What was the problem with the allison engine?

[davparlr]

I have been confused by the allison V-1710 engine. In 1940, It appears to roughly be equivalent to the Merlin and the DB-601A, around 1050-1150 hp, and in 1945 was generating 1600 hp in the P-38/P-82. But it was always associated with poor altitude performance. Wouldn't this be cured by an adequate compressor ala Merlin, and wouldn't this have corrected the problem, for, say, the P-51A? Why wasn't this accomplished?

 

[Readie]

It was an inferior design to the Merlin. That's why the P51 received Britain's finest aero engine.
Cheers
John

 

[fastmongrel]

It was an inferior design to the Merlin. That's why the P51 received Britain's finest aero engine.
Cheers
John

There was nothing wrong with the Allison it was in no way inferior to the Merlin. Rolls Royce were able to use the genius of Stanley Hooker and build better and better superchargers and improve altitude performance but that was very nearly the only advantage the Merlin had. Allison was a relatively small company that was swamped by the work heaped on it, a lot of its time was spent getting the installation right on the P38.

 

[Readie]

Allison had been leaning heavily towards exhaust-driven turbochargers instead of the more common mechanically-driven superchargers, feeling that their added performance more than made up for the added complexity. Thus little effort was invested in equipping the V-1710 with a reasonable supercharger, and when placed in aircraft designs like the P-39 or P-40 which lacked the room for a turbo the engine suffered tremendously at higher altitudes. It was for this reason in particular that the V-1710 was later removed from the P-51 Mustang and replaced with the Rolls-Royce Merlin.

The Merlin was a better developed aero engine and more appropriate for the needs of WW2 fighters. Mind you, the lack of power was not Allison's fault but,the US Army's thing about turbo chargers.

The Army had earlier decided to concentrate on turbosuperchargers for high altitude boost, believing that further development of turbochargers would allow their engines to outperform European rivals using superchargers. Turbosuperchargers are powered by the engine exhaust and so do not draw power from the engine crankshaft, whereas superchargers are connected directly by gears to the engine crankshaft. Turbosuperchargers do increase the exhaust back-pressure and thus do cause a decrease in engine power, but the power increase due to increased induction pressures more than make up for that decrease. Crankshaft-driven superchargers require an increasing percentage of engine power as altitude increases (the two-stage supercharger of the Merlin 60 series engines consumed some 230-280 horsepower at 30,000 ft). General Electric was the sole source for research and production of American turbosuperchargers during this period.
Turbosuperchargers were indeed highly successful in U.S. bombers, which were exclusively powered by radial engines. The P-47 fighter had the same combination of radial engine (R-2800) and turbosupercharger and was also successful, apart from its large bulk, which was caused by the need for the ductwork for the aft-mounted turbosupercharger.
However, mating the turbocharger with the Allison V-1710 proved to be problematic. As a result, designers of the fighter planes that utilized the V-1710 were invariably forced to choose between the poor high-altitude performance of the V-1710 versus the increased problems brought on by addition of the turbosupercharger. The fates of all of the V-1710 powered fighters of World War II would thus hinge on that choice.

Technically clever, the turbosupercharger.

The P-38 was the only fighter to make it into combat during World War II with turbosupercharged V-1710s. The operating conditions of the Western European air war – flying for long hours in intensely cold weather at 30,000 feet revealed several problems with the turbosupercharged V-1710. These had a poor manifold fuel-air distribution and poor temperature regulation of the turbosupercharger air, which resulted in frequent engine failures (detonation occurred in certain cylinders as the result of persistent uneven fuel-air mixture across the cylinders caused by the poor manifold design). The turbosupercharger had additional problems with getting stuck in the freezing air in either high or low boost mode; the high boost mode could cause detonation in the engine, while the low boost mode would be manifested as power loss in one engine, resulting in sudden fishtailing in flight. These problems were aggravated by suboptimal engine management techniques taught to many pilots during the first part of WWII, including a cruise setting that involves running the engine at a high RPM and low manifold pressure with a rich mixture. These settings can contribute to overcooling of the engine, fuel condensation problems, accelerated mechanical wear, and the likelihood of components binding or "freezing up."
Details of the failure patterns were described in a report by General Doolittle to General Spatz in January 1944. In March 1944, the first Allison engines appearing over Berlin belonged to a group of P-38H pilots of 55FG, engine troubles contributing to a reduction of the force to half strength over the target. It was too late to correct these problems in the production lines of Allison or GE, and so the P-38s were steadily withdrawn from Europe until they were no longer used for bomber escort duty with the Eighth Air Force by October 1944. A few P-38s would remain in the European theater as the F-5 for photo reconnaissance.

