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

[Ivan1GFP]

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?

Hello Wuzak,
I don't really know why the gun weights are different. I am GUESSING it has something to do with whether the gun weight includes the synchronizing mechanism or not. The same equipment must certainly be in place, but may be listed in another category. Note that there is "Miscellaneous Equipment" listed for the aircraft with the lighter cowl guns but not for the regular P-39D.
As for the pilot weight differences, it must be a diet thing....
Actually, I believe it is just an unrealistic weight allowance for the early US models of Airacobra. A typical American pilot with Parachute is probably going to weigh a bit more than 160 pounds. My Son is 17 and thin as a stick but at 124 pounds, I don't think he could come in under 160 pounds with flight suit, some basic equipment and a parachute.
This has been a subject of jokes I sometimes tell my friends who may not know better:
Germans must be heavier than Americans because their weight allowance for crew is 100 KG or 220 pounds rather than the typical 200 pounds for US crew. On the other hand, Japanese must be tiny because their weight allowance is only 75 KG.
Maybe the Airacobra was designed to be flown by Japanese?!? The cockpit seems kind of small with a typical US pilot.

Hello P-39 Expert,
I was under the impression that you believed that the P-39D could be brought down to a Normal Loaded weight of 7100 pounds.
That would be the implication if the British had knowingly increased the weight of what became the P-400 to reduce its performance.... But this doesn't seem to be the case if both the US spec P-39D and British spec P-400 are nearly identical in weight.
In comparing climb rates between A6M2 and P-39D, I believe we should be looking at Military Power because that is what would be used in an actual fight.
If your claim is that there is no real difference there, then there should have been no performance advantage at altitude for the A6M2 at all because neither aircraft would be using Military Power or equivalent until the fight began.
Such is not the case as shown by the test of A6M2 versus P-39D-1:

Climb from sea level to five-thousand (5,000) feet indicated. Take-off was accomplished in formation on signal. P-39D-1 was drawing 3000 RPM and seventy (70) inches manifold pressure. Engine started to detonate so manifold pressure was reduced to fifty-two (52) inches. P-39D-1 left the ground first and arrived at five-thousand feet just as Zero was passing four-thousand (4,000) feet indicated. Fifty-two (52) inches manifold pressure could be maintained to four-thousand-five-hundred (4,500) feet indicated. At five-thousand (5,000) feet indicated from a cruising speed of two-hundred-thirty (230) miles per hour indicated the P-39D-1 had a marked acceleration away from Zero. Climb from five-thousand (5,000) feet to ten-thousand (10,000) feet at the respective best climbing speeds, (thus eliminating zoom) P-39D-1 reached ten-thousand (10,000) feet indicated approximately six (6) seconds before Zero. At ten-thousand (10,000) feet indicated, from a cruising speed of two-hundred-twenty (220) miles per hour indicated, P-39D-1 still accelerated away from Zero rapidly. Climbing from ten-thousand (10,000) feet to fifteen-thousand (15,000) feet, both airplanes maintained equal rates of climb to twelve-thousand five-hundred (12,500) feet. Above this altitude the Zero walked away from the P-39D-1. At fifteen-thousand (15,000) feet indicated, from a cruising speed of two-hundred-ten (210) miles per hour indicated the P-39d-1 accelerated away from the Zero slowly.

Climb from fifteen-thousand (15,000) feet indicated to twenty-thousand (20,000) feet indicated the Zero took immediate advantage and walked away from P-39D-1. At twenty-thousand (20,000) feet indicated at a cruising speed of two-hundred (200) miles per hour indicated, and from a starting signal for acceleration, the Zero momentarily accelerated away from P-39D-1. It took P-39D-1 thirty (30) seconds to catch up and go by Zero.

Climb from twenty-thousand (20,000) feet to twenty-five thousand (25,000) feet was not completed as P-39D-1 was running low on gasoline.

Climb from sea level to twenty-five thousand (25,000) feet indicated. Take-off was accomplished in formation on signal, P-39D-1 left the ground with 3000 RPM and 55 inches manifold pressure. P-39D-1 maintained advantage of climb for take-off to fourteen-thousand eight-hundred (14,800) feet indicated. Above this altitude P-39D-1 was left behind reaching twenty-five-thousand (25,000) feet indicated approximately five (5) minutes behind Zero. At twenty-five-thousand (25,000) feet indicated from a cruising speed of one-hundred-eighty (180) miles per hour indicated Zero accelerated away from P-39D-1 for three (3) ship lengths. This lead was maintained by the Zero for one (1) minute and thirty (30) seconds and it took the P-39D-1 thirty (30) more seconds to gain a lead of (1) ship length.


Hello Glider,
The speed issue is the one that comes up the most. Of the aircraft you listed, the only one that was slower that was still being produced at the time would have been the Hurricane. The others were already out of production and the Kittyhawk was a bit faster at least in the way the RAF was using it.
As for the angle of climb,
the P-39D was achieving its best climb at about 160 MPH
Later models of P-39 typically climbed at 170 MPH.
I can't find the best climbing speed of A6M2, but for A6M3-32, it was 146 MPH

- Ivan.

