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The XP-39 & pre-war turbo persuits....


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#1 Wuzak

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Posted 07 January 2010 - 01:46 PM

Since the other thread has been diverted largely to the tactics and roles the P-39 was used for by the Soviets I thought I would start a fresh thread to discuss the issue.

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.

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#2 GregP

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Posted 08 January 2010 - 03:28 AM

Hi Wuzak,

The P-39's original installation was not perfect and I don't know of one that was until they were well understood. It was also nothing that could not have been fixed, but a fix was not even tried because the turbo was deleted by the government.

Don Berlin DID build a turbo P-40 and was allowed to have it flown. It performed excellently.

I don't believe I even mentioned the YP-37, which was interesting, but could never have been a fighter. It was, in today's parlance, a "technology demonstrator."

The XP-39 had a 2-stage turbosupercharger on the port side of the central fuselage. It did not have two separate turbos, not matter what sources might say. The Army dictated the tricycle landing gear, not Bell.

After wind tunnel tests, there was some aerodynamic cleanup and the turbosupercharger was deleted and replaced by a single-stage, single speed supercharged engine. The weight grew considerably due to fitting both armament and armor, all without any other redesign or increase in wing area.

Bsically, Bell was screwed by the government. They have a habit of doing that to vendors of weapons, and the habit continues today. The government added so much to the Raptor that if balooned to 60,000 pounds! Good thing it has a LOT of engine thrust!

Bell had no other choice than Allison at the time because there was no other choice available in the U.S.A. The Merlin was in its infancy. Recall the XP-39 FLEW in 1939 ... so it was designed well before that time with an American engine already selected, and the choices for liquid-cooled American V-12's were limited to the Allison and perhaps an outdated Curtiss engine. Clearly the Allison was better, but it was also limited in power at the time, and the turbocharger was needed for altitude performance.

When it was deleted, all possibilities for altitude were eliminated.

#3 Red Admiral

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Posted 08 January 2010 - 09:39 AM

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).


Empty weight of the XP-39 was 3995lb which increased to 4530lbs with the XP-39B. YP-39B weight rose to 5042lb with the addition of armament. P-39C was the first production version with a weight of 5070lb but still no armour or self sealing tanks. The P-39D added self sealing tanks and some armour which pushed up weight to 5462lb. P-39E had a more powerful two stage V-1710 (1325hp) and revised wings but weight went up to 6936lb (?). It was fast at altitude but climbed like a heavy bomber.

So actually a 37% increase in empty weight with no increase in power or wing area. Its hardly a surprise that performance fell.

Saying that Bell was screwed over by the government is a bit much. You've got to design an aircraft with a margin for growth. To contrast with the above, the Spitfire's empty weight doubled between the prototype and the Seafire 47 with no real changes in handling and a constant power increase saw performance massively increase. Designing the P-39 without armament or armour or equipment is a failure on Bell's part.

The problem wasn't just the lack of the turbocharger (even avoiding the discussion that it didn't really fit and added a lot of drag) it was the increase in weight. The P-39E had a two stage engine producing more power and at a higher altitude along with the armour and armament. It still didn't exceed 400mph at altitude but took over 9 minutes to reach 20,000ft and handled like a brick. Power alone isn't going to solve the P-39's problems.

#4 Wuzak

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Posted 08 January 2010 - 01:03 PM

Hi Wuzak,

The P-39's original installation was not perfect and I don't know of one that was until they were well understood. It was also nothing that could not have been fixed, but a fix was not even tried because the turbo was deleted by the government.


Hi Greg,

The installation may have taken time and effort to fix, but the AAF wanted an aircraft pronto...


Don Berlin DID build a turbo P-40 and was allowed to have it flown. It performed excellently.

I don't believe I even mentioned the YP-37, which was interesting, but could never have been a fighter. It was, in today's parlance, a "technology demonstrator.".


Still not sure about the turbo P-40. Anybody know any more about this?

The X/YP-37 were the direct precusrsos to the P-40. The P-37 and P-40 were both basically a P-36 with the Allison installed. I think that the X/YP-37 is often confused as a turbocharged P-40.

In the P-37 what was the pilot's area in the P-36 was occupied by the radiator and air-to-air intercooler. The cockpit was, thus, moved further back to a position which many pilots found unacceptable (destroyed much of the pilots vision). The oil radiators were mounted in the nose, under the spinner.