If my life depended upon it, I would choose the reliable Merlin
Cheers
John

 

[davebender]

Allison had been leaning heavily towards exhaust-driven turbochargers instead of the more common mechanically-driven superchargers

I think this is the main problem. The U.S. Army Air Corps bet on the wrong horse. If they had emphasized mechanically driven superchargers as Britain and Germany did the Allison engine might have turned out just fine.

 

[Shortround6]

Oh boy, were to begin.

Allison had been leaning heavily towards exhaust-driven turbochargers instead of the more common mechanically-driven superchargers, feeling that their added performance more than made up for the added complexity.

Allison had very little to do with the exhaust-driven turbochargers, they were government specified equipment and government supplied equipment as were the turbo controls. The exhaust-driven turbochargers were supplied by General Electric. Both th exhaust turbines and the intake air compressor. GE had been supplying ALL US aircraft manufacturers with supercharger designs and parts until about 1937. Allison had done sub-contract work for GE making impellers and such but had no supercharger design staff of their own. The ARMY felt that the turbo was the way to go, Allison and other companies just did what the Army told them to do or paid them to do. See the Continental I-1430 and Lycoming O-1230 for a few other examples of the Army projects.

edit>The turbo set up was a two stage supercharger, the turbo supplied sea level air pressure to the carburetor deck up to the rated altitude of the supercharger. the second impeller (compressor) in the system was between the carburetor and the intake manifold and was gear driven.
P&W flies a two stage mechanical system in 1939 and is is adopted for use in the F4F-3 in late 1939 but production difficulties force some F4F-3s to be fitted with single stage engines as F4F-3As.
Bristol has used a two stage system (mechanical driven) to set some worlds records in the late 1930s but aside from a few experimental Wellingtons no WW II production Bristol engine uses a two stage system.
Rolls-Royce doesn't get the 60 series Merlin into production until 1942.
For aircraft use two stage superchargers of any type (turbo or mechanical) were mighty scarce in the late 30s or even the first couple of years of the war.
Hookers work on superchargers doesn't affect anything until the introduction of the Merlin XX and 45 engines.<edit

Thus little effort was invested in equipping the V-1710 with a reasonable supercharger, and when placed in aircraft designs like the P-39 or P-40 which lacked the room for a turbo the engine suffered tremendously at higher altitudes.

With the entire US supercharger knowledge held by one company until 1937 it is rather amazing that the US did as good as it did in 1939-41 with superchargers. Allison, P&W and Wright all decided at different times in 1937-38 to design their own superchargers as it was becoming obvious that the GE designs weren't that good. The Supercharger on the Allison was actual just about equal to the supercharger used on the Merlin III or Merlin X. There were a few detail differences that kept the Allison from performing quite as well at altitude as the early Merlins but they had little to do with the actual supercharger itself.

It was for this reason in particular that the V-1710 was later removed from the P-51 Mustang and replaced with the Rolls-Royce Merlin.

This was done because R-R was the second company to get a 2 stage mechanical supercharger into production. Yes Allison was behind the curve in making a two stage mechanical supercharger.

The Merlin was a better developed aero engine and more appropriate for the needs of WW2 fighters. Mind you, the lack of power was not Allison's fault but,the US Army's thing about turbo chargers.

That is a rather simplistic way of looking at it. The Allison was perfectly capable of making power, it had trouble making power at altitude. At low altitudes using similar levels of boost there wasn't that much to chose between the two engines.

Turbosuperchargers were indeed highly successful in U.S. bombers, which were exclusively powered by radial engines. The P-47 fighter had the same combination of radial engine (R-2800) and turbosupercharger and was also successful, apart from its large bulk, which was caused by the need for the ductwork for the aft-mounted turbosupercharger.
However, mating the turbocharger with the Allison V-1710 proved to be problematic. As a result, designers of the fighter planes that utilized the V-1710 were invariably forced to choose between the poor high-altitude performance of the V-1710 versus the increased problems brought on by addition of the turbosupercharger. The fates of all of the V-1710 powered fighters of World War II would thus hinge on that choice.