 

[Shortround6]

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?

The guns themselves are the same weight, what may be included in the gun weight may have changed. Cocking/charging mechanism. Firing system. Synchronizer? gun heater and so on.

The P-400's pilot is a real world pilot instead of a fantasy world pilot. Not only does the pilot weight include a 20lb parachute,it includes all the "flying togs" a pilot would wear, including life vest. The only way you are going to get a 160lb dressed/equipped pilot is if you start with 120-130lb naked (or skivvie clad) pilots.

 

[Ivan1GFP]

Hello Shortround6,
Let's see if I understand this correctly....
The 1150 HP Allison engine did not pass its type test until January 1941.
This is the SAME month as the initial delivery of P-39C aircraft with this engine that is not yet in production.
A month later, the P-39D deliveries begin with the same engine which is or isn't in production????

Is this a case of fighters that can't be flown at full power and have ballast in place of guns?

- Ivan.

 

[Shortround6]

The P-39 1150hp engine did not pass it's type test until Jan 9th, The F type engine used in the P-40D&E didn't start it's model test until Feb 17th 1941.

The 1150hp E series (Airacobra) engine was in production at the very end of 1940. Apparently the US army allowed production to start before a successful type or model test. This was a gamble for both the Army and the manufacturer. remember that Allison had to bring back 228 of the long nose engines and rework them at Allison's expense (16 or 18 still not reworked in the middle of March 1941) because production and deliveries started before a successful type/model test.

The Army had contracted for 5 F series 1150 hp engines (short nose P-40) back in Jan of 1940 to expedite aircraft testing. One engine went to North American for the NA-73X, one went to Curtiss for the P-40D, two others went to Curtiss for the XP-46 and XP-46A. the fifth engine was supposed to be used for the model test but the engine that completed the model test had a different serial number. The model test was delayed so they could incorporate changes in the crankcases and the connecting rods that other test engines had shown to be needed.

Allison did an amazing job going from a small shop to a major manufacturer but it was a number of years behind the Merlin and they were still sorting the engine/s out while undertaking massive factory expansion and making hundreds of engines per month.

 

[P-39 Expert]

Hello Wuzak,
I don't really know why the gun weights are different. I am GUESSING it has something to do with whether the gun weight includes the synchronizing mechanism or not. The same equipment must certainly be in place, but may be listed in another category. Note that there is "Miscellaneous Equipment" listed for the aircraft with the lighter cowl guns but not for the regular P-39D.
As for the pilot weight differences, it must be a diet thing....
Actually, I believe it is just an unrealistic weight allowance for the early US models of Airacobra. A typical American pilot with Parachute is probably going to weigh a bit more than 160 pounds. My Son is 17 and thin as a stick but at 124 pounds, I don't think he could come in under 160 pounds with flight suit, some basic equipment and a parachute.
This has been a subject of jokes I sometimes tell my friends who may not know better:
Germans must be heavier than Americans because their weight allowance for crew is 100 KG or 220 pounds rather than the typical 200 pounds for US crew. On the other hand, Japanese must be tiny because their weight allowance is only 75 KG.
Maybe the Airacobra was designed to be flown by Japanese?!? The cockpit seems kind of small with a typical US pilot.

Hello P-39 Expert,
I was under the impression that you believed that the P-39D could be brought down to a Normal Loaded weight of 7100 pounds.
That would be the implication if the British had knowingly increased the weight of what became the P-400 to reduce its performance.... But this doesn't seem to be the case if both the US spec P-39D and British spec P-400 are nearly identical in weight.
In comparing climb rates between A6M2 and P-39D, I believe we should be looking at Military Power because that is what would be used in an actual fight.
If your claim is that there is no real difference there, then there should have been no performance advantage at altitude for the A6M2 at all because neither aircraft would be using Military Power or equivalent until the fight began.
Such is not the case as shown by the test of A6M2 versus P-39D-1:

Climb from sea level to five-thousand (5,000) feet indicated. Take-off was accomplished in formation on signal. P-39D-1 was drawing 3000 RPM and seventy (70) inches manifold pressure. Engine started to detonate so manifold pressure was reduced to fifty-two (52) inches. P-39D-1 left the ground first and arrived at five-thousand feet just as Zero was passing four-thousand (4,000) feet indicated. Fifty-two (52) inches manifold pressure could be maintained to four-thousand-five-hundred (4,500) feet indicated. At five-thousand (5,000) feet indicated from a cruising speed of two-hundred-thirty (230) miles per hour indicated the P-39D-1 had a marked acceleration away from Zero. Climb from five-thousand (5,000) feet to ten-thousand (10,000) feet at the respective best climbing speeds, (thus eliminating zoom) P-39D-1 reached ten-thousand (10,000) feet indicated approximately six (6) seconds before Zero. At ten-thousand (10,000) feet indicated, from a cruising speed of two-hundred-twenty (220) miles per hour indicated, P-39D-1 still accelerated away from Zero rapidly. Climbing from ten-thousand (10,000) feet to fifteen-thousand (15,000) feet, both airplanes maintained equal rates of climb to twelve-thousand five-hundred (12,500) feet. Above this altitude the Zero walked away from the P-39D-1. At fifteen-thousand (15,000) feet indicated, from a cruising speed of two-hundred-ten (210) miles per hour indicated the P-39d-1 accelerated away from the Zero slowly.