Posted Image

In this side view you can see the turbo of the XP-39 sitting below the rear of the engine, just ahead of the leading edge. You can also see the left exhaust manifold, which is feeding the exhaust forward and down. The right side exhaust loops over the reduction gear housing (the long nose version) and joins the left side one on its way to the turbo.



The XP-39 had a 2-stage turbosupercharger on the port side of the central fuselage. It did not have two separate turbos, not matter what sources might say. The Army dictated the tricycle landing gear, not Bell.

After wind tunnel tests, there was some aerodynamic cleanup and the turbosupercharger was deleted and replaced by a single-stage, single speed supercharged engine. The weight grew considerably due to fitting both armament and armor, all without any other redesign or increase in wing area.


The XP-39 had a single single-stage turbocharger mounted underneath the fuselage and engine. The turbo was the Form 10 turbocharger, in which the intake air was drawn from the centre of the shaft (in an attempt to cool the central bearing) up to the compressor wheel. This caused the drive shaft to be long, which then caused frequent bearing failures.

Posted Image

In that picture you can just make out the turbo protruding directly below the engine. The scoop on the left side is to direct air across the intercooler. The slots in the leading edge of the wing are to direct air to the radiator and oil coolers.

NACA apparently did some work on improving the aerodynamics of the turbo installation, but still recomended the deletion of the turbo.

Quite silly of the government to require a warplane to have armour and armaments!



Bell had no other choice than Allison at the time because there was no other choice available in the U.S.A. The Merlin was in its infancy. Recall the XP-39 FLEW in 1939 ... so it was designed well before that time with an American engine already selected, and the choices for liquid-cooled American V-12's were limited to the Allison and perhaps an outdated Curtiss engine. Clearly the Allison was better, but it was also limited in power at the time, and the turbocharger was needed for altitude performance.


Merlins were in their infancy, but were in production. They still had a few issues, but by 1939 they were being fielded in Hurricanes and late in 1939 Spitfires.

While the US still had the official isolationist policies it was hard for the AAC to scrimp together a budget to cover such aircraft as the P-37, P-38, P-39 & P-40, as well as the earlier Bell X/YFM-1. Spending money on a foreign engine was not even a remote possibility.

Allison too was struggling. With all the devlopment work they had done on the V-1710 and the multiple different configuartions the army and airframe manufacturers had dreamt up they were losing money and had yet to be awarded a production contract.

The P-40 was a vital for Allison. Without it and the altitude rated engine Allison's production prospects were slim - and very dependent on GE's ability to make a reliable turbocharger. The contract for the P-39 followed.

#5 Wuzak

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Posted 08 January 2010 - 01:17 PM

Empty weight of the XP-39 was 3995lb which increased to 4530lbs with the XP-39B. YP-39B weight rose to 5042lb with the addition of armament. P-39C was the first production version with a weight of 5070lb but still no armour or self sealing tanks. The P-39D added self sealing tanks and some armour which pushed up weight to 5462lb. P-39E had a more powerful two stage V-1710 (1325hp) and revised wings but weight went up to 6936lb (?). It was fast at altitude but climbed like a heavy bomber.

So actually a 37% increase in empty weight with no increase in power or wing area. Its hardly a surprise that performance fell.

Saying that Bell was screwed over by the government is a bit much. You've got to design an aircraft with a margin for growth. To contrast with the above, the Spitfire's empty weight doubled between the prototype and the Seafire 47 with no real changes in handling and a constant power increase saw performance massively increase. Designing the P-39 without armament or armour or equipment is a failure on Bell's part.

The problem wasn't just the lack of the turbocharger (even avoiding the discussion that it didn't really fit and added a lot of drag) it was the increase in weight. The P-39E had a two stage engine producing more power and at a higher altitude along with the armour and armament. It still didn't exceed 400mph at altitude but took over 9 minutes to reach 20,000ft and handled like a brick. Power alone isn't going to solve the P-39's problems.


The P-39E ended up acting as a test aircarft for the P-63.

