Supercharger Development & Aircraft Design Policy (USAAC)

[pbehn]

I know from some of your previous posts that you are an experienced bike racer. I also know that you know the difference between single stage and two stage superchargers.
I have no first hand knowledge of how a two stage system with a mechanical stage suffers from "lag" in it's turbo auxiliary stage. I would suspect that the throttle response is better than a turbo alone. I have read that the P-38 may have suffered and that there were two methods to get around it. The discredited high rpm, low boost cruise which used more fuel, wore out the engines quicker, over cooled the intake mixture and kept the large GE turbos at low rpm (and a GE turbo is absolutely huge compared to a modern car or bike turbo). or the preferred low rpm, high boost method of cruise, the turbo is spinning thousands of RPM higher, the intake mixture is not puddling in the manifolds and there is a fair amount of boost available to increase power (torque) as the the throttle is opened (or prop control moved?) to accelerate the engine/props.

You also have how the propeller acts as it goes from cruise to high speed. I believe (but could very well be wrong) that the prop is in coarse pitch (or close to it) to give the most speed for a low rpm. when the throttle is opened up and rpm increased the prop should go to a finer pitch as the engine gains rpm and then as the power increases and the speed increase the prop should go back to coarse pitch as high speed is approached?

Most of the people trying to compare modern cars/bikes to WW II engines seem to forget the aircraft engines were operating in rather thin air insead of the "soup" that most cars and bikes operate in. The AIrcraft engine at 20,000ft is only getting about 53% as much "air" (by weight) per cubic ft as the engine at sea level.

I agree completely, my point was that it is a sensible choice to put a turbo behind the pilot in a P-47, putting a turbo in the back of a car or bike may work "just" but would be very difficult to drive.

 

[K5083]

Dragon dog: "Wow - all those flaws and yet it was a top line (forAAF) Interceptor, escort fighter, fighter bomber, recon (both tactical and high-altitude), air superiority figter. "

Just down the road from me is Mount Farm, used to be home for the USAAF photo recce aircraft. They preferred Spitfires. P-38 was pretty disappointing as a fighter in the 8th. It did fine against enemy planes with a 80mph speed disadvantage in the SWPA, but not in the ETO. That's because it wasn't a top fighter in terms of the state of the art. Your list of roles to illustrate versatility includes four fighter distinctions which do not require versatility. It would fly in the same config for each of them. And as mentioned it wasn't a top camera ship either. Because all the things it could do were done as well or better by a lashup single made by NAA from the allied parts bin (yes, I know, P-51 is more than that) and by a largely private venture Republic design with a navy engine, it may represent the best the AAC-specified fighters could do, but it was the most mediocre fighter with the most expense, the steepest learning curve and the least development potential.

What this says mostly is that the USAAF succeeded mostly in spite of its procurement process, not because of it.

 

[tomo pauk]

If I may.

...
Because all the things it could do were done as well or better by a lashup single made by NAA from the allied parts bin (yes, I know, P-51 is more than that) and by a largely private venture Republic design with a navy engine, it may represent the best the AAC-specified fighters could do, but it was the most mediocre fighter with the most expense, the steepest learning curve and the least development potential.

What this says mostly is that the USAAF succeeded mostly in spite of its procurement process, not because of it.

Despite the diclaimer - now it seems that P-51 was just a step over an improvisation?
The P-47B and subsequent were not a fruit of private venture - USAAC regarded that P-47A (the never-flown fighter with non-turbo V-1710) and P-44 (R-2180A + turbo) will not cut it, and preferred a fighter with turboed R-2800 instead of the two.
Thunderbolts never used Navy engine(s).
P-38 have had a lot of development potential, both via improved aerodynamics and more engine power.

 

[Shortround6]

Not all photo reconnaissance was the same.

P-38s ( or F series) could (but not always did) carry an array of cameras the single engine planes could not.
Both Cameras and film evolved during WW II.

 

[IdahoRenegade]

SIlly the designers back then: they opted to put some distance between engine and turbine so the exhaust gasses can cool a bit, instead of bolting the turbine on the engine block and wait for tubine itself to desintegrate in mid-air due to temperature overload.

From an engineering/efficiency standpoint, that doesn't make a lot of sense. Turbochargers are "thermal devices", they essentially extract waste power (in the form of heat) from the exhaust to drive the compressor impeller. That is one of their main advantages over mechanical superchargers that consume engine horsepower (and fuel) to drive the compressor. Extending the distance between the engine and turbo, and allowing the exhaust to cool, makes the whole system less efficient. In addition, all that ducting to the turbo is transfering heat to other portions of the aircraft-most likely not a desirable situation.

