XP-39 II - The Groundhog Day Thread

[P-39 Expert]

You have obviously never done weight and balance on a real aircraft, or actually worked on a P-39, or you would know that much of what you wrote above is not correct. Worse still, as many of the pilots on the forum will tell you, a stall caused by aft CG on many aircraft is stable meaning you cannot regain control. Regardless of power or control inputs the aircraft remains stalled and drops like a brick with almost zero forward speed. I have gone to two funerals caused by this mistake. Both pilots had carefully calculated there gross weight CG but forgot to check their low fuel, low speed, gear down CG.

You will note in your own writing that the Soviets removed the IFF to compensate for the removal of the 30 cals. There is nothing else heavy back there to remove except the oil tank and that is aft of the fuselage joint line. There is a tie down kit but that weighs about 5 or 6 lbs so is not going to help. Yes you could move the oil tank (and coolant tank) into the wing leading edge like the Hurricane did but that introduces all sorts of other problems. And you can remove the radios from above the engine (they and the engine are actually aft of the CG which is just above 2 = fuel, 9 = guns and 10 = wing ammo in the diagram the below and all forward of the CG) but where would you fit them?
\View attachment 597251

I never said the Soviets removed the IFF radio in the tail to compensate for removing the .30s. The .30s were located in the wing on/near the CG and their removal would require no compensation of any kind whatsoever. The removal of the 120lb IFF radio from the tail apparently caused no CG problem at all since the Soviets didn't remove anything from the nose to compensate. The P-39 was able to be balanced despite all the different propeller, armament, armor, supercharger and radio combinations used.

On any aircraft if you remove 100lb from the very front of the aircraft you must compensate by removing weight from the rear.
If the weight you remove aft of the cg is only 50 lb then it must come from twice the distance from the CG that the removed weight was. etc etc etc

I have never seen anything from Bell saying the designed the P-39 to take the ASB used on the P-63. The coolant tank takes up all that space behind the engine on a P-39 and there is also structure there, including the fuselage bolt line, that would prevent an ASB being fitted. Find a flying example near you and go visit it as soon as restrictions allow. The lack of free space in that powerplant will surprise you and clearly show that there is no way it could fit an ASB.

They did design it with a turbo but that went under the engine.

View attachment 597249
This diagram shows the engine and supercharger outline but does not show the starter or generator or any of the mass of plumbing and "small" parts that extend from the rear of the engine. The oil tank is aft of the bolt line but the bulkhead is shaped to allow part of it to extend into the engine bay. There is no way an ASB would fit there.

Please expand the above for more comments and drawings.

The aux. stage supercharger occupied the same space as the coolant tank. On the P-63 (and the XP-39E) the coolant tank was moved forward to above the engine just aft of the pilot and flattened somewhat in shape to fit. In this diagram there is a bulkhead between the oil tank and the coolant tank, and the oil tank is clearly completely behind that bulkhead, as it was on the P-63. The area occupied by the coolant tank on the P-39 was exactly the same size as the area occupied by the aux. stage supercharger on the P-63.

The IFF radio is the black rectangle in the tail. It is actually farther away from the CG than the nose armor. Remove the 100lb nose armor and the IFF radio and the P-39 is balanced. Or just move the IFF radio up above the engine behind the pilot nearly on the CG and accomplish the same thing.

 

[buffnut453]

I think we are letting national pride intrude on the discussion of how mechanically adept the Americans were.

Not from me. I am, after all, American. I'm as patriotic as the next man. I just want to do so with clear-eyed comprehension, cognizant of the widest-possible breadth of facts rather than a nationalistic agenda that perpetuates myths based on false assumptions.

It is not a question of how good a small group of engineers or designers were.

Entirely agree. However, there persists this perception that countries other than America all had a "small group of engineers".

It is a question of how many troops (a percentage of total troops) had driven or worked on motor vehicles (or boats or industrial machinery).

What this means to an army (any army) is how much training is required to turn recruits into drivers or mechanics/fitters.
The army is going to put the troops selected for those jobs through some sort of training course and not just turn them loose.
How many hours or weeks should the course/s be given the average level of knowledge the troops bring from civilian life?

