct instant for firing each shot, not about occasionally interrupting the automatic fire. Calculations show that a gun firing at 1,200 rpm would have its firing rate slowed by an average of 12.5% depending on the synchronization type and aircraft.

There were complaints by U.S. airmen about the slow synchronized rate of fire (ROF) for the P-39 nose-mounted browning .50 Cal (12.7 mm) when compared to other fighter aircraft. The P-39 nose-mounted machine guns averaged around 300 rounds per minute which equated to only 5 rounds per second (rps) for each gun in the P-39 cowl. Synchronizing the .50 Cal. Browning P-39 nose guns caused about a 37% reduction in the rate of fire compared to unsynchronized guns. This speed was slow when compared to other fighters. As a comparison, the P-40B cowl guns averaged around 425 r/m. The discrepancy between the P-39's .50 Cal. Browning ROF nose gun and other US fighters synchronized ROF in the P-39's reduction gear-driven design, unlike the other propeller drive sync designs.

 

[FLYBOYJ]

HI Dave -

I've been looking high and low and I can't find that piece about a synchronization failure on the P-39, I'm sure it was mentioned during a pilot interview. I found the same statement about the ROF from this site:

Bell P-39 Airacobra - fighter

The complaints by U.S. airmen about the slow synchronized rate of fire for the browning .50 Cal (12.7 mm) P-40B cowl guns averaging around 425 r /m made the P-39 look bad at 300 r /m while the Soviet synchronized 12.7 mm UBS fired over 800 r /m each!

 

[FLYBOYJ]

For what it's worth...

https://www.aviatorsdatabase.com/wp-content/uploads/2013/07/P-39-Airacobra.pdf

 

[daveT]

HI Dave -

I've been looking high and low and I can't find that piece about a synchronization failure on the P-39, I'm sure it was mentioned during a pilot interview. I found the same statement about the ROF from this site:

Bell P-39 Airacobra - fighter

The complaints by U.S. airmen about the slow synchronized rate of fire for the browning .50 Cal (12.7 mm) P-40B cowl guns averaging around 425 r /m made the P-39 look bad at 300 r /m while the Soviet synchronized 12.7 mm UBS fired over 800 r /m each!

thanks I also found that statement. You are most likely correct that it was during an interview

 

[A.G. Williams]

I have an article on aircraft gun synchronisation on my website here: https://www.quarryhs.co.uk/Synchro.pdf

This includes the following:

A practical example of the effect of synchronisation is graphically provided by comparative tests held by the USN in 1926/7 of the .30 inch (7.62 mm) M1921 and .50 inch (12.7 mm) M1921, both on a test stand and in synchronised mountings. These also shed some light on the differences between claimed and actual rates of fire, and between different installations of the same gun. The .30 had a claimed RoF of 1,200 rpm, but proved capable of between 800 and 900 rpm on the test stand. When synchronised, the RoF went down to an average of 730 rpm (a fall of about 15%), with a range of between 667 and 818 rpm for different installations and propeller speeds. The .50 had a claimed RoF of 600 rpm, and did rather well to achieve between 500 and 700 rpm, depending on the recoil buffer adjustment (although a contemporary British report put this at 400-650 rpm, the difference possibly caused by belt drag when installed), but this fell to an average of 438 rpm when synchronised, varying between 383 and 487 rpm. As the synchronised guns were adjusted for maximum RoF, this represented a reduction of around 37%. There is no inherent reason why a larger calibre weapon would suffer a bigger reduction in RoF, so the synchronisation conditions must have been better suited to the .30 in gun's natural RoF.

 

[cammerjeff]

Just a thought but how much closer to the propellers hub were the Machine guns mounted on a P-39 than on a P-40 or P-51? If they were significantly closer to the hub that would mean the blades were passing by the guns at a slower rate, possibly requiring a longer interruption of the firing cycle?

This could have an effect on the firing rate of the P-39's guns?

 

[ThomasP]

I do not know if the following information might shed light on the issue re the P-39's .50 cal synchronized gun ROF, but here it is:

P-400/P-39D/F
V-1710-35(E4) engine.
Propeller reduction gearing was .556:1 and synchronizer gearing was .556:1, both relative to crankshaft speed.

P-39K/L
V-1710-63(E6) engine and P-39M with V-1710-83(E18) engine.
Propeller reduction gearing was .500:1 and synchronizer gearing was .500:1, both relative to crankshaft speed.

P-39N/Q
V-1710-85(E19) engine.
Propeller reduction gearing was .449:1 and synchronizer gearing was .449:1, both relative to crankshaft speed.

All of the above P-39 engine arrangements have the following entry re the Gun Synchronizer Impulse Generator:

When compared with the other airframes fitted with the V-1710 and synchronization gear:

P-40B/C
V-1710-33(C15) engine.
Propeller reduction gearing was .500:1 and synchronizer gearing was .500:1, both relative to crankshaft speed.

