XP-39 II - The Groundhog Day Thread

[Ivan1GFP]

I was just about to comment on the same thing. The confusion is between two stage and two SPEED superchargers.

 

[P-39 Expert]

from an old thread with some edits, edits are in italics.

1, no test report the XP-39 did fly 390mph.
2. reports of continued oil overheating and/or high temperatures causing at least one flight to be cut short.
3, At least one report of high coolant temperatures.
4, reports or accounts of modifications done to oil and radiator ducts. Which did not solve the problem
5, accounts of how the AAC wanted a new drive shaft installed in the plane because of vibration problems or worries if the engine misfired. Until new drive shaft was fitted engine rpm was restricted to either 2600or 2700rpm. The New drive shaft was not fitted until after the plane went to Langley. No full power flights?
6 time available.
a, first flight April 6th, 15/20 minutes.
b 2nd flight April 7th cut short due to oil temp
c. 3rd and 4th flights on April 22 total 47 minutes.
d 5th flight April 23, Nose wheel fails after manual lowering of landing gear, 1 hour 40 minutes total flying time in 5 flights.
E. Taking out the 1st flight and 3rd & 4th flights we have either 33 or 38 minutes to split between flights 2 and 5. While I have not seen how long flight 5 was accounts say they spent at least some time trying to get the landing gear down.
F. for the accounts that claim the XP-39 did 390mph on it's first flight, consider that the flight, depending on who wrote the account was either 15 or 20 minutes. In that short space of time our intrepid test pilot (James Taylor) would have had to take off, get the landing gear up, climb to 20,000ft, perform the speed run, descend (dive?) back down to ground level, lower flaps and landing gear, line up on the run way and land the plane. Taylor would have had to do this in plane that had never been flown before although taxi tests up to 80mph had been performed several months earlier.
G. Birch Mathew's book 'Cobra' says
"Taylor climbed into the diminutive interceptor, warmed the engine and lifted of uneventfully for a 20 minute flight on 6 April 1939. The only difficulty he experienced was an elevated oil temperature. The next day Taylor demonstrated the airplane for General Hap Arnold. He was forced to cut the flight short because once again oil temperatures shot up causing oil pressure to plummet."
No mention of a high speed run in this account.
e. accounts do not say what flying or tests were done in May of 1939. could have been some.
f. Plane is delivered to Langley on June 6th. After arrangements made at the end of April.
7, Langley claimed that the oil cooling problem was resolved or minimized during initial flight tests by using a higher drag duct than originally fitted.

So, if the XP-39 did do 390mph or anywhere near to it it had to be done in May, (repeating the above none of the April flights were long enough to get to 20,000ft, do a high speed run and get back down on the ground again.) with a high drag oil cooler duct, a radiator that had problems (like at 350mph the radiator was supposed to have 16,900 cubic ft of air going through but only 10,250 cu ft was needed to cool the engine) and the engine was limited to 90% or less of rated rpm, unless the pilot disobeyed instructions.

Agree with you and doubt the X-P39 ever made 390mph. Was possible on a 20 minute flight if the turbocharged engine actually generated 1150HP at 20000' without any of the temperatures redlining, but again I doubt it actually happened. Probably no calibrated test instruments (like on official govt/military tests), just the airspeed indicator reading then being converted to TAS after the flight.

The turbocharged XP-39 was a Rube Goldberg contraption that never could have been a combat plane. Historians say deleting the turbocharger was the biggest mistake in the P-39 program, but in reality the AAF made the right decision. It allowed the P-39 to be in production before the war started for the US. The biggest mistakes in the P-39 program were not controlling the weight (easily done) and not installing the two stage V1710-47 or -93 in a standard P-39D/F as early as possible.

 

[P-39 Expert]

This rather depends on almost suicidal test pilots.
What other aircraft in all of WW II established record setting climbs to altitude and did high speed runs in the first two hours of testing?

Let alone the first 20 minutes?