Would the reason to do that be limited to the metallurgy of the day, or perhaps the turbocharger design (not incorporating cooling) not allowing the turbo to survive at higher temperatures? Modern automotive turbo systems focus on keeping exhaust runs to the turbo short to maximize efficiency. But the materials of today are far from those of the 1940s.

I suspect a big part of the issues with turbocharger system packaging wasn't the turbocharger (it really wasn't that big-though managing heat was an issue) but rather the intercooler system. The air to air intercoolers of the day were huge (especially the P-47s). And to keep the pressure drops in both the intake and exhaust plumbing manageable with those long lengths of ducting, the diameter of those ducts were large.

I've always been impressed with the turbo installation on the '38 (and the bombers for that matter). The TC was mounted such that the outer surface/exhaust side of the turbo was approximately flush with the outside of the cowling, with reasonable airflow around the turbo to help cool it. This also meant that there was no downstream resistance downstream of the turbo impeller. The '47s install always seemed a bit "kluged" with the turbo located all the way back in the rear of the fuselage. This means long ductwork with lots of pressure drop, lots of heat dumped into the fuselage, and an exhaust pipe downstream of the turbo, leading to backpressure at the turbo (though probably minimal).

I always felt that both the '47 and '38 could have benefited from air to liquid intercoolers, rather than air to air. Coolant has a much higher specific heat than air, allowing the placement of a small(ish) HE in the induction air stream, rather than ducting all that compressed air to the intercooler. The secondary HE can be packaged where most efficient aerodynamically. It's a lot easier to run a couple small coolant lines than ductwork.

 

[Shortround6]

by a largely private venture Republic design with a navy engine,

Hmmm, The Army B-26 bomber was entering squadron service well over a year before the F4U let alone the F6F entered Squadron service. Were the B-26 engines navy engines?

A lot of rumor/innuendo and not much fact in that post.

it also disregards timing. How many photo recon Spitfires were in Australia or the SWP in the summer and fall of 1942, how may PR SPits or PR Mustangs were in North Africa in late 1942 or 1943? It took until late (Sept/Oct) 1944 for the Number of P-51s built to exceed the number of P-38s built. yes the P-51 was better but for 2 1/2 years the P-38 did an awful lot of work. and continued to do so through the fall and winter and spring of 1944/45.

 

[tomo pauk]

From an engineering/efficiency standpoint, that doesn't make a lot of sense. Turbochargers are "thermal devices", they essentially extract waste power (in the form of heat) from the exhaust to drive the compressor impeller. That is one of their main advantages over mechanical superchargers that consume engine horsepower (and fuel) to drive the compressor. Extending the distance between the engine and turbo, and allowing the exhaust to cool, makes the whole system less efficient. In addition, all that ducting to the turbo is transfering heat to other portions of the aircraft-most likely not a desirable situation.

Would the reason to do that be limited to the metallurgy of the day, or perhaps the turbocharger design (not incorporating cooling) not allowing the turbo to survive at higher temperatures? Modern automotive turbo systems focus on keeping exhaust runs to the turbo short to maximize efficiency. But the materials of today are far from those of the 1940s.
...

There you go - indeed, it is advantageous to have turbocharger as close to engine if the turbo can stand temperature build-up. In late 1930s/early 1940s, with engine powers already being 1100-1300 HP, the metalurgy of the day was not judged to provide a relaible answer to the problem, thus turboes were located several feet, if not several yards away from engine itself. It will took either self-cooling (hollow) blades for turbines, or, better yet, advances in metalurgy in order to circumvent the problem,

Hollow blades were used on Wright 800TSB turbo (~1000 produced for the Curtiss SC, and a handful for the XF2M-1), and on turbo used on BMW 801J, plus on German jet engines. Ford patented a 2-stage turbocharger with hollow blades for their 1650 cu in V12 aero engine (in 1941?).

 

[thunderbird]

double post

 

[drgondog]

Dragon dog: "Wow - all those flaws and yet it was a top line (forAAF) Interceptor, escort fighter, fighter bomber, recon (both tactical and high-altitude), air superiority figter. "

Just down the road from me is Mount Farm, used to be home for the USAAF photo recce aircraft. They preferred Spitfires. P-38 was pretty disappointing as a fighter in the 8th. It did fine against enemy planes with a 80mph speed disadvantage in the SWPA, but not in the ETO. That's because it wasn't a top fighter in terms of the state of the art. Your list of roles to illustrate versatility includes four fighter distinctions which do not require versatility. It would fly in the same config for each of them. And as mentioned it wasn't a top camera ship either. Because all the things it could do were done as well or better by a lashup single made by NAA from the allied parts bin (yes, I know, P-51 is more than that) and by a largely private venture Republic design with a navy engine, it may represent the best the AAC-specified fighters could do, but it was the most mediocre fighter with the most expense, the steepest learning curve and the least development potential.

