Groundhog Thread Part Deux - P-39 Fantasy and Fetish - The Never Ending Story (Mods take no responsibility for head against wall injuries sustained) (4 Viewers)

[tomo pauk]

Makes one wonder what the British were thinking when they specified a 7850# plane that should have weighed 7100#. I believe they were ordering ground attack planes since they also ordered P-38s without turbos (or handed propellers).

British were not ordering ground-attack planes, neither from USA nor from UK proper..
In 1941 and before, RAF was not interested in that type of aircraft. Further, P-38 and P-39 they contracted will be lousy ground-attack aircraft due to absence of bomb racks.

Then when the German invasion didn't materialize the British no longer wanted the P-400 or the turboless P-38.

They also didn't wanted turboed P-38 by time those materialized.

 

[P-39 Expert]

I don't believe Bell was promising anything once the British got the weight way up to where they knew the plane wouldn't meet spec.

Even after numerous weight increases and the British tests showing the lower speeds they still placed their final order for 505 more. Everybody knew about the extra weight and lower performance. Wasn't a secret.

Point is, the weight should have been a lot lower as shown by the performance of the P-39C at 7100#. Compare it to a Spitfire V. Spitfire V in 1941 was only good for 365-375mph. At 7100# the P-39 would do 379mph and outclimb the Spitfire which only weighed 6600#. That's about the same performance as a Spitfire V except 500# heavier on a less powerful engine.

The bottom line is the British specified a plane too heavy to meet spec and then tried to cancel the order. The main reason they didn't want the plane was they won the Battle of Britain in late 1940 and there would be no German invasion. Now their internal fighter production would be sufficient and they for sure didn't want to pay for them.

 

[MIflyer]

The tubo was deleted because it actually reduced performance as compared to just a single stage single speed mechanically supercharged V-1710. The turbo installation caused too much drag and the very small size of the P-39 meant there was no place they could put it and not cause that problem.

 

[Shortround6]

I don't believe Bell was promising anything once the British got the weight way up to where they knew the plane wouldn't meet spec.

Source for this?

Even after numerous weight increases and the British tests showing the lower speeds they still placed their final order for 505 more. Everybody knew about the extra weight and lower performance. Wasn't a secret.

Source for this?

Point is, the weight should have been a lot lower as shown by the performance of the P-39C at 7100#. Compare it to a Spitfire V. Spitfire V in 1941 was only good for 365-375mph. At 7100# the P-39 would do 379mph and outclimb the Spitfire which only weighed 6600#. That's about the same performance as a Spitfire V except 500# heavier on a less powerful engine.

Source for this

The bottom line is the British specified a plane too heavy to meet spec and then tried to cancel the order. The main reason they didn't want the plane was they won the Battle of Britain in late 1940 and there would be no German invasion. Now their internal fighter production would be sufficient and they for sure didn't want to pay for them.

I would really love a source for this one.

The French had 165 planes on order in the spring of 1940 (some sources say 170, some say the number changed to 170 when the British took the order over.

Now lets see if we can figure out who knew what when.

"The XP-39B resumed flight trials on November 25, 1939. Empty weight had grown from from 3995 lbs to 4530 lbs, and normal gross weight was up to 5834 pounds from 5550 pounds, and the aircraft STILL didn't have any armament."

The XP-39 was powered by a V-1710-37 (E5) engine rated 1090 hp at an altitude of 13,300 feet.

