The P-39's turbo-charger?

I've seen it debated that it couldn't be included in the production aircraft because of it's over-heating . What are your opinions? Any good sources on the matter?

 

Interesting topic, I have not come across any sources that explain why exactly it was not used except simply that the equipment was banned from export to the UK. The lack of this turbo-supercharger did hamper the P-39's performance heavily, causing it to have poor performance over 15,000 ft.

 

Well as I understand it the fighter was capable of 390 MPH with a climb rate of around 4,0000 FPM at a normal fighter wieght. That's pretty impressive for 1939-1940. I'm sure alot of later fighters would have had their hands full with that performance with turbo. Be real nice to find some good hard info.

 

Here is an interesting section on the elimination of the turbosupercharger in the Airacobra:

The initial XP-39 tests went quite well, and the only problem that was encountered being some engine overheating difficulties. At first, it was thought that the overheating problems might be due to bad ventilation, and the left-hand supercharger and the right-hand radiator intakes and exhausts were both enlarged. However, this did not cure the problem, and it was found later that the problem was easily cured by a simple change in the structure of the oil system. With this change, the XP-39 was accepted for production with an initial order for twelve service-test YP-39s (Bell Model 12) and one YP-39A in April 1939. Serials of the YP-39s were 40-027/038. The YP-39A (40-039) was to have been powered by a high-altitude V-1710-31 engine of 1150 hp.

In the meantime, the XP-39 underwent a series of full-scale wind-tunnel tests in NACA's wind tunnel at Langley Field, Virginia. After the tests, the XP-39 was returned to Buffalo for revisions. The rebuilt XP-39 emerged as the XP-39B. Most of the changes were improvements in the streamlining of the airframe. The cockpit canopy was changed to a longer and lower shape. Changes were made to the wheel doors. The oil cooler and radiator intakes were moved from the fuselage right side to the wing roots. The wing span was decreased from 35 feet 10 inches to 34 feet, and length was increased from 28 feet 8 inches to 29 feet 9 inches.

The most serious change, however, was the elimination of the turbosupercharger, and its replacement by a single-stage geared supercharger. This change was a result of a shift in philosophy on the part of the USAAC. The USAAC believed that the widths of the Atlantic and Pacific Oceans made the USA virtually immune from high-altitude attack by enemy bombers. Therefore, the development of high-altitude interceptors was curtailed in favor of strike fighters optimized for low-level close support. The 1150 hp V-1710-17 (E2) of the XP-39 was replaced by a V-1710-37 (E5) engine rated 1090 hp at an altitude of 13,300 feet. The carburetor air intake was mounted in a dorsal position just behind the cockpit, where it was to remain throughout the Airacobra production run. (emphasis mine)

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 removal of the turbosupercharger was to have fateful consequences for the future of the Airacobra. Although the Allison engine was more reliable and more easily service when the turbosupercharger was eliminated, the engine only performed well at low and medium altitudes and lost power quite rapidly at altitudes over 15,000 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. However, there was an increase in low-altitude maneuverability because of the reduced wing span, and the decrease in low-altitude performance was only marginal.
　
See:

Bell XP-39 Airacobra

and here is the link to Joe Baugher's excellent site on all the P-39s:

http://home.att.net/~jbaugher1/p39.html

 

The question is why didn't the USAAF just have Bell reinstate the turbocharger on the aircraft? If it came off, it could be put back.

I suspect the answer to this lies in other issues with the P-39, the biggest being its relatively short range. Another would likely be its rather light and mixed armament. The standard models had the 37mm, two .50 and four .30 "paint chippers" giving it a decidely light amount of firepower. The 37 proved problematic in service and was frequently replaced by a Hispano 20mm instead (the P-400). This would frequently leave a P-39 with just two functioning .50 machineguns as effective armament.

But, the P-39 did give good service as a fighter in many areas that were backwaters to the war. Panama, Iceland, the Aleutians, many island atolls on the shipping routes to Australia, etc. As a frontline fighter for the USAAF it just didn't have the range and altitude capacity necessary to fight the kind of war that service was engaged in.

On the other hand, in the hands of the Soviets performing as it was designed to, as a low altitude support aircraft, it proved exceptionally good. I think that mitigates alot of the negatives that were heaped on it during and after the war.

 

I agree with you in the hands of the soviets it became one of the biggest tank killers of German armor in the war. I think the problem lied in its design and as you said range. When it was in service with Britain it was considered hard to service by the Brits. I believe this along with newer fighters in development spelled doom for any future potential this fighter could of had. Also as stated before the armament was such a mix that this may have hurt it as well. I have no idea why Bell decided not to put the turbocharger back into service because they were being used on P-47's. unless they decided that the P-39 was obsolete to them and they were wanting to stay in the competition in fighter development.