What this says mostly is that the USAAF succeeded mostly in spite of its procurement process, not because of it.

Methinks you are trying too hard. pull up a list of fighters that went operational that a.) could accept and deliver a 2000 pound torpedo - even if not used, b.) first to accept plumbing and pylons to add 60, 75, 110, 150, 165, 330 gallon fuel tanks, c.) Capable of and actually delivering 2x2000 pound bombs, d.) capable of and operationally deployed with bombardier and Norden bombsight, e.) capable of installing radar and radar operator for night missions, f.) capable of escorts >1000 mi radius and 2500+ mile ferry, g.) deployed operationally as both tactical and long range high altitude recon, h.) typically carrying 2000+ bomb load plus rocket stubs for tactical CAS, I.) performed as fast climbing short AND long range interceptor.

Yes it had a long gestation period - but it was, along the Mustang I the first US operational fighter to reach combat and stay the duration to the end of WWII.

Personally I favor the Mustang but I have to tip my hat to the P-38 for doing what was required of it - and more - as my personal choice for most versatile fighter of WWII. It was never modified for carrier ops or true stabilized and controllable dive bombing role - but I'm having a hard time understanding why you don't think it was 'versatile'??

 

[drgondog]

Dragon dog: "Wow - all those flaws and yet it was a top line (forAAF) Interceptor, escort fighter, fighter bomber, recon (both tactical and high-altitude), air superiority figter. "

Just down the road from me is Mount Farm, used to be home for the USAAF photo recce aircraft. They preferred Spitfires. P-38 was pretty disappointing as a fighter in the 8th.
The ETO was not a happy time for P-38 until post March 1944, but after that it was an excellent platform for air to air and air to ground and high altitude - long range recon - and used to the EOW. It could go farther than ay Spit. The preference at Mt. Farm for Spit can be traced to the first deployment of the F-4 all the way to the Mid J F-5

It did fine against enemy planes with a 80mph speed disadvantage in the SWPA, but not in the ETO. That's because it wasn't a top fighter in terms of the state of the art.
See comments above. Operationally the P-38J-15 was a game changer and the -25 with boosted ailerons and dive flaps enabled the 55th and 479th to have spectacular days before converting to Mustangs in Summer 1944.

Your list of roles to illustrate versatility includes four fighter distinctions which do not require versatility. It would fly in the same config for each of them. And as mentioned it wasn't a top camera ship either. Because all the things it could do were done as well or better by a lashup single made by NAA from the allied parts bin (yes, I know, P-51 is more than that) and by a largely private venture Republic design with a navy engine, it may represent the best the AAC-specified fighters could do, but it was the most mediocre fighter with the most expense, the steepest learning curve and the least development potential.

Hmmm, how well did any fighter you have in mind to compare vs P-38 flew well (or at all) on one engine - a nice characteristic in SWP, or accepted a bombardier/Norden sight to lead a FG on a level bombing mission? Or tested for, and capable of, delivering a torpedo or a radar mod w/operator in back seat?

What this says mostly is that the USAAF succeeded mostly in spite of its procurement process, not because of it.

No argument there but has nothing to do with versatility for different missions across all theatres (except Ost Front).

 

[pbehn]

Just down the road from me is Mount Farm, used to be home for the USAAF photo recce aircraft. .

If living close to military airfield confers special expertise, everyone in Kent and East Anglia is a forum guru. I can walk to three former WW2 airfields and on a bicycle could reach well over a dozen.

 

[P-39 Expert]

I think the configuration for the P-38 was chosen partly because of scarce funds at the time. They called it an interceptor to get into a new category and didn't use a single long fuselage/tail arrangement to keep the Army from turning it into a bomber or transport to save money through commonality. Just an opinion.

Agree that turbos worked well in multi engine planes because of the extra room in the nacelle for the turbo. Trying to shoehorn a turbo into a single engine plane is destined to look like a Thunderbolt. Mechanical superchargers are better for single engine planes. Less space, less ducting, less heat for almost the efficiency.

The Lightning was a cutting edge technical achievement at the time. The turbos kept it from being ready for combat until 1943 (Dec. '42). It had all the measureables (speed, climb, range, ceiling, armament) but I wouldn't want to be in combat against the Luftwaffe in one. LW are just as fast, climb just as well and can dive away anytime they want. Big combat disadvantage.

