# USA: P-38, P-39, P-40, P-47 (and P-51)



## gjs238 (May 31, 2015)

Did the US spread its resources too thin on four (five including the P-51) fighters?
Should the US have concentrated resources on one or two, rather than four/five?

The P-51 probably fits into this equation as well.
The P-51 probably could have have accomplished the work of all four/five.

PS: I'm thinking pre Pearl Harbor decision making.
Not re-jiggering things around later.


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## fubar57 (May 31, 2015)

I know nothing of the numbers (climb, dive rate, hp., etc) but I think it was a case of make do with what you had at the time. The -39 and -40 were all the U.S. had until the -47. The -38 was great in the Pacific because of the vast water expanses and that secure feeling the pilots had with the spare engine, The -47 was and is good fighter until the -51 came along and then it became the U.S. ground pounder as well. This is just my opinion and those who know the numbers will be along shortly.

Geo


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## tomo pauk (May 31, 2015)

Timing should explain a lot. The P-40 was initially a P-36 airframe with V-12 engine as a retrofit, similar what Italians did later. It helped to timely solve the US problem of not having decent fighters in production and service, with other powers having a headstart due to the preparations to the war (or a commitment in a war).
The P-39 was the 'next gen' fighter, that was to sport a big firepower, along with great altitude performance. The later part didn't pan out, the installation of the turbo was bugged, the drag was too big, so the Bell company was suggested (told?) to ditch the turbo, do some rearranging so the USAF can have a new timely strea of 'good enough fighters, rather than to wait several years for might-have-been turbo P-39s.
The P-38 was of the same generation with P-39, it's turbo installation was far more streamlined affair (tough with it's own caveats), hence the performance was good. The on-board fuel % was less affected once the s-s tanks were added, the big drop tanks provided another boost of usability that it took years for other US fighters to match.
P-47 was partially an insurance against the V-1710 being a flop (Packard Merlin partially served the same purpose) - all previous 'new' US fighters were based around the Allison.
The P-51 came out of a blue, the story is well known, and initially it did not offered anything to the USAF, at least they though it. The capability to accept the 2-stage Merlin (along with other advantages over existing fighters) quickly moved the P-51 in priority list, with new Merlin to be produced also by Packard.


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## Shortround6 (May 31, 2015)

Tomo has pretty well covered it. 

It takes several years to go from even prototype to widespread squadron service. 

The P-47 was ordered off the drawing board in in Sept of 1940. First prototype flew May 6th 1941, first _production_ example is finished Dec 21st 1941. Curtiss is building 250-285 P-40s a month in the fall of 1941, Bell is building 128-198 a month and Lockheed is is getting going with 128 built in Nov-Dec. NA builds 138 Mustangs in in 1941. 

Republic takes a while to sort out the P-47 and gets up to 61 a month in Aug of 1942 up form 38 in July. Curtiss is having an off month (change in production models?)only 143 but comes back with 416 in Sept, they had been making over 340 a month in March, April and May. Bell has a good month in Aug and builds 309 P-39s. another change over month in Sept sees none followed by 3 in Oct and then 268 in Nov. Aug was a bad month for P38s with just 80 built after 170 in July and 132 in Sept. Aug sees Mustang production falling off as the switch is made to A-36 production. 

1942 production totals are:
P-38.....1479 planes
P-39.....1932 planes
P-40.....3854 planes
P-47......532 planes
P-51......634 planes but does not include A-36s. All but a few experimental aircraft have Allison engines. 

Giving up on the P-47 and P-51 and trying to upgrade the P-39 and P-40 would have been flogging already dead horses by the time you get to 1943. NOT building P-39s and P-40s in 1941-42 and some of 1943 leaves with not enough planes to cover the areas of combat you are involved in. Late production P-39s and P-40s were pretty much lend lease aircraft.


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## GrauGeist (May 31, 2015)

And like in another thread, removing any of these aircraft would have caused a huge imbalance in the historical timeline.

The aircraft the U.S. had at the onset of it's involvement helped to hold the enemy in check until the new/improved fighters could get into service, like the F4F and P-40, for example. Without them, there was nothing the U.S. had in it's inventory that could at least challenge the Japanese on relatively equal terms.

The P-40 was able to provide a force of resistance against German and Italian fighters in the MTO until more capable fighters could be brought to the theater to challenge the Axis.

The P-39 (and P-400) did add to the fight in the Pacific, in spite of it's short-comings and later went on to serve the Russians well in the Eastern war.

The problem with trying to evaluate and make decisions before the war is impossible, as they didn't have a crystal ball to see how these designs would hold up against the enemy under various combat conditions. It was only after first blood was drawn, were they able to get a clearer picture of how to get a better design to overcome the enemy's aircraft...


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## drgondog (May 31, 2015)

In my opinion the choice you are asking is whether one or more should not have been developed.

I would say all needed to be developed as we should not have put all future capability to change mission with an airframe that looked great based on aerial warfare concepts in 1939 but prove to be inadequate in 1943.