Climb from fifteen-thousand (15,000) feet indicated to twenty-thousand (20,000) feet indicated the Zero took immediate advantage and walked away from P-39D-1. At twenty-thousand (20,000) feet indicated at a cruising speed of two-hundred (200) miles per hour indicated, and from a starting signal for acceleration, the Zero momentarily accelerated away from P-39D-1. It took P-39D-1 thirty (30) seconds to catch up and go by Zero.

Climb from twenty-thousand (20,000) feet to twenty-five thousand (25,000) feet was not completed as P-39D-1 was running low on gasoline.

Climb from sea level to twenty-five thousand (25,000) feet indicated. Take-off was accomplished in formation on signal, P-39D-1 left the ground with 3000 RPM and 55 inches manifold pressure. P-39D-1 maintained advantage of climb for take-off to fourteen-thousand eight-hundred (14,800) feet indicated. Above this altitude P-39D-1 was left behind reaching twenty-five-thousand (25,000) feet indicated approximately five (5) minutes behind Zero. At twenty-five-thousand (25,000) feet indicated from a cruising speed of one-hundred-eighty (180) miles per hour indicated Zero accelerated away from P-39D-1 for three (3) ship lengths. This lead was maintained by the Zero for one (1) minute and thirty (30) seconds and it took the P-39D-1 thirty (30) more seconds to gain a lead of (1) ship length.


Hello Glider,
The speed issue is the one that comes up the most. Of the aircraft you listed, the only one that was slower that was still being produced at the time would have been the Hurricane. The others were already out of production and the Kittyhawk was a bit faster at least in the way the RAF was using it.
As for the angle of climb,
the P-39D was achieving its best climb at about 160 MPH
Later models of P-39 typically climbed at 170 MPH.
I can't find the best climbing speed of A6M2, but for A6M3-32, it was 146 MPH

- Ivan.

I do believe the P-39D weight could be reduced to 7162#. 5462# empty, 1700# load (pilot 200, fuel 720, guns/ammo 580, armor plate/glass 130, oil 70).

In comparing climb rates, military power was used until the 5 minute limit was reached. In the "climb from sea level to 25000ft indicated" paragraph the "P-39D-1 maintained advantage in climb for take-off to 14800' indicated." Above this altitude the D-1 was left behind..."

The 14800' was when the Allison reached the 5 minute limit for military power (3000rpm) and had to reduce power to normal power 2600rpm. That's when the Zero climbed away. The 5 minute limit was increased to 15 minutes mid year 1942. If this engine had the 15 minute limit then the D-1 would have climbed with the Zero all the way up.

 

[Shortround6]

I do believe the P-39D weight could be reduced to 7162#. 5462# empty, 1700# load (pilot 200, fuel 720, guns/ammo 580, armor plate/glass 130, oil 70).

Somehow you have reduced the empty weight of the P-39 by several hundred pounds. The P-39D going 5523lbs, the P-400 -5550lbs the P-39D-2 was 5658, the the P-39K was also 5658 and the P-39L porked up to 5733. These weights for the later aircraft include radios, oxygen and de-icer fluid. The weights for the early aircraft do not include oxygen or radios or flares (night landing?)

The 14800' was when the Allison reached the 5 minute limit for military power (3000rpm) and had to reduce power to normal power 2600rpm. That's when the Zero climbed away. The 5 minute limit was increased to 15 minutes mid year 1942. If this engine had the 15 minute limit then the D-1 would have climbed with the Zero all the way up.

Military power for the Allison engine used in the D-1 aircraft in this test was achieved at 42in. NOT the 52in used in this test which would be WEP or as much as the engine could stand on the day of the test without detonating. The engine could not hold the 52in above 4500ft while climbing but the test makes no mention of the pressures used at the higher (over 5000ft) altitudes, either for the 5000ft increment climbs or the acceleration tests.

the 2nd test was a straight climb to 25,000 starting using 3in more manifold pressure than the previous test (55in) which is 13 in over military power.

While it was normal to throttle back in normal standardised testing after 5 minutes they sometimes exceeded time limits in combat testing or combat evaluations. This test does not say either way.
I don't know when the test flying was done but the date on the report is Dec 1942, which means the 15 time limit had been in place for several months. The attempt (failed due to detonation) to use 70in of MAP shows that they were NOT limiting this test to even recommended WEP settings if the engine would tolerate it as the WEP rating for this engine was 56in.

