P51 with Allison turbo

[bob44]

Thoughts and ideas about this.
As we know, the 51 already had pretty good range, but lacked high altitude performance with the Allison.
Clearly, adding the turbo would require some modifications and probably a redesigned airframe.
What if the US Army would have pushed this from the start? Or did they plan/test any of this?
Could the 51 still be as fast and with a low drag airframe?

 

[drgondog]

No place for the turbo. Allison finally built a decent (sub par to Merlin) supercharger for the 1710-143 in 1945 but it had significant detonation issues at max boost. Still, had that engine been available in 1942, the P-51A would have been the best fighter in the world.

 

[GregP]

The P-51 could have been a very good one with a proper Allison / Turbo setup, but the issues with the turbo, intake manifold, and British fuel weren't really solved in the P-38 until the P-51's started getting to Europe. At that late date, they COULD have fitted a turbo to an Allison-engine P-51, but I believe Drgondog is right when he says there was no room in the existing fuselage, so something would have to change.

So, I believe they COULD have stretched it and shoehorned a turbo in, but I have no idea what this would do to drag or maneuverability since I don't know where the turbo would have gone in the final configuration. There may have been a way; the P-47 had a turbo and did well.

At that point, however, the Merlin-engine Mustang was doing well and I'd have to say that while I believe it could have been made to work, the question I'd need answered from a production and/or military standpoint is, "What would it gain us if we did that?" Suppose the new Allison-turbo installation performed exactly as well as the Merlin-engine P-51. Then we'd have wasted the development time and money since we already had the Merlin-engine P-51.

Of course, if there would have been an improvement, then the question might be, is the improvement worth the expense and effort to develop the Allison-turbo P-51? At this point in time, I'd have to say that the effort, while possible, would not have been worth it. It would have been better to take the Allison-turbo configuration and develop a new fighter, if we had needed one. In the event, we really didn't and the war was won, at least from the US side, with the P-51 / P-47 in Europe, the P-38 / P-40 / P-47 and the med and Pacific, and the Naval fighters in use. Another fighter might well have helped, but it would have to a good one to be developed, produced, trained on, deployed, and then have been better than what was actually fielded before the war wound down and the end was in sight. After VE Day, I'm sure development of new piston types wound down rapidly since the Europe-based fighters could be mostly moved to the Pacific and would have been a HEAVY reinforcement.

All in all, the P-51 did quite well with the Merlin (and still does). I am a big Allison fan and while the Allison engine-turbo could have been developed in a single-engine fighter, I am not sure the benefit would have been worth the development time, effort, cost, etc. Now if the Merlin had NOT been available to us, the Allison-turbo would have been REQUIRED (or an Allison with a multi-stage supercharger), but that is a what-if that didn't occur and I'd not care to predict any developments that might have been possible.

I WILL say that P-38 pilots who have been to Joe Yancey's shop said that after the issues were solved, they had no trouble with the P-38 at all except for the extra maintenance required from a twin over a single. The engine type would not play into that, two engines of ANY sort require more maintenance than one. I believe they COULD have come up with simpler engine/prop controls for the P-38 the way the Fw 190 team did for that fighter. It had a single-lever throttle and considerably eased the combat workload on the pilots. Configuring a P-38 for combat from cruise configuration rapidly was difficult. but was a necessary skill for the P-38 pilot.

It may well have been the same for the new Allison-turbo fighter pilot, too, unless different philosophies had been followed in engine control design.

I personally think the XP-40Q had great potential, maybe even WITH a turbo, but it was not proceeded with, so any postulation in that direction cannot be substantiated and is another "what if" that cannot be settled in a debate of existing evidence.

 

[tomo pauk]

Thoughts and ideas about this.
As we know, the 51 already had pretty good range, but lacked high altitude performance with the Allison.
Clearly, adding the turbo would require some modifications and probably a redesigned airframe.
What if the US Army would have pushed this from the start? Or did they plan/test any of this?
Could the 51 still be as fast and with a low drag airframe?