And it cost at least twice as much as a single engine plane, twice the maintenance, twice the fuel, twice the pilot workload but it wasn't twice as capable. By the time Lockheed got most of the bugs out of them (P-38L) they were being relegated to ground attack in Europe. Fine for the Pacific, maybe the Med, but not Europe. Hindsight tells me we would have been better off making twice as many single engine planes.

 

[pbehn]

Rolls Royce considered the use of turbos for the Merlin , I found this on a little know web site. http://wwiiaircraftperformance.org/merlin-lovesey.pdf

 

[GreenKnight121]

Not all photo reconnaissance was the same.

View attachment 563919

P-38s ( or F series) could (but not always did) carry an array of cameras the single engine planes could not.
Both Cameras and film evolved during WW II.

Nice photo of the camera in the nose of a P-80A recon bird (F-80 Shooting Star) - I assume you included it for contrast to the P-38's suite of cameras?

 

[fubar57]

Actually the camera is in an RF-80A from this site...Every Man A Tiger 8th/15th Homepage

 

[Shortround6]

Sorry guys, mis-captioned picture on one website.

 

[IdahoRenegade]

Dragon dog: "Wow - all those flaws and yet it was a top line (forAAF) Interceptor, escort fighter, fighter bomber, recon (both tactical and high-altitude), air superiority figter. "

Just down the road from me is Mount Farm, used to be home for the USAAF photo recce aircraft. They preferred Spitfires. P-38 was pretty disappointing as a fighter in the 8th. It did fine against enemy planes with a 80mph speed disadvantage in the SWPA, but not in the ETO. That's because it wasn't a top fighter in terms of the state of the art. Your list of roles to illustrate versatility includes four fighter distinctions which do not require versatility. It would fly in the same config for each of them. And as mentioned it wasn't a top camera ship either. Because all the things it could do were done as well or better by a lashup single made by NAA from the allied parts bin (yes, I know, P-51 is more than that) and by a largely private venture Republic design with a navy engine, it may represent the best the AAC-specified fighters could do, but it was the most mediocre fighter with the most expense, the steepest learning curve and the least development potential.

What this says mostly is that the USAAF succeeded mostly in spite of its procurement process, not because of it.

It was the first 400mph fighter. It was one of the first fighters that combined a usefully long range, with good performance at all altitudes. It was the only long range, high performance fighter capable against the best of the enemy fighters in the allied arsenal until December 1943 when the 51B finally saw combat. It could and did carry a 4000 bomb load (what other fighter did that? I think the Corsair claimed a 4000lb rating on late versions, but don't know that they ever flew combat that way). It made more power than most (all?) mechanically supercharged fighters at high altitudes, throughout the war. It had one of the most effective gun packages of the war (not limited by convergence of wing mounted guns). It was a tough fighter, able to take damage, with the redundancy of a 2nd engine to bring you home with a dead engine over enemy territory.

No doubt the Mustang was a great fighter and did a number of things very well. But is showed up relatively late in the war (in a high altitude-capable variant anyway), was relatively fragile (at least compared to the '38, '47, Corsair and Hellcat). It had a much lower bomb carrying capability. And of course, one hit anywhere in the cooling system and it wasn't likely to make it home.

 

[IdahoRenegade]

Simply stated - Material Division under leadership of Oliver Echols, did not believe that a two speed-two stage supercharged engine was feasible. In-line engines weren't even considered with enough regard to experiment until Allison passed the 1000 Hp bench testing about the time the RFP for P-38 and P-39 emerged - driven by Kelsey - and as described adequately by Kelsey quotes in Bodie's P-38 work. The Ge Turbo was required at that time to push the P-38 performance envelope above 15,000 feet enough to qualify as a high altitude 'Interceptor'. The P-39 was described by Kelsey as 'the small solution' and never achieved promised capability until the P-63 limped out with the Allison auxiliary second stage - requiring a major redesign to accommodate the engine.

Secondly, the only promising in-line engine manufacturer that's actually developed a reliable supercharged engine was Allison. Allison was underfunded by GM, and starved for R&D funding, but there is no evidence that Allison believed they could develop such an integral 2s/2s engine if given the funding.

As to many references to Kelsey as a visionary, history doesn't support him as ever being an advocate of the P-51 until he was out from under Echols and in ETO operations where the P-38 was under-performing to the P-51B.