Our enemies never developed the heavy bomber strategic capability that both the P-47 and P-38 were designed to intercept. The Mustang was overlooked by the AAF until spring 1942 and was modified into a great escort fighter with escort range deemed impossible in 1941 when we went to war.

The P-40 was an adequate fighter on all theatres and available in numbers to bride the combat introduction of both the P-47 and P-38 but had to be kept in front deployment until the production numbers enable replacement of both the P-40 and P-39 starting in mid 1943.

The timeline for the P-47, P-51 and P-38 was within months of entry date had the AAF killed the P-39 and P-40 before Pearl harbor which would not been a very good idea


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## gjs238 (May 31, 2015)

OK, there's a lot here, let's pick one for now...



tomo pauk said:


> The P-39 was the 'next gen' fighter, that was to sport a big firepower, along with great altitude performance. *The later part didn't pan out, the installation of the turbo was bugged, the drag was too big, so the Bell company was suggested (told?) to ditch the turbo, do some rearranging* so the USAF can have a new timely strea of 'good enough fighters, rather than to wait several years for might-have-been turbo P-39s.



At this point, what if the Bell company is told - this thing bites...
Make licensed P-40's, P-38's, P-51's, etc.


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## GrauGeist (May 31, 2015)

gjs238 said:


> OK, there's a lot here, let's pick one for now...
> At this point, what if the Bell company is told - this thing bites...
> Make licensed P-40's, P-38's, P-51's, etc.


Without the P-39, U.S. forces may have lost Henderson field to the Japanese, who were determined to take it back. The fierce ground attack by the P-39 gave U.S. ground forces the edge they needed to hold their ground and force the Japanese back.


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## gjs238 (May 31, 2015)

GrauGeist said:


> Without the P-39, U.S. forces may have lost Henderson field to the Japanese, who were determined to take it back. The fierce ground attack by the P-39 gave U.S. ground forces the edge they needed to hold their ground and force the Japanese back.



Perhaps the US would have fared better with more P-40's.


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## GrauGeist (May 31, 2015)

gjs238 said:


> Perhaps the US would have fared better with more P-40's.


I honestly don't think that the P-40 could have replaced the P-39. The P-39 was a capable adversary to the Japanese fighters, like the A6M...it wasn't better, but it was well armored and could at least hold it's own in a fight AND the P-39 had the firepower to rip a KI-43 or A6M to shreds. The only shortcoming to the P-39 in early 1942, was that the Japanese had experinced pilots while the American pilots were green, yet the early encounters between the two were standoffs.

Like I mentioned earlier, the P-39 was able to perform ground attack against Japanese ground forces and naval units, like transports with a greater success than the P-40 was capable of.

Removing the P-39 would have left a large void in the timeline. Especially, for example, in the critical year of 1942 where the P-39 was making valuable contributions to a desperate situation. You would have to find well over 1,000 other types to fill that hole and hope they could deliver the same results...


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## kool kitty89 (May 31, 2015)

gjs238 said:


> Did the US spread its resources too thin on four (five including the P-51) fighters?
> Should the US have concentrated resources on one or two, rather than four/five?
> 
> The P-51 probably fits into this equation as well.
> The P-51 probably could have have accomplished the work of all four/five.



Don't forget all the Navy fighters too: F2A, F4F. F4U, F6F, F8F. And of those, timeline and performance wise, the Corsair is more at home with the USAAF fighters. (or rather, it'd have an easier time fitting in as a front line Army fighter than Navy, as it did land based USMC -with some gains from omitting wing folding, tail hook, and other naval requirements)

Granted, the army's fixation (and inconsistent policies) on turbocharging and focus on their hyper engine project didn't help matters either. (had at least some impact on killing a turbo P-39, seems to have killed even prototypes of a turbo P-40 -follow on to the P-40 and less successful YP-37, and definitely crippled Allison's auxiliary supercharger development) Similar policies may have prevented potential Army interest in the Corsair as well. (I don't believe Vought ever approached the army with such a proposal, but I'm more posing the possibility of it being curtailed or rejected on similar grounds ... that and the P-47 ended up vindicating the utility of a turbocharger for high alt work; still the XF4U flew a full year earlier and had medium/high altitude performance better than any US fighter flying in 1940 other than the YP-38 -and had fewer development problems and greatly superior roll rate)

For Navy fighters, there's really no substitute for the F6F. (barring hypothetical -untested historically- wing modifications to the corsair, like slots to correct the tip-stall and related torque roll and spin issues -fixed slots would add drag, of course and slats are a much bigger engineering undertaking)

The F4U is also limited in engine availability, though Army interest (and funding) in that may have forced P&W to shift priorities and/or expand production capacity earlier on.






tomo pauk said:


> The P-39 was the 'next gen' fighter, that was to sport a big firepower, along with great altitude performance. The later part didn't pan out, the installation of the turbo was bugged, the drag was too big, so the Bell company was suggested (told?) to ditch the turbo, do some rearranging so the USAF can have a new timely strea of 'good enough fighters, rather than to wait several years for might-have-been turbo P-39s.


