Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained) (3 Viewers)

[P-39 Expert]

Source for the 292lbs? about 30lbs seems to have disappeared when the planes were in US service. Armor for the oxygen perhaps?

The P-39 armor never got down to 140lbs. On a P-39Q-1 it was still 193lbs.




Pretty much irrelevant to the discussion at hand. Another member had suggested that Britain wanted the Airabora Is for invasion barge busting. Given the ordering of the planes before the BoB even started that seems unlikely and given the armament of the AIracobras as ordered (drum fed 20mm) they were not any better than a number of any other aircraft for this job.




You have yet to show a reference or source that says the British specified this weight. Please do so.

You can put up all the graphs you want of planes that don't really apply to the discussion at hand. A Spitfire running at less than full throttle, The P-39K with its 1325hp engine won't show up until the fall of 1942, as for the P-39C, even the USAAC wasn't dumb enough to order more than 20 of them and that was before France fell. They changed the order for the last 60 to the P-39D which weighed, surprise 7697lbs, with 104 US gallons of fuel. add another 96lbs if the internal tanks are full.



The P-39 was never a 5000lb airplane, ever. It was 'designed' for gross weight of 5500lbs, The XP-39 was changed to 5,855lbs before it was finished. when it showed up at Wright Field it weighed 6,104lbs, This was in April of 1939. It was already 10% overweight and had no guns.

Please give list of these changes to the British contracts and source/s



You have the dates wrong or are confusing them, the British added 505 planes to the contract in May or June of 1940, not 1941. Now this may have been a letter of intent, or provisional contract or some other temporary agreement that was not finalized until June of 1941 but all parties (Bell, Allison, the USAAC, and the British) were moving forward under the assumption that the British were ordering 675 planes, that is making allocation of materials, long lead items and whose aircraft (British or American ) would be delivered (hopefully) in which months. Hundreds of extra P-39s were allocated to the British in the Spring of 1941 under lend lease and many of those were the P-39D-1s and D-2s.

Can you give us a list of British agents ( or at least a memoir from one) doing industrial espionage in the US at this time or is this another conclusion you you have come to with no evidence to back it up?

Birch Matthews in "Cobra! Bell Aircraft Corporation 1934-1946" is very specific that the final revision (and increase) to the contract was in June 1941 after testing the P-400 in April.

 

[Ivan1GFP]

Thanks Shortround6,
That P-39Q manual is the same one I was using for calculations and, as I commented earlier, I suspect that its numbers do not really reflect the CoG issues of the earlier Airacobra we are discussing here.

Since you already provided a link to the manual, I won't repost the data from the diagram and chart listing the weights and moment arms of the equipment.
Early last year, I took the numbers from the chart and entered them into a spreadsheet so that I could see what would happen under various load conditions and what MIGHT happen if the loads were varied a bit in location.

The "Arm" for each item of equipment appears to be the offset from the Fuselage Horizontal Reference Line - Station Zero.
From dimensional drawings of the Airacobra, it can be seen that:
Wing Root Leading Edge is at Station 106.15 inch
LE Wing Root to LE MAC is 5.41 inch, therefore LE MAC is at Station 111.56 inch.
MAC is 80.64 inch.

From this, it can be calculated that with aircraft in empty equipped (Basic Weight) condition, CoG is at 30.16% MAC.
In Loaded condition, CoG is at 28.1% MAC.
Both of these numbers are quite ordinary.
Note that these numbers are calculated with the Landing Gear UP which would be the expected configuration for maneuvering.
Gear Down would shift the CoG very slightly forward and improve stability.

Assuming the Basic Weight is the low limit of the aircraft weight in the air, then it can be seen that Fuel load has a pretty negligible effect on CoG. It is less than two inches ahead of the CoG.
The Wing Armament is also very close to the longitudinal CoG (about 3 inches ahead) so whether the normal load of 300 rounds per gun or maximum load of 1000 rounds per gun was carried would not affect CoG very much.