 

I've always found the P-39 a factinating plane. For all it's faults I think it's advancements were far ahead of the curve. Tricycle landing gear, mid engine, full vbiew canopy(almost) and it was arguably the first plane to be build as a weapons system from the begining. I think the internal fuel capacity was 87 gallons so even with a 75 galllon drop tank it's not going too far from home. It worked well as a ground attack plane when it's base was just behind the front lines as in Russia, Guadalcannal, New Guinnea, Italy. Some Q's were sent to Tarawa as well after the island fell to the Marines. It was never going to fly accross the channel and seriously take on the Germans. The British took them on as a nimble, 400mph fighter (salesmanship) and flew one mission to France and the plane was withdrawn from service and returned to the US which in turn sent them to the Pacific were they flew as the P-400. One other problem it suffered from was the 3 different calibers of weaons. The 37mm cannon had a tendency to knock out the compass which was not a good thing in combat. When firing all weapons at the same time the differnt trajectories from the cannon, 50's and 30's took a lot of finesse from the pilot to even get some of the rounds on target let alone a twisting Zero. It's a tribute to the skilled pilots that got kills in this plane.

 

The .50's and the 37mm had similar trajectories. The .30 in US use were often removed simply because the "paint chippers" weren't worth the weight or effort to use. The 37mm had a bad tendency to jam after just one or two rounds. To recock it the pilot had to reach between his legs (no this isn't something dirty) bending over (so he can see where he's going), grab the cocking handle and then pull this with about 50 to 60 lbs of force up to about even with his ear. This would reset the 37 and allow it to fire. That wasn't going to happen in combat.....

 

The Soviets used P-39s mostly as fighters and thought well of them in that function.

 

Unfortunately these are all myths, and they caused major issues. Very few overall tank kills were with aircraft, in fact, only 1 in 20 German tanks were killed from the air. The thing is, by Russian standards the kill ratio of the P-39 was quite good, as their own weapons had abysmal kill rates.
When it came to British use, all aircraft of the type, single engine attack aricraft, performed terribly. There was nothing they could really do with them, they would send them over the channel and they would be cut up pretty badly. Only fighters could withstand the hostile airspace over the channel, at least during the day.

When it came to its guns, they were actually pretty good. Concentration of fire was important, and the 37 was highly effective.

Oh, and it handled very well.

I think rumors doomed this aircraft in US service more than anything else. It served very well in Russian service, there is really no reason it would not have worked in US service.

 

I recall in a book on Guadalcanal published in the 50's, the author recalled a story about the P39's that were on the Island. They were not wanted as they could not fight at altitude. The author also related how when the army asked manufacturer about a supercharger, Bell said "it's not necessary". Something along those lines. I could dig up the book if anyone's interested.
From Allied Fighters of WWII-Bill Gunston:" First flown as a company prototype in 1939. Unique in having a nosewheel-type landing gear and engine behind the pilot.Propeller driven by a long shaft under the pilots seat and reduction gear in the nose which also contains a big 37mm canon firing through the hub.....Britain purchased it in 1940 and June 1941 saw the first Airacobra arrive.No 601 Sqn did poorly with it and failed to keep the unusual aircraft serviceable, but the ASAAF used it in large numbers. 9,588 were built and were used with fair success in the Mediterranean and Far East. Some 5,000 being supplied to the Soviet Union, mainly through Iran. Succeeded in production in 1944 by the P-63 Kingcobra." Interestingly, it was used by the Italian Co-Belligerent AF.
The P 400 were ex RAF planes with the 20mm Hispano canon....There was a joke back then: the P400 - a P40 with a zero in it's tail.

 

There were P-39 and there were P-39.

The ones on GC were P-400, the UK export version with a different oxygen system, so they could not climb above 15k feet.

Then there were the early P-39 the USAAF used on NG and in the Solomons. They climbed like a P-38 until they reached 12k feet. At that altitude they could make a bit more than 360mph. A very good speed actually but once they climed higher engine power ans speed dropped like a stone.

The Russians got the P-39 N/Q. Not only did it have a more poweful engine but the top speed increased to 375mph at 14k feet and could be maintained up to 20k feet. Above that the speed dropped but slowly. The plane was also fully debugged, no jamming 37mm gun any more. Ohh, and it was never used as a tank killer. The 37mm gun had a very low MV, ok for killing planes with HE shells but not useful for firing AP. The idea of the P-39 having been a tank killer is the result of the wrong translation of the russian phrase for "air superiority mission". Translated word by word its: "coverage of ground forces". Someone unfamiliar with russian terminology thought that meant the same as CAS and the misunderstanding was borne.

With regard to the turbocharger, my sources say it just didn´t work right. That sounds reasonable, turbos were extremely complicated.