We can recall that, when going for Merlin installation, whole cooling system was redesigned, along with the part of the fuselage 'containing' the coolers. Radiators themselves were now of rectangular shape, vs. circular shaped ones in Allison Mustang. Intercooler radiator was also incorporated. Similar thing would've happened if it was decided to incorporate turbocharger. The turbo itself should not require any more volume than fuselage tank (85 gals). The exhaust piping will be going each side of fuselage, before entering the turbine.
Provided that NAA pulls this thing in a timely manner, in early 1943 the resulting Turbo Mustang should have at least 1325 HP, from ~10000 ft to 25000 ft (about twice as much with non-turbo Allison at 25000 ft?). That should provide plenty of performance between 20-30 kft, exactly where needed to confront LW in the ETO. In second half of 1943, that would be 1425 HP up to 25000 ft. Hopefully 420 mph should be attained at 25000 ft, or about 20 mph more than Fw-190A-5/A-6.

 

[nuuumannn]

Would the extra weight and complexity of the turbo installation be worth the effort, when excellent performance could have been attained by fitting a Griffon to the P-51 with less redesign of the airframe than what fitting a turbo-supercharger unit would have done? It didn't take RR or Packard long to get a Merlin engined Mustang flying after initial proposals.

 

[tomo pauk]

Weight of a turbo-supercharged V-1710 (engine, turbo, cooling, inter-cooler, oil system) was in the ballpark with weight of 2-stage Merlin (engine, all of the cooling systems). The complexity would be only half of of the complexity of P-38 powerplant. Since the turbo-V-1710 was giving circa twice power when compared with single stage V-1710, I'd venture to say that it would be worth it. Once in late 1943, it will provide 1600 HP at 25000 ft, more with 150 PN fuel from Spring of 1944.
The Griffon engine P-51 sounds fine. What was not fine was availability of the Griffons for the USA, and, prior 1944, you have only single-stage Griffon in limited production. Such a Griffon Mustang should be faster than Spitfire XII - maybe 20-30 faster? So maybe 410-420 mph at 20-25000 ft - should've beaten the Fw-190A-5/A-6.
The Merlin Mustang was flying before 1943, again the US production of 2-stage Merlins was lagging enough for NAA having stocks of airframes waiting for engines in mid-1943.

 

[drgondog]

Remember the cockpit floor in the A/B/C model sat on top of the wing, the left hand side of the cockpit was occupied by aileron/rudder trim controls. Air intake would have to extend outside the existing lines and ducting would interfere with wheel and gear uplock as well as hole the main spar.

So, it seems that the intake duct would have to be placed just aft of the 85 gallon fuel tank and turbo would be located between fuel tank and tail wheel. Permanent weight of a couple of hundred pounds would create enough aft cg re-location that the fuselage tank capacity would have to be eliminated - as well as all future Berlin escort missions.

 

[tomo pauk]

Admittedly, shoehorning a turbo should represents a challenge. There were several pipes leading from engine to the cooler 'bay' (2 for each oil radiator, 'Prestone' radiator and inter-cooler radiator for Merlin Mustangs). We also have fuel, oxygen and hydraulic lines between cockpit and wing; wing tank adds another fuel pipe there. In case the turbo V-1710 is installed, only one 'pipe' (air duct, actually) emanating from inter-cooler would be passing under the cockpit. That duct will be of greater cross section than both of inter-cooler pipes, but hopefully not to much?

As for the fuselage tank and CoG issues, it would be too much to ask anything close to 85 gals. With only wing tanks and 2 x 75 DTs, the Merlin Mustangs were good for 460 miles at 25000 ft. We should have about the same for this turbo contraption? Less, more?
460 miles is indeed not enough for Berlin and back missions, but it's almost 100 miles further than next best escort of 1943 was capable for, enough for missions vs. Sttutgart, Schweinfurt and Hamburg.

 

[davparlr]

I think the P-51 design was a snug, efficient package. The turbo installation should be maybe two-thirds the size of the P-47's? It probably could be done for the A but not having a extended range tank would certainly hamper it escort mission and may have some lumps and bumps. Still, in 1941-42, it certainly could have been a threat to the Germans over a wide range of western Europe. Of course it may have spurred the Germans to develop the advanced Bf-109s and Fw-190D-9 earlier and maybe other advanced designs and be prepared for the deep penetration daylight bomber raids with escorts in 1944. Things change when things change.

 

[gjs238]

Other than the de Seversky/Kartveli designs, did anyone else locate the turbo aft of the cockpit in a single-engine plane?
The others I've seen attempted to locate the turbo up forward.

 

[tomo pauk]

I think the P-51 design was a snug, efficient package. The turbo installation should be maybe two-thirds the size of the P-47's?