The 37 mm cannon ammunition is pretty far forward but there is only 60 pounds of weight for 30 rounds which was typical load from the P-39D onwards. (The P-39C only carried 15 rounds.)

The .50 Cal cowl MG ammunition was typically 200 rounds and for the P-39Q would weigh 124 pounds. Expending this ammunition would have the greatest effect on aircraft trim.
Now here is where life gets interesting.
Most sources I have found list the ammunition capacity as 200 rounds per gun.
Some of the aircraft tests from WW2 Aircraft Performance site list the aircraft as carrying 250 (!) rounds.
The Russian Airacobra manual which can be found on our site lists the ammunition capacity as 270 rounds per gun.

This extra ammunition might be one factor that made CoG issues less of a problem when the Soviets flew the Airacobra.

- Ivan.

 

[Ivan1GFP]

OoooKaaay.
Fit one of the first "bubble' canopies on a fighter and then fill the back of the bubble with "stuff" so you can't see out of it

Hello Shortround6,
Before coming to that conclusion, it might be worthwhile to look over some photographs of the Airacobra.
The Airacobra Pilot sits relatively high in the cockpit which can be seen in numerous photographs and videos.
It was a fairly small cockpit. The typical pilot sat with his eyes very nearly at the level of the top of the window in the door.
At that angle, one can cram in quite a bit of stuff behind the cockpit in the little deck area above the engine and not obscure any vision at all.
Also keep in mind that on some of the later models of the Airacobra, the geniuses of design decided to replace the slab of armour glass behind the pilot with a piece of nice opaque metal armour, so it would not make a difference anyway.

- Ivan.

 

[Shortround6]

Birch Matthews in "Cobra! Bell Aircraft Corporation 1934-1946" is very specific that the final revision (and increase) to the contract was in June 1941 after testing the P-400 in April.

You are correct, that is what is printed it the book. However it makes no sense, especially for your theory that the British deliberately specified a plane that was too heavy so they could get out of the contract.

The French ordered, or expressed an intent/opened,
negations for 165 planes in the fall of 1939, there is a provision in the contract that should France fall or otherwise default Britain will take over the contract. The deal is not signed until April 10th 1940 Larry Bell returns to Buffalo with a 2 million dollar check, quite possibly saving the company from bankruptcy.

In June France falls and Britain takes over the contract. Here is where the narratives start to diverge. Narrative A says the British increased the total number of aircraft to 170 (some people say it was already 170 but 5 aircraft aren't that big a deal) and that 170 on order would remain the number of the contract until the following year.
Narrative B says that after France fell Britain not only took over the contract (as it was obligated to do) but ordered an additional 505 aircraft.

for both Narratives the US Army orders about 389-394 additional P-39Ds to go with the original 60 on order.

Narative B says that Bell had about 1600 aircraft on order worth 60 million as of Jan 1941. Numbers from either narrative don't line up with this.
Dec 1940 is When Churchill tells Roosevelt that Britain is effectively broke and cannot pay for any more supplies. According to Narrative A the British only have 170 aircraft on order at this time.
March 11th is when the lend lease bill is passed/signed and Britain no longer has to pay for supplies.

The P-400/Airacobra performance trial with the Special is flown at the end of April.

Narrative A has the British signing a contract for the 505 P-400s on June 20th 1941 and paying for them cash. No narative says these were lend lease aircraft.

Narrative B has the British, on June 11th of 1941 ordering 494 P-39D-1 aircraft and P-39D-2 aircraft under lend lease.(not all may have been built).