General Malin Craig was Army Chief of Staff and retired days before the Germans attacked Poland, not a staunch supporter of AAF vision but also not regarded as a meticulous bean counter looming over Arnold's shoulder regarding approved budgets for R&D. Simply stated again, Arnold drove R&D for AAF while under Malin and basically throughout WWII. Bomber development was number one priority until events in Europe cast doubts of unescorted bombers being able to carry out AWPD-1 to destroy enemy war making capability. Turbo supercharged engines, whether on interceptors or high altitude bombers were central to specifications for new development. The only other alternatives considered were to boost in-line power to continue to raise the ceiling of singles stage engines. The Continental, etc. were attempts that repeatedly failed to drive down weight and volume as well as achieve reliability requirements.

Materiel Command had a miserable record for developing high performance fighters capable of long range escort to support AWPD-1 and -2. Lack of in-line engine development in the US was central to the failures.

The '38 was literally designed around two turbocharged V-1710s-and the specification that drove that requirement was based on Kelsey's vision into future requirements for a fighter. "Conventional" AAC wisdom had "pursuits" with ~300-400 lbs of light machine guns, perhaps with a HMG or small cannon. Modest range and speed. The focus of the USAAC was on bombers, not pursuits. Kelsey wanted a 1000 lb weapons capability (IIRC-need to crack out Bodie's book again). 2 hours at rated power was specified. Top speed in excess of 360mph. Climb to 20,000 feet in (IIRC) about 6 minutes. None of these specs had been achieved previously, and the only engine available in '38 when Kelsey wrote the spec was the V-1710, and both he and Johnson recognized that two engines would be required to do the job. I'm curious-just when did the R-2800 become available, and when/how did the turbocharger development with it take place? I'm assuming it was a little behind the 1710, but honestly don't know. One point of interest-according to Kelsey, the '38 was designed as a versatile fighter right from the start, not as an "interceptor". He created the interceptor name/classification as a way to get around the AAC limits and restrictions of the day.

Turbocharging offered a number of advantages over mechanical supercharging (along with disadvantages). Mechanical superchargers take a lot of power to drive-I believe I've seen around 250hp discussed for the Merlin (correct me if I'm way off base-could well be). Lets assume for the sake of argument that that is true-for a net output of 1400hp, you need the engine to produce 1650 (to provide the power to drive the SC). Which means the engine is stressed more. The intake charge has to be compressed that much more, heating the charge more. The structure of the engine needs to be built around that. And of course you burn the fuel of a 1650hp motor, not a 1400hp one. The turbo, being powered from "waste" energy, doesn't have those issues-your 1400hp engine puts out 1400hp. Fuel economy is theoretically better. Mechanical SCs were typically gear driven with a transmission to shift gears (at least in 2-speed ones). That gave peak power at the point where you shifted into the next higher gear-with power dropping off as altitude increased (and boost of the fixed-speed SC decreased). Turbos were infinitely variable in terms of compressor output. (poorly worded, let me explain). The turbocharger's boost was set by a wastegate, which adjusted the exhaust "energy" driving the turbine blade. Fully closing the wastegate drove all exhaust gas through the turbo, opening it bypassed it. Boost was regulated by the degree to which the waste gate was opened. In theory at least, peak power could be maintained from sea level up to the maximum altitude where the turbo could still produce rated boost (assuming no other limitations, such as intercooling capacity).

Think of the 2-speed supercharger vs the turbo as a 2-speed powerglide transmission vs a CVT.

Theoretical advantages, vs the design implementation are often two different things, as are related technological developments of the time. What has impressed me so much about the development during the war was the engineering and manufacturing that was done with the tools of the day. They didn't have computer aided design. Finite element analysis. Computational fluid dynamics. CNC machine tools. Yet the engineers, designers and machinists of the day did some amazing things. Hard to believe the design of planes like the Spit, 109, '38, etc, took place less than 40 years after the first Wright flyer.

 

[Sid327]

Found this among an old collection....

 

[drgondog]

No doubt the Mustang was a great fighter and did a number of things very well. But is showed up relatively late in the war (in a high altitude-capable variant anyway), was relatively fragile (at least compared to the '38, '47, Corsair and Hellcat). It had a much lower bomb carrying capability. And of course, one hit anywhere in the cooling system and it wasn't likely to make it home.

Not quibbling than Merlin cooling system was more vulnerable than R-2800 but the Mustang had about the same loss rate per sorties as the Corsair in Korea for CAS role. Go figure. As to being more vulnerable than P-38? Maybe during a mechanical failure of say the engine - but statistics show that the P-38 lost more P-38s in ETO when measured against achievements for high threat airfield strafing - huge target, two sources of coolant/engine vulnerability to fire plus two high speed turbos for catastrophic failure when hit hard.

Although the Mustang was limited to 2x1000 pound bombs, it had a greater tactical radius than the P-38, P-47, Hellcat, Corsair for the same (or less) bombload.