" Even in spite of the improved streamlining, the XP-39B suffered a severe degradation in high-altitude performance. Maximum speed fell from 390 mph at 20,000 feet to 375 mph at 15,000 feet, and it now took 7.5 minutes to reach 20,000 feet rather than five minutes. "

Quotes are from Joe Baugher but he makes the common mistake that says the XP-39 flew at 390mph with the turbo, it didn't. Period, end of story. More Bell advertising hype.

then " The XP-39B was damaged in a belly-landing at Wright Field, Ohio on January 6, 1940. It was repaired and resumed flying, later to be demonstrated at Bolling Field in Washington, DC. Unfortunately, the XP-39B was destroyed in an accident after only 28 flying hours. "

" The first YP-39 (40-027) was flown on September 13, 1940 with the 1090 hp V-1710-37 (E5) engine driving a Curtiss Electric propeller. It differed externally from the XP-39B primarily in having a wider-chord vertical tail. The first few YP-39s were initially flown without armament, but subsequent machines were fitted with a 37 mm cannon with 15 rounds, a pair of 0.5-inch machine guns with 200 rounds per gun, and two 0.30-inch machine guns with 500 rounds per gun. All of these guns were mounted in the nose. Some armor protection was provided for the pilot. Empty and normal loaded weights rose to 5042 pounds and 7000 pounds, respectively. "
"the performance of the YP-39 dropped to a maximum speed of 368 mph at 15,000 feet. An altitude of 20,000 feet could be attained in 7.3 minutes. Service ceiling was 33,300 feet. "

No (or little) armor and no self sealing tanks. Please note that this this first flight was AFTER the British had ordered the 505 additional aircraft.

So Bell Knew about at least some of the weight gain and the fact that the P-39/P-400 was NOT a 400mph airplane.

The first P-39C flew in Jan of 1941.
"The P-39C was powered by a 1150 hp Allison V-1710-35 engine. Weights were 5070 pounds empty, 7075 pounds gross (combat weight), and 7300 pounds maximum takeoff. Maximum speed was 379 mph at 13,000 feet. An altitude of 12,000 feet could be reached in 3.9 minutes. "

Accounts vary, The P-39D gets to the first Army units sometime between Feb and April of 1941. British test pilot flies the tricked out P-400 April 27th 1941.
We also have to conclude that Bell knew the plane was missing the contract performance numbers or they wouldn't have spent so much time working over that one example.

March saw the signing of lend lease and further British orders for lend lease Airacobras were placed but more important to us (or me) is that 3 of the used P-39Cs are declared lend lease and sent to England and arrive July 3rd 1941.
Joe Baugher again" Although the test pilots praised the general ease of handling of the aircraft, the maximum speed was a shocking 33 mph lower than that anticipated. The fighter proved to be definitely inferior to the Hurricane and Spitfire in climb rate and ceiling, and the 750-yard takeoff run of the Airacobra excluded its operation from some smaller fighter airfields. There was universal shock and dismay among the RAF personnel. What had gone wrong? Bell Aircraft executives later sheepishly admitted that their performance figures had been based on the unarmed and unequipped XP-39 prototype, which weighed a ton less than the armed and equipped P-39C. "

And the P-39C has little armor, no BP glass and no self-sealing tanks, they were sent over to aid in training/familiarization for the P-400?Airacobra Is that began arriving in just a few weeks.

From Baugher again, your sources may differ
"The Airacobra I was powered by an Allison V-1710-E4 twelve-cylinder V in-line engine rated at 1150 hp for takeoff. Weights were 5462 pounds empty and 7845 pounds normal gross. Maximum speeds were 326 mph at 6000 feet, 343 mph at 10,000 feet, 355 mph at 13, 000 feet, 341 mph at 20,000 feet. Initial climb rate was 2040 feet per minute. With an internal fuel capacity of 100 Imp gal the Airacobra had an endurance of 1 hour 20 minutes at maximum continuous cruising speed at 6000 feet, 1 hour 5 minutes at 12,000 feet, and 1 hour 35 minutes at 20,000 feet. The true airspeeds at these altitudes were 287 mph, 327 mph, and 308 mph, respectively. Under most economical cruise conditions, the endurance increased to 3 hours 20 minutes, the relevant speeds being 183 mph at 6000 feet, 217 mph at 12,000 feet, and 215 mph at 20,000 feet. Under maximum continuous climb conditions, it took 15 minutes to reach 20,000 feet. The operational ceiling was considered to be about 24,000 feet, although there was a marked decrease in performance above 20,000 feet. At the Airacobra's rated altitude of 13,000 feet, it was 18 mph faster than the Spitfire VB. However, the speed fell off rapidly above that height, and the two planes were almost exactly matched at 15,000 feet. At 20,000 feet, the Spitfire VB was 35 mph faster and at 24,000 feet it was 55 mph faster. The ground run of the Airacobra during takeoff was 2250 feet, as compared with 1470 feet for the Hurricane II and 1590 feet for the Spitfire V. "

