I have just been reading some parts of Whitney's Vees for Victory, and this has given me some more insight than previously.
Let's start at the beginning:
I've heard that it wasn't included in production aircraft because of over heating problems. Your Opinion? Any good sources?
According to Whitney a NACA report stated that at its climbing speed of 160mph the XP-39 had approximately 75% of the required cooling air flow over the radiator. At 350mph it was estimated that the cooling airflow was 50% more than required. In other words the engine was either not cooled enough or cooled too much.
The problem wasn't with overheating per se, rather the limited intercooling arrangements. It simply wasn't possible to cram a turbosupercharged V12 into a small aeroplane. You need to get up to P-47 size for all the ducting and intercoolers.
In terms of the intercooler (an air-air type) the airflow was found to be 1/3-1/4 of the air flow required. This would have meant that the Allison would have found it difficult to make the rated power due to detonation.
So, yes, cooling was a very large issue in the XP-39's performance.
The other main factor was drag.
NACA found that the ducting arrangements for engine cooling radiator, oil radiator and the intercooler were poor, and produced much more drag than was necessary.
The canopy was tall and produced extra drag.
The turbocharger was mounted beneath the engine, with the turbine wheel protruding in an attempt at providing some extra cooling to that critical component. This was found to produce significant drag. A similar system was used in the Curtiss X/YP-37. albeit with a different system of manifolding.
The engine manifold was rather simple - 4 collector pipes at the engine exhaust (3 cylinders each) with vertical pipes doen to a manifold or plenum which fed the turbine wheel.
Deleting the turbocharger limited the P-39s performance at altitude. However, that wasn't the real problem. The circa 20% increase in weight over the prototype was more problematic, generally destroying all around performance (some doubt has been cast over the validity of the XP-39 performance figures as well). The centre of gravity shifts didn't help the case either, making it rather dangerous.
In terms of weight the XP-39 wasn't a featherweight. In fact it was heavier than the production P-39s at over 6,000lbs (2722kg) when the AAC weighed it prior to testing. That is about 1000lb (454kg) heavier than the Spitfire Vb, the earlier Mk of Spitfire which were contemporary to the XP-39 were lighter. The P-39Q had a dry weight of 5350lb (2426kg).
NACA estimated the maximum performance to be 340mph @ 20,000ft with the engine at its rated power of 1150hp.
In response 'Hap' Arnold made, in 1939, several recommendations which included reverting to manually operated flaps, specified a critical altitude for the plane of 15,000ft (determined from observations of the European war) and streamlining the fuselage to suit a pilot of 5'8" and 160lbs!
NACA's wind tunnels were able to refine the design to improve performance, but their recommendations were to:
- Improve streamlining of wheel well doors
- Lower the Canopy to improve streamlining
- Remove the turbosupercharger due to high installation drag
- Relocate carburetor scoop from the left side of the fuselage to just behind the canopy
- Install the altitude rated engine
This is basically what was done for the production P-39. This both reduced airframe weight and improved performance.
The Allison engine was a propulsion system, not just an engine. When they deleted the turbocharger, it rendered the Allison a low to medium altitude engine package since the turbocharger was the primary element in performance above 15,000 feet.
Not strictly correct.
The turbocharger system was an AAC requirement. Allison desired to make altitude rated engines, but since the AAC were paying the bills they pretty much had to do what they were told.
By the time of the XP-39 Allison had built all of 15-20 engines, none of which had ever been run on the bench or in type testing with a turbocharger. The turbochargers were government furnished equipment to the airframe manufacturers, who would then design their own installation.
It would appear that Allison were never involved in designing engine installations for the aircraft. Instead they adapted their systems as required - engine mounted reduction, remote reduction gear with extension shaft, engine mounted reduction gear with prop extension shaft, and so on.
Later in the war Don Berlin was offered the chance to build a P-40 with the turbo system. It flew VERY well at 25,000 feet and was as fast as a Mustang. Yes, it was still obsolescent, but it had the high-altitude performance to make a difference. Ditto the P-39. When flown with the turbo power package it was designed with, it was a great fighter.
As far as I can tell the only turbocharged Allison powered models made by Curtiss were the X/YP-37s. These had some similar features to the XP-39, particularly in the turbocharger and intercooler mounting. The Turbocharger was fitted below the engine, with the turbine housing protruding into the airstream. The intercooler and radiators were all mounted behind the engine, fed by ducting in one side and out the other. All this cause considerable drag.
Both the XP-37 and XP-39 were fitted with the GE "Form 10" turbocharger. These had considerable reliability issues, with main bearing failure being a frequent occurence, and often presented a significant fire risk. Later on the XP-37 was fitted with a new turbo type with revised bearing arrangement and which was closer to the final production spec.
The YP-37s used the production spec GE B-2 turbocharger. There were still reliability issues with this unit, mainly with the control systems, sometimes leading to overspeeding turbines, which could potentially damage the engine, as well as a catastrophic turbine failure.
In the case of the P-40 and the P-39, if both designers had been apprised from the beginning taht no turbocharger would be installed, the designs would have been different from those that flew since there would be no need to incorporate the turbo system requirements into the aircraft. That would eliminate the need for air duct area and some heat protection, too.
Both companies built single aircraft with the turbo systems, but that's all they were allowed.
Don Berlin designed the P-40 around the altitude rated V-1710. There was no thought to do otherwise. The P-39 was, obviously, extensively redesigned around the altitude rated engine, and was the better for it.
The XP-46 was also designed around the altitude engine, the Xp-53 and XP-60 were intended to use different engine (the IV-1430 and Merlin respectively).
Anyway, after all these detours, I will simply say that the turbocharger installation in the Bell P-39 did not have any problems other than those generated in Washington, D.C. . All that decision did was to remove a potentially great fighter from the fray and repalce it with a decent low-to-medium altitude unit that nobody except the U.S.S.R wanted. Too bad it didn't get a chance and too bad it didn't get a decent cannon to go along with the turbocharger, too.
The turbo installation in the X/YP-37 and XP-39 was basically woeful. The turbo itself was seriously flawed, being totally redesigned, changining to a layout that has very much persisted to this day, and presented a fire risk and had a major chance of catastrophic failure.
It wasn't included by a stroke of the pen by the US War Material Board. The U.S.A. was short of Tungsten and the metal we had was allocated to the bombers and to one fighter, the Lockheed P-38 Lightning. After all, they needed at least ONE fighter to escort the bombers, didn't they?
It is interesting to note that continued turbocharger development in the USA may have been due to the development of the Allison V-1710. The heads of the USAAC were very much bomber boys in the '30s, and the power required for their high altitude bombers would have to be provided by liquid cooled engines equipped with turbochargers, the prevailing view being that air cooled types could not take the boost pressures required. In any case the situation was that Allison were to provide power for the XB-15, but they didn't have any engines ready at the time.
Both the P-39 and P-38 were designed as interceptor aircraft. That is they were expected to climb quickly to altitude to meet enemy aircraft relatively close to base. They were never envisioned as escort fighters. In fact, it wasn't until late 1943 that it finally dawned on some of the USAAF's 8th AF bomber command that escort fighters were absolutely necessary for the bombing campaign. Prior to that the powers-that-be belived that no persuit type was capable of escorting bombers over the ranges required whilst maintianing the performance and manoeuvrability of a persuit.
In addition to the P-39 (which used two turbos of the size that suit an Allison) there was also the P-47 which used larger turbos to suit the capacity of the R-2800.