#6 Wuzak

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Posted 08 January 2010 - 01:47 PM

fwiw,

Graham White states in Allied Aircraft Pistone Engines of WWII that al B and C model turbochargers had single stage compressors and single stage turbines, including the B-5. Dan Whitney is one of his references on his chapter about turbochargers.

As I understand it there were some experimental two stage compressor turbochargers built late in the war, one such being used for teh experimental V-3420 installation in teh XB-19 and XB-39.

Also, Dan Whitney in Vees for Victory notes:

It is clear from recently located NACA test reports on the XP-39 that it was not meeting the contracted performance guarantees. While it has been reported that the aircraft was able to climb from takeoff to 20,000 feet in five minutes, and that the maximum speed at that altitude was 390 mph, with the airplane weighing 5,550 pounds, the data does not show it. Birch Matthews, aviation historian and author who has researched Bell Aircraft and the P-39 extensively, reports that he has never found a source document confirming this maximum speed. Furthermore, given that General Arnold was hurredly arranging to have the NACA put the airplane in its wind tunnel for drag reduction tests only a month after the first flight, suggests that all was not well.



#7 Wuzak

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Posted 08 January 2010 - 02:02 PM

Improvements in the P-39 turbocharger installation...

We can make a few suggestions with hind sight.

First thing I probably would consider is to mount the turbo in the fuselage where the intercooler was in the XP-39. Some additional ducting to keep the turbine cool may be necessary, but that could be achieved without much drag penalty, I believe. The turbine outlet and wastegate could then exhaust directly below the tail of the aircraft, providing some residual thrust.

The compressor outlet would pass through a water-to-air intercooler before going into the carburettor and the engine stage supercharger. The water-to-air intercooler would be similar to what was used on two stage Merlins and some two stage Allisons, and is chosen over the air-to-air type because it is more compact and allows for the turbo to be mounted quite close behind the engine. It also allows the heat to be removed to the radiator matrix, which is mounted below the engine and fed by leading edge ducts in the wing root. This cooling arrangement was quite neat for the P-39, having been much refined from the XP-39.


PS If the government deleted the turbo from the P-39 to save the resources for the bombers, then how did the P-38's two turbos get through?

#8 GregP

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Posted 09 January 2010 - 01:47 AM

Methinks the XP-39 had a two-stage B-5 Trubosupercharger ... :o , not a single-stage unit. However, you are free to disagree.

There was only one built and it crashed, so all we have are references. Here is one, but there are more that say the same thing.

http://home.att.net/...ugher1/p39.html

In any case, I believe the P-39 was never given half a chance to be developed into the fighter that was needed. I already KNOW what history says, so there is little enough evidence to bolster the P-39. I'll stick by my opinion and I assume you will stick by yours, too.

We seem to disagree on several things, but we both love the planes, huh?

#9 Wuzak

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Posted 09 January 2010 - 06:30 AM

Yes, I looked at Baugher's site. I also looked at wiki, which also presented the data - 390mph top speed and climb to 20,000ft in 5 minutes. They also have the same data about the turbocharger being a 2 stage unit mounted in the left side of the fuselage.

Thing is I am looking at a photo of the XP-39, a better quality than the one above, and the turbo is clearly poking out at the bottom.

Also, two stage supercharging was just staring to be experimented with at about that time. We also know that all production GE turbos, at least until near the end of the war, had single stage compressors. So, if the unit in the XP-39 was a two stage compressor unit then it was highly experimental and not proceeded with.

Remember that turbocharging was still in the development atges. The turbo fitted to the XP-39 was, according to Whitney, the same unit as fitted to the XFM-1 and the XP-37.

You can see the wastegate pipe of the turbo on the side view of the XP-37 behind the turbo, just ahead of where the wing blends into the fuselage.

This picture

Posted Image

shows a cutaway of a typical GE turbocharger. The Turbosupercharger and the Airplane Power Plant.

No doubt you have some turbos at the museum that you could check out.

Some confusion may exist about the two stage compression, people assuming that this happens in the turbo, when in fact the turbo acts as the first stage of a two stage compression system with the engine supercharger.

Also, as the turbocharger was, at the time, intended only for altitude compensation and not to actually boost the engine, the pressure ratio was low (with a critical altitude of 20,000ft) so a two stage compressor in the turbo would have been of dubious benefit. The CM-2 turbocharger was developed later for high altitude operations, and had a two stage compressor and a critical altitude of 40,000ft.