 

[MIflyer]

Note that the British did not build a high altitude version of the Whirlwind, Beaufighter, or Typhoon - or a two stage supercharged Spitfire until confronted by the FW-190. The Lightning I was designed to use the C series engines employed by the Tomahawk, in order to aid commonality. They did not do high altitude daylight bombing and were not focused on high altitude operations such as the USAAF was.

 

[Shortround6]

Note that the British did not build a high altitude version of the Whirlwind, Beaufighter, or Typhoon - or a two stage supercharged Spitfire until confronted by the FW-190. The Lightning I was designed to use the C series engines employed by the Tomahawk, in order to aid commonality. They did not do high altitude daylight bombing and were not focused on high altitude operations such as the USAAF was.

I agree with just about all of this.
We should note however that in late 1940 and well into 1941 the Merlin XX and 45 engines were pretty much in class of their own when it came to "high" altitude performance. The Russian AM-35 engine and the two stage R-1830s in the Wildcats were the only real competition. The US was certainly interested in higher altitudes but the turbo charged P-43s and early P-38s weren't really combat ready. I would also note that the Merlins were being produced in far larger numbers than the Russian and American engines at this time.

 

[wuzak]

Note that the British did not build a high altitude version of the Whirlwind, Beaufighter, or Typhoon - or a two stage supercharged Spitfire until confronted by the FW-190. The Lightning I was designed to use the C series engines employed by the Tomahawk, in order to aid commonality. They did not do high altitude daylight bombing and were not focused on high altitude operations such as the USAAF was.

The 2 stage Spitfire was being developed from before the first encounter with the Fw 190, based on the Mk III.

The encounters with the Fw 190 led to the interim Mk IX program, which was initially a Mk V with the Merlin 61, as the definitive Mk VII/VIII versions were developed.

 

[P-39 Expert]

Source for this? Birch Matthews "Cobra! Bell Aircraft Corporation 1934-1946"


Source for this? Ditto.



Source for this wwiiaircraftperformance.org



I would really love a source for this one. Just common sense.

The French had 165 planes on order in the spring of 1940 (some sources say 170, some say the number changed to 170 when the British took the order over.

Now lets see if we can figure out who knew what when.

"The XP-39B resumed flight trials on November 25, 1939. Empty weight had grown from from 3995 lbs to 4530 lbs, and normal gross weight was up to 5834 pounds from 5550 pounds, and the aircraft STILL didn't have any armament."

The XP-39 was powered by a V-1710-37 (E5) engine rated 1090 hp at an altitude of 13,300 feet.

" Even in spite of the improved streamlining, the XP-39B suffered a severe degradation in high-altitude performance. Maximum speed fell from 390 mph at 20,000 feet to 375 mph at 15,000 feet, and it now took 7.5 minutes to reach 20,000 feet rather than five minutes. "

Quotes are from Joe Baugher but he makes the common mistake that says the XP-39 flew at 390mph with the turbo, it didn't. Period, end of story. More Bell advertising hype.

then " The XP-39B was damaged in a belly-landing at Wright Field, Ohio on January 6, 1940. It was repaired and resumed flying, later to be demonstrated at Bolling Field in Washington, DC. Unfortunately, the XP-39B was destroyed in an accident after only 28 flying hours. "

