P-38 vs P-82

[Shortround6]

Was there anything inherent in the Allison's design that made it difficult to etract the kind of performance obtained from the Merlin - by which I mean was the Merlin a 'hotter' engine - or was it more a matter of supercharger technology?

about the ONLY thing in the Allison's design (and it was by no means inherent) was the use of a higher compression ratio which limited usable boost. Any other differences were due to the supercharger technology.

It was estimated that the Allison could have made about 10% more "power" if the compression ratio was dropped to 6:1 like was done on the last models (and used on all Merlins). Of course fuel economy (range) would have suffered.

USAAF reluctance to approve WEP settings until late 1942 also skews the official figures for the first part of the war.

 

[drgondog]

about the ONLY thing in the Allison's design (and it was by no means inherent) was the use of a higher compression ratio which limited usable boost. Any other differences were due to the supercharger technology.

It was estimated that the Allison could have made about 10% more "power" if the compression ratio was dropped to 6:1 like was done on the last models (and used on all Merlins). Of course fuel economy (range) would have suffered.

USAAF reluctance to approve WEP settings until late 1942 also skews the official figures for the first part of the war.

Shortround - whether technically correct or not there is considerable discussion regarding Allison's stubbornees in refusing to design and install backfire screens a la Merlin to protect the engine from backfiring into the blower case above 60" (for the XP-51J w/1710-119 or the P-82E w/1710-143/145). The Air Force specified 90" but Allison refused, did not try to solve the problem per Rolls, refused to attempt to modify per NAA experiment...

Whether Allison could or could not modify their -119 to -143/145 to meet spec is a moot point but it is a detailed account in Ray Wagner's book..

 

[Timppa]

Excerpt from "Mustang Designer- Edgar Schmued and the Development of the P-51" by Ray Wagner:

As the Air Force Historical Office case history explained in great detail, "The major problem in the F-82 program was the failure of the government-furnished engine".
A multitude of engine troubles so disrupted the program that total costs rose from $35 million to over $50 million, and usable aircraft were over a year late getting into service. Only one plane had been delivered by November 1947, when North American had to request an increase in partial payments, which was granted because the airframes, finished but minus engines, had to be stored at the former Vultee plant at Downey.

This cost rent, maintenance of the delicate electrical systems, insurance, and plant protection.

Schmued describes the central problem:
"The engine we had intended for this design was a Merlin. The United States Air Force was tired of paying a $6,000 royalty to England for each Merlin engine built in this country by Packard, on royalty basis. So they decided, then, to substitute an Allison V-1710 for the Merlin.

Now the Merlin engine had a very high rating, 2,270 hp with 90-inch manifold pressure, and the Air Force told the Allison to duplicate this performance. It was obvious that the way their engine was built was not suitable for these high manifold pressures. The British built a backfire screen into their engine, which made it run properly. But the Allison people refused to do that.

To help the situation along, we actually modified an Allison engine with the backfire screen that worked fine. But then, the secretary of defence was powerful enough to override the Air Force and told Allison not to do anything. Which, of course, left the F-82 with a rating far below that of a good trainer. The manifold pressure was reduced to 60 inches to keep this thing from backfiring into the blower case and damaging the engines. This was a very sad situation, because it really ruined the project. The secretary of defence (James Forrestal) favored General Motors and, I think, he had a good idea how to protect the Allison people.

The Allison people were already on the way to build jet engines and did not really like to go back and build reciprocating engines. That pretty nearly ended the project.
We flew each of these airplanes with the Allison engine at high manifold pressure, and pretty nearly on every flight, we lost the engine for sure and sometimes it was dangerous enough to lose the airplane and the pilot. It was really pathetic to see a good design simply ruined by politics and the lack of cooperation by the Allison people in building a good engine."

Actually, all of the airplanes were shop-completed by April 30, 1948, as per contract, but the final delivery with engines was not made until April 12, 1949. Even then a critical shortage of parts grounded many aircraft and 33 hours of maintenance were required for each hour flown.

 

[GregP]

If anyone out there is an Allison owner and wants factory Allison backfire screens, we stock them and have plenty for sale. We don't use them. If anyone wants to race, our "Reno" version of the Allison G-6 can run to 115" of manifold pressure without backfire screens at 3600 rpm, without the Aux-stage supercharger.

 

[Shortround6]

Allison spent a lot of time and effort getting rid of backfire screens on earlier engines as the pressure drop through the older screens was worth 1000ft of altitude or more.
There were also a number of maintenance issues with the old screens including build up of fuel dyes and general crud which lowered performance with increased hours and in some cases actual increased the risk of back fires. (combat use/maintenance vs test results?)
Perhaps the Merlin type screens were much better.
The G-6 engines in the F-82 were originally supposed to have some sort of Bendix port fuel injection system which would greatly reduce or eliminate backfire problems, since it was not ready they went to the Bendix Speed Density Carburetor and this model Allison engine may have been the ONLY engine to use this system, which apparently gave quite a bit of trouble and may not have been really ready for service use.