There is no record of the plane making that fantastic climb to 20,000ft. like on what day?

There was no 47 minute flight, two flights in one day equeled 47 minutes.
The plane was plagued with overheating issues at this time, mostly oil, and they spent 15 days trying to sort that out between flights 2 and 3? and yet we are supposed to believe that they hammered the crap out of the engine on the first flight to set all these marvelous performance records?

Bell was lying like 2020 politician (pick one from either/any side) back in 1939. The XP-39 was supposed to weigh 5550lbs gross, it didn't, it was weighed at Wright field at 6,104lbs. Unless they added over 500lbs to the useful load that means the Bell was off by 500lbs on the empty weight Bell had admitted before hand that the gross weight had climbed to 5,855lbs before the plane was completed.

NACA figured that the XP-39 was almost a flying parachute when they got it. The oil cooling problem had NOT been fixed and was causing even more drag than the original set up, the radiator was a disaster, the intercooler was only good for about 25% down to 12% of the cooling needed, and nobody has mentioned the fact that the engine was not allowed to run at full rpm/power on the days in question.

See Bell's lies to the British about performance and the special aircraft needed to even get to 3% under the contract performance to keep the contract going.

Larry didn't lie, he promised 400mph from a 6000lb plane. This was before the need for self sealing tanks and armor plate was known. Bell stopped promoting the 400mph figure as the P-400 production contract proceeded and weight gradually grew to 7850lbs.

 

[GregP]

Cme on, Tomo. You are deliberately misunderstanding my post.

The engine-driven supercharger, especially driven at a lower speed, wasn't good for much more than 15,000 feet, if that much. The population of non-turbocharged P-39s proves that without a doubt.

The turbocharger, although running effectively only when the waste gate was closed or partly-closed, was the high-altitude boost part of the system that was added onto the low-altitude engine-driven stage compression. We both know that.

By calling them the low-altitude and high-altitude stages, I very certainly didn't mean that only one stage was used at a time and they changed at some altitude. Noboldy in here thinks that.

 

[P-39 Expert]

Hello All.

There is no doubt that Larry Bell was a bit loose with the facts when advertising the new and wonderful P-39 to the British but there are a few other factors to consider that don't seem to get brought up in these discussions.

The test conditions under which Bell conducted tests and the British did were a bit different.
First of all, for British service, a snow screen was added to the supercharger intake.
This was an obstruction and caused a noticeable loss in performance.
For later US tests, the backfire screens were removed from the Allison engines. Were they still in place during the British tests?
Many US aircraft at the time used the practice of declaring a partial fuel and ammunition load as "standard" for normal loaded weight.
As an example, even though the P-39D could carry 120 Gallons of Fuel, 100 Gallons was considered the standard load.
Although the .50 cal guns in the nose could carry 270 rounds per gun, seldom did anyone load more than 200..... Except the Soviets.
The wing guns could carry up to 1000 rounds per gun but "standard" was only 300.

In reading through descriptions of the aircraft in the British tests, it seems that they did not get the instructions and loaded their aircraft to capacity.
Also, the substitution of a 20 mm cannon for the 37 mm sounds like a serious weight reduction but it really was not. The aircraft with the 20 mm carried two pieces of "cheek armour" that were not found in the 37 mm aircraft. This was probably also to help maintain proper CG but with everything else and with other equipment substitutions, the British aircraft were heavier.

Another odd thing is that for climb and maximum speed tests, the throttle settings were quite a bit lower than what the engine was capable of.

No doubt there was some additional Larry Bell "Magic" with testing aircraft with no armament or armour or very early versions but there were definitely a few more factors involved.

- Ivan.

P-39 weight was always confusing. Actually the early P-39D and the much later P-39Q-1 empty weights were about the same. AHT lists the D at 5523lbs and the Q-1 at 5680lbs but the Q-1 had the additional IFF radio that weighed about 120lbs. Empty weights of all the production P-39 models (D/F/K/L/M/N/Q) were about the same.