" The first YP-39 (40-027) was flown on September 13, 1940 with the 1090 hp V-1710-37 (E5) engine driving a Curtiss Electric propeller. It differed externally from the XP-39B primarily in having a wider-chord vertical tail. The first few YP-39s were initially flown without armament, but subsequent machines were fitted with a 37 mm cannon with 15 rounds, a pair of 0.5-inch machine guns with 200 rounds per gun, and two 0.30-inch machine guns with 500 rounds per gun. All of these guns were mounted in the nose. Some armor protection was provided for the pilot. Empty and normal loaded weights rose to 5042 pounds and 7000 pounds, respectively. "
"the performance of the YP-39 dropped to a maximum speed of 368 mph at 15,000 feet. An altitude of 20,000 feet could be attained in 7.3 minutes. Service ceiling was 33,300 feet. "

No (or little) armor and no self sealing tanks. Please note that this this first flight was AFTER the British had ordered the 505 additional aircraft. Per Birch Matthews the final revision to the contract and the increase of 505 planes was signed June 20, 1941 AFTER the disappointing April tests and the altered P-400 with the smaller tail and filled/sanded seams.

So Bell Knew about at least some of the weight gain and the fact that the P-39/P-400 was NOT a 400mph airplane. Both Bell and the British knew this, certainly after the April tests.

The first P-39C flew in Jan of 1941.
"The P-39C was powered by a 1150 hp Allison V-1710-35 engine. Weights were 5070 pounds empty, 7075 pounds gross (combat weight), and 7300 pounds maximum takeoff. Maximum speed was 379 mph at 13,000 feet. An altitude of 12,000 feet could be reached in 3.9 minutes. "

Accounts vary, The P-39D gets to the first Army units sometime between Feb and April of 1941. British test pilot flies the tricked out P-400 April 27th 1941.
We also have to conclude that Bell knew the plane was missing the contract performance numbers or they wouldn't have spent so much time working over that one example.

March saw the signing of lend lease and further British orders for lend lease Airacobras were placed but more important to us (or me) is that 3 of the used P-39Cs are declared lend lease and sent to England and arrive July 3rd 1941.
Joe Baugher again" Although the test pilots praised the general ease of handling of the aircraft, the maximum speed was a shocking 33 mph lower than that anticipated. The fighter proved to be definitely inferior to the Hurricane and Spitfire in climb rate and ceiling, and the 750-yard takeoff run of the Airacobra excluded its operation from some smaller fighter airfields. There was universal shock and dismay among the RAF personnel. What had gone wrong? Bell Aircraft executives later sheepishly admitted that their performance figures had been based on the unarmed and unequipped XP-39 prototype, which weighed a ton less than the armed and equipped P-39C. " A chart is attached showing performance of the P-39K (representative of the V-1710-35 powered early P-39s) vs. the Spitfire V and P-39C.

And the P-39C has little armor, no BP glass and no self-sealing tanks, they were sent over to aid in training/familiarization for the P-400?Airacobra Is that began arriving in just a few weeks. A revised P-39C with self sealing tanks and armor still weighed 7100# since the new tanks held 300# less fuel. A 7100# fully equipped P-39 was very viable.

From Baugher again, your sources may differ
"The Airacobra I was powered by an Allison V-1710-E4 twelve-cylinder V in-line engine rated at 1150 hp for takeoff. Weights were 5462 pounds empty and 7845 pounds normal gross. Maximum speeds were 326 mph at 6000 feet, 343 mph at 10,000 feet, 355 mph at 13, 000 feet, 341 mph at 20,000 feet. Initial climb rate was 2040 feet per minute. With an internal fuel capacity of 100 Imp gal the Airacobra had an endurance of 1 hour 20 minutes at maximum continuous cruising speed at 6000 feet, 1 hour 5 minutes at 12,000 feet, and 1 hour 35 minutes at 20,000 feet. The true airspeeds at these altitudes were 287 mph, 327 mph, and 308 mph, respectively. Under most economical cruise conditions, the endurance increased to 3 hours 20 minutes, the relevant speeds being 183 mph at 6000 feet, 217 mph at 12,000 feet, and 215 mph at 20,000 feet. Under maximum continuous climb conditions, it took 15 minutes to reach 20,000 feet. The operational ceiling was considered to be about 24,000 feet, although there was a marked decrease in performance above 20,000 feet. At the Airacobra's rated altitude of 13,000 feet, it was 18 mph faster than the Spitfire VB. However, the speed fell off rapidly above that height, and the two planes were almost exactly matched at 15,000 feet. At 20,000 feet, the Spitfire VB was 35 mph faster and at 24,000 feet it was 55 mph faster. The ground run of the Airacobra during takeoff was 2250 feet, as compared with 1470 feet for the Hurricane II and 1590 feet for the Spitfire V. "