Mustang Mk I
V-1710-39(F3R) engine.
Propeller reduction gearing was .500:1 and synchronizer gearing was .500:1, both relative to crankshaft speed.

A-36A
V-1710-87(F21) engine.
Propeller reduction gearing was .500:1 and synchronizer gearing was .500:1, both relative to crankshaft speed.

If we assume that the information posted up-thread by daveT (re the ROF for the P-40B nose guns is approximately 425 rpm) and by A.G. Williams (re the ROF range of 383 - 487 the British reported) then we get the following theoretical ROFs:

P-40B/C__________425 rpm
P-400/P-39D/F____473 rpm
P-39K/L__________425 rpm
P-39N/Q_________ 382 rpm
Mustang Mk I_____425 rpm
A-36A___________ 425 rpm

For a range of 382 - 473 rpm.

Maybe?

 

[daveT]

I have an article on aircraft gun synchronisation on my website here: https://www.quarryhs.co.uk/Synchro.pdf

This includes the following:

message sent regarding this issue

 

[daveT]

I do not know if the following information might shed light on the issue re the P-39's .50 cal synchronized gun ROF, but here it is:

P-400/P-39D/F
V-1710-35(E4) engine.
Propeller reduction gearing was .556:1 and synchronizer gearing was .556:1, both relative to crankshaft speed.

P-39K/L
V-1710-63(E6) engine and P-39M with V-1710-83(E18) engine.
Propeller reduction gearing was .500:1 and synchronizer gearing was .500:1, both relative to crankshaft speed.

P-39N/Q
V-1710-85(E19) engine.
Propeller reduction gearing was .449:1 and synchronizer gearing was .449:1, both relative to crankshaft speed.

All of the above P-39 engine arrangements have the following entry re the Gun Synchronizer Impulse Generator:
View attachment 677872

When compared with the other airframes fitted with the V-1710 and synchronization gear:

P-40B/C
V-1710-33(C15) engine.
Propeller reduction gearing was .500:1 and synchronizer gearing was .500:1, both relative to crankshaft speed.
View attachment 677873
View attachment 677877

Mustang Mk I
V-1710-39(F3R) engine.
Propeller reduction gearing was .500:1 and synchronizer gearing was .500:1, both relative to crankshaft speed.
View attachment 677874

A-36A
V-1710-87(F21) engine.
Propeller reduction gearing was .500:1 and synchronizer gearing was .500:1, both relative to crankshaft speed.
View attachment 677875

If we assume that the information posted up-thread by daveT (re the ROF for the P-40B nose guns is approximately 425 rpm) and by A.G. Williams (re the ROF range of 383 - 487 the British reported) then we get the following theoretical ROFs:

P-40B/C__________425 rpm
P-400/P-39D/F____473 rpm
P-39K/L__________425 rpm
P-39N/Q_________ 382 rpm
Mustang Mk I_____425 rpm
A-36A___________ 425 rpm

For a range of 382 - 473 rpm.

Maybe?

by all information that I have, the P-39Q was the slowest RoF for the synchronized guns and pilots complained about it. The question is what made it the slowest? It appears the design was at fault and different from the others.

 

[daveT]

still looking for P-39 .50 cal. synchronization problems.
some information that I have found:
Poor maintenance was the number one cause of synchronization problems, but there were other causes. The same type of gun could vary quite significantly depending on age, maintenance, ammunition, and environment.

Machine guns might vary their rate of fire depending on the ammunition used. Inaccuracies in bullet manufacturer and quality meant that some bullets would fire at the wrong time. Tiny variations in ammunition quality could cause bullet synchronization problems. Special "Red Label, Synchronized Guns" ammunition was sometimes used in aircraft synchronized cowl machine guns. This type of ammunition had better quality control and tighter tolerances than standard ammunition.

With precise timing requirements, changes in the environment could result in small changes to the synchronization. Temperature changes in the metal parts caused thermal expansion/contraction and resulted in small changes to the synchronization. The effect of the low temperatures experienced at high altitude in congealing the gun lubricants and on the variable G-forces consequent on maneuvering also could cause small changes to the synchronization.

Propeller speeds also varied during the flight. While you might think that it wouldn't matter since the cam rotated at the same speed as the propeller, the firing of the gun took a length of time that was independent of the propeller speed. With some systems, the pilot had to keep an eye on the tachometer indicating the engine speed to know when it was safe to fire. Aircraft without constant-speed propellers also suffered considerable variations in RoF depending on the engine speed.

There were complaints by U.S. airmen about the slow synchronized rate of fire for the P-39 nose-mounted browning .50 Cal (12.7 mm) when compared to other fighter aircraft. According to the ordnance chief of the 68th fighter squadron on Guadalcanal and other islands in the Pacific, P-39s, and P-40s both had synchronized guns using hydraulic synchronizers. The synchronizers only worked properly within a certain engine RPM range. He stated that several pilots came back with holed and dinged propellers because they fired outside that RPM range.