P-51 was a great package. Not too a tight one, NAA was able to shove in a decent fuel tank aft the pilot - should have enough space for turbo-supercharger in a part of the volume used for that tank. As for the turbo weight - the P-47s 'engine accessories' (turbo inter-cooler included) weighted 950-980 lbs, while the weight for P-38-J's turbo+accessories was 940 lbs (for two engines).

It probably could be done for the A but not having a extended range tank would certainly hamper it escort mission and may have some lumps and bumps. Still, in 1941-42, it certainly could have been a threat to the Germans over a wide range of western Europe. Of course it may have spurred the Germans to develop the advanced Bf-109s and Fw-190D-9 earlier and maybe other advanced designs and be prepared for the deep penetration daylight bomber raids with escorts in 1944. Things change when things change.

The P-51 is out for 1941. In 1942, the Germans have their share of problems with Fw-190, particularly with it's engine (full power was allowed from Oct 1942), so the Allied aircraft going 400 mph at 25000 ft will be able to compete there (i'm not sure that RAF will attempt long range daylight escort back then?). In 1943, the Fw-190 is okay, but the turbo P-51 can do better. The another LW fighter, Bf-109, has it's own engine-related issues in 1943, it can do circa 410 mph. In early 1944, the Jumo-213 is not as trouble free as in early 1945, and V-1710 has 1600 HP at 25000 ft.
The availability of long range fighter early on should spur similar development of P-47 (wing drop tanks already in 1943?), Spitfire (more emphasis production of Mk. VIII? rear fuel tank earlier for the Mk.IX, not in late 1944? rear fuel tank for Mk.VIII, even if it's only 30 gals?), Typhoon/Tempest (Tempest starting with both LE tanks, instead of only one)?
Third issue are the pilots - who would fly the early Doras in 1944, if the LW is attrited already in 1943?

 

[nuuumannn]

Thanks for the answer Tomo, I guess I don't see any real performance advantages fitting a turbo engined Allison would offer, bearing in mind the complexities and alterations required for doing so would add when good altitude performance for the P-51 was gained by fitting a two-speed-two-stage Merlin. The secret to the Mustang's success was its aerodynamically efficient airframe. I can't really see any real gain from doing this over what was actually investigated (the British did consider building a Griffon engined variant) and carried out, other than having to carry out considerable redesign on the fuselage, which would take time - not an ideal situation in wartime.

 

[tomo pauk]

Against the 2-stage Merlin, the turbo V-1710 will offer next to nothing in terms of performance. On the other hand, the US-built 2-stage Merlin was not available in quantity before second half of 1943, meaning the Merlin Mustangs were available in penny packets prior 1944 (only 2 Groups in Feb 1944, vs. ~15 Groups of P-47 in same time?). Quote from US hundred thousands: "July 1943: only 173 Merlin engines have been received by North American when 534 P-51B airframes have been completed".
Contrary to turbo V-1710 vs. 2-stage Merlin situation, we can note that single stage V-1710 was offering maybe 700-750 HP at 25000 ft (with exhaust thrust and ram effect accounted for). Ie. the turbo V-1710 was offering almost twice of that already in 1943, while available in quantity - that would be it's sole, but crucial advantage vs. 2-stage V-1650.
As for redesigns of the fuselage - the service P-51 endured redesigns of it's rear fuselage at least twice during war time - once it was the complete change of it's cooling system to cater for Merlin, second time it was when bubble top was added. The rear fuselage internals were re-hashed to allow for fuselage tank to be installed.
The lightweight Mustangs (F/G/H) were featuring a whole new airframe vs. P-51/A/B/C/D/K - yep, everything was changed: wing, fuselage, fuel system, tail, undercarriage, canopy.

 

[nuuumannn]

That might be the case, but since you comparing later P-51s and the changes made to produce them, then modifying these to fit an Allison turbo surely would be unecessary and is a pointless comparison since with the Packard Merlin the Mustang could produce the performance that a theoretical Allison turbo could. The P.51H was/is regarded as one of the fastest production piston engined fighters.

The other factor was time; would incorporating an Allison turbo have taken place in a similar time frame? Surely if there was no benefit in performance and the time frame was not going to get the aircraft in service sooner, what would be the point? The Mustang offered excellent performance in 1942/3 at low altitudes, but the RAF had a high altitude fighter in the Spitfire Mk.IX in 1943. The Americans had the P-38 and P-47 in service. The other issue was, just how far advanced was Allison in modifying their engine to incorporate a turbo unit? Was there much research done already and was such an engine fit ready for service use? I doubt it. Fitting a Packard Merlin was a far safer bet; the engine was tried and tested and the modifications could be done without too much alteration to the airframe in a workable time frame.