So we have several major discrepancies.
For the theory that the British deliberately specced the P-400 to be too heavy, Narrative A really doesn't look good as they had only paid for 170 planes (or were obligated to pay for). If they wanted to save money just don't order the 505 extra planes in June of 1941. Germans are helping the Italians in Yugoslavia and Greece. They started sending a few small units to North Africa in Feb. They would attack the Soviet Union on June 22 so one can assume that the British had pretty well written off the possibility of the Germans invading anytime soon (many, but not all, of the barges had returned to normal commercial traffic).
It would also have to assume that the British had decided they didn't even want the ex French aircraft if they were changing the specs with the intent to get out of the contract in the fall of 1940 when German intentions/capabilities were not so clear cut.
In August of 1941 the USAAC ordered 1800 P-39G/model 26s. These would become P-39K,L and Ms.

For the British to load up the the P-400 so it won't meet the performance specifications the British have to know that lend lease is a done deal (end of February at the soonest?) .
They have to have a lot more than the original 170 on order. and it has to be done in a few months so they can pocket the cash and replace the canceled planes with lend lease ones.

What happened is that the British took all of the original 170 plus around another 42 or more, Shipped a bunch to russia and gave the rest back to the US and also either sent the lend lease planes to Russia or let the US take them over.

 

[Greyman]

Thank you,

So we may assume (with all that entails) that the British specified the armor around the oxygen tank/s ?

Was there a difference in the oxygen systems that the British felt that they needed such protection?
AHT says the US P-39s used a low pressure system with 1 to 4 bottles while the P-400 export version got a higher pressure system.

Was the British system (or high pressure system) particularly vulnerable to gun fire?
Bottles exploded when hit or ruptured violently?

Not sure who specified it. Might have been a British, French or US addition.

I'd note Fighter Command had said "The two oxygen bottles in the nose have armour protection and these might be deleted if wire-wound bottles were used." Perhaps safer O2 bottles were used in later variants when the armour was deleted.

One thing I've noticed is that the British don't seem to have made mention of the 'tumble' problem that the VVS and USAAF had -- and the British only dealt with the version with that forward O2 armour ...

Bottles can explode when hit, though it seems to heavily depend on where they are hit. I have a good chunk of information on this subject but I haven't gone through it. I know George Beurling specifically mentioned he tried to hit enemy aircraft in the O2 bottles.

Check out this Wellington that had a bottle struck by a 20-mm.

 

[Shortround6]

Thank you.
as mentioned before the US used a low pressure system with a different regulator.
Perhaps the US bottles didn't rupture as badly when hit???
Or like you say, later bottles were of heavier construction and didn't need armor (although the heavier bottle/s would eat up some of the savings on armor)

 

[P-39 Expert]

Thanks Shortround6,
That P-39Q manual is the same one I was using for calculations and, as I commented earlier, I suspect that its numbers do not really reflect the CoG issues of the earlier Airacobra we are discussing here.

Since you already provided a link to the manual, I won't repost the data from the diagram and chart listing the weights and moment arms of the equipment.
Early last year, I took the numbers from the chart and entered them into a spreadsheet so that I could see what would happen under various load conditions and what MIGHT happen if the loads were varied a bit in location.

The "Arm" for each item of equipment appears to be the offset from the Fuselage Horizontal Reference Line - Station Zero.
From dimensional drawings of the Airacobra, it can be seen that:
Wing Root Leading Edge is at Station 106.15 inch
LE Wing Root to LE MAC is 5.41 inch, therefore LE MAC is at Station 111.56 inch.
MAC is 80.64 inch.

From this, it can be calculated that with aircraft in empty equipped (Basic Weight) condition, CoG is at 30.16% MAC.
In Loaded condition, CoG is at 28.1% MAC.
Both of these numbers are quite ordinary.
Note that these numbers are calculated with the Landing Gear UP which would be the expected configuration for maneuvering.
Gear Down would shift the CoG very slightly forward and improve stability.

Assuming the Basic Weight is the low limit of the aircraft weight in the air, then it can be seen that Fuel load has a pretty negligible effect on CoG. It is less than two inches ahead of the CoG.
The Wing Armament is also very close to the longitudinal CoG (about 3 inches ahead) so whether the normal load of 300 rounds per gun or maximum load of 1000 rounds per gun was carried would not affect CoG very much.