Loaded weights varied because of the differences in .30cal ammunition and armor plate, both of which were included in the "load" total and not the empty weight. .30cal ammunition boxes held 1000 rounds per gun but normal load was 300 rounds per gun, a difference of about 200lbs. That's why you sometimes see 7650lbs and 7850lbs quoted al loaded weight. The armor plate varied from about 265lbs on the P-400 to about 195lbs on the later N and Q models.

The British went a little crazy with armor on the P-400 by armoring the oxygen bottles etc. Compare that 265lbs to the contemporary P-40E with 111lbs of armor plate and glass. Deleting the 100lb nose armor that didn't protect anything and a few small pieces outside of the rear armored glass would get the armor plate/glass to a more reasonable 130lbs while still providing excellent protection.

 

[tomo pauk]

Cme on, Tomo. You are deliberately misunderstanding my post.

The engine-driven supercharger, especially driven at a lower speed, wasn't good for much more than 15,000 feet, if that much. The population of non-turbocharged P-39s proves that without a doubt.

The turbocharger, although running effectively only when the waste gate was closed or partly-closed, was the high-altitude boost part of the system that was added onto the low-altitude engine-driven stage compression. We both know that.

By calling them the low-altitude and high-altitude stages, I very certainly didn't mean that only one stage was used at a time and they changed at some altitude. Noboldy in here thinks that.

It might be far harder to misunderstand your post if you didn't invent the terms like 'low-altitude stage' or 'high-altitude stage'.

 

[P-39 Expert]

Cme on, Tomo. You are deliberately misunderstanding my post.

The engine-driven supercharger, especially driven at a lower speed, wasn't good for much more than 15,000 feet, if that much. The population of non-turbocharged P-39s proves that without a doubt.

The turbocharger, although running effectively only when the waste gate was closed or partly-closed, was the high-altitude boost part of the system that was added onto the low-altitude engine-driven stage compression. We both know that.

By calling them the low-altitude and high-altitude stages, I very certainly didn't mean that only one stage was used at a time and they changed at some altitude. Noboldy in here thinks that.

Just because the single stage Allison had a critical altitude of around 15000' didn't mean it stopped running above that altitude. The -85 in the P-39N was rated at 1125hp at 15500' (call it 1150hp @ 15000') but it had a service ceiling of 38500' and would outclimb pretty much everything in 1943 except a Spitfire IX. Service ceiling was about the same as the two stage Hellcat, Corsair and Thunderbolt in 1943.

 

[P-39 Expert]

Which other manufacturer was as desperate as Bell to get an order to avoid going bust? Which then extends to how desperate a test pilot might be to keep his job that he'd agree to make such a flight. And then you have bravado and just plain guts. During the second flight of the Hawker Typhoon prototype, chief test pilot Philip Lucas noticed the fuselage had started to suffer a structural failure, with a tear large enough to push his hand through opening in one side. Lucas stubbornly refused to bailout, the accepted procedure in such a situation, and saved the prototype. Whilst he was awarded the George Medal for his bravery, he admitted in later years that one possible cause of the failure was some flight manouveres "not in the official test plan".
The British would have rejected the whole order if Bell hadn't have been able to meet the "within 3% of 400mph" requirement. As I already mentioned, the British Direct Purchase Commission to the US weren't exactly the best and brightest, so I'm not surprised they signed some bad contracts. As it was, they were under pressure to buy just about anything. The P-40 snuck under the barrier because they arrived reasonably early and were judged suitable for the Middle East. By the time the P-400s arrived in the UK, we had loads of Spitfire Vs and the Hawker Typhoon was supposed to soon be sweeping the Luftwaffe from the sky. The Bf109F meant nothing without good altitude performance was going to be used as a fighter in the UK. The Desert requirement was already being met by the mediocre but trusted Tomahawk, and a "much-improved" Kittyhawk was being promised by an equally dishonest Curtis, leaving no role for the troublesome P-400s. Not surprisingly, the RAF jumped at the chance to offload the P-400s to the Russians.

