Supercharger Development & Aircraft Design Policy (USAAC)

[thunderbird]

Cooda, wooda, shooda... hindsight is wonderful!

Modern hotrodders know that intercooler turbocharger or supercharger are bolt on components, but that concept seems to have been unknown in WWII. The merlin two-stage, 2 speed intercooled supercharger should have been installed to an Allison engine perhaps as simply as developing an adaptor plate, or better yet, by adapting the Allison to the merlin supercharger, ie, changing the bolt patterns in the Allison block to match the R-R supercharger/intercooler assembly.

Modern turbocharger installations keep the exhaust path as short as possible and sometimes wrap the entire exhaust system to retain as much exhaust energy as possible. Not sure why the Americans put so much distance between their engines and their turbochargers. The P-39 turbochargers were very close to the Allison engine, but NACA scientists belittled it out of existence, criticizing everything rather than looking for solutions.

 

[tomo pauk]

Cooda, wooda, shooda... hindsight is wonderful!

Modern hotrodders know that intercooler turbocharger or supercharger are bolt on components, but that concept seems to have been unknown in WWII.

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.

The merlin two-stage, 2 speed intercooled supercharger should have been installed to an Allison engine perhaps as simply as developing an adaptor plate, or better yet, by adapting the Allison to the merlin supercharger, ie, changing the bolt patterns in the Allison block to match the R-R supercharger/intercooler assembly.

Cooda, wooda, shooda?

Modern turbocharger installations keep the exhaust path as short as possible and sometimes wrap the entire exhaust system to retain as much exhaust energy as possible. Not sure why the Americans put so much distance between their engines and their turbochargers. The P-39 turbochargers were very close to the Allison engine, but NACA scientists belittled it out of existence, criticizing everything rather than looking for solutions.

Americans (and Germans) 1st invented the hollow turbine blades (so turbine can cool itself), and then slapped the newly fanged turboes back on the R-1820 aboard the ~1000 of Curtiss SC naval aircraft. Too bad that was in 1944, those funky Germans and Japanese didn't wanted to wait to 1944 to start the war.
Turbo installation on P-39 was draggy as hell; turbo, along with plethora of coolers managed to up the Cd0 of aircraft by 50%. Bell (and US AAC) tried to stuff 1.5 liter of hardware into a 1 liter box, no wonder it will not work as hoped. Too bad Bell and USAAC haven't contacted NACA while XP-39 was yet in mock-up stage.

 

[Shortround6]

There is a huge difference in metallurgy between even the 1990s and 1942 let alone today's metallurgy.

Allison built a compound engine where the exhaust gas was routed to a turbine that was connected to the crankshaft, at high levels of boost they easily exceeded 1750 degrees F which was the limit of the turbine unit (in 1945) and had to use water-methanol injection into the exhaust manifolds to cool the exhaust before it reached the turbine and causing it to fail.

P-38s went through either 3 or 4 different turbochargers, some with higher rpm limits and perhaps higher temperature limits. They were exposed to the air in order to cool the turbines.

 

[K5083]

"P-38 was a very versatile aircraft. "

No it was compromised in every role it undertook by the turbo installation. It coulda been a Hornet, even with Allisons. BUT, no internal stowage, mass spread along the span, no fuselage space, a lot of problems inherent to the config, without even starting on various operational problems which were eventually fixed.

 

[wuzak]

Modern hotrodders know that intercooler turbocharger or supercharger are bolt on components, but that concept seems to have been unknown in WWII. The merlin two-stage, 2 speed intercooled supercharger should have been installed to an Allison engine perhaps as simply as developing an adaptor plate, or better yet, by adapting the Allison to the merlin supercharger, ie, changing the bolt patterns in the Allison block to match the R-R supercharger/intercooler assembly.

The Merlin supercharger was integral with the Merlin engine, it was not a bolt-on component.

The same could be said for the Allison supercharger.

The V-1710 auxiliary supercharger was designed as a bolt as Allison preferred their engine to be as modular as possible.

Allison were able to run a V-1710 with the Merlin supercharger, but it had to be driven remotely.

Modern turbocharger installations keep the exhaust path as short as possible and sometimes wrap the entire exhaust system to retain as much exhaust energy as possible. Not sure why the Americans put so much distance between their engines and their turbochargers. The P-39 turbochargers were very close to the Allison engine, but NACA scientists belittled it out of existence, criticizing everything rather than looking for solutions.

Cars need the turbo to be as responsive as possible, as the rpm changes quite a lot. In contrast WW2 era aircraft piston engines operated at constant rpm. The biggest rpm changes occur when the pilot changes from a cruise setting to a combat setting, or from combat to WEP. But once in that setting, the rpm stays constant.

That is why the variable pitch propellers used on most combat types in WW2 are referred to as "constant speed" types.