The 37 mm cannon ammunition is pretty far forward but there is only 60 pounds of weight for 30 rounds which was typical load from the P-39D onwards. (The P-39C only carried 15 rounds.)

The .50 Cal cowl MG ammunition was typically 200 rounds and for the P-39Q would weigh 124 pounds. Expending this ammunition would have the greatest effect on aircraft trim.
Now here is where life gets interesting.
Most sources I have found list the ammunition capacity as 200 rounds per gun.
Some of the aircraft tests from WW2 Aircraft Performance site list the aircraft as carrying 250 (!) rounds.
The Russian Airacobra manual which can be found on our site lists the ammunition capacity as 270 rounds per gun.

This extra ammunition might be one factor that made CoG issues less of a problem when the Soviets flew the Airacobra.

- Ivan.

Excellent analysis. Any idea how much the brass weighed on the ammunition, because it was held inside the armament bay after firing instead of being ejected overboard.

 

[Shortround6]

The empty primed case weighed up to about 880 grains/57 grams (about 0.125lbs) there was tolerance of -50 grains which we can ignore here.

So the plane kept about 50lbs of brass inside with all the .50 cal ammo fired (for 200rpg), plus the weight of the links.

 

[P-39 Expert]

You are correct, that is what is printed it the book. However it makes no sense, especially for your theory that the British deliberately specified a plane that was too heavy so they could get out of the contract.

The French ordered, or expressed an intent/opened,
negations for 165 planes in the fall of 1939, there is a provision in the contract that should France fall or otherwise default Britain will take over the contract. The deal is not signed until April 10th 1940 Larry Bell returns to Buffalo with a 2 million dollar check, quite possibly saving the company from bankruptcy.

In June France falls and Britain takes over the contract. Here is where the narratives start to diverge. Narrative A says the British increased the total number of aircraft to 170 (some people say it was already 170 but 5 aircraft aren't that big a deal) and that 170 on order would remain the number of the contract until the following year.
Narrative B says that after France fell Britain not only took over the contract (as it was obligated to do) but ordered an additional 505 aircraft.

for both Narratives the US Army orders about 389-394 additional P-39Ds to go with the original 60 on order.

Narative B says that Bell had about 1600 aircraft on order worth 60 million as of Jan 1941. Numbers from either narrative don't line up with this.
Dec 1940 is When Churchill tells Roosevelt that Britain is effectively broke and cannot pay for any more supplies. According to Narrative A the British only have 170 aircraft on order at this time.
March 11th is when the lend lease bill is passed/signed and Britain no longer has to pay for supplies.

The P-400/Airacobra performance trial with the Special is flown at the end of April.

Narrative A has the British signing a contract for the 505 P-400s on June 20th 1941 and paying for them cash. No narative says these were lend lease aircraft.

Narrative B has the British, on June 11th of 1941 ordering 494 P-39D-1 aircraft and P-39D-2 aircraft under lend lease.(not all may have been built).

So we have several major discrepancies.
For the theory that the British deliberately specced the P-400 to be too heavy, Narrative A really doesn't look good as they had only paid for 170 planes (or were obligated to pay for). If they wanted to save money just don't order the 505 extra planes in June of 1941. Germans are helping the Italians in Yugoslavia and Greece. They started sending a few small units to North Africa in Feb. They would attack the Soviet Union on June 22 so one can assume that the British had pretty well written off the possibility of the Germans invading anytime soon (many, but not all, of the barges had returned to normal commercial traffic).
It would also have to assume that the British had decided they didn't even want the ex French aircraft if they were changing the specs with the intent to get out of the contract in the fall of 1940 when German intentions/capabilities were not so clear cut.
In August of 1941 the USAAC ordered 1800 P-39G/model 26s. These would become P-39K,L and Ms.