 

[P-39 Expert]

Somehow you have reduced the empty weight of the P-39 by several hundred pounds. The P-39D going 5523lbs, the P-400 -5550lbs the P-39D-2 was 5658, the the P-39K was also 5658 and the P-39L porked up to 5733. These weights for the later aircraft include radios, oxygen and de-icer fluid. The weights for the early aircraft do not include oxygen or radios or flares (night landing?)




Military power for the Allison engine used in the D-1 aircraft in this test was achieved at 42in. NOT the 52in used in this test which would be WEP or as much as the engine could stand on the day of the test without detonating. The engine could not hold the 52in above 4500ft while climbing but the test makes no mention of the pressures used at the higher (over 5000ft) altitudes, either for the 5000ft increment climbs or the acceleration tests.

the 2nd test was a straight climb to 25,000 starting using 3in more manifold pressure than the previous test (55in) which is 13 in over military power.

While it was normal to throttle back in normal standardised testing after 5 minutes they sometimes exceeded time limits in combat testing or combat evaluations. This test does not say either way.
I don't know when the test flying was done but the date on the report is Dec 1942, which means the 15 time limit had been in place for several months. The attempt (failed due to detonation) to use 70in of MAP shows that they were NOT limiting this test to even recommended WEP settings if the engine would tolerate it as the WEP rating for this engine was 56in.

I'v always seen 5462# as empty weight for the P-39D (not the heavier D-1 which was a virtual copy of the P-400 or the D-2). The P-39F at 5409# was the same as the D except for the hydromatic propeller which weighed 50# less than the Curtiss electric. The L was a little heavier than the K because it reverted temporarily to the heavier Curtiss electric propeller. These were the last models to use the Curtiss Electric propeller, the M, N and Q all used the hydromatic propeller.

 

[Shortround6]

I don't why or when but different "items" wound up in different categories at different times. Like at some point on P-40s the armor shifted from armament provisions (which is separate from the guns or ammo) to it's own catagory.

It may also depend on where (what publication or document) you are taking your empty weights from.
From a design standpoint the radio/s are part of the payload. They are not part of the structure or the powerplant and aren't really needed for the airplane to get off the ground, Different Customers will have requirements for different radios.
However from a pilot's perspective (pilot's manual) the radio is part of the fixed equipment like the instrument panel. The Japanese and Russians may have removed some of theirs but I don't believe the US pilots did. The radios (at least the basic radio in the P-39, the IFF may be different) became part of the basic weight for the pilots.
The less stuff you have in the list of stuff on the weights and balance charts the simpler they are to use (less mistakes).

Once you have added certain things in, they are hard to take out. Like structure. A 7,000lb plane stressed for 8/12 Gs service/ultimate has what I would call figures of merit (they may have a real name) of 56,000/84,000. If you increase the gross weight of the plane to 8000lbs you either beef up the structure or accept G limits of 7/10.5 Gs service/ultimate. While they did relax the standard for the Mustang I am not so sure they would do it for the P-39 at an earlier date. Once you have tooled up to build the heavier structure then building light structure planes becomes nearly impossible. The Mustang required quite extensive revision to go to the H model. Curtiss never built a "light" P-40, they built strippers where they pulled out operational equipment and used lightweight accessory parts like Magnesium wheels and aluminium radiators. Now perhaps Bell could have resorted to the same things if the supply had been sufficient or need great enough.

I will note that according to one account the P-39 had been designed to use a fair amount of magnesium castings instead of aluminum and perhaps aluminum radiators.
However the state of the art in Magnesium casting and finishing (unprotected magnesium corrodes much quicker than aluminum) and the aluminium radiators not really being ready for volume production in 1938-39 (or even 1941) ended those avenues and that is part of the reason that the prototype was 10% overweight.
There is sometimes a fine line between designing a state of the art aircraft and one that is made out of unobtainium.
Bell had also gotten caught by the US enthusiasm for integral fuel tanks. Just seal up parts of the wing and voila, fuel storage using already existing structure, no weight wasted on seperate tanks. The Republic P-43, the Vought Kingfisher (not sure about wing tanks on the F4U), Brewster Buffalo and Consolidated PBY all started with integral tanks, there may be others.
When they had to go to self sealing tanks they had no existing tanks to take-out, help offset the weight of the new tanks.

 

[P-39 Expert]

I don't why or when but different "items" wound up in different categories at different times. Like at some point on P-40s the armor shifted from armament provisions (which is separate from the guns or ammo) to it's own catagory.

It may also depend on where (what publication or document) you are taking your empty weights from.
From a design standpoint the radio/s are part of the payload. They are not part of the structure or the powerplant and aren't really needed for the airplane to get off the ground, Different Customers will have requirements for different radios.
However from a pilot's perspective (pilot's manual) the radio is part of the fixed equipment like the instrument panel. The Japanese and Russians may have removed some of theirs but I don't believe the US pilots did. The radios (at least the basic radio in the P-39, the IFF may be different) became part of the basic weight for the pilots.
The less stuff you have in the list of stuff on the weights and balance charts the simpler they are to use (less mistakes).