 

[tomo pauk]

No it was compromised in every role it undertook by the turbo installation. It coulda been a Hornet, even with Allisons(1). BUT, no internal stowage(2), mass spread along the span(3), no fuselage space(4), a lot of problems inherent to the config, without even starting on various operational problems which were eventually fixed.

1- ?? Spitfire coulda been Mustang, but it was not.
2- It was abe to fit guns, ammo, fuel, tricycle U/C, radios, and other bits & pieces within the airframe.
3- Darn it, what a fault.
4- Indeed?

Out of all shortcomings of P-38 (and there was half a dozen), you've harped on the non-existent ones.

 

[GreenKnight121]

The turbo ... made the P-47 so big.

A good part of P-47's size depended on it's big R-2800 engine, big fuel tankage, and it's battery of HMGs with plenty of ammo. Hi alt operation necesitates a big wing, too.
See F4U and F6F - big engine, big wing, wide fuselage, heavy gun firepower, heavy payload, no turbo (bar prototypes).

The largest-volume fuselage component of the P-47 was the total turbocharger installation.

 

[GregP]

Hi Shortround,

I have had lunch with Dan Whitney and am well acquainted with Vees for Victory. That said, the V-1710 had a power section, a supercharger section, and a nose section. The main difference between the E and F series engines was simply the nose section. one had a prop spindle and the other drove a driveshaft. I think the dirigible engine drove the power section development, and resulted in the original engine's basic form as it got operated. The V-12 layout was from the Army spec, but the development money for RUNNING the engines was mostly Navy money, unless I misunderstand (possible).

 

[tomo pauk]

The largest-volume fuselage component of the P-47 was the total turbocharger installation.

We can take a look on the cross-section and cutaways of the F6F, in order to compare. Where the P-47 have had turbocharger and intercooler, the F6F have had basically nothing (bar radio and wires/bars for control of empenage). The F6F also featured long 'beard', needed for ram air intake, intercoolers and oil cooler. Add the thick belly, too, while the wings were of greater area by 11%.
On the other hand, the P-47 carried more fuel by a large margin (even before we facture in the P-47N), and more firepower.

 

[Deleted member 68059]

Modern hotrodders know that intercooler turbocharger or supercharger are bolt on components, but that concept seems to have been unknown in WWII.

I dont know what this sentence means.

1) Who is "modern hotrodders"
2) They know what ? That they can phone up Garrett and buy a turbo? - GE mass produced them in tens of thousands in WW2.
3) Turbochargers were put on aero engines in WW1, and intercoolers were used on the Bristol 1936 altitude record plane, with 2-stage superchargers.

 

[K5083]

Hi Tomo,
I am comparing the P-38 as built with what any other designer might have done. Of course it is speculative because no other operational twin used the best available piston engines with a single-seat sizing. There were a lot of designs, but nobody got them made. There's nothing about a DH Hornet that could not have been done with earlier merlins in 1941. But Lockheed famously examined six configurations for the P-38 and the conventional twin was rejected. The one they chose meant no fuselage volume, no internal bomb stowage (you can lift 4000lbs but you have to live with the drag and the loss of drop tank fuel.), no place for a decent second seat if required, or decent radar (yes I know the radar came along, that was lucky). The mass along the span meant a really much higher rolling moment of intertia making a quarter-second delay in roll. Doesn't seem much but it is widely mentioned as a disadvantage. The use of a bomber/transport wing profile. The miserable potential for a bigger prop because of short gear (probably soluble, but never solved.). There just is nothing good about the twin-boom that makes it preferable to a normal twin, given tricycle gear.

 

[Shortround6]

I would note that as far as "bolt on components" goes, GE only made a few different turbo chargers at any one time (they did make succeeding models of roughly the same size in each series) as in the P-38, B-17 and B-24s all used "B" series turbo chargers. Earlier the Curtiss Conqueror engine used a "bolt on" turbo in a variety of airframes to gain altitude performance.
The Curtiss Conqueror engine was unsupercharged in most of it's models and installations and the turbo added little or nothing to the power rating at sea level. The turbo did allow the engine to make it's sea level power rating at around 15,000ft.

Curtiss Hawk with Conqueror engine with "bolt on" turbo.

 

[tomo pauk]

Hi Tomo,
I am comparing the P-38 as built with what any other designer might have done. Of course it is speculative because no other operational twin used the best available piston engines with a single-seat sizing. There were a lot of designs, but nobody got them made. There's nothing about a DH Hornet that could not have been done with earlier merlins in 1941. But Lockheed famously examined six configurations for the P-38 and the conventional twin was rejected. The one they chose meant no fuselage volume, no internal bomb stowage (you can lift 4000lbs but you have to live with the drag and the loss of drop tank fuel.), no place for a decent second seat if required, or decent radar (yes I know the radar came along, that was lucky). The mass along the span meant a really much higher rolling moment of intertia making a quarter-second delay in roll. Doesn't seem much but it is widely mentioned as a disadvantage. The use of a bomber/transport wing profile. The miserable potential for a bigger prop because of short gear (probably soluble, but never solved.). There just is nothing good about the twin-boom that makes it preferable to a normal twin, given tricycle gear.

