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There has been some discussion about how different in performance were the P-39Q and N. They did differ in armament, the two .30 cal. machine guns in each wing being replaced by a .50 cal. mounted in lower-wing gun pods. I checked wind tunnel testing concerning the extra drag of these gun pods in NACA report L5A30 to verify. NACA concluded that the CD of the airplane (in this case, a P-63) was increased by 0.0010, which amounted to a -6 mph change in level speed. So if we add 6 mph to the speed of a P-39Q we will pretty much have the speed of an average P-39N. But then again, those four missing .30 cal. machine guns had to reduce drag slightly as well. So the addition of 4-5 mph is a reasonable amount to add, but never more IMHO.
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Altitude SpeedMPH RPM b.h.p. Man.Pr.Hg. Oil Cooler ShutterPosition Prestone Shutter Position
*16,100 389.5 3000 1125 46.7 Flush 6 turns from W.O.
**9,700 398.5 3000 1420 59.8 Flush 6 turns from W.O.
2,700 358.0 3000 1330 57.0 Flush 6 turns from W.O.
30,100 353.0 3000 655 26.7 Flush 6 turns from W.O.
Airplane does not meet Air Corps cooling requirements at any of these powers.
On the test of the P-39N the report says the oil cooler shutters "flush" and Prestone sutter 6 turns from W.O. (as above) followed by "
Airplane does not meet Air Corps cooling requirements at any of these powers. " How is this allowed was a valid test?
It was a general question, it seems to me that cooling drag was reduced by not cooling the engine. Overheating on tests and in service seems to be a feature on the P-39.I see your point and it is well taken. I plan to create a speed chart of the N model using the P-39Q test results instead. There is no mention of cooling problems so the results are probably more reliable. I will add 6 mph to the speed at the various heights in order to recreate the actual speeds that a typical P-39N would have attained. Hopefully we will have a better picture of the P-39N's capabilities after that.
On the test of the P-39N the report says the oil cooler shutters "flush" and Prestone shutter 6 turns from W.O. (as above) followed by "
Airplane does not meet Air Corps cooling requirements at any of these powers. " How is this allowed was a valid test?
Sounds good, I would even by the 6mph just stop the argument
Sounds good, I would even go with -6mph to avoid the argument.
I would say that the increased weight will affect climb more than the drag will affect speed.
I see your point and it is well taken. I plan to create a speed chart of the N model using the P-39Q test results instead. There is no mention of cooling problems so the results are probably more reliable. I will add 6 mph to the speed at the various heights in order to recreate the actual speeds that a typical P-39N would have attained. Hopefully we will have a better picture of the P-39N's capabilities after that.
Shutters were open in climb and closed in level flight, as a layman I would have thought the engine overheating is as invalid as using uncertified boost or revs on the test.
The test may have been valid (unless done on a highly polished, specially prepared aircraft).
It is the conclusions that may not be valid.
You don't know what is going to work and what won't until you test.
The P-39 seems to have had chronic cooling problems, get it sorted out for one level of power and the Allison (or the Army) upped the power rating/s the problems came back at the higher power levels.
A number of other planes went through the same thing. Spitfires got ever larger radiators/oil coolers.
In my opinion (from reading about 601 Squadron during those dark Airacobra days), what killed the aircraft for the RAF was chronic unserviceability, and they were unwilling to put in the mountain of effort that was required to fix it - while the Soviet NII and VVS made a special case out of the P-39 and went all-out to turn it into a real fighter. No small feat - but they sure did it.
A great read here:
Early Cobra's in Soviet Aviation
The first party of 20 aircraft (all series AH, from 599 to 677) arrived in 22d ZAP in the period between late December 1941 and early January 1942. Their arrival was taken very seriously. The Scientific Research Institute (NII) of the VVS sent a group of specialists to the regiment, from which was formed a separate team (22d ZAP order no. 7 of 2 January 1942).
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The assembly of the first party of aircraft was begun in mid-January. This author does not know the specific site of the assembly and test flight of the first Airacobra. The literature only makes reference to a large airfield on the outskirts of the town. Drawing from the basing of 22 ZAP, the site was one of three airfields: Kineshma, Ivanovo-South, or Ivanovo-North.
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Typical deficiencies for foreign aircraft surfaced from the first days of its use: reduced outflow of oil from the lubrication system, the remnants of which quickly froze in the cold weather. The technicians quickly had to rework the lubrication system by means of installing additional oil taps [as in "faucets"] and also to create a special collector for the simultaneous induction of hot air into the crankcase, reduction box, radiator, and other areas that required warming before starting the engine. This collector successfully withstood testing and was recommended for implementation in the employment of the airplane.
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After completion of testing, approximately in April 1942, the Airacobra I fighter began to be issued to combat units. The procedure for re-equipping a unit with new aircraft was standard. A fighter air regiment that had suffered losses at the front turned over its remaining serviceable aircraft to neighboring units. The personnel of this regiment were sent to the 22d ZAP. Here over the course of 1-2 months the regiment was re-trained, reconstituted in personnel up to TOE [table of organization and equipment] level, issued new equipment, and then returned to the front. Losses in aircraft of the new type were also made up by deliveries from the same 22d ZAP.
It was fast. Properly stripped down, the P-39 was faster than a Bf 109E at low altitude and arguably, maybe even Bf 109F. No Soviet planes could match this down low (the MiG 3 could at high altitude only but that really didn't matter in combat along the Russian Front).
*snip*
I think ceiling and range were the two biggest performance issues preventing the P-39 from doing well in the Pacific. i suspect the 'twitchyness' and training were the major issues in the Med.
*snip*
S
I would say that the increased weight will affect climb more than the drag will affect speed.
You can find the weights on the P-39Q and P-39K/L weight charts.
Basically it was 99lbs for four .30 cal guns and 150lb for two .50s.
Weight of ammo was 262lbs for the K/L (300rpg for the .30s) and 370lbs for the Q.
I beg to differ on the twitchiness angle. Twitchiness is usually a low speed/high AOA phenomenon, unless workmanship or shoddy materials has weakened flight controls. The Soviets praised the high speed controllability of the 'Cobra and used tactics that took advantage of it. The twitchiness comes into play when you are foolish enough to get into a high G, energy draining knife fight with a Zero at tropical air densities and altitudes where your aircraft is running out of steam. This also is where dealing with the three separate trajectories of your gun package is at its most vexing.I suspect that the 'twitchyness' affected quite a bit P-39 as a gun platform. That, in pair with the lousy armament, put P-39 clearly at a disadvange in a fighter to fighter combat, unavoidable in ETO and PTO, while in RTO, in which the Luftwaffe lost the vast majority of his transport force, Ju 52 and He 111, flying relatively low and with small escorts, could have been easy game.
Could you explain further what you mean by "operational"? Both the P-47D and F4U-1A were operational by this time. What am I missing here? Also, you did asked people to "help you out".
I generally do agree with you but I suspect that the 'twitchyness' affected quite a bit P-39 as a gun platform. That, in pair with the lousy armament, put P-39 clearly at a disadvange in a fighter to fighter combat, unavoidable in ETO and PTO, while in RTO, in which the Luftwaffe lost the vast majority of his transport force, Ju 52 and He 111, flying relatively low and with small escorts, could have been easy game. Luftwaffe had to keep a difficult balance between fighters defending German towns in ETO and fighters escorting transports in RTO. End 1942, with P-39 in numbers, Stalingrad.