The P-38 gets a bit of a bum rap because it helped pioneer the turbo-superchager in combat. Turbo-ed P-43 with their radial engines had their share of turbo woes as did early B-17s. It took a while to straighten out the turbo in ALL US aircraft.

The P-38 was the only fighter to make it into combat during World War II with turbosupercharged V-1710s. The operating conditions of the Western European air war – flying for long hours in intensely cold weather at 30,000 feet revealed several problems with the turbosupercharged V-1710. These had a poor manifold fuel-air distribution and poor temperature regulation of the turbosupercharger air, which resulted in frequent engine failures (detonation occurred in certain cylinders as the result of persistent uneven fuel-air mixture across the cylinders caused by the poor manifold design). The turbosupercharger had additional problems with getting stuck in the freezing air in either high or low boost mode; the high boost mode could cause detonation in the engine, while the low boost mode would be manifested as power loss in one engine, resulting in sudden fishtailing in flight. These problems were aggravated by suboptimal engine management techniques taught to many pilots during the first part of WWII, including a cruise setting that involves running the engine at a high RPM and low manifold pressure with a rich mixture. These settings can contribute to overcooling of the engine, fuel condensation problems, accelerated mechanical wear, and the likelihood of components binding or "freezing up."
Details of the failure patterns were described in a report by General Doolittle to General Spatz in January 1944. In March 1944, the first Allison engines appearing over Berlin belonged to a group of P-38H pilots of 55FG, engine troubles contributing to a reduction of the force to half strength over the target. It was too late to correct these problems in the production lines of Allison or GE, and so the P-38s were steadily withdrawn from Europe until they were no longer used for bomber escort duty with the Eighth Air Force by October 1944. A few P-38s would remain in the European theater as the F-5 for photo reconnaissance.

Part of the problem was change in the allowable "ingredients" for 100/130 fuel which changed it's volatility. This was known as a potential problem in the spring/summer of 1943 and Allison worked on a new intake manifold to help solve the problem, as it would affect ALL Allisons, not just the turbo-ed ones. It went into production in the fall of 1943 and was being fitted to new engines in Nov of 1943. Many manifolds were shipped overseas for refitting in service engines.
The Army's initial design of turbo control was not well thought out, it sought to control the turbo by measuring the pressure in the exhaust ducts or close to the turbine. By maintaining a constant exhaust back pressure they thought this would regulate the intake boost. It was slow responding and subject to freezing, as anybody you has seen the plume of water vapor coming out of a tail pipe on a cold day might have guessed. The turbo control was changed to a system that measured the intake pressure and adjusted the turbo waste gates accordingly. Please note that this was an "Army" or government furnished piece of equipment and not supplied by either Allison (or any other engine maker) or even by GE.
The problems had been sorted out/solved by the spring of 1944 and all new P-38s from Dec of 1943 on (which may take weeks or months to get to a combat theater) have most of the basic engine problems solved. Mis rigged turbo controls and bad piloting technique (against the directions of both Allison and Lockheed) do cause problems. Decision in favor of the P-51 is made before operational experience with corrected P-38s can be accumulated. It was still the right decision but given a few more weeks it might have been a bit tougher to make. The P-38H that made that flight over Berlin was being built side by side with P-38Js because of a shortage of the intercoolers used in the J. First Js are delivered in Sept of 1943.

In post war civil use Merlins were fitted with an intake charge heater to solve a similar fuel mixture distribution problem in cruising flight.

 

[V-1710]

The above posts are correct. It really came down to most Allsions having a simple single stage centrifical blower, while the Merlin used a two-stage supercharger. The V-1710 was indeed designed to be used with a turbocharger for high altitude performance as it was in the YP-37, XP-39, YB-38 and the P-38's. Since the P-40 was intended to be a low altitude ground support fighter, a single stage non-turbo V-1710 was deemed sufficient. Also, when the role of the P-39 was changed from high altitude interceptor to ground attack, the turbocharger was deleted. Beyond that, the V-1710 was a fine design, very rugged and tolerated abuse better than most any water cooled aircraft engine. In fact, Allison powered P-40's were more reliable than the Merlin powered versions in North Africa because the Allison was less prone to damage from sand ingestion. Of course, the Allison's downdraft carburetor and intake mounted on top of the cowling helped, as opposed to the Merlin using an updraft carburetor and intorporating the air intake in the radiator air scoop, below the propellor.