For the British to load up the the P-400 so it won't meet the performance specifications the British have to know that lend lease is a done deal (end of February at the soonest?) .
They have to have a lot more than the original 170 on order. and it has to be done in a few months so they can pocket the cash and replace the canceled planes with lend lease ones.

What happened is that the British took all of the original 170 plus around another 42 or more, Shipped a bunch to russia and gave the rest back to the US and also either sent the lend lease planes to Russia or let the US take them over.

Big question I always had is, how in the world did the British think the P-400 would be competitive (achieve the performance guarantees) at 7850# when the SpitfireV weighed 6450# and had a more powerful engine? The whole exercise seems doomed from the start. The planes did get built, they sure got used by the AAF and the Russians, and Bell got the $2million advance which probably saved them financially, but the rest of that procurement exercise was crazy.

 

[P-39 Expert]

The empty primed case weighed up to about 880 grains/57 grams (about 0.125lbs) there was tolerance of -50 grains which we can ignore here.

So the plane kept about 50lbs of brass inside with all the .50 cal ammo fired (for 200rpg), plus the weight of the links.

Then a little more for the 37mm cases.

 

[Shortround6]

Big question I always had is, how in the world did the British think the P-400 would be competitive (achieve the performance guarantees) at 7850# when the SpitfireV weighed 6450# and had a more powerful engine? The whole exercise seems doomed from the start. The planes did get built, they sure got used by the AAF and the Russians, and Bell got the $2million advance which probably saved them financially, but the rest of that procurement exercise was crazy.

The process was crazy, but it is what can happen when you have a new and inexperienced company trying out novel concepts/ideas.
Mix in the French, with their inclination to the weird and wonderful in engineering, and then British, who seem to have been along for the ride. The only prople who seem to have any real handle on things was the USAAC, who did change their contracts.

Some of the 'steps' as laid out in Birch Mathews book look a bit strange.

Date..........................empty.....................useful load.......................gross......................remarks
Feb 1940.................4,524...........................1,325..............................5,849......................1-cannon, 2 machine guns (1)
Mar 1940................4,715...........................1,285..............................6,000.......................2 machine guns added to each wing (2)
May 1940...............5,139...........................1,841..............................7,000.......................wing gun caliber increased (3)
Jul 1940..................5,383............................1,974..............................7,350.......................armor and self sealing tanks added (4)
Jan 1941.................5,406............................2,060..............................7,466.......................minor empty and useful load changes
Jun 1941.................5,548............................2,087.............................7,655.......................Engine and fixed equipment weight changes
Jul 1941...................5,550...........................2,087..............................7,637.......................Actual weight of Serial number AH 621

Which is very pretty but leaves out a lot and/or makes no sense.

The XP-39 weighed 4,545lbs empty, had a 1,559lb useful load and grossed 6,104 lbs. Yes in had the weight of the turbo installation but it also had no guns.

Some sources say the XP-39B (same airframe, turbo removed and other details changed) grossed around 5845lbs. this was the ONLY p-39 airframe in existence in the fall/winter of 1939/40. it also had no guns. See:

Note (1) how they expected to add even a 20mm cannon (around 160lbs with full drum of ammo) and two 12.7/13mm machine guns (another 260lbs) and NOT increase the gross weight is beyond me.

Note (2) more confusion. Empty weight does NOT include the weight of the guns (that would be empty equipped). Now perhaps the increase in "empty" weight covers the gun mounts/braces, gun servicing doors and some other details? but you just added 80-100lb of gun weight to the useful load (and at least 70lbs of ammo weight at 300rpg) while cutting the useful load by 40lbs.
Interesting factoid here, Useful load of a MK I Spitfire with a fixed pitch prop was 1337lbs. Guns, ammo, fuel, oil, radio, pilot&parachute. Seeing a problem with useful load of the French P-400 here????