#10 ChrisMcD

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Posted 09 January 2010 - 10:16 AM

Remember that turbocharging was still in the developmentstages.
Also, as the turbocharger was, at the time, intended only for altitude compensation and not to actually boost the engine.


Bit of a sweeping statement.

Following their work on the R for the Schneider Trophy Rolls were well aware of the benefits of low level supercharging when they came to design the Merlin - which is why it was only 27 litres and significantly lighter than a lot of comparable engines.

Also, why they were one of the first companies to supply large quantities of two stage supercharged engines with the results we all know.

Mind you, I have a feeling that the Twin Wasps in the Wildcat were two stage quite early on as well

#11 Wuzak

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Posted 09 January 2010 - 11:06 AM

Bit of a sweeping statement.

Following their work on the R for the Schneider Trophy Rolls were well aware of the benefits of low level supercharging when they came to design the Merlin - which is why it was only 27 litres and significantly lighter than a lot of comparable engines.

Also, why they were one of the first companies to supply large quantities of two stage supercharged engines with the results we all know.

Mind you, I have a feeling that the Twin Wasps in the Wildcat were two stage quite early on as well



Yes, supercharging had been used to boost the engine's performance for many years, in both aviation and automobile engines. The turbocharger, however, was still in development at the time of the XP-39 (ie 1938/9). The XP-37, which flew at the beginning with a similar, if not the same, turbo, hardly got off the ground for all the turbo problems.

The turbo's only reason for existance to start with was to maintain sea level pressure to a critical altitude. In some later installations the turbo was used to boost pressure beyond sea level pressure. The job of boosting that sea level pressure for the means of increasing power was left to the engine stage supercharger.

In terms of two stage superchargers Pratt & Whitney probably had the first production engine so equipped - the R-1830s. This did, indeed, go into the F4F, but that was still a year away from production and service. Merlin twin stage engines did not go into production until late 1941 or early 1942.

The Merlin benefitted from a 2 speed supercharger drive about 1940, and from the influence of Sir Stanley Hooker a couple of years before that.

The R used a boost pressure which was way above the normal for the time. I wouldn't call it a low level boosting.

Also interesting is that the 1931 R weighed 1640lbs (744kg) and produced 2500hp. The Merlin 61 weighed the same and made about 1800hp when using +21psi boost and 150 grade fuel. Of course the Merlin was more suitable for sustained use.

#12 Wuzak

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Posted 09 January 2010 - 11:11 AM

Just because you mentioned it:

Posted Image

The Rolls Royce R.

Even though this is a 2240cid (36l) engine, it is very much dominated by the massive double sided supercharger at the back.

Pinched from http://www.k5054.com/

Edited by Wuzak, 09 January 2010 - 11:14 AM.


#13 Red Admiral

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Posted 09 January 2010 - 11:28 AM

The Rolls-Royce R engine was boosted to 18lb. A more normal value from around the period is 3-6lb.

The difference between the R and the late Merlins is interesting, very similar power levels but the R is a much higher capacity (37L at up to 3400rpm). The most powerful R giving around 2750hp at 21lb boost on ethanol. R capacity is 125800L/min whilst 81000L/min for the Merlin. Slightly higher boost with the Merlin, 2300hp at 30lb. Given the difference in capacity, it really shows the level of supercharger development over the 1930s.

#14 Wuzak

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Posted 09 January 2010 - 11:29 AM

Methinks the XP-39 had a two-stage B-5 Trubosupercharger ... :o , not a single-stage unit. However, you are free to disagree.


Yes...I do disagree!

From Graham White, Allied Aircraft Piston Engines of World War II,

The unit [turbocharger] was made up of three sections; the turbine, the compressor, and the oil pump bearing assembly. The Type Bs and Type Cs used single stage, centrifugal compressors driven by a sing stage axial-flow turbine.




In any case, I believe the P-39 was never given half a chance to be developed into the fighter that was needed. I already KNOW what history says, so there is little enough evidence to bolster the P-39. I'll stick by my opinion and I assume you will stick by yours, too.


That is debatable. In the other thread there has been much disagreement on whether the P-39 made a good fighter or not. It definitely turned out to be a neat looking aircraft (more so than the XP-39).