Once you have added certain things in, they are hard to take out. Like structure. A 7,000lb plane stressed for 8/12 Gs service/ultimate has what I would call figures of merit (they may have a real name) of 56,000/84,000. If you increase the gross weight of the plane to 8000lbs you either beef up the structure or accept G limits of 7/10.5 Gs service/ultimate. While they did relax the standard for the Mustang I am not so sure they would do it for the P-39 at an earlier date. Once you have tooled up to build the heavier structure then building light structure planes becomes nearly impossible. The Mustang required quite extensive revision to go to the H model. Curtiss never built a "light" P-40, they built strippers where they pulled out operational equipment and used lightweight accessory parts like Magnesium wheels and aluminium radiators. Now perhaps Bell could have resorted to the same things if the supply had been sufficient or need great enough.

I will note that according to one account the P-39 had been designed to use a fair amount of magnesium castings instead of aluminum and perhaps aluminum radiators.
However the state of the art in Magnesium casting and finishing (unprotected magnesium corrodes much quicker than aluminum) and the aluminium radiators not really being ready for volume production in 1938-39 (or even 1941) ended those avenues and that is part of the reason that the prototype was 10% overweight.
There is sometimes a fine line between designing a state of the art aircraft and one that is made out of unobtainium.
Bell had also gotten caught by the US enthusiasm for integral fuel tanks. Just seal up parts of the wing and voila, fuel storage using already existing structure, no weight wasted on seperate tanks. The Republic P-43, the Vought Kingfisher (not sure about wing tanks on the F4U), Brewster Buffalo and Consolidated PBY all started with integral tanks, there may be others.
When they had to go to self sealing tanks they had no existing tanks to take-out, help offset the weight of the new tanks.

You are correct, the items included in Empty or Empty Equipped can be different for different planes, even comparing the Weight and Balance Chart from the pilot's manual. Radio equipment was normally included in the Empty category.

 

[Ivan1GFP]

I do believe the P-39D weight could be reduced to 7162#. 5462# empty, 1700# load (pilot 200, fuel 720, guns/ammo 580, armor plate/glass 130, oil 70).

In comparing climb rates, military power was used until the 5 minute limit was reached. In the "climb from sea level to 25000ft indicated" paragraph the "P-39D-1 maintained advantage in climb for take-off to 14800' indicated." Above this altitude the D-1 was left behind..."

The 14800' was when the Allison reached the 5 minute limit for military power (3000rpm) and had to reduce power to normal power 2600rpm. That's when the Zero climbed away. The 5 minute limit was increased to 15 minutes mid year 1942. If this engine had the 15 minute limit then the D-1 would have climbed with the Zero all the way up.

Hello P-39 Expert,
Your figure for the empty weight for the P-39D is 5462 pounds.
The number I found is 5523 pounds. I believe it is quite possible for BOTH numbers to be correct..... if you take them in context.
Note that for the items that are directly accounted for there are quite a few that are often seen In other weight breakdowns that are missing in the listing I found for the different Airacobra models:
1. Trapped Oil - Note that only Usable Oil is listed here in the disposable loads. There may also be other trapped fluids such as hydraulics for brakes and the like. I know the Airacobra is a mostly electric ship.
2. Engine Coolant
3. Oil supply for the Propeller Reduction Gear up front. Note that unlike most other aircraft, this one has its own oil system for this purpose. Driving this extra pump might also explain minor power differences between E series Allisons and corresponding F series.

In my opinion, IF this was the difference between your empty weight and mine, it appears to be rather small.
The most detailed manual seems to be the Russian version, so it might be worthwhile to go looking there for details.
You list the weight of P-39F as being 53 pounds less. Does this include any additional equipment and oil volume required to run a hydromatic type propeller?

You state that the P-39D-1 was heavier than a P-39D. My understanding of the differences is that the D-1 added a fin fillet that was retrofitted to most D airframes and the 37 mm cannon was replaced with a 20 mm Hispano. Other than that, they should be identical. There should have been more detail differences especially in armour between P-39D-1 and P-400.
The P-39D-2 obviously would weigh more because it had a different engine (V-1710-63 as compared to V-1710-35).

The test description I posted is obviously against the A6M2 captured in the Aleutians.
As Shortround6 already pointed out, the folks flying the P-39D-1 were pushing as hard as they could.
70 inches Hg was never authorized for ANY of these aircraft AND they were using it for Take-Off!
Even then, the test report notes that they could not sustain even War Emergency Power (52 inches Hg) above 4500 feet and considering what the climb rate would have been, it was most likely the supercharger running out of capacity and not a time limitation. This was pretty typical for the -35, -39, -63, and -73 engines.
Note that the A6M2 in this test also had an "Overboost" power setting but it was never used in any of the tests.
Another interesting detail is that the automatic mixture control on this aircraft was not working.
The starting speeds chosen for acceleration tests also suggest that the intent was to be fairly close to the maximum level speed of the A6M2 to make the American fighters look better.