I wouldn't be too hard on the P-400. While it wasn't going to go 400mph (the British made sure of that by adding all that weight) it would have had excellent performance for the time if equipped properly. AHT lists the empty weight as 5523lbs including radio. Load could have easily been reduced to 1520lbs (pilot 160, 20mm+2x.50calMG+ammo 430, armor plate/glass 130, fuel 720 for 120gal, and oil 71) for a loaded weight of 7046lbs. The 20mm cannon only held 60rounds so add another 60 rounds to get to 120 rounds total for only 32lbs more making loaded gross weight 7078lbs. A fully equipped warplane with self sealing fuel tanks, armor plate, cannon and heavy machine gun armament.

The contemporary P-39C was tested in July 1941 weighing 7075lbs with the exact same aerodynamics, engine and propeller (Mike Williams site). Same as the P-400. Compared to the contemporary Spitfire MarkV at 6450lbs:
Top speed P-39C was 379mph at 13000' MarkV 347mph at 13000'
Top speed P-39C 375mph at 20000' MarkV 375mph at 21000'
Climb P-39C 3720feet per minute at 12500' MarkV 3090fpm at 12500' Climb at 3000rpm for P-39C, 2850rpm for MarkV.
Climb P-39C 1900fpm at 25000' MarkV 1820fpm at 25000'
P-39C (P-400) had 20% more fuel than the MarkV.
Armament for the P-400 was a little lighter (1x20mm+2x.50cal vs 2x20mm+4x.30cal) but the P-400 was all centerline fire while MarkV was wing mounted converging fire.

P-400 wouldn't go 400mph but it was darn sure more than a match for the contemporary British Spitfire MarkV.

 

[wuzak]

The contemporary P-39C was tested in July 1941 weighing 7075lbs with the exact same aerodynamics, engine and propeller (Mike Williams site). Same as the P-400.

The Army discovered almost immediately that the P-39C was not combat ready, since it lacked armor and self-sealing tanks.

Bell P-39C Airacobra

 

[wuzak]

It would appear that the first 3 British Airacobras were delivered in P-39C spec, except for the 37mm being replaced by the 20mm.

These arrived in July 1941, and were the ones tested and found to be wanting.

EDIT: Corrected the year.

 

[Shortround6]

Larry didn't lie, he promised 400mph from a 6000lb plane.

Problem was that the XP-39 showed up at Wright field weighing 6,104lb gross..........................without guns.
He could not deliver an 6000lb plane with guns.

 

[P-39 Expert]

It would appear that the first 3 British Airacobras were delivered in P-39C spec, except for the 37mm being replaced by the 20mm.

These arrived in July 1943, and were the ones tested and found to be wanting.

Was using the govt/military test of the P-39C because it had the same engine, propeller, weight and drag as a properly equipped P-400 without all the extra weight. P-400 equipped properly had excellent performance for the time.

 

[P-39 Expert]

Problem was that the XP-39 showed up at Wright field weighing 6,104lb gross..........................without guns.
He could not deliver an 6000lb plane with guns.

Like I said, Bell didn't promote the P-400 as a 400mph airplane after production weight growth began.

 

[Ivan1GFP]

Hello P-39 Expert,

P-39 weight was always confusing. Actually the early P-39D and the much later P-39Q-1 empty weights were about the same. AHT lists the D at 5523lbs and the Q-1 at 5680lbs but the Q-1 had the additional IFF radio that weighed about 120lbs. Empty weights of all the production P-39 models (D/F/K/L/M/N/Q) were about the same.

The airframe of the production P-39 didn't really change all that much. Most of it was a matter of swapping engines and propellers which would change the empty weight a bit. The weight difference between the D and Q is most likely due to engine and propeller differences because later engines were heavier and propellers also got slightly bigger. The IFF radio you mentioned is a bit of a distractor because it would most likely be included in the "load" just as a regular radio and armament would.