Even though the Sixty Series Merlin was not in production by Packard when the idea was first put forward, both Roll-Royce and Packard toyed with the idea of the Merlin XX being fitted to the Mustang as an interim because of feared shortages of Merlin 61s that were to be allocated to Spitfires. In mid 1942 Wilfred Freeman was in discussion with Hives of Roll-Royce about a 'Super Mustang' powered by a Griffon 61;

"We must get the North American Company to go absolutely full out on a Super Mustang with the Griffon 61, so as to get the best of both aircraft design and engine design with backing at the highest level and unstinted help in the way of drawing office staff and jig and tool makers, we might beat all records for getting a new aircraft developed and into production." Freeman to Hives dated 30 June 1942.

 

[GregP]

You think the Allison turbo offers next to nothing compared with a 2-stage Merlin? How is that possible?

It might not offer much in the way of advantages, but would be about as fast and offer about the same horsepower, perhaps more higher up ... depending on the turbo setup. I think they would have been quite competitive with one another. The Allison also had better fuel consumption.

When I think of the P-82, I know the Merlin-powered unit was 7 mph faster (482 mph versus 475 mph), but the Allison-powered unit had a service ceiling about 3,200 feet higher (42.200 feet versus 39,000 feet), so it was making good HP up high and would have been competitive. Still, as I said earlier, I don't think the P-51 development would have offered enough significant advantages to make it a worthwhile endeavor over the Merlin-engined Mustang.

If piston development had continued after WWII, these engines, the Merlin and the Allison (along with the Griffon), would probably have been very useful. That they weren't was mostly due to the arrival of jet engines.

 

[nuuumannn]

It might not offer much in the way of advantages, but would be about as fast and offer about the same horsepower, perhaps more higher up ... depending on the turbo setup. I think they would have been quite competitive with one another. The Allison also had better fuel consumption.

To be honest, I don't know if it would have comparable performance to a Merlin engined Mustang or not, Greg; my point is, would the effort in converting the Allison Mustang to take a turbo be worth it and could it be done in a similar time frame as putting a Merlin or Griffon in? Would it be as capable as the P-51 was in terms of performance and range? I don't believe the end result would have been significantly better than what was produced and I think it would have taken longer to get into service. The Sixty Series Merlin and Griffon were ready and in production in mid 1942 - converting the Mustang to take the Merlin didn't take too much in the way of modification. Could the work that Rolls-Royce, Packard and North American carried out to get the Merlin engined Mustang in service have been done in the same time frame by Allison developing a turbo with North American? I doubt it.

 

[GregP]

OK, I understand your point, and agree that it probably would not have been worth the effort since the Merlin-engined Mustang was a pretty good unit and still is. I think the Allison-turbo thing COULD have been done and could have resulted in a good aircraft, though probably not quite looking like a Mustang as we know it. I'd want the issues with the P-38 turbo installation ironed out first ... and then perhaps some packaging could have been figured out.

I don't think a WWII-era turbo would have been as compact a package as the Merlin since compact turbos under the hood of something like a Subaru WRX STI weren't invented yet, but the plane could have been built and they probably could have figured out a way to make if fly to Berlin and back as an escort. Nobody thought we could really intercept Yamamoto with P-38's either, but we did at extreme range.

The thing is, I anticipate good performance, but no major improvement over the Merlin package, so I don't think it would have been worth the effort at the time since the Merlin P-51 was a good one right out of the box. So maybe we agree here, and think similarly about the exercise. Perhaps I just say it differently.

Unless Allison had done an independent, 2-stage integral supercharger unit similar to the Rolls Royce unit, the V-1710 was absolutely tied to the turbocharger as the government specified the high-altitude boost system to be. Today we'd probably put two small turbos in, one on each side, but that wasn't a possibility in 1942 - 1944. The technology of the time favored the design of the Merlin 2-stage supercharger for compactness and lower weight for an engine package to be used at higher altitudes in a single-seat fighter.

It would be a VERY interesting exercise today to take an Allison and a Merlin and use modern technology to get max performance. I'd bet they would be very close, as they were at low altitudes in WWII. I majorly fault the US government for requiring the turbo, but Allison also bears some blame for not seeing the light and moving ahead with what was needed for the war. Maybe the real culprit was unavailability of money for the 2-stage development.