- Ivan.

 

[P-39 Expert]

Hello P-39 Expert,
Your figure for the empty weight for the P-39D is 5462 pounds.
The number I found is 5523 pounds. I believe it is quite possible for BOTH numbers to be correct..... if you take them in context.
Note that for the items that are directly accounted for there are quite a few that are often seen In other weight breakdowns that are missing in the listing I found for the different Airacobra models:
1. Trapped Oil - Note that only Usable Oil is listed here in the disposable loads. There may also be other trapped fluids such as hydraulics for brakes and the like. I know the Airacobra is a mostly electric ship.
2. Engine Coolant
3. Oil supply for the Propeller Reduction Gear up front. Note that unlike most other aircraft, this one has its own oil system for this purpose. Driving this extra pump might also explain minor power differences between E series Allisons and corresponding F series.

In my opinion, IF this was the difference between your empty weight and mine, it appears to be rather small.
The most detailed manual seems to be the Russian version, so it might be worthwhile to go looking there for details.
You list the weight of P-39F as being 53 pounds less. Does this include any additional equipment and oil volume required to run a hydromatic type propeller?

You state that the P-39D-1 was heavier than a P-39D. My understanding of the differences is that the D-1 added a fin fillet that was retrofitted to most D airframes and the 37 mm cannon was replaced with a 20 mm Hispano. Other than that, they should be identical. There should have been more detail differences especially in armour between P-39D-1 and P-400.
The P-39D-2 obviously would weigh more because it had a different engine (V-1710-63 as compared to V-1710-35).

The test description I posted is obviously against the A6M2 captured in the Aleutians.
As Shortround6 already pointed out, the folks flying the P-39D-1 were pushing as hard as they could.
70 inches Hg was never authorized for ANY of these aircraft AND they were using it for Take-Off!
Even then, the test report notes that they could not sustain even War Emergency Power (52 inches Hg) above 4500 feet and considering what the climb rate would have been, it was most likely the supercharger running out of capacity and not a time limitation. This was pretty typical for the -35, -39, -63, and -73 engines.
Note that the A6M2 in this test also had an "Overboost" power setting but it was never used in any of the tests.
Another interesting detail is that the automatic mixture control on this aircraft was not working.
The starting speeds chosen for acceleration tests also suggest that the intent was to be fairly close to the maximum level speed of the A6M2 to make the American fighters look better.

- Ivan.

Thanks Ivan.

The D-1 was about 200# heavier than the D.

The F was about 50# lighter than the D because the only difference in the two planes was the new Hydromatic propeller on the F which was about 50# lighter than the Curtiss Electric propeller on the D.

Regarding the test with the Zero, in my opinion the reduction in climb rate was because of the required reduction from 3000rpm (takeoff and Military) to 2600rpm (normal or max continuous) at 5 minutes. By mid '42 this 5 minute restriction was increased to 15 minutes. This test used the heaviest early P-39 model (7850#), obviously the 5 minute restriction was still being observed, War Emergency Power was just being implemented on Allison engines at the time of this test, and it's not known if this D-1 still had backfire screens.

 

[Shortround6]

That is a strange concluslion to reach from reading the test report.

Climb from sea level to five-thousand (5,000) feet indicated. Take-off was accomplished in formation on signal. P-39D-1 was drawing 3000 RPM and seventy (70) inches manifold pressure. Engine started to detonate so manifold pressure was reduced to fifty-two (52) inches. P-39D-1 left the ground first and arrived at five-thousand feet just as Zero was passing four-thousand (4,000) feet indicated. Fifty-two (52) inches manifold pressure could be maintained to four-thousand-five-hundred (4,500) feet indicated. At five-thousand (5,000) feet indicated from a cruising speed of two-hundred-thirty (230) miles per hour indicated the P-39D-1 had a marked acceleration away from Zero. Climb from five-thousand (5,000) feet to ten-thousand (10,000) feet at the respective best climbing speeds, (thus eliminating zoom) P-39D-1 reached ten-thousand (10,000) feet indicated approximately six (6) seconds before Zero.

Now please note that they are using a manifold pressure well into WEP range (and in fact tried to exceed the WEP rating) on take-off.
The climb was interrupted at 5,000, which should have been reached in 2 minutes or less. The "book" rating for this engine was 56in at 4300ft. Perhaps this engine wasn't running quite right? they were testing at more than 59 degrees F? some other problem?
The planes then leveled off and assumed the pre-arranged "cruise" speed for the acceleration test, Engine would not be running at either WEP or Military power at this point (thus resetting the clock for the 5 minute rule?) Going to full throttle on signal to do the acceleration test, then throttling back to best climb speed before doing the 5,000 to 10,000ft climb (again should be done in under two minutes) so even at whatever pressure the engine would give them they have not run out the 5 minute clock before setting up for the acceleration test. Once again there is a brief period (or two) of less than full throttle as planes set up for the acceleration test and then as they slow down to best climb speed after the acceleration test. Process is repeated for the 10,000 to 15,000ft climbs and the 15,000ft to 20,000ft climbs.