Please see above for additional comments. Yep, a 7845 pound gross weight will have a detrimental effect on performance. The British did everything in their power to make this plane as heavy as possible so it wouldn't meet spec. The British version weighed 7845#, the AAF version P-39C weighed 7100# and the Spitfire V weighed 6450#. See the attached chart below. The Spitfire V is in red, the P-39C is in black, and P-39K climb at 3000rpm (military power) is green.

 

[Glider]

Note that the British did not build a high altitude version of the Whirlwind, Beaufighter, or Typhoon - or a two stage supercharged Spitfire until confronted by the FW-190. The Lightning I was designed to use the C series engines employed by the Tomahawk, in order to aid commonality. They did not do high altitude daylight bombing and were not focused on high altitude operations such as the USAAF was.

I disagree in some way with the first part of this. Westland did develop a high altitude version of the Whirlwind as the Westland Welkin which entered limited production, is clearly based on the same design principles as the Whirlwind.
The idea of developing a high altitude version of the Typhoon and Beaufighter both of which were designs that were low / medium altitude aircraft was never going to happen. Besides, by the time the need arose the RAF had the Spit VII and Mosquito both of which were good for high altitude combat.

 

[Glider]

Please see above for additional comments. Yep, a 7845 pound gross weight will have a detrimental effect on performance. The British did everything in their power to make this plane as heavy as possible so it wouldn't meet spec. [quote\]
One day, probably in a far off distant time you might do one of two things:-
a) supply some evidence that the RAF had a real thing about the P39 and wanted to destoy it's credibility (and supply a reason for this behaviour)
b) agree that the P39c wasn't combat capable without armour and self sealing fuel tanks

 

[tomo pauk]

I disagree in some way with the first part of this. Westland did develop a high altitude version of the Whirlwind as the Westland Welkin which entered limited production, is clearly based on the same design principles as the Whirlwind.
The idea of developing a high altitude version of the Typhoon and Beaufighter both of which were designs that were low / medium altitude aircraft was never going to happen. Besides, by the time the need arose the RAF had the Spit VII and Mosquito both of which were good for high altitude combat.

Welkin was not a high-altitude version of Whirlwind, but a brand new aircraft - new wing of thicker TtC ratio and strong enough to sustain 2-stage Merlins, new were at least central and front fuselage, undercarriage, new cooling systems to cater for much greater cooling load + intercoolers, new fuel system. Old tail will not suffice for much greater engine power.
Neither Typhoon nor Beaufighter were purposely-designed as low/medium altitude aircraft.
Granted, RAF have had enough of hi-alt fighters in it's toolbox, any day between 1920s on, including ww2.

 

[Glider]

Welkin was not a high-altitude version of Whirlwind, but a brand new aircraft - new wing of thicker TtC ratio and strong enough to sustain 2-stage Merlins, new were at least central and front fuselage, undercarriage, new cooling systems to cater for much greater cooling load + intercoolers, new fuel system. Old tail will not suffice for much greater engine power.

Please note that I said design principles.
Your comments could apply to a number of aircraft that were radically redesigned such as the P51D to P51H or Me109E to 109F

 

[tomo pauk]

Please note that I said design principles.
Your comments could apply to a number of aircraft that were radically redesigned such as the P51D to P51H or Me109E to 109F

You've said:
Westland did develop a high altitude version of the Whirlwind as the Westland Welkin which entered limited production, is clearly based on the same design principles as the Whirlwind.

so I think my comment is valid.
But, yes, P-51H was a brand-new aircraft vs. P-51D (to the point where only non-NAA items were the same, like MGs or gauges); the Bf 109F seems to be using same fuselage from firewall back as the 109E, but indeed th wing, cooling systems, tail and nose were new.