That's a nice idea but it doesn't work in reality. You can't guarantee that every entrant has the same basic level of experience, nor can you tailor courses to suit the needs of an individual class (one class might be packed with motor mechanics while the next is filled with clerks). There's also the risk that new recruits may have learned bad habits in their prior experience. Regardless of incoming skills, you have to start from square one and give every recruit the same training with an assumed zero level of starting knowledge. Certainly, that means some highly-skilled recruits will be bored out of their tiny minds but the training has to be of sufficient duration and tempo that a student with no prior knowledge can reasonably be expected to graduate. It's exactly the same today. I've run military training courses for many years and you have to assume zero knowledge to ensure they've all been training the "right way".

The American advantage was not that they were smarter or more mechanical adept 'naturally' but that they had much more experience driving/maintaining cars and machinery on average
per 1000 recruits. than other armies.
How good you were rebuilding a turret lathe in civilian life gives you some advantage in working on truck/tank but perhaps not as much as a guy who worked in a gas station and had seen any number of minor problems.

Ok...let's take a look at things from the other end of the telescope. The 1930s didn't happen in isolation: they built on existing structures from the preceding decades. The US was largely unaffected by the Great War while the European nations were impacted massively. Again, I'll focus on the UK because it's the nation I know best.

In 1918, the British Army operated enough vehicles powered by internal combustion engines to provide one vehicle per 350 of the British population (and that doesn't include the vehicles operated by the Royal Navy or the Royal Air Force). The vast majority of those vehicles were trucks, ambulances and motorbikes which weren't scrapped at the end of the Great War; they were demobilized and put to civilian use. In personnel terms, the Army Service Corps had 314 Motor Transport Companies, each of around 400 personnel...and that was just the establishment, not including those in training, or those who had been demobbed but still capable of working.

The Royal Engineers had a personnel strength in 1918 of almost 300,000 men. Not all of them were technical but a great many were. There were Electrical and Mechanical Companies who maintained generators, pumps and plumbed electric lines in the trenches. There were Signals Companies attached to each Division (and the British Army had 70+ Divisions by the end of the War) to operate and maintain wireless, telegraph and other communication systems in the field. There were also Railway Companies, Light Railway Companies, Tramways Companies, and Transportation Works Companies (for maintaining vehicles...in addition to those belonging to the ASC which owned the MT Companies).

In 1918, the Royal Air Force was the largest air arm the world had ever seen, operating 22,000 aircraft--all powered by internal combustion engines--with a force of around 300,000 personnel, the vast majority of whom had a technical background. The RAF also made extensive use of motor vehicles which were critical to help the RAF keep pace with the rapid advances made in 1918 (my cousin served on 11 Sqn and moved airfields 4 times in 6 weeks towards the end of the war. These were not short trips and all personnel and equipment were moved in trucks. As the most technologically advanced service, the RAF had also adopted wireless for communicating within aircraft formations and from aircraft to ground forces for CAS coordination. All of this technology and innovation came was enabled because of the technical proficiency of the personnel.

On the Home Front, the entire national production switched over to making war materiel. Factories were repurposed and many jobs previously held by men were being taken by women who drove motorbikes, trucks, ambulances, and cars. They maintained and operated machinery, including precision work, gaining skills and proving their abilities so that the 1918 push for women's suffrage became an irresistible force. Britain had the largest rail network per capita in the world. If the US had cars, the UK had railways and, while rail is a mass transit system in which passengers are scarcely aware of the machine they're using, the industry itself employed literally hundreds of thousands of men who understood machinery and were mechanically-minded.

I've already mentioned the interwar fishing fleet and the many thousands of local fishermen who operated their own powered trawlers (again, internal combustion engines during this timeframe). None of the above has even touched the massive proportion of the British workforce engaged in maritime shipping. Yes, many were dockhands. But there were countless machinists, engineers and other technical staff, ontop of those employed in shipyards.

When the Great War ended, the UK Government's biggest focus was finding "jobs for heroes." It seems likely that well over 1.5 million former soldiers and airmen (not including Navy or Merchant Marine personnel) came home from war with technical skills, a great many related to internal combustion engines, electric power, wireless and telecommunications. That's 10% of the entire male workforce, not including the men who never wore uniform but also had the same or similar technical skills, nor does it include the hundreds of thousands of women who learned to drive and maintain machinery during WW1. Given the logistical demands of the Great War, it's not unreasonable to assume that at least another 1.5 million Home Front citizens developed similar technical skills directly as a result of the War.