Also, with regards to the P-38's performance over Europe, there was a problem with too low oil temperatures, cause bo too large an engine oil cooler. That was not an issue in the Pacific.

 

[Shortround6]

I would really, really like somebody to come up with some proof that the P-40 was designed or intended to be a "low altitude ground support fighter".
If was intended to be such a thing they did a remarkably poor job of it. The Army had already decided they wanted air-cooled engines in attack planes. Even the P-26 with it's 600hp Wasp engine had a bigger bomb load than the P-40 prototype and early production versions. Two cowl mounted .50 cal guns with 200rpg is a pretty poor ground attack armament. They used a version of the Allison with the highest possible gear ratio for the supercharger that Allison offered at the time, this cut available power at sea level in favor of power at 11,000-12,000ft and higher. Better "low altitude" performance could have been had by using the supercharger gears from early P-38s.

The Mission of the P-39 was not changed. The original turbo charger installation was just plain bad with an inter-cooler that severaly limited performance. There was no room inside the aircraft for a proper inter-cooler set up and by switching to a NON-TURBO version the reduction in drag was good for an increase in speed of 30-40mph at all altitudes up to 15,000ft with the turbo plane only being faster at around 20,000ft and up.

Just like the YP-37 and the P-40, the Army realized that SERVICE turboed p-39s were, if they were lucky, at least a year further down the road than the non-turboed planes. Considering the state of the USAAF and the types and numbers of planes in hand or likely to show up in the next year or two going for non-turboed aircraft meant having at least SOME SORT of MODERN fighter even if they weren't the best compared to a possibly great (or possibly not great) fighter still in testing if they had to go to war.

Your choice, Going to war in P-40B&Cs with Es starting to show up and early P-39s (which had a LOT of troubles to begin with) or go to war with turbo P-39s/P-40s still in testing and service squadrons still using P-35s/ P-36s in hand fulls with some squadrons still using P-26s?

 

[Shortround6]

I think this is the main problem. The U.S. Army Air Corps bet on the wrong horse. If they had emphasized mechanically driven superchargers as Britain and Germany did the Allison engine might have turned out just fine.

The German superchargers weren't much better (or worse) than anybody else's. Supercharger design did not stay static during the war. The performance of the supercharger, measured in pressure ratio achievable and efficiency, was improved buy most, if not all countries as the war went on. The German superchargers didn't offer any better performance than the allied ones except maybe for brief periods as advantages see-sawed back and forth as many other fields experienced.

 

[MIflyer]

It's true that the lack of a suitable supercharger was the problem with the V-1710. And it is also true that the USAAF focused on turbosuperchargers as the answer to high altitude power following that famous GE Pike's Peak test in 1918. Turbos were excellent for bombers but you could not put a turbo in a small fighter aircraft, one of the main reasons being the incrased boost mandated a intercooler or aftercooler.

But the inexplicable things are:

1. Once it became obvious from the XP-37 and the XP-39 that you could not count on turbos for fighters Allison could have made a two-speed supercharger for the V-1710 very easily. It might not have been as good as the Merlin's but it would have added valuable performance to the P-39, P-40, and P-51. In fact, Allison could have done this very easily because unlike the Merlin the crankcase, reduction gears, and supercharger accessory case of the Allison were separate parts. The supercharger accessory case for the V-1710 was great for production since the same one could be used in all of the versions and only a change in impeller and gear ratio had to be made. BUT by sticking with that one design Allison doomed the V-1710 to single speed. By 1941 everybody but everybody was going to at least two speed superchargers.

2. When Allison added an auxillary mechanical supercharger stage to the V-1710 they left out an intercooler or aftercooler. The reason is clear - it is too hard to fit one in a sleek fighter airframe IF you make it an air to air cooler like was used in everything else BUT the Merlin 60 series. Have you seen the intercooler of a B-17? It is large and buried in the wing - not that much of a problem for a big bomber. Have you seen the intercooler for a P-47? It is huge and buried in the fuselage where the 2nd seat of its ancestor was (the P-35 was virtually a 2 seat airplane). That was the clear incredible genius of Sir Stanley Hooker. He solved the problem of getting all that draggy air in and out of the aftercooler by going to a liquid to air cooler. The fact that Allison and no one else did not copy that stunningly obvious idea is simply incredible. Using a liquid cooler would have solved the P-38's intercooler problems, made the 2 stage V-1710 as used in the P-63 and F-82 a more viable powerplant, and would have been useful in otehr engine installations as well, for everything from the P-61 to the B-28 to the Republic Rainbow.