Note (3) and an increase in empty weight of 444lbs but empty weight does not include the weight of the guns. Blaming any meaningful weight difference on changing the gun caliber from 7.5mm to 7.7mm is bogus. Increasing the useful load by 1000lbs so you can actually carry the guns, ammo, pilot and fuel at the same time is understandable. but there is where the performance starts to go to pot. How much of this was being communicated to the British I have no Idea (or if claims of an improved Allison with more power would restore performance?)
Please note that the YP-39s being delivered in the late fall and winter of 1940 had 1090hp V-1710-37 engines and not the 1150hp V-1710-35 of later Airacobras.

Note (4) we finally get close to reality, the empty weight goes up 194lbs (which doesn't quite cover the weight of the self sealing tanks which would be included in empty weight and useful load (which would include armor) goes up 133lbs. but they may have traded off fuel to cover the rest of the weight difference?

 

[P-39 Expert]

I told you those wing guns were heavy.

I couldn't reconcile those numbers either. But at the end of the day the P-39 could have easily weighed 5400# empty (with self sealing tanks) and 7100# gross (with armor plate/glass). A very different plane performance wise.

 

[Ivan1GFP]

Big question I always had is, how in the world did the British think the P-400 would be competitive (achieve the performance guarantees) at 7850# when the SpitfireV weighed 6450# and had a more powerful engine? The whole exercise seems doomed from the start. The planes did get built, they sure got used by the AAF and the Russians, and Bell got the $2million advance which probably saved them financially, but the rest of that procurement exercise was crazy.

Hello P-39 Expert,
There was a lot of "Crazy" going on at that time if you think about it.
In April 1940, the British were willing to contract with North American to build an entirely new fighter that didn't even exist as a paper design. This was early in the Battle of Britain and the outcome wasn't so sure. The fact that things turned out pretty well is a nice story but from the British viewpoint, there wasn't any way they could predict things would go so well.

As for getting decent performance at 7850 pounds with a 1150 HP engine, that sounds very much like what that new fighter design achieved. The Airacobra is a very slick design and LOOKS like it should be very fast. There was plenty of advertising hype at the time that claimed it was a 400 MPH fighter. There were articles in a few aviation magazines and even one in Popular Mechanics.

My own belief is that this argument about the increased gross weight of the Airacobra isn't really getting at the real cause of the issue.
From what I have seen in test reports, a weight increase doesn't usually affect the maximum speed of an aircraft very much.
While there IS an effect, it tends to be small, perhaps 2-5 MPH in the typical case.
In the case of the Airacobra, the difference between P-39C and British spec Airacobra is 15-20 MPH: 350+ MPH versus around 370 MPH. Something else is going on besides a weight increase.
Weight increases affect Climb Rate and Service Ceiling to a much greater extent.

My belief is that the place to look is at the changes to the engine configuration and perhaps operating parameters that may have reduced power or critical altitude. The backfire screens as an intake obstruction would have had some effect. The snow screens that were on the British Airacobra were only on their aircraft and it appears that although the maximum engine output was not decreased, the aircraft's critical altitude was reduced to about 11,000 feet which is 2000 feet below similar aircraft without either of those obstructions in place.
A couple of the US tests also show that the pilot was using manual mixture control and getting another 3 MPH and 25 HP over the typical "Auto-Rich" setting. Can this be taken as a representative test result? Would the British test pilot have done something similar?

- Ivan.

 

[P-39 Expert]

The gross weight was extremely important for the P-39/400 because it affected the climb/ceiling much more than the speed. A weight savings of 100# only increased speed about 1mph. A weight savings of 100# increased climb rate about 120fpm. As you can see if the weight savings was 800# (7850-7050) speed would only increase about 8mph (not significant), but the rate of climb would increase about 1000fpm (very significant). Add 1000fpm to the nominal climb rate of 2700fpm and you have a very fast climbing plane.