In other tests it only takes a P-39D 2.8 minutes to go from 15,000ft to 20,000ft even at 2600rpm. So if they started at 15,000ft and were allowed to use full throttle ( because the engine had started full throttle after a cooling off period) you aren't going to come close to the 5 minute limit being a problem.

The ONLY part of this test were the 5 minute limit might have come into play was on the 2nd flight (after they refueled). where the P-39D took off using 55in of MPA ( one inch less that full WEP). The test does not say one way or the other. The test does not give the date/s of the flights, just the date of the report which is AFTER WEP being approved and AFTER the Military power rating being extended to 15 minutes. Military power on this engine was 42in.

Please note that a P-39K was rated (in the manual) as making it to 15,000ft in 5.9 minutes at 7400lbs and 6.3 minutes at 7800lbs and that is using 3000rpm all the way and on a 32 degree F day. Chart says to add 10% to the time to altitude for every 20 degrees F above freezing so even a 72 degree day could be 1 minute 15 seconds slower.

 

[Ivan1GFP]

Thanks Ivan.

The D-1 was about 200# heavier than the D.

The F was about 50# lighter than the D because the only difference in the two planes was the new Hydromatic propeller on the F which was about 50# lighter than the Curtiss Electric propeller on the D.

Regarding the test with the Zero, in my opinion the reduction in climb rate was because of the required reduction from 3000rpm (takeoff and Military) to 2600rpm (normal or max continuous) at 5 minutes. By mid '42 this 5 minute restriction was increased to 15 minutes. This test used the heaviest early P-39 model (7850#), obviously the 5 minute restriction was still being observed, War Emergency Power was just being implemented on Allison engines at the time of this test, and it's not known if this D-1 still had backfire screens.

Hello P-39 Expert,
Where was the extra 200 pounds added from the P-39D to P-39D-1?
Note that with substitution of a 20 mm HS 404 cannon in place of the 37 mm cannon, there was a loss of 111 pounds for the gun and about 28 pounds for ammunition so for the D-1 to be 200 pounds heavier, it needs to have about 340 pounds of additional equipment. The engine, other armament and armour should be the same between the two or do you know that not to be the case?

Regarding the test against Koga's Zero, note that the Airacobra crew made no pretense of following the limitations of the operating manual as can be seen by attempting to run at 70 inches Hg. They also were not running at Military power but instead were running at War Emergency until the supercharger was unable to maintain boost at 4500 feet.
There is no indication that the Airacobra was run at Max Continuous for these tests.

Note that historically, the Airacobra always seemed to have trouble doing intercepts above about 15,000 feet and its performance here seems to fit in pretty well with known history.

My belief is that the fact that the aircraft could even be flown at 70 inches Hg is a pretty good indication that neither snow screens nor backfire screens were installed. Note that the famous Allison memo points out that they did not believe it was possible to achieve 70 inches Hg without a significant ram AND over-revving the engine.

I do not believe there is any mention of a gross weight of the P-39D-1 used in the test and the fact that it was running low on fuel at one point during the test may be an indication that it was not carrying a full fuel load. (?) Fairly or unfairly, these folks were trying to make the Airacobra look good relative to the captured A6M2.

- Ivan.

 

[Shortround6]

I do not believe there is any mention of a gross weight of the P-39D-1 used in the test and the fact that it was running low on fuel at one point during the test may be an indication that it was not carrying a full fuel load. (?) Fairly or unfairly, these folks were trying to make the Airacobra look good relative to the captured A6M2.

That or trying to figure out how to beat the A6M2

 

[P-39 Expert]

Hello P-39 Expert,
Where was the extra 200 pounds added from the P-39D to P-39D-1?
Note that with substitution of a 20 mm HS 404 cannon in place of the 37 mm cannon, there was a loss of 111 pounds for the gun and about 28 pounds for ammunition so for the D-1 to be 200 pounds heavier, it needs to have about 340 pounds of additional equipment. The engine, other armament and armour should be the same between the two or do you know that not to be the case?

Regarding the test against Koga's Zero, note that the Airacobra crew made no pretense of following the limitations of the operating manual as can be seen by attempting to run at 70 inches Hg. They also were not running at Military power but instead were running at War Emergency until the supercharger was unable to maintain boost at 4500 feet.
There is no indication that the Airacobra was run at Max Continuous for these tests.

Note that historically, the Airacobra always seemed to have trouble doing intercepts above about 15,000 feet and its performance here seems to fit in pretty well with known history.

My belief is that the fact that the aircraft could even be flown at 70 inches Hg is a pretty good indication that neither snow screens nor backfire screens were installed. Note that the famous Allison memo points out that they did not believe it was possible to achieve 70 inches Hg without a significant ram AND over-revving the engine.