 

[wuzak]

Please see above for additional comments. Yep, a 7845 pound gross weight will have a detrimental effect on performance. The British did everything in their power to make this plane as heavy as possible so it wouldn't meet spec. The British version weighed 7845#, the AAF version P-39C weighed 7100# and the Spitfire V weighed 6450#. See the attached chart below. The Spitfire V is in red, the P-39C is in black, and P-39K climb at 3000rpm (military power) is green.

Why would the British, who were in desperate need for front line fighters, reject an aircraft which out-performed the Spitfire?

The Spitfire V had a higher critical altitude, so it is not surprising that the P-39 lightweight special out-climbed it at low altitude.

 

[Shortround6]

You've said:
Westland did develop a high altitude version of the Whirlwind as the Westland Welkin which entered limited production, is clearly based on the same design principles as the Whirlwind.

so I think my comment is valid.
But, yes, P-51H was a brand-new aircraft vs. P-51D (to the point where only non-NAA items were the same, like MGs or gauges); the Bf 109F seems to be using same fuselage from firewall back as the 109E, but indeed th wing, cooling systems, tail and nose were new.

The P-51H used pretty much the same wing, at least aerodynamicly, did it not?
Yes they got rid of the little wing root extensions (which helped house the landing gear) but wasn't the airfoil the same? or slightly stretched to straighten out the leading edge?
Wing span the same, (or within a 1/4 in) wing area is within 1-2 sq ft. fuselage is a 2 feet longer and the tail is taller (but a lot of planes got bigger/longer tails as they got more powerfuel engines.
Yes you can't make a P-51H out of a P-51D airframe (or at least not without a lot of work) but they are pretty close to each other.

The Whirlwind and Welkin were not anywhere near as close.

it is not quite as bad as claiming the Lockheed P-58 Chain Lighting is clearly based on the same design principles as the P-38 Lighting but the Welkin was a lot further from the Whirlwind than the P-51H was from the P-51D.

Welkin and Whirlwind

The Welkin was almost double the weight of the Whirlwind.

 

[jetcal1]

The P-51H used pretty much the same wing, at least aerodynamicly, did it not?
Yes they got rid of the little wing root extensions (which helped house the landing gear) but wasn't the airfoil the same? or slightly stretched to straighten out the leading edge?
Wing span the same, (or within a 1/4 in) wing area is within 1-2 sq ft. fuselage is a 2 feet longer and the tail is taller (but a lot of planes got bigger/longer tails as they got more powerfuel engines.
Yes you can't make a P-51H out of a P-51D airframe (or at least not without a lot of work) but they are pretty close to each other.

The Whirlwind and Welkin were not anywhere near as close.

it is not quite as bad as claiming the Lockheed P-58 Chain Lighting is clearly based on the same design principles as the P-38 Lighting but the Welkin was a lot further from the Whirlwind than the P-51H was from the P-51D.
View attachment 531536

View attachment 531537
Welkin and Whirlwind
View attachment 531538

The Welkin was almost double the weight of the Whirlwind.

Well, there is a family resemblance.
(Don't laugh, I can see Schumed and the P-51 in a F-5 profile. Or Ted Smith in both a B-26 and the Aerocommander.)

 

[MIflyer]

The Spitfire III was not intended to be a high altitude version, at least not in the same sense as the two stage supercharged Merlin versions. The Spit III was to be powered by the Merlin XX rated at 1,480 hp at 12,250 ft. The Spitfire IV was to be powered by the early single stage supercharged Griffon - still not a high altitude engine.