More importantly less weight increases ceiling as well as climb rate. A planes "combat" ceiling was the altitude at which the plane could no longer climb at at least 1000fpm. Rate of climb normally decreases with altitude so once the "combat" ceiling was reached rate of climb slowed gradually above that altitude reaching the absolute ceiling when climb rate is zero. The P-400 with the ubiquitous drop tank's combat ceiling was around 18000'. The Japanese bombers and fighters flew well above that altitude, bombers coming in at between 18000' and 22000', sometimes a little higher. A combat ceiling of 18000' means that while you can then climb up to 22000' it is a long, slow and laborious process that is hard on your engine (at full power) and exposes you to opposing fighters that can climb faster. Not much fun, at all. This is what the term "low altitude fighter" means. With drop tank you have a real hard time getting up to where the opposition is. The ultimate disadvantage. See the graph in post #388.

But at 7100# the P-39/400 with drop tank would have a "combat" ceiling of about 23000' at normal power and 31000' at full power after the drop tank is dropped. So you could easily climb at normal power with drop tank up to 23000' and patrol for incoming bombers. When bombers are sighted the drop tank is jettisoned and you can climb up to 31000' before reaching "combat" ceiling. Since not much combat occurred above 26000' you had plenty of climb to handle pretty much any situation. And that rate of climb will be better than any opponent at all altitudes in '41-'42. Huge advantage. P-39-P/400 is now a high altitude plane.

 

[Ivan1GFP]

Hello P-39 Expert,

First of all, it seems to me that the main complaint about the Airacobra was that its speed was too low. British testing put it at just over 350 MPH while the original expectation was a 400 MPH fighter. Note that when the Mustang came along, it was doing about 380 MPH but didn't climb any better because its engine power was identical and it was about as heavy or even more so and, yet the British didn't have problems with it and continued operating the ones they had until the end of the war while they got rid of the Airacobra.

Since we seem to also be discussion Airacobra versus Japanese aircraft:
For initial climb rate, the P-39D tests show 2720 fpm which is almost identical to the 2750 fpm climb rate achieved in testing the Aleutian A6M2. The critical altitude of the P-39D without backfire screens is about 13,000 feet. The critical altitude of the A6M2 according to their manual is 4200 Meters or 13,780 feet. (Pretty close, huh?) So there should not be much of a difference., right?
Both aircraft had single speed single stage superchargers.

You keep commenting on how there was a great weight difference between the P-39D and P-400.
I have attached a table with the values I believe to be correct for both aircraft. I collected the data early last year when I was working on a project involving a P-39D/F. I believe most of the data came from America's Hundred Thousand.
I added a "Normal Loaded Weight" column this morning with the partial ammunition and fuel loads in that configuration.
Engine Oil weight should also be reduced a bit also but I do not have the numbers handy to do that and I do not see that it would make any substantial difference. The weights of the two aircraft seem to be very close to me.
Can you explain why my numbers are wrong and why you believe that the weights really should be 7100 versus 7800 pounds?
Other than "Miscellaneous Equipment", I do not see anything that can not be explained pretty easily.

- Ivan.

 

[wuzak]

Why are the cowl guns heavier on the P-39D than the P-400 or P-39D-2?

And why is the P-400's pilot heavier?

 

[P-39 Expert]

Thanks Ivan,

Main complaint from the British was speed because it was below the contract guarantee. Actually it was about the same as their Spitfire V, faster below 18000' and not more than 10mph slower above. P-39 vs Allison Mustang: Mustang weighs 8400-8600#, will always be outclimbed by a P-39 at 7650#. Mustang was faster by 10-15mph.

A standard P-39 at 3000rpm would climb with a contemporary Zero for five minutes, then the P-39 had to throttle back to normal power for the remainder of the climb. That's when the Zero "walked away" from the P-39. The five minute military power limit was increased to 15 minutes in mid '42, after that it should climb with a Zero all the way up.

I hope I didn't confuse you, I was comparing the weights of the P-39/400 (about the same) with the P-39C which weighed 7100#. It's just a way to compare performance based on weight since the P-39/400 had the same aerodynamics, engine and propeller, just different weights.