The altitude performance problem is, obvioulsy, one sticking point. Another way to get altitude performance without using a two stage supercharger or turbocharger was to have a two speed supercharger drive - something Allison never developed. The two stage Allisons developed were too big for the P-39 (hence the P-63 being larger), a turbocharged version may have been possible, but it would have required some more detail work.


We seem to disagree on several things, but we both love the planes, huh?


Well, yes, it's great to be able to disagree and not be abused (as I have been in other forums in teh past).

Planes are great and the best era is the mid '30s to late '40s, when the plane went from a slow biplane, through monoplanes, including some of the most famous ofa ll time, and into the jet age.

#15 Wuzak

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Posted 09 January 2010 - 11:30 AM

Methinks the XP-39 had a two-stage B-5 Trubosupercharger ... :o , not a single-stage unit. However, you are free to disagree.


One thing we have in common is the ability to produce typos!

#16 Wuzak

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Posted 09 January 2010 - 11:54 AM

The Rolls-Royce R engine was boosted to 18lb. A more normal value from around the period is 3-6lb.

The difference between the R and the late Merlins is interesting, very similar power levels but the R is a much higher capacity (37L at up to 3400rpm). The most powerful R giving around 2750hp at 21lb boost on ethanol. R capacity is 125800L/min whilst 81000L/min for the Merlin. Slightly higher boost with the Merlin, 2300hp at 30lb. Given the difference in capacity, it really shows the level of supercharger development over the 1930s.


I think you'd agree that +18psi was astronomical for 1931.

Those numbers are for air usage?

Remember that the flow rates you are giving are for air at pressures above atmospheric.

Assuming the boost numbers are for sea level, the free air delivery (flow of standard sea level pressure air) to the engines are 277,700l/min (+18psi) and 303,100l/min (+21psi) for the R and 246,600l/min for the Merlin (assuming 3000rpm).

A Mk 66 Merlin was tested (accidentally) at 3300rpm and +30psi boost and made 2380hp. That is a free air delivery of 271,300l/min. The RM.17Sm at 3150rpm and +36psi boost used 293,700l/m of air and made over 2600hp.

Sir Stanley Hooker must get most of the credit for the improvement of RR superchargers in the late '30s, whilst Rod Banks made the fuel that allowed such boost pressures in the R and many engines afterwards.

#17 GregP

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Posted 10 January 2010 - 03:02 AM

You know, Wuzak, I have read several places that the XP-39 was a 2-stage turbocharger.

However, per your conjecture, it is possible the authors were intending the "2-stage" term to mean the supercharger was the first-stage of boost, and the turbocharger was the second-stage or auxiliary-stage of boost.

I have been unable to locate a cutaway of a GE B-5 turbocharger, but I'll ask at the Museum next weekend. Maybe we HAVE a B-5 unit lying around ... I really haven't looked for one. For all I know, the turbo in our P-47 might be a GE B-5. So, I'll ask and get back with you.

I really have no stake in whether or not it was single or two-stage, but it would be nice to find out and clear it up. I still believe the P-39 was doomed by the deletion of the turbo, but ... that is another question.

Next, about the Don Berlin turbo P-40, I have never read that the turbo P-40 was newly-built. I assumed it was modified for some test flights and then returned back to stock and reissued to the Air Corps, unless the airframe converted was a Bell company hack. I have never seen the serial number described, and have not seen a lot of details on it either. I just recall reading about it on a couple of occasions and talking about that with a former Curtiss employeee some years back.

He confirmed it flew and flew quite well but, like most people, I think the P-40 was obsolescent by 1942, much less 1944, and it was probably just to satisfy Don Berlin's curiosity.

Also, it is possible one of the XP-40Q models was fitted with a turbo for a flight or two, and maybe that is what was being described. Most company records are pretty good about what was built and delivered, but almost non-existent about things that were tried during aircraft testing and evaluation. In other words, the "experiments" are not always well documented, and were never even mentioned in official company records in a lot of cases.

Yes I generate a LOT of typos! :D I apologize for that, but I'm really not a great typist.

Last, when we read about the things that happened on the 1930's and 1940's, we are reading what someone wrote, and their opinions may be no better than anyone else's. Maybe they didn't even witness the events they were writing about at the time. So, I suppose there are mistakes enough to go around for everyone.