These demobbed soldiers didn't just go back to pitchforking hay into the back of horse-drawn wagons. They went back to the cities (where most of them came from, anyhow) and into major growth industries related to automation and technology. Electricity and telephones were just being rolled out to the wealthy in 1913. After the war, there was an explosion in the use of these new technologies, with consequent increasing demand for generators (and staff to operate them), both as primary power sources and to provide back-ups. The demobbed trucks flooded the haulage market to the extent that use of horses went into rapid terminal decline in urban areas. The vast increase in road haulage also put the rail network under stress, and by the 1930s large sections were already being closed because they were no longer profitable.

These Great War veterans were the backbone of British industry in the 1930s....and yet you're telling me that, because they didn't own a car, they weren't as well-versed in things mechanical as their American counterparts?

Britain's servicemen in WW2 were the sons of these WW1 veterans and, like their fathers, the VAST majority came from industrialized, urban areas. They came from families where working with machinery was just second nature. It's what they grew up doing. My uncle served in the RAF as an engine fitter. As previously related, he didn't own a car until the 1950s...but he knew how to strip down an engine before he even joined up.

As stated earlier, the biggest flaw in the "American farmer-mechanic" myth is the assumption that other countries had similar demographics. The vast differences in the degree of urbanization between the US and the UK alone should make people question the assumption, before we consider the disparate proportions of the respective workforces relative to agriculture and manufacturing.

I am immensely proud of America's accomplishments during WW2. The spin-up was remarkable and turned our nation into the world's first Super Power. However, we do ourselves no favors if we denigrate other countries, particularly our Allies, by pushing cosy narratives that promote perceived superiority where none exists.

 

[FLYBOYJ]

This means that for every pound of weight saved the rate of climb increases by 1.2fpm. In other words, if you reduce the weight of a P-39 by 300lbs the rate of climb increases by 360fpm.

And again, that is a linear calculation based on that one aircraft. You cannot guarantee that for all P-39s especially if you were to remove that weight in the field. Removing weight in the nose may require ballast in the tail and again this depends where the Mean Aerodynamic Chord falls to give the best flying qualities (whether you're looking for speed or maneuverability). Yes, the weight removal will give you the performance, but there's a limit on what you can do in the field.

Yes the plane had to be in balance after the items were removed. Bell proved they could do that even with varying weights for propellers, nose cannons, aux. stage superchargers, rear armor plate and radio equipment in the tail. The plane was balanced on all those different forms.

This was a very "doable" exercise at the factory but as this discussion started out about doing this in the field to a production airframe, you're looking at 2 different scenarios that will not yield the same results and could actually be dangerous. As stated many times, the P-39 had a vertical and horizontal C/G which is not very common for fixed wing aircraft. In the field you can remove weight but may have to add ballast to remain within C/G limits, and even then I don't think the P-39 is an aircraft you really want to be tail heavy.

I'd like to see this play out on an actual weight and balance chart and it the end see where everything falls into place with a "production" equipment list.

 

[Milosh]

I'd like to see this play out on an actual weight and balance chart and it the end see where everything falls into place with a "production" equipment list.

 

[FLYBOYJ]

The IFF radio is the black rectangle in the tail. It is actually farther away from the CG than the nose armor. Remove the 100lb nose armor and the IFF radio and the P-39 is balanced. Or just move the IFF radio up above the engine behind the pilot nearly on the CG and accomplish the same thing.

You don't know that until you know the arm and moment!!!! And you also have to calculate that in vertical datum!

 

[FLYBOYJ]

So let's start here...

 

[Shortround6]

Not from me. I am, after all, American. I'm as patriotic as the next man. I just want to do so with clear-eyed comprehension, cognizant of the widest-possible breadth of facts rather than a nationalistic agenda that perpetuates myths based on false assumptions.

The number of cars per 1000 people is not a false assumption, it is a fact. However it is just one fact out of many.

Entirely agree. However, there persists this perception that countries other than America all had a "small group of engineers".

