Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained)

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I have learned several languages, some well enough to earn money for my knowledge. I have learned that understanding a language is more than knowing which word endings to use, or the proper spelling of words. It is best to know what the other person is trying to say even when he is not saying or spelling it correctly.
 
Ahhhh, pigeon English!

At las', Ahh unerstand.

But Ah DO speak P-39: Yes we have no fuel to speak of. Yes, we cannot fight at 25,000 feet. Wait! 7.620 metres! And, the "39" in P-39 is probably in feet, so shouldn't it be a P-7,89 instead of a P-39? Different language, huh? Might take a fortnight or so to understand it.
 
I'm going to start a thread explaining why the other Bell product, The FM-1 Airacuda could have won the war all by itself if only the Brass had accepted it.
I mean, if one 37mm cannon is good, then two is even better.

1622255644655.png
 
A short history, by me, of the armament in the P-39.

In 1936-37-38 when the project was first conceived and worked on (Requirement X-609 was issued March of 1937) the 37mm cannon was not the first choice.
A theoretical/hypothetical 25mm gun was being bandied about. The only actual 25mm automatic gun suitable for aircraft use was French and it was on the secrets list.
The US did buy and test the 23mm Danish Madsen gun but it had a low rate of fire and an unreliable feed mechanism. The US Ordnance dept. was working on a .90 caliber gun but it was far from ready.
Early drawings of the Bell Model 4 show a cannon with a large pan magazine on top. The 37mm was chosen as an interim weapon as it was actually "available". Captain Kelsey, headof the pursuit dept was not happy with 37mm because of the low velocity and low rate of fire.
Available is in quotes because according to one source only 45 had been delivered by Jan of 1941 when Colt firearms shut down production to retool to allow higher production. This took about one year even with the help of some engineers for Springfield Armory. In Early 1942 demand for the 37mm outstripped supply although this was soon corrected.

With this as a back drop the French and British decision to use the 20mm Hispano was pretty much a no-brainer. As an added complication the 37mm was on the US secrets list or at lest the "do not export" list for part of 1940 at the least. So for the French and British in 1940 it was the 20mm Hispano or nothing.

Low production of the 37mm lead to the use of the 20mm Hispano in some US P-39Ds in addition to the ex British P-400s.

The US P-39C used the 37mm with just a 15 round feed, in part because there were two .30 cal guns in the cowl in addition to the two .50 cal guns.

The US .50 cal of 1936-40 was not the .50 cal of 1942 and later. It topped out at 600rpm unsynchronized using short belts and under no (actually 1) G load. Long belts (many planes had never actually been tested with full length belts) slowed the guns down and trying to fire while turning and pulling "G"s was also a problem, not only with rate of fire but with jams.
The ammo was also not the same, this has been gone over in many other threads (if not this one earlier) and used slightly heavier bullets at under 2600fpm velocity.
1940 was a year of change for the US .50 cal. At some point they boosted the rate of fire to 800rpm, still for the unsynchronized guns although their rate of fire should have improved.
The US also changed the ammo, using a slightly lighter bullet and a different propellent they got the velocity up to nearly 2900fps for shorter time of flight and much improved striking power. However this was pretty much a kinetic energy weapon. The US never approved a high explosive projectile and the Incendiary round may have been scarce. The British disliked it enough to design their own.

Which brings us to the .30 cal and .303 guns. As noted above the P-39C had two small guns in the cowl. When these were moved to the wing and another gun added in each wing the 37mm ammo was raised to 30 rounds. While the British did technically order the P-400 with wing guns they actually inherited the French version which had 7.5mm MAC 34 guns in the wings. French ammo outfit is unknown. The British were just introducing the De Wilde bullet for the .303 at the time they took over the French contract and ordered more. British were also just getting the 20mm Hispano into service so it was something of an unknown. The British didn't get any P-40s delivered to England until after the P-39 orders were placed so the American .50 cal and it's ammunition were also something of an unknown. British tests of P-40s showed a lower rate of fire for the cowl .50s than expected when they did show up.

As far as one 37mm and two synched .50s being adequate armament goes. The US was ordering P-40Ds with four free firing wing guns and provisions for under wing 20mm cannon pods before they sighed the contract for the P-39D with the four .30 cal wing guns.
British were in process of buying the Mustang I with four .50s (two in the cowl and one in each wing free firing) and four .303 guns. British were also ordering F4Fs with six .50 cal wing guns. How much of this was based of the 600rpm (or under) .50 cal guns I don't know.