And you are right Shortround. The P-40, P-39 and Allsion P-51 were NOT meant to be low altitude fighters. Take a look at the performance curves and you will see that the superchargers were optimized for about 15,000 ft, that being a happy medium for a single speed supercharger. In contrast the V-1710 in the A-36A was optimized for around 5,000 ft. And the "cropped" Merlin 45's used in the Spitfire V LF versions were also optimized for low altitude and made that airplane what was described as the hottest Spitfire of all below 10,000 ft.

And take a look at the performance data for the V-1650-7 and the V-1710 with turbo. The Allision was 400 pounds lighter (not counting the turbo) and put out more power at a higher altitude.

I was going to follow up my 2001 Airpower magazine article on WWII supercharging with one that explanined all of this but the mag went out of business. So thanks to y'all for letting me rant!

 

[Shortround6]

I have been confused by the allison V-1710 engine. In 1940, It appears to roughly be equivalent to the Merlin and the DB-601A, around 1050-1150 hp, and in 1945 was generating 1600 hp in the P-38/P-82. But it was always associated with poor altitude performance. Wouldn't this be cured by an adequate compressor ala Merlin, and wouldn't this have corrected the problem, for, say, the P-51A? Why wasn't this accomplished?

For an Allison to make 1600hp at 27,000ft it needed about 60in of manifold pressure. At 27,000ft the air pressure is 10.16in according to one old chart so you need a supercharger with a pressure ratio of 5.9 to one. This is beyond the capability of a single stage supercharger of any type. So you are into a two stage supercharger set up of some kind. And with that kind of pressure ratio you need an inter-cooler or after-cooler or the intake air will be too hot.

The two stage Merlin was designed with an especially compact two stage supercharger in the same housing and it used a liquid after-cooler. The 1st stage was always turning and used power/heated the intake mixture even at low altitude were it wasn't needed. Air to air inter-coolers are larger and bulker but more resistant to combat damage. By placing the the 1st stage a bit further away from the engine it is possible to use a different drive system than the common shaft used by the Merlin 60 series.
It is also possible to have the carburetter between the stages. This makes for a bulker installation but allows the first stage to be either de-clutched or idled freeing more power for take-off or low altitudes.

Allison started work on a two stage mechanical system in 1938 but due to the small size of the staff and the pressure to produce what was already tested it had a low priority. It was also very much learn as you go because one of Hooker's claims to fame was that he recognized very early on that some of the formulas in text books about supercharger design were wrong.

The P-51B had a 7in vertical splice in the fuselage to accommodate the Merlin and it's associated inter-cooler radiator and larger coolant radiator.

 

[Shortround6]

Nice post MIflyer.

One point needs a bit of clarification though.

1. Once it became obvious from the XP-37 and the XP-39 that you could not count on turbos for fighters Allison could have made a two-speed supercharger for the V-1710 very easily. It might not have been as good as the Merlin's but it would have added valuable performance to the P-39, P-40, and P-51. In fact, Allison could have done this very easily because unlike the Merlin the crankcase, reduction gears, and supercharger accessory case of the Allison were separate parts. The supercharger accessory case for the V-1710 was great for production since the same one could be used in all of the versions and only a change in impeller and gear ratio had to be made. BUT by sticking with that one design Allison doomed the V-1710 to single speed. By 1941 everybody but everybody was going to at least two speed superchargers.

Two speed superchargers don't do a lot for high altitude performance. This can be seen on a few Merlins. The Merlin III used an 8.588 gear for it's supercharger. The Merlin X with a two speed drive used a 6.389 gear for low speed and a 8.75 gear for high for the same supercharger as used on the III. Altitude performance improved about 1000-1500ft but take off power was improved from the 880hp of the III to 1075hp (a 22% improvement) on 87 octane fuel. Likewise while a Merlin 45 used a 9.089 gear the Merlin XX (with the same supercharger) used 8.15 and 9.49 gears. While altitude performance was slightly improved the low gear set allowed about another 100hp for take-off/low altitude flying at the same RPM and boost pressure.