Well, considering that the US had over 2 1/2 times the population of the UK in 1939/40 (and much larger than Germany and almost 3 times the population of Italy, any industrialized country had a small number of engineers compared to the US. Not saying the US engineers were smarter, just that there were more of them.

Do we really want to get into nitpicking the size of fishing fleets? Just look at US population and the size of the US coast.
Now for the US considering the size of it's population the percentage of men with fishing boat experience would be less than the UK, even if the total number was higher.

Same for railroads. US went into WW II (2 years late) with 41,000 locomotives, 2 million freight cars and 230,000 miles of track.

Factory workers?
From Wiki " In 1937 the UK produced 379,310 passenger cars and 113,946 commercial vehicles"
In 1937 production numbers for the top eight US makers were
Ford........................942,005
Chevrolet.............815,375
Plymouth.............566,128
Dodge...................295,047
Pontiac.................236,189
Buick.....................220,346
Oldsmobile........200,886
Packard...............122,593

trucks/commercial vehicles not included nor the rest of US production.

For Germany in 1937 they built 267,910 cars, 79,16 trucks and 171,239 motorcycles which helps to point out the misconception that Germany was highly motorized.
Italy, France and Japan were even further behind.

These Great War veterans were the backbone of British industry in the 1930s....and yet you're telling me that, because they didn't own a car, they weren't as well-versed in things mechanical as their American counterparts?

car ownership may have nothing to do with or it may have something to do with it. The chances of an american being well versed (or at least somewhat versed) is better but certainly not an absolute.
Story from grandfather when he was working in the clock/watch shop. Man comes in and asks the best way to clean the works of an old clock (most of which used brass works, gears, shafts and frame). My Grandfather tells him to take the works out of the case and boil it in a pot of water to get rid of the accumulated grease/oil and dirt. Man says thanks and leaves.
The Man comes back a couple of days later spitting mad that my grandfathers advice had ruined his clock. Come to find out the man had a clock with the works made of wood.
Wooden gears, shafts and frame and he had gone ahead and boiled it in a pot of water.
Some people should not be let loose near mechanical devices even if they can drive a car

 

[wuzak]

The IFF radio is the black rectangle in the tail. It is actually farther away from the CG than the nose armor. Remove the 100lb nose armor and the IFF radio and the P-39 is balanced. Or just move the IFF radio up above the engine behind the pilot nearly on the CG and accomplish the same thing.

You don't know that until you know the arm and moment!!!! And you also have to calculate that in vertical datum!

Isn't the IFF something you probably want to keep, except in Russian service?

Also, many items have been identified as being removed by the Russians. The nose armour is not one of them.

 

[Ivan1GFP]

The Allies didn't recognize the Ki-43 as a distinct type until 1943. Prior to that point, all single-engine Japanese fighters with retractable undercarriage were Zeros, with the variation for the A6M3 identified when it entered service because of its clipped wingtips (and, of course, the Allied reporting names that were associated).

Hello buffnut453,

I am not disputing that fact. I am just wondering that if the pilots were noting a difference in the cowl size and nothing else, then perhaps there was nothing else to note and there are a LOT more detail differences between the Ki 43 and A6M.
Some of those differences are a lot more obvious than the size of the engine cowl such as the presence or absence of wing guns.

It is sort of like telling me that this is a different woman because the colour of her eyes is different but not pointing out that she is also six inches taller and 50 pounds heavier.

- Ivan.

 

[Ivan1GFP]

Hello P-39 Expert,

You are correct on the reduction gear, but the 2:1 was also used on the D-2 with the -63 engine. I really can't be sure about the M propeller, some sources say the Curtiss prop was used, some say the Aeroproducts prop was used. I haven't read anywhere that either prop was larger then the 10'4" prop used on the earlier models.

The P-39K also used a 2:1 reduction with a -63 engine.
Detail & Scale 63 states that an Aeroproducts propeller of 11 feet 1 inch was used on the P-39M. I don't know if other sources give greater detail on the specifics about the propeller. I was gathering information on P-39 when I started working on a project a couple years ago. I got a bit stuck because I could not find a really good fuselage drawing.

Regarding P-39M propeller diameters, what you just stated directly contradicts your earlier statement which I have reproduced below. You need to make up your mind which story you are going with.