 

[Greyman]

I really don't think the issue was speed. The RAF used Hurricanes, Mohawks, Tomahawks, Kittyhawks, and Whirlwinds for months if not years after kicking the Airacobra to the curb. The Airacobra was as fast or faster than all of these aircraft.

The aircraft wasn't finished, didn't work (as 601 Squadron and the VVS pointed out), and to bring it to an acceptable level required more effort than the RAF was willing to put up with.

 

[Glider]

Thanks Ivan,

A standard P-39 at 3000rpm would climb with a contemporary Zero for five minutes, then the P-39 had to throttle back to normal power for the remainder of the climb. That's when the Zero "walked away" from the P-39. The five minute military power limit was increased to 15 minutes in mid '42, after that it should climb with a Zero all the way up.

While I don't know how a Zero would compare against a P39 it could climb at a steeper angle than the Hellcat which gave the Zero an advantage in that type of combat. My suspicion is that the P39 would be in a similar situation.

 

[Shortround6]

Hello P-39 Expert,
There was a lot of "Crazy" going on at that time if you think about it.
In April 1940, the British were willing to contract with North American to build an entirely new fighter that didn't even exist as a paper design. This was early in the Battle of Britain and the outcome wasn't so sure. The fact that things turned out pretty well is a nice story but from the British viewpoint, there wasn't any way they could predict things would go so well.

As for getting decent performance at 7850 pounds with a 1150 HP engine, that sounds very much like what that new fighter design achieved. The Airacobra is a very slick design and LOOKS like it should be very fast. There was plenty of advertising hype at the time that claimed it was a 400 MPH fighter.
My belief is that the place to look is at the changes to the engine configuration and perhaps operating parameters that may have reduced power or critical altitude. The backfire screens as an intake obstruction would have had some effect. The snow screens that were on the British Airacobra .

Things with the Allison were even stranger than what you list in 1940-41.
While Allison built 1149 engines in 1940 according to one source, a whopping 68 of those engines were completed in the first six months of the year. Allison was engaged in a dispute with the Army over the long nose C-15 engine, Allison wanted to rate it at 1040hp at 14,200ft while the Army wanted 1090hp at 13,200ft. The engine would actual make either goal for a short time. the problem was that Allison wanted the engine rated at 1040hp for take off and to have 1040 as the max power up to the 14.200ft altitude, The Army wanted wanted 1090hp from take-off to 13,200ft.
Allison began type testing the C-13 engine on Aug 11th 1939. By May 11th they were on the sixth attempt and the fourth engine. The Army Material division agreed to one more attempt at 1090hp military and 960hp normal rating and if unsuccessful they would try Allisons recommendation of 1040hp military and 930 normal. The engine failed in hour 79 at the #8 crank cheek. Finally on Aug 15th 1940 another test was started which the engine successfully completed at 1040hp military and take-off and 930hp normal.
I am sure Allison was offering 1150hp engines for later delivery but they had no type tested engine of that power available in 1940 although a few developmental engine may have been available.

In fact, while Allison was offering an 1150hp engine for sale to foreign governments in May of 1940 the engine did not go into production (in P-39 form) until Dec 26th 1940 ( and that is going into production, not first engine delivered) and it passed it's type test Jan 9th 1941. However several parts were still unsatisfactory and the lower crankcase casting and extension drive shaft engine end center bearings had to be modified.
The first 13 YP-39s were provided with 16 engines using the unmodified crankcases and were operated at reduced power settings until they could be sent back to the factory and modified. The XP-39B crashed on Jan 6th 1940 and was rebuilt, to be crashed again on Aug 6th 1940, Only 28 hours of flight time had been accumulated before it was written off.
The first of 13 YP-39s was delivered Sept 6th and the last Dec 16th 1940. The first few carried no armament.
The first P-39C flew in Jan 1941 and it was obvious that these were test/famirazation aircraft as the bulk of the contract had been shifted to combat capable P-39Ds 7-8 months earlier.