I do not believe there is any mention of a gross weight of the P-39D-1 used in the test and the fact that it was running low on fuel at one point during the test may be an indication that it was not carrying a full fuel load. (?) Fairly or unfairly, these folks were trying to make the Airacobra look good relative to the captured A6M2.

- Ivan.

If they were trying to make the Airacobra look good, why didn't they use a P-39N? It was just becoming available about the time of these tests, was faster at all altitudes (all P-39s were) and climbed much better at all altitudes.

 

[Peter Gunn]

Wait, from what I've read here, the P-39 could out climb the Allison powered Mustangs, was only ~10MPH slower and had almost the same range and if outfitted properly, could intercept bombers at 31,000 feet.

Yet somehow North American hoodwinked the Brits into buying as many Allison Mustangs as they could build (and would have kept buying them as long as NAA was building them) meanwhile, these same Brits conspired to add weight and put the kibosh on this wonderful little Bell fighter because... reasons.

Either the Brits have some serious 'splainin to do or Bell was just totally incompetent at politicking and greasing the correct palms while NAA was apparently a past master at same...

 

[Kevin J]

Wait, from what I've read here, the P-39 could out climb the Allison powered Mustangs, was only ~10MPH slower and had almost the same range and if outfitted properly, could intercept bombers at 31,000 feet.

Yet somehow North American hoodwinked the Brits into buying as many Allison Mustangs as they could build (and would have kept buying them as long as NAA was building them) meanwhile, these same Brits conspired to add weight and put the kibosh on this wonderful little Bell fighter because... reasons.

Either the Brits have some serious 'splainin to do or Bell was just totally incompetent at politicking and greasing the correct palms while NAA was apparently a past master at same...

This is simply not true. The Bell fighter had a list of issues to sort out and it simply lacked the range of the Allison Mustang. The Soviets took them, sorted out the problems and unlike the Americans operated it successfully until the end of the war. They didn't need it as a fighter with high altitude capability or range. Over half the Soviet top scoring aces flew it. As for the Americans, it simply lacked the range to be useful.

 

[Ivan1GFP]

If they were trying to make the Airacobra look good, why didn't they use a P-39N? It was just becoming available about the time of these tests, was faster at all altitudes (all P-39s were) and climbed much better at all altitudes.

Hello P-39 Expert,
My belief is that this report was intended to show how US fighters compared favorably to A6M2. To show how a P-39N performed better was not useful because that is not what was in service at the front lines at the time. This report is trying to convince folks flying the older P-39D that they have a competitive fighter.
If one does not notice the details of the test, the summary might be: The acceleration and speed of the Airacobra is better at all altitudes and decreasing slightly past 20,000 feet. The climb rate of Airacobra is superior at all altitudes below 15,000 to 20,000 feet and greatly superior near sea level.
If one looks at the details, one can see that the Airacobra is using War Emergency Power or even higher to achieve its low altitude performance while the A6M2 never uses more than "Military Power" with an engine that does not have functioning automatic mixture control and has some fit and finish problems and is (IMO) about 10 MPH slower than an operational example would be. On overboost, there should be an even greater speed difference.
If the intent had been to compare actual acceleration, they might have started the test runs at a much lower speed such as 150-175 MPH TAS and see how quickly the A6M2 pulls away.

Hello Shortround6,
These folks were trying hard to beat the A6M2 in these tests regardless of whether the tactics would work or not in the field.
The interesting thing is that although the P-39 is supposed to be substantially faster, it didn't seem to be so hard to catch according to combat reports. Perhaps it had something to do with much of its speed coming from the ram effect you mentioned earlier.

- Ivan.

 

[special ed]

If I may chime in, Saburo Sakai in his book describes just such a combat and reports how well flown the American and his P-39 were. The P-39 jock may have survived against a lesser experienced Japanese pilot.

 

[Shortround6]

This is simply not true.

I beleive it was sarcasm, doesn't need to be true literally.

The Bell fighter had a list of issues to sort out and it simply lacked the range of the Allison Mustang.

It lacked a few other things besides range, like speed. In some tests the Allison Mustang was 20mph faster than P-39 with an equivalent engine.
which means things like at 13,000ft and doing about 370mph the Mustang can still climb (slowly), the P-39 can't unless it slows down a bit.

The Soviets took them, sorted out the problems and unlike the Americans operated it successfully until the end of the war. They didn't need it as a fighter with high altitude capability or range.

Depends on what you mean by operated successfully. The Americans certainly "operated" P-39s during 1942/43/44 and in combat zones. They just didn't push them to very front as they had better planes available, in addition to different tactical needs. The Americans did not suffer any large number of operational losses(non combat) or planes losing their way (faulty compass) so a lot of the early operational problems were solved by the Americans also. Of the 9500 Airacobras built the Russians got about 1/2, yes the Americans used a fair number as advanced trainers but P-39s were not phased out of combat until Aug/Sept of 1944 in US combat units. They were being used mostly for ground support.