Bell said in writing that the nose armor plate was not needed for ballast/balance on the P-39M. The M was an early model with the same weight and weight distribution as previous and later models. They were able to balance the plane with larger (heavier) propellers and different nose cannons that differed in weight by 140lbs. Bell designed the P-39 to take larger (heavier) three blade and four blade propellers and an auxiliary stage supercharger behind the engine that weighed 175lbs. They certainly were able to maintain proper balance with any or all of these items installed. The nose armor certainly could have been deleted and balance maintained.

Guess I wasn't clear, didn't mean to imply that the 20mm was used in the M. The 20mm was used in the P-400 and the D-1. Point I was making was that the 140lb weight difference was compensated for by Bell to make prior and subsequent P-39 models balanced. Why would you say that I am trying to bait and switch?

Your two statements above which I have quoted simply do not agree.
You state that the P-39M didn't need the nose armour because a larger (heavier) propeller and cannon that weighed 140 pounds more could make up the difference.
The problem is 140 pounds more than WHAT?
ALL of the preceding models of the P-39 back to the P-39F (F, J, K, L) carried the same 37 mm cannon with only minor differences in weight as the P-39M did.

- Ivan.

 

[buffnut453]

Hello buffnut453,

I am not disputing that fact. I am just wondering that if the pilots were noting a difference in the cowl size and nothing else, then perhaps there was nothing else to note and there are a LOT more detail differences between the Ki 43 and A6M.
Some of those differences are a lot more obvious than the size of the engine cowl such as the presence or absence of wing guns.

It is sort of like telling me that this is a different woman because the colour of her eyes is different but not pointing out that she is also six inches taller and 50 pounds heavier.

- Ivan.

Entirely agree. I'm always dubious of aircraft descriptions derived from the heat of combat. I think it highly unlikely that anyone could objectively determine different cowling opening diameters. I suspect it was just the impression the pilot(s) perceived in the split second they saw it...and that perception could be entirely skewed by stress.

If pilots were so good at aircraft recognition, we wouldn't have Hurricanes being mistaken for Spitfires, Me 109s for He113s, or even noting Me109s over Malaya in 1941!

 

[GrauGeist]

The Allies didn't recognize the Ki-43 as a distinct type until 1943. Prior to that point, all single-engine Japanese fighters with retractable undercarriage were Zeros, with the variation for the A6M3 identified when it entered service because of its clipped wingtips (and, of course, the Allied reporting names that were associated).

From a screenshot of a USAAF identification chart from 1942.
Please note in the upper left corner the two fighters: "Zeke" and "Oscar".
In the heat of battle, you have a split second to realize you're being bounced by an enemy fighter.
Do you:
A) stop to be sure of the aircraft type
B) wait to see if it has any particular identifying features
C) shoot it down and try and figure out what it was later in the action report.

A and B will get you killed.
C is what happened most often and due to the strong similarity between the two, confusion is very understandable.

 

[MiTasol]

Please expand the above for more comments and drawings.

The aux. stage supercharger occupied the same space as the coolant tank. On the P-63 (and the XP-39E) the coolant tank was moved forward to above the engine just aft of the pilot and flattened somewhat in shape to fit. In this diagram there is a bulkhead between the oil tank and the coolant tank, and the oil tank is clearly completely behind that bulkhead, as it was on the P-63. The area occupied by the coolant tank on the P-39 was exactly the same size as the area occupied by the aux. stage supercharger on the P-63.

Now let us put those three errors you keep making permanently to bed.

• The aux. stage supercharger occupied the same space as the coolant tank.
• the (P-39) oil tank is clearly completely behind that bulkhead, as it was on the P-63.
• The area occupied by the coolant tank on the P-39 was exactly the same size as the area occupied by the aux. stage supercharger on the P-63.

P-39 rear fuselage. I cannot find my -2, -3 &- 4 for the P-39 aircraft so the first drawing is a P-39 rear fuselage from an on line IPL (-4) that is missing 90% of its pages. Note that the oil tank extends forward from the bulkhead and is partially restrained by curved strap across the front of the bulkhead.

The following diagrams are from the Airacobra Design Analysis in the May 43 edition of Aviation magazine and clearly show the shape of the P-39 rear fuselage front bulkhead and the cutout that the oil tank extends through and that the oil tank is mounted hard forward in the rear fuselage. Furthermore is mounted at an angle causing a significant volume of it to extend forward into the engine bay.