While Allison proposed further developments of the V-1710 at this time they were also heavily involved in production of the J-33 Jet engine.

There may have been a variety of reasons as to the V-1710-143/145 problems and lack of solutions, how many were factored in and to what extent I have no idea.

 

[GregP]

Regarding the post about the Allison not being cleared for more than 60" of MAP and needed backfire screens, I don't know where this stuff comes from, but the North American P-82 powered by the Allison V-1710-143 / 145 didn't need "backfire screens," they were eliminated in the C-15 engines of the 1930's. Saying the Allison could not pull more than 60 inches of manifold pressure without backfiring or reaching a detonation limit is just wrong. It was cleared for 82" of MAP at takeoff and 101" at WER, where it made 2,250 HP. Several were flown at 144" of MAP without any trouble.

Go to eBay and buy a pilot's manual or download one. If the engines were -143 / -145, they had no trouble at all with 60" of MAP. They still don't today and easily go to more than 100" of MAP when we run one in.

 

[drgondog]

Regarding the post about the Allison not being cleared for more than 60" of MAP and needed backfire screens, I don't know where this stuff comes from, but the North American P-82 powered by the Allison V-1710-143 / 145 didn't need "backfire screens," they were eliminated in the C-15 engines of the 1930's. Saying the Allison could not pull more than 60 inches of manifold pressure without backfiring or reaching a detonation limit is just wrong. It was cleared for 82" of MAP at takeoff and 101" at WER, where it made 2,250 HP. Several were flown at 144" of MAP without any trouble.

Go to eBay and buy a pilot's manual or download one. If the engines were -143 / -145, they had no trouble at all with 60" of MAP. They still don't today and easily go to more than 100" of MAP when we run one in.

Greg - right or wrong Schmeud was pretty emphatc and deeply disappointed that the Allison 1710-143/145 was mandated to replace the Merlin and equally vocal about the power issues and the cause. He felt the potential of the P-82 was compromised.

So, you can believe or disbelieve - I have no dog in this race one way or the other but admit I was suprised when I followed up on Timppa's comments and found that the source and the quote was dead on with no embellishment.

 

[GregP]

I don't doubt for one minute Schmued said what is claimed. What I said was the Allison was never limirted to 60" of MAP and backfire screens were eliminated in the C15 engine early on in the V-1710 line. By the time of the P-82, they hadn't been used for YEARS. People have favorites and perhaps Schmeud's favorite was the Merlin. As it happens I like the Merlin a lot and it deserves its laurels. The Allisons early on had issues but they were fixed and it, too, deserves to have its laurels that were fairly earned.

Instead, I find old myths that are flat wrong being posted in here and nobody else seems to object or even know the truth. Since we build them and support them today, we happen to have most of the facts about them along with some very unusual Allison factory tooling which is required for a good buildup of the V-1710. When built by the book, they are reliable, long-lasting, and run very well. We have no unhappy customers.

Have we had failures? Yes; things break. We've only had one quit in the air and it was a broken cam drive shaft ... luckily right over the airport, so no damage. It was flying again in a short time and still is. The other failures manifested thenselves in slight loss of oil pressure and slight roughness ... and the pilots returned to base, landed, and started troubleshooting. So, I personally have bo experience with the maligned, weak, troublesome Allison; quite the reverse.

I also know some people who flew the P-82 and really liked it, Allisons and all. I know the Merlin units were a few mph faster and don't dispute it, though I'd have to see the flight report to be convinced that they were at WER. I think they were at Military power and let it go there. The few mph difference are meaningless since the jets that were replacing them were more than 100 mph faster and, if they were to actually have been used in WWII, then either the Merlin or the Allison units would have faster than 99% of their opponets of a piston variety. Alas, they didn't make the war and almost didn't make the peace either.

Sorry Schmeud didn't like the Allison, but it did run well and did fly in the P-82, and the P-82 was a versatile aircraft in either form. I daresay, they didn't maintain the Merlin P-82's any better than the Allison uunits, and probably didn't lament the passing of the P-82 or even piston fighters in general when they were gone. I do and am a big fan of the piston fighters. At the time of the completion of the V-1710 productionm, I daresay that most if not all of the Allison designers were working on jet engines.

Thankfully, we have enough engines and parts to make about 100 Allison V-1710's, so maybe they'll be represented in the world a bit longer. Our engine backlog is 20 orders, so I think they will be around to dispell the rumors for quite some time going forward.

