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Thread: P-39 vs P-40

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    Senior Member gjs238's Avatar
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    P-39 vs P-40

    Why did the P-39 perform, or seem to perform, so much more poorly than the P-40 - particularly in the early war years?

    Both were V-1710 powered, sans turbocharger with single stage/single speed supercharger.

    I know the P-39 had short endurance.
    But what else?


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    Quote Originally Posted by gjs238 View Post
    Why did the P-39 perform, or seem to perform, so much more poorly than the P-40 - particularly in the early war years?
    I don't think it did
    the P-39 didn't get much of a look-in with western air forces but the Soviet Union employed it in numbers and with it Soviet pilots regularly mixed it up with the Luftwaffe. It fell short with later versions of the Bf109 and the arrival of the Fw190 but that was later, rather than earlier in the war. Soviet pilots were impressed with its low-level performance, manoeuvrability, heavy armament and structural strength.

    It's also worth noting that of the top 5 Soviet aces, P-39 drivers occupied slots 2, 3 and 4.

    It had handling problems, I think spinning was one of them. In its initial configuration, it had the altitude of the P-38 but curiously, Bell thought they could improve the type by shortening the wings, lengthening the fuselage and lowering the canopy profile; this impinged on the available space and supercharging fell back on a single-stage arrangement. This was quite unlike the P-40 which never had the altitude performance from the outset.

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    Senior Member davebender's Avatar
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    It had handling problems, I think spinning was one of them.
    Spinning is about the worst possible problem to have when you are flying at low altitude. I suspect some pilots were afraid to fly the P-39 at max ability for fear of a low altitude spin.

    Low endurance is a huge problem, as the Luftwaffe discovered during the Battle of Britain.

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    Quote Originally Posted by davebender View Post
    Spinning is about the worst possible problem to have when you are flying at low altitude.

    I suspect some pilots were afraid to fly the P-39 at max ability for fear of a low altitude spin.

    Low endurance is a huge problem, as the Luftwaffe discovered during the Battle of Britain.
    Never doubted it for a minute

    The Soviets seemed to get the best out of it

    Low endurance was a huge problem for the western Alllies until late 1944 but of course, by then the P-39 had been supplanted by better designs. It was not quite such a problem for the Soviets, who principally used it to:

    Protect ground units from enemy aircraft
    Escort bombers
    Suppress AAA in the area of bombers
    Reconnaissance
    Free hunt
    Attack soft targets (i.e. troops, convoys, supply dumps, railroads, airfields, barges or other small naval craft)
    Protect high-value friendly targets (i.e. bridges, amphibious landing forces, reserves, command and control, major cities, etc)


    rather than long-range escort duties or even just hopping across the Channel. That's not to say that range wasn't an issue, more is clearly better but if the Soviets were doing all of this then they were obviously locating all of their P-39 units just behind the action on the ground and largely negating any endurance issues.
    Last edited by Colin1; 06-21-2009 at 07:45 AM.

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    Senior Member gjs238's Avatar
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    Endurance:
    If I understand correctly, the plane was submitted as a response to Circular Proposal X-609 - didn't that contain any criteria for endurance?

    Turbocharger:
    Mounting the turbocharger close to the engine, as was done on the P-38 and bombers, makes for a clean, simple and compact package. The P-39 seemed to provide a great opportunity for this.
    I always read about how the mid-engine placement was ideal for the nose mounted cannon and a streamlined profile, but I can't help but feel that turbocharger placement was a great beneficiary of this layout.

    P-39 vs. P-40 statistics:
    Do we know how the P-39 compared to the P-40 statistically?
    - Early war years in Pacific theatre.
    - In Soviet Union.

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    Think the Soviets also took some of the armor out as well as dumped a few machine guns. Made it a lighter bird, closer to the original spec.

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    Quote Originally Posted by gjs238 View Post
    Turbocharger:
    Mounting the turbocharger close to the engine, as was done on the P-38 and bombers, makes for a clean, simple and compact package. The P-39 seemed to provide a great opportunity for this.
    I always read about how the mid-engine placement was ideal for the nose mounted cannon and a streamlined profile, but I can't help but feel that turbocharger placement was a great beneficiary of this layout
    The problem
    with turbocharger arrangements is the hefty volume of ducting to and from the engine and the double problem is where would you put it all in the P-39? It sat behind the pilot in the P-47 (a substantially bigger bird than the P-39) but with the engine behind the pilot in the P-39, it clearly can't go there; Bell needed to weigh up the pros and cons of their lower-profile canopy vs the choice of single or two-stage supercharging, it might have had a different history altogether with the western Allied airforces.

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    Senior Member gjs238's Avatar
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    Quote Originally Posted by Colin1 View Post
    The problem
    with turbocharger arrangements is the hefty volume of ducting to and from the engine and the double problem is where would you put it all in the P-39? It sat behind the pilot in the P-47 (a substantially bigger bird than the P-39) but with the engine behind the pilot in the P-39, it clearly can't go there; Bell needed to weigh up the pros and cons of their lower-profile canopy vs the choice of single or two-stage supercharging, it might have had a different history altogether with the western Allied airforces.
    The prototype XP-39 flew with the turbocharger and performed well.
    The P-38 mounted the turbo behind the engine in the nacelle nicely.
    Extensive ducting is not required.

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    Quote Originally Posted by gjs238 View Post
    The prototype XP-39 flew with the turbocharger and performed well.
    The P-38 mounted the turbo behind the engine in the nacelle nicely.
    Extensive ducting is not required.
    My apologies
    I was under the impression the XP-39 flew with two-stage supercharging

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    Senior Member Juha's Avatar
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    IIRC there was capacity promlem in turbocharger production, bombers needed it also. At least part of USAAF hierarcy saw the main function of US fighters as protection of USA beaches against invasition, so low altitude work, so no need to put turbo into all fighters.

    Juha

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    Senior Member gjs238's Avatar
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    Quote Originally Posted by Juha View Post
    IIRC there was capacity promlem in turbocharger production, bombers needed it also. At least part of USAAF hierarcy saw the main function of US fighters as protection of USA beaches against invasition, so low altitude work, so no need to put turbo into all fighters.

    Juha
    There were plenty of turbochargers for the P-38 & P-47, both of which reached operational status after the P-39.
    Decision seemed to be made by NACA for aerodynamic reasons.

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    Senior Member Juha's Avatar
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    You might well be right, but the fact that there was plenty of turbos available later doesn't prove in itself that the supply was adequate earlier when the decision was made. But as I wrote, you might well be right.

    Juha

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    Senior Member davebender's Avatar
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    Plenty of turbochargers for the P-38 & P-47

    The P-38 and P-47 cost twice as much as many other fighter aircraft. Which suggests to me that turbochargers and their associated ducting were very expensive. Not even the U.S. can afford to turbocharge the entire fighter force.

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    I got to talk to a pilot severial years ago who flew both the P39 and P40 in combat. For him the biggest differance was in reliability. He said that the some part of the P39 electical system failed on evey mission, and everything was electic. he said the P40 was ok and dependable. He had also flew P36s before the war and finished the war in a P51, which he considered vastly better then anything else.

  15. #15
    Senior Member gjs238's Avatar
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    Quote Originally Posted by davebender View Post
    The P-38 and P-47 cost twice as much as many other fighter aircraft. Which suggests to me that turbochargers and their associated ducting were very expensive. Not even the U.S. can afford to turbocharge the entire fighter force.
    Sure didn't stop them from turbocharging the bombers.
    I doubt this is why NACA removed the turbo from the XP-39.
    And the P-38 needed TWO turbos per plane.

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