P-63 rear fuselage from SRM (-3) and IPL (-4). Note that all three illustrations show the oil tank spherical support is mounted back from the bulkhead, and the oil tank is mounted behind the bulkhead to allow space for the ASB. NOte also the cutout in the P-63 bulkhead is somewhat larger than the cutout in the P-39 bulkhead.

From the above it is abundantly clear that the only way to fit and ASB in a P-39 is to delete the oil tank and carve a large hole in the rear fuselage front bulkhead. Either and both of these changes would render the aircraft inoperative.

QED Allison engines with ASBs attached cannot be installed in any production P-39 airframes because

• The aux. stage supercharger occupied a larger space as than the coolant tank.
• the (P-39) oil tank is clearly not completely behind that bulkhead, as it was on the P-63.
• The area occupied by the coolant tank on the P-39 was not exactly the same size as the area occupied by the aux. stage supercharger on the P-63.

 

[windhund116]

From a screenshot of a USAAF identification chart from 1942.
Please note in the upper left corner the two fighters: "Zeke" and "Oscar".
In the heat of battle, you have a split second to realize you're being bounced by an enemy fighter.
Do you:
A) stop to be sure of the aircraft type
B) wait to see if it has any particular identifying features
C) shoot it down and try and figure out what it was later in the action report.

A and B will get you killed.
C is what happened most often and due to the strong similarity between the two, confusion is very understandable.

View attachment 597326

Correct. It's like trying to ID what pistol a robber has, at 50 yards, while running your @ss off. The only thing I noticed is a lot of Nakajima made fighter planes have straight leading edges. Whereas the Zero has a tapered leading edge. The Oscar has a more slender fuselage. But, at 300MPH turning 3Gs, at 1000 yards... good luck with a positive ID.

I believe that by this time of the the SWPA action (late 1942-43) both Imperial Navy and Army fighters used the same loose finger-four formation. Similar layered attack strategies. So, it would be hard to tell from a distance, just looking at the formations --- whether it was IJNAF or IJAAF.

 

[Elmas]

Entirely agree. I'm always dubious of aircraft descriptions derived from the heat of combat. I think it highly unlikely that anyone could objectively determine different cowling opening diameters. I suspect it was just the impression the pilot(s) perceived in the split second they saw it...and that perception could be entirely skewed by stress.

If pilots were so good at aircraft recognition, we wouldn't have Hurricanes being mistaken for Spitfires, Me 109s for He113s, or even noting Me109s over Malaya in 1941!

Or Ki-61 mistaken for Macchi 202...

 

[Milosh]

Germans said they shot down, or were shot down by Spitfires but were actually Hurricanes. Course this could have been German fighter pilot snobbery.

 

[P-39 Expert]

So let's start here...

View attachment 597318

Delete No. 12. This chart is for the P-39Q, this item weighed 100lbs on most earlier models.
Move the IFF radio from the tail cone forward to just aft of No. 18. IFF radio not shown on this drawing, weighed approximately 120lbs, located about midway between No. 17 and the leading edge of the horizontal stabilizer. Or delete the IFF radio altogether and save another 120lbs, as the Russians did.

 

[buffnut453]

From a screenshot of a USAAF identification chart from 1942.
Please note in the upper left corner the two fighters: "Zeke" and "Oscar".
In the heat of battle, you have a split second to realize you're being bounced by an enemy fighter.
Do you:
A) stop to be sure of the aircraft type
B) wait to see if it has any particular identifying features
C) shoot it down and try and figure out what it was later in the action report.

A and B will get you killed.
C is what happened most often and due to the strong similarity between the two, confusion is very understandable.

View attachment 597326

Agree with your comments about split second decisions. That's exactly why I don't trust pilot reports. It's not a criticism of the pilots, just a reflection of the operational environment.

BTW, that recognition chart is NOT from 1942. The Rex floatplane wasn't introduced until 1943. In fact the presence of the Norm puts it well into 1944. The only operational deployment for Norms took place 1 June thru 12 August 1944. First encounter with US forces was 15 June when 2 were destroyed by a US Navy task force.