 

[CobberKane]

about the ONLY thing in the Allison's design (and it was by no means inherent) was the use of a higher compression ratio which limited usable boost. Any other differences were due to the supercharger technology.

It was estimated that the Allison could have made about 10% more "power" if the compression ratio was dropped to 6:1 like was done on the last models (and used on all Merlins). Of course fuel economy (range) would have suffered.

USAAF reluctance to approve WEP settings until late 1942 also skews the official figures for the first part of the war.

So drop the compression ratio back, stick on a decent supercharger and (with a bit of development) the Allison would have run with the Merlin?

 

[GregP]

Late model Allison DID run with the Merlin, and at about the same altitudes. They could have used a better supercharger and Allison could have designed one, but was turned down by the Army Air Corps on the two or mroe times it was proposed by Allison. Likely it was turned down the first time due to the pre-war situation in the U.S.A. and later during the war because the Merlin was available and a one-year or longer development was not seen as being better than using the Merlin. Whatever the reason, the Merlin was a superb engine that was not without its faults, too. The Allison was a great engine, too, but lagged the Merlin in altitude capability due to decisions made by our own government. So basically, they got what they dictated.

Dropping the compression ratio would have been a good move. If we ever build a Reno Allison, it will likely have a compression ratio of about 5.0 to 1. For wartime use, 6.0 isn't a bad place to be given the superchargers of the time. I'll look forward to hearing Shortround's reply, too.

Though I love the Allison, the single-stage units weren't the equal of the Merlin at higher altitudes. Later models were. I don't want to take anything away from the Merlin, a truly great engine. Just want to get the truth about the Allison.

Fortunately, nobody these days operates either one at high altitude, so the single-stage Allisons have no disadvantage today in warbird operations and actually have a lot of advantages. Owners like VFR fighter flying, not boring holes in an IFR sky. Maybe they go IFR going to, say, Oshkosh or some other airshow. Otherwise, they fly in severe clear and have fun playing fighter pilot.

 

[CobberKane]

Late model Allison DID run with the Merlin, and at about the same altitudes. They could have used a better supercharger and Allison could have designed one, but was turned down by the Army Air Corps on the two or mroe times it was proposed by Allison. Likely it was turned down the first time due to the pre-war situation in the U.S.A. and later during the war because the Merlin was available and a one-year or longer development was not seen as being better than using the Merlin. Whatever the reason, the Merlin was a superb engine that was not without its faults, too. The Allison was a great engine, too, but lagged the Merlin in altitude capability due to decisions made by our own government. So basically, they got what they dictated.

Dropping the compression ratio would have been a good move. If we ever build a Reno Allison, it will likely have a compression ratio of about 5.0 to 1. For wartime use, 6.0 isn't a bad place to be given the superchargers of the time. I'll look forward to hearing Shortround's reply, too.

Though I love the Allison, the single-stage units weren't the equal of the Merlin at higher altitudes. Later models were. I don't want to take anything away from the Merlin, a truly great engine. Just want to get the truth about the Allison.

Fortunately, nobody these days operates either one at high altitude, so the single-stage Allisons have no disadvantage today n warbird operation and actually have a lot of advantages. Owners like VFR fighter flying, not boring holes in an IFR sky. Maybe they go IFR going to, say, Oshkosh or some other airshow. Otherwise, they fly in severe clear and have fun playing fighter pilot.

All pretty reflective of the times, I guess; immediately prior to the war the Brits knew damm well what was coming and were doing everything they could to get ready for it, including loosening the purse strings for things like engine development. On the other side of the big ditch the isolationist lobby still had their heads firmly in the sand and as a result American investment in technology lagged. I suppose come 1944 when the USAAF was oloking for a high performing, high altitude aero-engine there wasn't much point in re-inventing the wheel by developing the Allison when the Merlin was already available as a finished product.

 

[FLYBOYJ]

Fortunately, nobody these days operates either one at high altitude, so the single-stage Allisons have no disadvantage today in warbird operations and actually have a lot of advantages. Owners like VFR fighter flying, not boring holes in an IFR sky. Maybe they go IFR going to, say, Oshkosh or some other airshow. Otherwise, they fly in severe clear and have fun playing fighter pilot.

Greg, great discussions and always great information, but the VFR/ IFR situation MAY be dictated by the FEDS in their operation letters to the owner/ operators. Furthermore, if you did have an IFR equipped recip warbird (and was blessed by the FAA to fly IFR), I don't know how IFR operations will bring any disadvantages to say Allison operation, even at higher altitudes. Maybe JimH has some input.

 

[Shortround6]

So drop the compression ratio back, stick on a decent supercharger and (with a bit of development) the Allison would have run with the Merlin?