Loaded weights varied because of the differences in .30cal ammunition and armor plate, both of which were included in the "load" total and not the empty weight. .30cal ammunition boxes held 1000 rounds per gun but normal load was 300 rounds per gun, a difference of about 200lbs. That's why you sometimes see 7650lbs and 7850lbs quoted al loaded weight. The armor plate varied from about 265lbs on the P-400 to about 195lbs on the later N and Q models.

There actually was a lot more differences than that to be found between different models and how each service used them.

The British went a little crazy with armor on the P-400 by armoring the oxygen bottles etc. Compare that 265lbs to the contemporary P-40E with 111lbs of armor plate and glass. Deleting the 100lb nose armor that didn't protect anything and a few small pieces outside of the rear armored glass would get the armor plate/glass to a more reasonable 130lbs while still providing excellent protection.

I don't remember how many times you have tried to push this same argument about deleting all the "unnecessary" armour in the nose of the Airacobra.
The problem with this idea is that the Airacobra in most versions ALREADY had a problem with the Center of Gravity migrating too far aft when the ammunition for the nose armament was expended.
Expending the .50 cal ammunition for the cowl guns seemed to cause the most problems and the total weight of all 400 rounds typically carried was only 120 pounds. The 37 mm cannon ammunition weighed 60 pounds but didn't have nearly as long a moment arm.
From pilot reports, aerobatics were not safe when these loads were expended and the CoG migrated aft.

NOW, you are suggesting removing 100 pounds of armour from the extreme nose of the aircraft. That would have an even longer moment arm and also cause CoG to shift aft. Considering how many different weight pieces of armour were fitted in the nose, my belief is that Bell used the armour as permanent ballast for the aircraft to bring the balance into proper range depending on installed equipment.
The extra pieces of cheek armour on the British Airacobras was the same type of thing.
When a 37 mm cannon is removed and replaced by a 20 mm, the missing 100-something pounds needs something to take its place to restore balance.

Some of the other pieces of armour on the Airacobra would not have been necessary on a more conventional design. The big piece of armour protecting the oil tank was in about as bad a place as possible for balance, but without it, the oil tank was exposed to gunfire from aft and not protected by any significant structure. Other pieces of the engine were also exposed but those were the consequences of a rear mounted engine.
The Soviets apparently thought the oil tank armour was so bad that they removed it for most of the spin tests.

There is no doubt the Airacobra carried more armament and armour than it should have for the amount of engine power that was available, but yanking out armour that looked useless wasn't going to help that much. It really needed a redesign which would move the CoG such as that proposed by the P-39E.

- Ivan.

 

[Shortround6]

I would note that the P-39 retained the fired .50 cal and 37mm casings, while 30 37mm casings may not have made that much difference 400 empty .50 cal casings would go about 840 grains (+/- 25 grains) each so about 48 lbs, weight of the fired primers is negligible (1 lb?) plus the weight of the links, also retained.

My own opinion is that the 100lbs of plate in front of the gear box did protect something, the Center of Gravity.

Further comments on P-39s after the XP-39 & XP-39B will be in a new thread.

P-39C-D & 400

 

[Ivan1GFP]

I would note that the P-39 retained the fired .50 cal and 37mm casings, while 30 37mm casings may not have made that much difference 400 empty .50 cal casings would go about 840 grains (+/- 25 grains) each so about 48 lbs, weight of the fired primers is negligible (1 lb?) plus the weight of the links, also retained.

Hello Shortround6,

This is interesting because I have heard that also, but have also had a P-39 owner describe the ejection ports for the nose guns. I wonder which is correct.

- Ivan.

 

[Shortround6]

can't find a picture of the ejection ports on P-39, doesn't mean they don't exist.
But I can find pictures of ejection ports on P-38s.

 

[fubar57]

First batch of P-39Cs, January 1941. In the foreground is s/n 40-2971. Looks pretty clean though I don't see any armament.