An estimate for the performance of the Allison with a two speed drive can be made by using the HP figures of the late Allison's with 9.60 drive ratio and for low altitude work the 7.88 gears.

By my calculations (which could be wrong) the Allison impeller tips were moving at 1193 FPS with the 9.60 gears while the Merlin XX with 9.49 gears had an impeller tip speed of 1272fps. Perhaps more could be gotten out of the Allison by changing to a higher ratio (that takes more power to turn) but it wasn't going to do much for performance over 20,000ft no matter what ratio is used. The air pressure at 20,000ft is 13.75in and the Allison needed 44.5in to make 1125hp for a pressure ratio of 3.23 which is asking a lot of any single stage compressor.

 

[renrich]

Dav, I appreciate you starting this thread. Many questions I have had for a long time are being answered by some well informed posters. Many thanks and keep up the good work, all!

 

[tomo pauk]

When comparing 1710 with DB 601/605, it might be OK to note that 601/605 were ~35 liter engines, and have had a supercharger tailored for such working volume. V-1710 have had 25% less working volume, thus a similar deficit in power output is to be expected for contemporary engines.

 

[Milosh]

It was an inferior design to the Merlin. That's why the P51 received Britain's finest aero engine.
Cheers
John

You do know that Allison engine parts are put in Merlins today because those parts were better than the R-R parts.

 

[V-1710]

What was the arrangement on the P-63 and P-82?

 

[Readie]

What was the arrangement on the P-82?

On 27 February 1947, a P-82B 44-65168 named Betty Jo and flown by Colonel Robert E. Thacker made history when it flew nonstop from Hawaii to New York without refueling, a distance of 5,051 mi (8,129 km) in 14 hr 32 min (347.5 mph/559.2 km/h). This flight tested the P-82's range. The aircraft carried a full internal fuel tank of 576 gal, augmented by four 310 gal tanks for a total of 1,816 gal . Also, Colonel Thacker forgot to drop three of his external tanks when their fuel was expended, landing with them in New York.
To this day, it remains the longest nonstop flight ever made by a propeller-driven fighter, and the fastest such a distance has ever been covered in a piston-engined aircraft (the record for the longest unrefueled flight by a propeller-driven aircraft of any type is held by the Rutan Voyager). The aircraft chosen was an earlier "B" model powered by Rolls-Royce Merlin engines

Sense prevailed in the end with RR Merlins.

Cheers
John

 

[Shortround6]

Sense prevailed in the end with RR Merlins.

Cheers
John

Common sense did prevail, mainly because the first flight of an Allison powered F-82 was 10 days before the record setting flight.

I would say that not trying a record setting long distance flight with an airframe engine combo that had only been flying a few days is common sense, aside from that it doesn't prove much of anything about the relative merits of the two "brands" of engines considering the vast number of different models and modifications both had.

 

[davparlr]

Dav, I appreciate you starting this thread. Many questions I have had for a long time are being answered by some well informed posters. Many thanks and keep up the good work, all!

Thanks. This certainly has been very informative.

 

[Shortround6]

What was the arrangement on the P-63 and P-82?

The engines used in the P-63 and P-82 had a second supercharger spaced out to the rear of the engine and the original supercharger, a short drive shaft and variable speed hydraulic coupling took up some of the space The P-63 did not use an inter-cooler and I am not sure that the P-82 did. They did use water injection but against a plane using both an intercooler and water injection that still leaves it behind. The engines were about 22 in longer than a single stage engine. Some versions mounted the carburetor and the engine supercharger (auxiliary supercharge blew into the carburettor) and some had the carburettor on the outside of the auxiliary stage with the auxiliary stage sucking through the carburettor. The first system worked better.
The P-63 engines were the so called "E" series just like the P-39 but had many changes including, on many P-63s a heavier crankshaft. The P-82s not only had the heavier crankshaft but different reduction gearing to the prop that allowed them to run at 3200rpm without overspeeding the prop tips. The P-82s used the "G" series Allison that was not used in any other production aircraft. The "G" series engines also used a 10 1/4 in impeller on the main supercharger instead of the 9 1/2 in impeller on the earlier engines.