Some American pilots may have been very glad to have the four wing .30s because the 37mm gun in the early American planes in 1942 was notorious for jamming in just a few shots.

another consideration for the French and British was that the 20mm Hispano only carried enough ammo for about 6 seconds of firing time, assuming the gun didn't jam first. perhaps they desired a plane that could stay in the fight longer, after the cannon ran out of ammo?
The French were building the D 520 fighter with the 20mm gun, four 7.5mm machine guns with 675rpg (?) with a 950hp engine. The P-39 had an 1150 hp engine, adding a pair of .50 cal guns shouldn't be that big a problem?
 
Marcel said:
I thought some of you had been here long enough to know that in order to stop talking to a brick wall, you just have to stop talking. But maybe I overestimated that?
Click to expand...
Actually I didn't mean it as a joke, I believe above is meant for both sides of the argument.

I do agree with P-39 Expert P-39 Expert to some extent that the P-39 gets a bad rap that it doesn't fully deserve. Actually I believe that is true for most aircraft that get a bad rap (Buffalo comes to mind). But at one point discussion makes no sense anymore. I see the discussion hardening, discussion runs around details and speculations and misinterpretations of eachother's words. It's just waiting until it turns ugly and personal and the mod team will have to step in to close threads and put some people on pauze time for a while. Sometimes you should just give it a rest, enjoy some polite discussions about other topics for some time.

And again, this is not solely aimed at P-39 Expert P-39 Expert , but it's meant for all of the participants of this circus.
 
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Hey GregP,

When you check the radio access on the P-39, I believe that you will find that the right side of the rear canopy is (was originally) hinged for easier access to the radio. I ran across this bit of info in one or another P-39 manual a number of years ago, unfortunately I do not remember which one. :( I think it was a maintenance manual, but am not sure.


re the P-39C climb rate:

Unless I am missing something, you did not factor the propeller efficiency (e) into your calculations. This factor is not included in the basic formula you are using. I usually use an e of .83 unless I have specific data on the propeller used.

Your PR value of 395.97 at Vclimb seems high to me. I get about 300-336 BHPe (depending on what drag factor I use) up to ROCmax critical altitude. I could be off on this. I used the low end of 300 BHPe for the P-39C since it was relatively light and clean for the tests.

If you remember the other discussion (up-thread) where the issue of the P-39C claimed ROC came up, I could not get it past about 3270 ft/min on the 1160 BHP listed in the test, without using the equivalent of early-war WEP ratings (1320 BHP) or by using a zoom climb. In order to get the claimed sustained ROC (3720 ft/min) we would need a value of 1 for e, which is not possible (obviously). The best WWII era value for e (at climbing speeds) that I have run across is somewhere around .85 (this was a Rotol 3- and 4-blade prop with Jablo blades, wide at the base, like those fitted to many Hurricanes and Spitfires/Seafires). There may be other props that attained this value but I have not run across any actual data that says so, at least not that saw service in WWII. Maybe some of the German wide blade types? Or the late-war US paddle types? As far as I am aware, the best e value for a conventional airplane prop of today's era is .91 (a 2-blade made by Hartzell).
 
Last edited:
Shortround6 said:
A short history, by me, of the armament in the P-39.

In 1936-37-38 when the project was first conceived and worked on (Requirement X-609 was issued March of 1937) the 37mm cannon was not the first choice.
A theoretical/hypothetical 25mm gun was being bandied about. The only actual 25mm automatic gun suitable for aircraft use was French and it was on the secrets list.
The US did buy and test the 23mm Danish Madsen gun but it had a low rate of fire and an unreliable feed mechanism. The US Ordnance dept. was working on a .90 caliber gun but it was far from ready.
Early drawings of the Bell Model 4 show a cannon with a large pan magazine on top. The 37mm was chosen as an interim weapon as it was actually "available". Captain Kelsey, headof the pursuit dept was not happy with 37mm because of the low velocity and low rate of fire.
Available is in quotes because according to one source only 45 had been delivered by Jan of 1941 when Colt firearms shut down production to retool to allow higher production. This took about one year even with the help of some engineers for Springfield Armory. In Early 1942 demand for the 37mm outstripped supply although this was soon corrected.