​

 

[P-39 Expert]

Hello Shortround6,

This is interesting because I have heard that also, but have also had a P-39 owner describe the ejection ports for the nose guns. I wonder which is correct.

- Ivan.

No ejection ports in the nose of any P-39. Spent shells were removed after each flight.

 

[P-39 Expert]

Hello P-39 Expert,



The airframe of the production P-39 didn't really change all that much. Most of it was a matter of swapping engines and propellers which would change the empty weight a bit. The weight difference between the D and Q is most likely due to engine and propeller differences because later engines were heavier and propellers also got slightly bigger. The IFF radio you mentioned is a bit of a distractor because it would most likely be included in the "load" just as a regular radio and armament would.



There actually was a lot more differences than that to be found between different models and how each service used them.



I don't remember how many times you have tried to push this same argument about deleting all the "unnecessary" armour in the nose of the Airacobra.
The problem with this idea is that the Airacobra in most versions ALREADY had a problem with the Center of Gravity migrating too far aft when the ammunition for the nose armament was expended.
Expending the .50 cal ammunition for the cowl guns seemed to cause the most problems and the total weight of all 400 rounds typically carried was only 120 pounds. The 37 mm cannon ammunition weighed 60 pounds but didn't have nearly as long a moment arm.
From pilot reports, aerobatics were not safe when these loads were expended and the CoG migrated aft.

NOW, you are suggesting removing 100 pounds of armour from the extreme nose of the aircraft. That would have an even longer moment arm and also cause CoG to shift aft. Considering how many different weight pieces of armour were fitted in the nose, my belief is that Bell used the armour as permanent ballast for the aircraft to bring the balance into proper range depending on installed equipment.
The extra pieces of cheek armour on the British Airacobras was the same type of thing.
When a 37 mm cannon is removed and replaced by a 20 mm, the missing 100-something pounds needs something to take its place to restore balance.

Some of the other pieces of armour on the Airacobra would not have been necessary on a more conventional design. The big piece of armour protecting the oil tank was in about as bad a place as possible for balance, but without it, the oil tank was exposed to gunfire from aft and not protected by any significant structure. Other pieces of the engine were also exposed but those were the consequences of a rear mounted engine.
The Soviets apparently thought the oil tank armour was so bad that they removed it for most of the spin tests.

There is no doubt the Airacobra carried more armament and armour than it should have for the amount of engine power that was available, but yanking out armour that looked useless wasn't going to help that much. It really needed a redesign which would move the CoG such as that proposed by the P-39E.

- Ivan.

SCR-535 IFF radio weighed 110lbs-130lbs and was included in the "Load" on P-39D-1/D-2 but included in empty weight of subsequent models K/L/M/N/Q. P-39D/F did not have IFF radio.

The 100lb nose armor plate was unnecessary since it protected the reduction gear which was not armored in any other AAF/Navy planes. P-39 already had armor plate directly in front the pilot mounted on the bulkhead separating the cockpit from the armament bay.

British clearly specified way too much armor plate/glass for the P-400. Total weight was 265lbs as compared to 111lbs for the contemporary P-40E.

The nose armor plate was present whether the nose cannon was 37mm or 20mm. The 37mm was 140lbs heavier than the 20mm so Bell obviously had the ability to adjust the COG.

The armor for the oil tank weighed only 29lbs and was absolutely necessary as it protected the oil tank and engine from the rear.

Regarding the P-39M Bell stated that the nose armor plate was not necessary for balance. The need to adjust weights for the COG was obviously a design necessity since from the beginning the P-39 was designed for larger heavier propellers (both 3 and 4 blade) and the possibility of the mechanical auxiliary stage supercharger located aft of the engine as used on the Allison -47 and -93.

The only factor that kept the early P-39 from being competitive with the contemporary SpitfireV and Me109F was excessive weight. This was easily corrected by deleting unnecessary armor plate (specified by the British) and the useless .30cal wing guns. The lighter P-39 had self sealing fuel tanks, armor plate/glass and cannon/heavy machine gun armament.