The Merlin 60 series of two stage two speed supercharged engines was developed as the result of the overflight of Ju-86P and R bombers that operated at too high an altitude for interception by RAF fighters. The British planned to build a version of the Wellington to emulate the Germans, initially to be equipped with radial engines with turbosuperchargers "in the American style" as described by Stanley Hooker. But they wanted a back up engine and that is when Sir Hooker did something utterly brilliant and combined the Merlin engine with the supercharger from the failed Vulture engine used for the Manchester, along with a liquid cooled aftercooler/intercooler. When the FW-190 showed up quite unexpectedly they hung the two stage Merlin on a Spit V airframe and produced the Spitfire IX, an airplane that many German fighter pilots thought was never equalled by the Luftwaffe.

The British only came up with very high altitude bomber and interceptor after the Germans embarrassed them into pursuing it. The fact that this effort was all but worthless to the Luftwaffe but sealed its doom with coming of the two stage Spitfires and Merlin Mustangs is downright delicious. In contrast the USAAF was working on incorporating turbosuperchargers from the day that General Electric hauled that experimental turbo hooked to a Liberty engine up the side of Pike's Peak and proved it would work.

The F4F and XP-37 were supposed to be high altitude fighters. The turbo in a P-36 airframe with a V-1710 engine worked, and boosted speed above 20,000 ft by 50 mph - but was rather grotesque and must have been terrible to fly.

 

[Shortround6]

Source for this? Birch Matthews "Cobra! Bell Aircraft Corporation 1934-1946"

How about some page numbers?

as for the order for 505, there seems to some confusion, Initial order was placed back in the late spring, early summer of 1940. final details. amendments and other details might very well not have been signed until May of 1941, but then why did Bell sign, knowing full well that the planes would not meet contract speed?

Bell had been through this with the US Army in the fall of 1939. When the French wanted to purchase the P-39/P-400 the Army tried to extract a number of concessions form Bell in return for permission to export. Much like Allison had to forgive $900,00o in past Army debt. Army wanted no increase in cost and no delays in deliver in regards to the 93 planes it had on order at the time. This might be considered fair except the Army also wanted a no cost change order for armor plate and armor glass. The Army also wanted no reduction in performance with these modifications. The Army also wanted the last 18 P-39Cs to have self sealing tanks in restructured wings and didn't want to pay for those either.
If Bell didn't want to absorb their cost the Armies position was that Bell could just add the cost of the improvements to the Army's planes to the cost of the french order.
"Birch Matthews "Cobra! Bell Aircraft Corporation 1934-1946" pages 156-157.

On April 3rd the Army and Bell came to agreement (with pressure from Roosevelt). The 13 YP-39s and the first 20 P-39Cs would be delivered as scheduled and without any modifications. However the final 60 would be delayed until Oct and Nov of 1941. These would incorporate the armor plate, armor glass and self sealing tanks. The Army also agreed to a "suitable adjustment of weights, performance, etc" ......."in consideration of the changes to be incorporated"
"Birch Matthews "Cobra! Bell Aircraft Corporation 1934-1946" pages 157-158.".

So Bell knew darn well that extra protection and increased weight would affect performance in March/April of 1940 and had negotiated a contract with the US Army to make those allowances. But they didn't do it with the British? Why the extensively modified test aircraft in April of 1941?

Please see above for additional comments. Yep, a 7845 pound gross weight will have a detrimental effect on performance. The British did everything in their power to make this plane as heavy as possible so it wouldn't meet spec. The British version weighed 7845#, the AAF version P-39C weighed 7100# and the Spitfire V weighed 6450#. See the attached chart below. The Spitfire V is in red, the P-39C is in black, and P-39K climb at 3000rpm (military power) is green.

We have asked before, what did the British specify that made the plane so much heavier than the US planes? Specific items of equipment please.

You do know that if you increase the gross weight of a plane by hundreds of pounds you also have to beef up the structure in order to maintain the same "G" loading and safety margin. I don't know if you need 5lbs of extra material or 50lbs but you need something.

I would really love a source for this one. Just common sense.

In other words you made it up?