 

[P-39 Expert]

Hello P-39 Expert,



The P-39K also used a 2:1 reduction with a -63 engine.
Detail & Scale 63 states that an Aeroproducts propeller of 11 feet 1 inch was used on the P-39M. I don't know if other sources give greater detail on the specifics about the propeller. I was gathering information on P-39 when I started working on a project a couple years ago. I got a bit stuck because I could not find a really good fuselage drawing.

Regarding P-39M propeller diameters, what you just stated directly contradicts your earlier statement which I have reproduced below. You need to make up your mind which story you are going with.





Your two statements above which I have quoted simply do not agree.
You state that the P-39M didn't need the nose armour because a larger (heavier) propeller and cannon that weighed 140 pounds more could make up the difference.
The problem is 140 pounds more than WHAT?
ALL of the preceding models of the P-39 back to the P-39F (F, J, K, L) carried the same 37 mm cannon with only minor differences in weight as the P-39M did.

- Ivan.

My statements agree exactly. I stated that the nose armor was not needed on the P-39M and that the M was an early model with similar weight and weight distribution as earlier and later models. All the P-39 models were balanced whether they had the heavier 37mm cannon or the lighter by 140lbs 20mm cannon.

Then I said that Bell was able to balance the plane with varying weights for different components located fore and aft of the CG. When I said the plane I was referring to all the P-39 models that had different components. I can see how this could have been misinterpreted.

Let me be absolutely clear: P-39s (and P-400s) had different propellers, nose armor, nose cannons, radios (in different locations) etc that had different weights. Bell was able to balance all the different models with all these different components. The nose armor could have been deleted and the plane could have been balanced.

 

[P-39 Expert]

Now let us put those three errors you keep making permanently to bed.

• The aux. stage supercharger occupied the same space as the coolant tank.
• the (P-39) oil tank is clearly completely behind that bulkhead, as it was on the P-63.
• The area occupied by the coolant tank on the P-39 was exactly the same size as the area occupied by the aux. stage supercharger on the P-63.

P-39 rear fuselage. I cannot find my -2, -3 &- 4 for the P-39 aircraft so the first drawing is a P-39 rear fuselage from an on line IPL (-4) that is missing 90% of its pages. Note that the oil tank extends forward from the bulkhead and is partially restrained by curved strap across the front of the bulkhead.

View attachment 597332

The following diagrams are from the Airacobra Design Analysis in the May 43 edition of Aviation magazine and clearly show the shape of the P-39 rear fuselage front bulkhead and the cutout that the oil tank extends through and that the oil tank is mounted hard forward in the rear fuselage. Furthermore is mounted at an angle causing a significant volume of it to extend forward into the engine bay.

View attachment 597331
View attachment 597330

P-63 rear fuselage from SRM (-3) and IPL (-4). Note that all three illustrations show the oil tank spherical support is mounted back from the bulkhead, and the oil tank is mounted behind the bulkhead to allow space for the ASB. NOte also the cutout in the P-63 bulkhead is somewhat larger than the cutout in the P-39 bulkhead.

View attachment 597329

View attachment 597328

View attachment 597327

From the above it is abundantly clear that the only way to fit and ASB in a P-39 is to delete the oil tank and carve a large hole in the rear fuselage front bulkhead. Either and both of these changes would render the aircraft inoperative.

QED Allison engines with ASBs attached cannot be installed in any production P-39 airframes because

• The aux. stage supercharger occupied a larger space as than the coolant tank.
• the (P-39) oil tank is clearly not completely behind that bulkhead, as it was on the P-63.
• The area occupied by the coolant tank on the P-39 was not exactly the same size as the area occupied by the aux. stage supercharger on the P-63.

When I said the aux. stage blower occupied the same space as the coolant tank, I meant that it occupied the same general location. Not that it was exactly the same size. It was larger than the coolant tank, but did fit in the location occupied by the coolant tank.

The P-39 oil tank is completely behind the bulkhead. It's base sat in the same type of cradle as the P-63, behind the bulkhead.

The aux. stage blower was installed in the XP-39E. The engine compartment was not enlarged to accommodate the aux. stage, it was the same length as the other P-39s and the P-63 as evidenced by the drawings in my post #236. The E model was longer because the tail section was longer, not the engine section as reported by other sources.