 

A little mechanic background talking here and a turbo that is powered from exhaust horsepower has several factors of heat problems in that it gains some heat from hot exhaust plus air compression going through the inlet also increases the intitial heat of combustion. Some times it is more efficient to give up "turbocharging" for just supercharging depending on the demands the engine makes and the ability to handle the heat so it is highly likely they did not give up total performance for whatever trade they made in not including the "turbochargers". I don't know anything about the particular engines however.

 

So the Russians used their Cobras as the Bell designers had intended?

BTW, mechanical supercharging was much more prevalent in WWII fighters than turbocharging

 

At least more so than the USAAF. In my opinion the P-39 was meant to be a point defence interceptor, heavy bomber interceptor to be specific. Yery small airframe, small fuel tanks but US standards, 37mm LV gun firing a powerful HE shell and a turbocharger. Looks like a bomber destroyer to me.

The single stage supercharger wasn´t nearly as good but it worked and was ok for 1940 when fighters had engines had critical altitudes around 15k feet.

 

The P38 already was the designated high altitude interceptor.

 

The Luftwaffe didn't have a high-altitude bomber in significant numbers, though they did produce a few aircraft long-range high altitude aircraft, the so-called Amerika Bombers (Me-264, Ju-290, Ta-400 and others). However these were produced in very limited numbers (there were about 65 Junkers Ju-290s, the rest were mostly prototypes; Amerika Bombers of all types amounted to about 73 machines) and those that actually flew were mainly used as maritime reconnaissance bombers over the Atlantic convoy routes, rather than as strategic bombers against Soviet cities and factories. The only significant high altitude threat in the Russian theatre of war was the He-177 (about 1169 produced from 1942 to 1945) however limited fuel and serious mechanical issues (one might say deadly mechanical issues) tended to keep these otherwise impressive bombers grounded. The overwhelming threat came from medium to low altitude tactical bombers like the He-111 and the Ju-88, and various close-support types like the Ju-87 and the Hs-129, which were designed to conform to the Luftwaffe's role as an integral part of the Wehrmacht. As a consequence of strategic decisions made before 1940 by the Reichsluftministerium and the Luftwaffe the Russians didn't face the kind of high altitude bombing that was inflicted on German by the Anglo-American bomber force. Therefore medium to low altitude interceptors like the P-39 were well-suited to the kind of air war the Russians had to fight.

 

The P-38 was good at high altitudes but not in 1940. The production of combat ready versions had just begun when the USA entered the war at the end of 41 and even they still had some bugs that weren't fully fixed until 43/44.

While the LW had no very long range bombers, the fact wasn't known in 1940. At that time the LW was vastly overestimated. FDR was still worried about trans Atlantic bombers in 41.

 

The Oldsmobile 37mm has worse anti-armour performance than the Browning .50 and they were never used as a tank killer by the Soviets (it was indeed designed specifically for bomber destroying prewar, anything more serious than an armoured car they just bounce off, sidenote it was the original intended armament for the P-38 too). The ShAP air forces needed escorts with a secondary fighter-bomber role which the Airacobra was good for since a big gun is the same thing as a bomb for that kind of work (so is rockets), if the Sturmoviks didn't get bounced they'd help clean up soft targets. They also used LaGG with a 23mm replacing the ShVAK 20mm (it has twice the warhead weight), but they were less available at first (but this was the reason the LaGG remained in production after the La-5 came out, which also did escort work for Sturmoviks but couldn't mount the 23mm and there was a delay on the B-20 it was supposed to have three of). This was for soft targets, but mostly to escort without having to assign IAP air force squadrons which were taken up in counter-air duties.

But the other thing about the Airacobra and lend lease aircraft in general was the routine fitment of good piloting equipment. Russians were only just beginning to fit simple single radios (unreliable, basic features and easily jammed) and the Airacobra had three good ones installed as standard, cockpit heating actually worked and navigational equipment was far superior (Lavochkin compasses were notoriously unreliable, as in visual navigation was used until the expense of radio navigational beacons stretching half the continent was made in 44). They were a cadillac of the skies compared to even the excellent La-5 and several aces infamously switched back to the airacobra after receiving better performing La-5FN in 1944 because the pilot equipment was still better, and they were getting good enough skill wise and had the sheer numbers to take up the performance slack in combat.

It was this luxurious emphasis which led to the airacobra being preferred by many aces, and a little reserved in how it was distributed. It's altitude limitations were no different to the Yak-1 or LaGG except they needed two gears to run out of puff at 15,000 feet so it was actually better on that score. But that didn't even matter so much because combat rarely exceeded that altitude and was commonly at 5-10,000 ft. since it was all revolving around close support and fighter-bomber interception over there. Some bomber interception went on, but it was never really particularly significant, germany never had a strategic air force and the one really good high alt fighter (the MiG, an amazing high alt performer in 1941), well it wound up being used as a fighter bomber and then went out of production so more Sturmoviks could be made. They just weren't needed.