Ironically, I agree that choosing a 'classic' layout would've been better, even if not for all of the reasons you've mentioned. My reasoning is that 'classic' should be lighter, cheaper/faster to make, and will leave less blind areas for the pilot. We can recall than Hornet was also without internal bomb stovage, so it might not be that of a shortcoming for a fighter. I'm not sure that fighters with their thin fuselages will happily accept the 4000 lb bomb, just becausee the wider fuselage of Mosquito did it, after the modification.
Granted, a 'classic' will leave more space for an extra crew member and/or fuel, however even the P-38 as-is was able to carry another crew member in it's NF version, the P-38L. The long nacelle, with probably double the internal space, never made it to production line (for better or worse). picture
Wing profile was same as used on the LaGG-3/La-5 & -7 series, and thinner than what was used on F4U, F8F, Typhoon, Me-110/210/410, Whirlwind, let alone Welkin. Compressibility problems arose due to venturi effect. between pod and nacelles, as well as on cockpit front and rear part. Unfortunately, the relatively starightforward modifications suggested by NACA in winter of 1941/42 never made it to production line (those were to raise critical speed from under 430 mph to beyond 470 mph) .
NACA report
P-38 prop diameter was 11'6" - bigger than on Merlin Mustang with 11'2"; the biggest prop for the Spitfire that I was able to find was 11'3" (Bf 109 prop was still smaller) . What the P-38 never received was a wide-blade prop, perferably a 4-bladed - I don't think that was fault of P-38's design.
Mass along the span was a problem with most of the twin-engined fighters - 2x1 ton worth of powerplant (engine, coolers, prop) will make instant rolling hard to achieve until the boosted ailerons were installed. That happened in 1944 for the P-38.

 

[drgondog]

"P-38 was a very versatile aircraft. "

No it was compromised in every role it undertook by the turbo installation. It coulda been a Hornet, even with Allisons. BUT, no internal stowage, mass spread along the span, no fuselage space, a lot of problems inherent to the config, without even starting on various operational problems which were eventually fixed.

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.

Expensive, complex, buggy through the early J - yes. Versatile - Yes. Better than any other fighter spawned by Materiel Command - yes (save - and debatable, versus the P-47 with respect to 'versatility'. Mustang doesn't count with respect to AAF MC development.

 

[pbehn]

The main drawback to a long distance between engine and turbo is turbo lag, not as much of an issue on an aircraft as it is on a car and motorcycle. A friend of mine tried racing a Kawasaki 750 turbo, it was faster than the conventional 900cc and 1000cc bikes of the time but had no "feel" at all coming out of corners or even on a wet straight.

 

[tomo pauk]

The main drawback to a long distance between engine and turbo is turbo lag, not as much of an issue on an aircraft as it is on a car and motorcycle. A friend of mine tried racing a Kawasaki 750 turbo, it was faster than the conventional 900cc and 1000cc bikes of the time but had no "feel" at all coming out of corners or even on a wet straight.

You'r friend's Kawasaki probably didn't have an engine-integrated supercharger, like it was the case on P-38, P-47, B-17, or what Ju 388 had with BMW 801J.

 

[pbehn]

You'r friend's Kawasaki probably didn't have an engine-integrated supercharger, like it was the case on P-38, P-47, B-17, or what Ju 388 had with BMW 801J.

No, it was a turbo, not aftermarket but a Kawasaki product. Kawasaki GPZ750 Turbo - Wikipedia

 

[Shortround6]

I believe what Tomo ment (and I could be wrong) was that the Kawasaki didn't have it's turbo feeding a mechanical drive supercharger like ALL American turbocharged aircraft engines of WW II did.

Yes the WW II turbos did suffer from lag, but there were plenty of other things contributing to poor throttle response if trying to accelerate from a low rpm cruise condition, like 300-500lb flywheels (otherwise known as propellers

 

[pbehn]

I believe what Tomo ment (and I could be wrong) was that the Kawasaki didn't have it's turbo feeding a mechanical drive supercharger like ALL American turbocharged aircraft engines of WW II did.

Yes the WW II turbos did suffer from lag, but there were plenty of other things contributing to poor throttle response if trying to accelerate from a low rpm cruise condition, like 300-500lb flywheels (otherwise known as propellers

I was responding to the previous post about position of turbos (not superchargers) and aftermarket stuff for cars and bikes. It just doesn't apply to WW2 aero engines. I don't know what the lag was in real terms for a P-47 maybe a few seconds or more but that is completely unacceptable on a car or bike.

 

[Shortround6]

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.