That is pretty much it.
All engine companies were not equal.
Allison had delivered 29 engines from 1930 to the end of 1938, and some engines back in WW I and some conversions of Liberty's from water cooled to air cooled in the late 20s. Wright had built about 8,000 Cyclone 9s by the end of 1938 and several thousand whirlwinds. P&W had produced over 8,000 engines. Rolls-Royce was on their 6th or 7th V-12 aircraft engine (depends on how you count the Peregrine) plus a few other types. Bristol hand been a dominate engine maker in the world market bu was slipping to Wright and P&W. Allison had under 600 employees TOTAL ant the end of 1938 and that includes the bearing division.
It is to Allisons credit (and General motors) that they did as well as they did in developing the engines the Air Corp asked for (a lot of other companies got nowhere) AND produced the numbers they did starting from such a small base.

In hind sight things could have been done better, but that is true of many engine (and airframe) companies. The Air Corp diverting the main effort into such things as pusher engines (with extension shafts) for the Airacuda, a different pusher engine for the XP-55, the V-3420 and a few other projects may have diverted too much effort from the main effort or primary engine from what was a small company. Let's not forget the the Army was $900,000 behind in payments to Allison for work already done in the Spring of 1939 and Allison had to "forgive" the dept in order to get permission to export the engine to France and England. There is only so much work a company can do on it's "own dime" without payment before the company goes out of business.

There are a lot of drawings of British proposed aircraft with pusher engines or engines buried in the fuselage with drive shafts and gear boxes lead to various locations but I don't know how much effort R-R actually put into such schemes, as in drive shafts and gear boxes actually designed and built vs just sketched.

"On the other side of the big ditch the isolationist lobby still had their heads firmly in the sand and as a result American investment in technology lagged"

To some extent but the need for better superchargers was dependent upon the available fuel. 87 octane will only support a certain supercharger/compression ratio combinations. The US was changing to 100 octane in the very late 30s but it was NOT the British 100 octane fuel (it had little or no improved performance in rich mode) and while this will support a higher limit it is still not at the level that 100/130 fuel will support.
This was part of the reason for the USAAC "obsession" with turbo charging. They were trying to "fool" the engine into thinking (acting) like it was at sea level when it was at 20-25,000ft. The turbo (and inter cooler) would supply sea level air pressure at not over 100 degrees F to the carburetor inlet of the engine. raise the inlet temperature at the same pressure and you need better fuel to prevent detonation. Raise the pressure at 100 degrees and you need better fuel to prevent detonation.

There were TWO aircraft at the 1939 Army fighter trials using two stage P-W mechanical drive superchargers. About 2 years before R-R flew one.

 

[GregP]

Hi FlyboyJ,

Actually, what I attempted to imply is that the owners of WWII fighters don't generally want to spend much time flying straight and level; they have Bonanzas and King Airs for that. They'd MUCH rather go out and play FIGHTER PILOT, as would I if I had one (don't I wish!). So ... I know of several warbirds with Merlin, Allison, and various radial power that have wonderful glass cockpits. One of the neatest I ever saw was a Hawker Hunter ... absolutely ALL glass. Surely they are IFR capable. Heck, if you equip an RV-8 with the right instruments, it is IFR capable and is actually flown that way at times by some local people I know. They are not deined IFR priveleges by the FAA.

I am not a warbird pilot and am not aware that a warbird, if IFR capable, might be barred from it in the U.S.A.; you are probably much more up-to-date on that than I am since I don't fly warbirds. If they ARE denied IFR flight, on what grounds? The warbirds that fall out of the sky are mostly due to racing collisions, airshow collisions or overstress like The Galloping Ghost, not to engine failures.

Even the P-40 could get to 30,000+ feet, but it wasn't making a lot of power when it did due to the single-stage supercharger.

Great reply Shortround.

I believe the main issue was not designing a 2-stage or even multi-speed supercharger, but the design of the impeller case that would mate to the V-1710 and house it while being light and fitting in the engine envelope that was acceptable to the customer. It isn't especially difficult, but does take engineering time and effort, and nobody wanted to pay for it at the time, even after Allison wrote off the almost $1M (in dollars of the day, not today's dollars) owed to them. So, they lost the engine rights, were not paid all that was due, and then the Air Corps wanted them to fund a further development for the same engine they refused to completely pay for to start with. In hindsight, even when they "bought" the right to export it, the war saw the virtual confiscation of the engine production by the government for our own use. Some were exported, to be sure, but nothing like what was dangled to and anticipated by Allison.

All in all, it was not a great example of government - industry cooperation, was it? Allison was a bit mistreated, but nothing like what was done to Jack Northrop ... but that's another thread!