With this as a back drop the French and British decision to use the 20mm Hispano was pretty much a no-brainer. As an added complication the 37mm was on the US secrets list or at lest the "do not export" list for part of 1940 at the least. So for the French and British in 1940 it was the 20mm Hispano or nothing.

Low production of the 37mm lead to the use of the 20mm Hispano in some US P-39Ds in addition to the ex British P-400s.

The US P-39C used the 37mm with just a 15 round feed, in part because there were two .30 cal guns in the cowl in addition to the two .50 cal guns.

The US .50 cal of 1936-40 was not the .50 cal of 1942 and later. It topped out at 600rpm unsynchronized using short belts and under no (actually 1) G load. Long belts (many planes had never actually been tested with full length belts) slowed the guns down and trying to fire while turning and pulling "G"s was also a problem, not only with rate of fire but with jams.
The ammo was also not the same, this has been gone over in many other threads (if not this one earlier) and used slightly heavier bullets at under 2600fpm velocity.
1940 was a year of change for the US .50 cal. At some point they boosted the rate of fire to 800rpm, still for the unsynchronized guns although their rate of fire should have improved.
The US also changed the ammo, using a slightly lighter bullet and a different propellent they got the velocity up to nearly 2900fps for shorter time of flight and much improved striking power. However this was pretty much a kinetic energy weapon. The US never approved a high explosive projectile and the Incendiary round may have been scarce. The British disliked it enough to design their own.

Which brings us to the .30 cal and .303 guns. As noted above the P-39C had two small guns in the cowl. When these were moved to the wing and another gun added in each wing the 37mm ammo was raised to 30 rounds. While the British did technically order the P-400 with wing guns they actually inherited the French version which had 7.5mm MAC 34 guns in the wings. French ammo outfit is unknown. The British were just introducing the De Wilde bullet for the .303 at the time they took over the French contract and ordered more. British were also just getting the 20mm Hispano into service so it was something of an unknown. The British didn't get any P-40s delivered to England until after the P-39 orders were placed so the American .50 cal and it's ammunition were also something of an unknown. British tests of P-40s showed a lower rate of fire for the cowl .50s than expected when they did show up.

As far as one 37mm and two synched .50s being adequate armament goes. The US was ordering P-40Ds with four free firing wing guns and provisions for under wing 20mm cannon pods before they sighed the contract for the P-39D with the four .30 cal wing guns.
British were in process of buying the Mustang I with four .50s (two in the cowl and one in each wing free firing) and four .303 guns. British were also ordering F4Fs with six .50 cal wing guns. How much of this was based of the 600rpm (or under) .50 cal guns I don't know.

Some American pilots may have been very glad to have the four wing .30s because the 37mm gun in the early American planes in 1942 was notorious for jamming in just a few shots.

another consideration for the French and British was that the 20mm Hispano only carried enough ammo for about 6 seconds of firing time, assuming the gun didn't jam first. perhaps they desired a plane that could stay in the fight longer, after the cannon ran out of ammo?
The French were building the D 520 fighter with the 20mm gun, four 7.5mm machine guns with 675rpg (?) with a 950hp engine. The P-39 had an 1150 hp engine, adding a pair of .50 cal guns shouldn't be that big a problem?
Click to expand...
As usual, a very informative, factual and well researched post. However I might disagree with your definition of "short". Just kidding.

Regarding the 30calMGs, AHT says their effective range was only 200 yards. Is that due to trajectory drop?

Regarding the 37mm cannon, supposedly the jamming problem was corrected with the P-39L model with the heating air exit louvers in the forward gun bay that allowed better flow of heated air from the cockpit that kept the cannon from freezing at altitude. Ideally in hindsight the 20mm cannon should have been used on all models prior to the L. The 20mm was used in over 1000 P-400s and P-39D-1s. I always thought the 20mm cannon with 120-150 rounds would have been the ideal solution. Seems doable since the P-38F had a 150 round belt fed magazine in production from March 1942. Just my opinion.
 
Thumpalumpacus said:
Just put "
Click to expand...
" at the end of the part you want to reference, and delete the rest. Or separate it into another paragraph. If you want to learn that we can take it to PM, it's easy but there's no sense in interrupting the flogging of a deceased equine.[/QUOTE]
Please PM me, I didn't realize my replies were a problem. Not sure when to use "quote" or "reply". Thanks in advance for your help.
 
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