 

[Shortround6]

The Spitfire III was not intended to be a high altitude version, at least not in the same sense as the two stage supercharged Merlin versions. The Spit III was to be powered by the Merlin XX rated at 1,480 hp at 12,250 ft

A lot depends on exactly what time period we are talking about.
A Merlin XX compared to a Merlin III was a high altitude engine. In Hurricane it would hold 6lbs of boost to around 23,000 ft in level flight compared to the Merlin III holding 6lbs of boost to about 17,000ft.

The F4F, even with the two stage supercharger was only good for about 1000hp at 19,000ft (no ram) which is certainly an improvement over the 1000hp at 14,500ft of the two speed R-1830 engine.

The Merlin III was good for 1310 hp at 9000ft and the Merlin XII was good for 1280hp at 10,500ft. so while not a 30,000ft engine it was better than just about anything else in service
in late 1940 and most of 1941.

 

[wuzak]

The Spitfire III was not intended to be a high altitude version, at least not in the same sense as the two stage supercharged Merlin versions. The Spit III was to be powered by the Merlin XX rated at 1,480 hp at 12,250 ft. The Spitfire IV was to be powered by the early single stage supercharged Griffon - still not a high altitude engine.

The XX had a higher rated altitude than the 45 used in the V.

The Griffon IIB wasn't that high altitude rated, but the Mk XII could still do ~390mph at 25,000ft.

The Merlin 60 series of two stage two speed supercharged engines was developed as the result of the overflight of Ju-86P and R bombers that operated at too high an altitude for interception by RAF fighters. The British planned to build a version of the Wellington to emulate the Germans, initially to be equipped with radial engines with turbosuperchargers "in the American style" as described by Stanley Hooker. But they wanted a back up engine and that is when Sir Hooker did something utterly brilliant and combined the Merlin engine with the supercharger from the failed Vulture engine used for the Manchester, along with a liquid cooled aftercooler/intercooler. When the FW-190 showed up quite unexpectedly they hung the two stage Merlin on a Spit V airframe and produced the Spitfire IX, an airplane that many German fighter pilots thought was never equalled by the Luftwaffe.

Not sure that the appearance, in very small numbers, was the reason that the Merlin 60 was being developed, nor the reason for the Wellington VI.

Hooker only tested the concept of the 2 stage supercharger with an impeller from the Vulture.

The first production versions of the 60 series engine had 11.5 inch impellers, while the Vulture's was 12 inches. Later 60 series Merlins had a 12 inch first stage impeller, but not the Vulture's.

Lord Hives suggested putting the Merlin 60 in a Spitfire very early on in development. The first Spitfire equipped with the 2 stage engine was the Mk III. This first flew in September 1941, after several months' conversion work.

This would be the basis of the Spitfire Mk VII/VIII.

The appearance of the Fw 190 pushed them into adapting the 60 series Merlin to the Mk V as an interim measure, since the VII/VIII were some way off production.

The British only came up with very high altitude bomber and interceptor after the Germans embarrassed them into pursuing it. The fact that this effort was all but worthless to the Luftwaffe but sealed its doom with coming of the two stage Spitfires and Merlin Mustangs is downright delicious.

The requirement for the Wellington V and VI came before the Ju 86P started flying over Britain, in small numbers and only as a reconnaissance aircraft. The Ju 86P bombers would not appear over Britain until 1942.

In contrast the USAAF was working on incorporating turbosuperchargers from the day that General Electric hauled that experimental turbo hooked to a Liberty engine up the side of Pike's Peak and proved it would work.

Rolls-Royce had tested a turbo Condor in the 1920s, and concluded that it was not worth the effort.

Bristol had experimented with turbos in the 1930s as well.

The F4F and XP-37 were supposed to be high altitude fighters. The turbo in a P-36 airframe with a V-1710 engine worked, and boosted speed above 20,000 ft by 50 mph - but was rather grotesque and must have been terrible to fly.

The top speed of the YP-37 was 340mph at 10,000ft.

Not a great success as a high altitude aircraft.