I volunteer at a flying museum, but that doesn't make me anymore right than someone who is just fond of old fighters, and so has read a lot about them. My only real advantage is that if we're all unsure (when we have differening opinions), I can probably ask someone around the museum who KNOWS. The trick is knowing who KNOWS and who is spouting BS!

#18 Wuzak

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Posted 10 January 2010 - 12:24 PM

The one reference I have which lists GE turbochargers and the aircraft they were used in suggests that only the XP-39 ran with a B-5 turbo. That was in Graham White's Allied Aircraft Pistone Engines of World War II.

Whitney says that the XP-39 was fitted with a Form 10 turbo, similar to the one fitted to the XFM-1 and originally to the XP-37. This differed from the production turbochargers in that the compressor inlet was facing toward the centre of the drive shaft, and not outward as on subsequent turbos. The theory behind this was to use the induction air to cool the bearing before entering the compressor. From what Whitney says this was less than successful.

GE's production turbos in WW2 were confined to two different series - the B and C series. These were specified to work with engines of certain power, ie airflow requirements. Your P-47 will be fitted with a C series or CH series (a C series with a higher critical altitude) turbosupercharger. Turbos used with Allison V-1710s (ie on the P-38) were B series turbos.

So, had the P-39 gone ahead with a turbo the same ones used in the various P-38 models would have been used.

#19 Wuzak

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Posted 10 January 2010 - 12:48 PM

Talking about turbos in this past week or so, it dawned on me that most turbos had critical altitudes of around 20,000 - 25,000ft. Up to that altitude they could maintain the sea level power rating, but after that power dropped off dramatically.

In this article about the P-38 (which I have yet to fully read) the dramatic drop off in performance after the critical altitude is shown in a graph about half way down th epage.

The P-39's supercharger critical altitude was, IIRC, about 12,000ft. Chosen, I guess, to not be too set to be high or low altitude.

This graph shows the performance of the Allison engined P-39D and P-51 and the Merlin powered P-40F (clearly a two speed engine at work). The graph only goes to 27,500 feet, but shows a less dramatic drop off of performance after critical altitude.

Changing the critical altitude of an altitude rated engine was a matter of gearing. Spin the supercharger faster to get a higher critical altitude, but lose out at lower altitudes (not sure how Alliosn controlled boost in their altitude rated engines - will have to look into that further). Hence Rolls Royce's move to two speed gear drives for the supercharger.

So I would submit that a turbocharged P-39 would most likely have vastly improved performance up to the critical altitude (ie in low to medium altitudes) of the turbocharger, but not much improvement, if at all, above that.

Edited by Wuzak, 10 January 2010 - 01:05 PM.


#20 GregP

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Posted 10 January 2010 - 08:08 PM

In the Allison 2-stage supercharger, the second stage was called the auxiliary stage. It had a fluid drive coupling like an automatic transmission in a car, and so was a continuously variable ratio.

I have mentioned Allison builder Joe Yancey before, and he has two of the Allison auxiliary superchargers in inventory. One of them is currently in the process of being rebuilt. It fits on the end of the engine, and looks like a second supercharger stage bolted on with a thick central shaft. The thick central shaft is the fluid coupling.

I don't know why he is choosing to overhaul it at this time, but will ask next week when I stop by the shop. I am assuming some imminent use, but it may be just to have one available in case someone decides to assault the Reno air races with an Allison.

Thanks for the article and graph. Where do you FIND these things?

The original P-40F was powered by a Rolls Royce Merlin 28 of some 1,300 HP. The production P-40F's were powered by Packard-built V-1650-1 Merlins of 1,300 HP @ 3,000 rpm @ sea level.They produced 1,120 hp @ 18,500 feet and had a single-stage, 2-speed supercharger with ratios of 8.15:1 and 9.49:1 . Compressionr atio was 6.00 : 1 .

I believe the only one still flyable, or even in existence, is the one restored by Judy Pay of Australia. The is serial number 41-14112, and can be seen flying around Australia today. I think is easily the prettiest P-40 built, but that is only my opinion. The lines are improved by the absence of the Allison-required air scoop on top of the cowling.

Edited by GregP, 10 January 2010 - 08:54 PM.





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