It happens in small shops with critical cash flow. I think General Motors could have provided the money / resources and just didn't out of corporate greed, but I might be off base there.

 

[tomo pauk]

That might be the case, but since you comparing later P-51s and the changes made to produce them, then modifying these to fit an Allison turbo surely would be unecessary and is a pointless comparison since with the Packard Merlin the Mustang could produce the performance that a theoretical Allison turbo could.

You may recall that I've never talked that a Turbo Mustang will offer better performance than Merlin Mustang. The only, yet important feature of 'my' contraption is that it can offer a workable high altitude and long range capabilities already in early 1943, in good numbers, while for the Merlin Mustang the Allies must wait until late 1943.

The P.51H was/is regarded as one of the fastest production piston engined fighters.

Agreed.

The other factor was time; would incorporating an Allison turbo have taken place in a similar time frame? Surely if there was no benefit in performance and the time frame was not going to get the aircraft in service sooner, what would be the point? The Mustang offered excellent performance in 1942/3 at low altitudes, but the RAF had a high altitude fighter in the Spitfire Mk.IX in 1943. The Americans had the P-38 and P-47 in service. The other issue was, just how far advanced was Allison in modifying their engine to incorporate a turbo unit? Was there much research done already and was such an engine fit ready for service use? I doubt it. Fitting a Packard Merlin was a far safer bet; the engine was tried and tested and the modifications could be done without too much alteration to the airframe in a workable time frame.

It would be easier if you've stated what Mustang is compared with Turbo Mustang, when talking about 'no benefit in high performance' Compared with historical Allison Mustangs, the turbo would offer significant boost in hi-alt performance.
RAF did have the Mk. VII/VIII/IX Spitfires, I doubt they would've declined an aircraft that can do a bit better, while having far better combat radius. We can recall that Spitfire V was mainstay of the RAF until when, January 1944?
The Americans did have the P-38, but not in big numbers - Gen. Kenney in Pacific was clamoring for P-38s, could not get them enough, so was forced to use the sub-par (in his words) P-40s and loathed the P-39s. Even in 1943, there were only 2 groups of P-38 in ETO (~100 fighters).
P-47B was not a combat-worthy aircraft, the P-47C was usable, but it took some time to have a usable combat radius from P-47. Last but not least, the US can buy 3 Turbo P-51 for 2 of P-38 or P-47, and build them faster than those, so they can replace the P-40s.
Allison was outfitting it's engines with turbo from 1941 (serial produced P-38, up to 1150 HP to 20-25000 ft), by mid 1942 the issues with turbo regulator were dealt with, while the engine power rose to 1325 HP (1st until 15000 ft, than until 25000 ft by late 1942). From March 1943, 1425 HP was available for new P-38H.

Even though the Sixty Series Merlin was not in production by Packard when the idea was first put forward, both Roll-Royce and Packard toyed with the idea of the Merlin XX being fitted to the Mustang as an interim because of feared shortages of Merlin 61s that were to be allocated to Spitfires. In mid 1942 Wilfred Freeman was in discussion with Hives of Roll-Royce about a 'Super Mustang' powered by a Griffon 61;

"We must get the North American Company to go absolutely full out on a Super Mustang with the Griffon 61, so as to get the best of both aircraft design and engine design with backing at the highest level and unstinted help in the way of drawing office staff and jig and tool makers, we might beat all records for getting a new aircraft developed and into production." Freeman to Hives dated 30 June 1942.

Thanks for that.
We can read that 1st prototype of the Spitfire XIV (= Griffon 61), former Spitfire VIII, was flown in October 1943, 1st serial produced Spits XIV were issued in May/June 1944? That would relegate the Griffon Mustang for 1945 service use?

 

[wuzak]

We can read that 1st prototype of the Spitfire XIV (= Griffon 61), former Spitfire VIII, was flown in October 1943, 1st serial produced Spits XIV were issued in May/June 1944? That would relegate the Griffon Mustang for 1945 service use?

The first VIIIG flew early in 1943, production XIVs machines were coming off the line by late 1943 and were entering service in January 1944.

 

[tomo pauk]

Thanks for clearing that out, I stand corrected.
The Griffon Mustang should've been a most interesting aircraft - 470-480 mph already in 1944?