After the XP-39's heavy redesign, there were no further attempts in a tightly integrated turbo installation on the P-39 at all. (there were some dorsal 'saddle' pack modules tested that apparently weren't worth the low altitude performance loss and weight gain, but nothing akin to the turbo embedded in the belly as on the XP-39)

A bunch of aerodynamic problems with the basic airframe got addressed due to the XP-39 tests, and it wasn't the turbocharger but the intercooler installation that was the problematic portion of the system. (too much drag, poor location and not enough cooling capacity) That issue probably wouldn't be effectively solved without developing a water-air intercooler to replace the bulky air-to-air arrangement. (surface cooling as the P-38 did with the wing leading edges- method might have worked too, but probably would have the same vulnerability and inadequacy issues as the P-38, and with the engine placement on the P-39, ducting all the way to wing leading edges might not have been practical so using fusealage and possible wing/wing root )

The Production P-39s also had the entire oil and coolant radiator system embedded in the belly, so turbocharger placement would have to account for that as well.


From what I understand, turbocharger use ended up strictly limited by 1940 and proposals to use them on the P-39 and P-40 were denied by the USAAF while the P-38 and P-47 were the exceptions there. This does make some sense given the P-38 ended up near the top of the USAAF's priority of the pre-war fighters (P-37, P-38, P-39, P-40) with the P-39 and P-40 relegated more to stop gaps. That may have made more sense if not for the delays in volume production and service readiness of the P-38. --Granted, this same logic should have incited more interest in aux supercharger development by Allison, but contradictory bureaucratic issues and misplaced spending allocation among other things might explain that. (that goes for both funds allocated by congress AND allocation/interest on the Army end of things as well -cases like the A-36 is a more obvious example of the Army wanting the plane but having to find workarounds for securing funds from congress)

I may be mistaken here, but it seems the US Army was more interested in big, powerful, potent heavy fighters than most other air forces, with less concern on production cost. (which did have some reason to it as I've argued for the cost to performance benefits for the P-38 vs P-40 and P-39 -including pilot survivability; the P-47 would obviously have major advantages there too; honestly it's similar arguments to those I've posed for the Fw 187's value vs the 109 ... and 110 and I suppose has some merit for B-17 vs B-18 as well)



> The P-51 came out of a blue, the story is well known, and initially it did not offered anything to the USAF, at least they though it. The capability to accept the 2-stage Merlin (along with other advantages over existing fighters) quickly moved the P-51 in priority list, with new Merlin to be produced also by Packard.


The P-51 seems to have been solely held back by perception and bureaucratic issues. Compared to the P-39 and P-40 it had obvious value from the start in all around performance and range using the same engines. And the Army may not have shown excessive interest of funded the project when it was on the drawing board, but once the prototypes were flying, interest and production orders were pretty forthcoming. (going so far as to use the A-36 derivative to expand production beyond available 'fighter' allocation)


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## Shortround6 (May 31, 2015)

The turbo P-39 was trying to put a quart into a pint pot. (1000ml into 500ml bottle for our metric friends). The radiators and oil coolers on the original XP-39 didn't work worth spit and needed larger ducts, the original inter cooler was too small (way too small) and wouldn't come close to doing the job needed. Without over a dozen extra cubic feet of volume in the fuselage there just isn't enough room. 

One can judge the likelihood of a turbo P-40 by looking at the XP-37, YP-37 and XP-60 fighter









The weight of the turbo is not a big problem. The Problem is the size of the intercooler and the size of the ducts needed to carry the intake air and the cooling air for the intercooler. It just won't fit without some major surgery. 
The extra drag on the P-39 mock ups cost around 40mph below 15,000ft. A P-39 could outrun a Zero at low altitude rather easily. With the turbo the new P-39T or P-40T may be able to out run them at high altitude but they will loose the ability to out run them at low altitude. 



> I may be mistaken here, but it seems the US Army was more interested in big, powerful, potent heavy fighters than most other air forces, with less concern on production cost.



The US, even before 1939/40, was interested in fighters with longer range than European fighters, in part simply due to the size of the United States. Trying to move fighters with a range of 400-500 miles around the US to meet threats in different areas was a major headache. There is a tale about one P-40 squadron moving from the west coast to the east coast (If I remember right) that took about two weeks to get _all_ its aircraft moved the entire distance. Between refueling stops, malfunctions and weather. The US policy (at least in the very late 30s) was to put the auxiliary or ferry tank _inside_ the fuselage. Want to stick a 55 gallon drum in a Spitfire or 109 and see what it does to the fuselage size? 

I an not saying it was a good idea or a bad one but there was more to it than simply saying the US liked big airplanes.


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## kool kitty89 (May 31, 2015)

Shortround6 said:


> Giving up on the P-47 and P-51 and trying to upgrade the P-39 and P-40 would have been flogging already dead horses by the time you get to 1943. NOT building P-39s and P-40s in 1941-42 and some of 1943 leaves with not enough planes to cover the areas of combat you are involved in. Late production P-39s and P-40s were pretty much lend lease aircraft.


Indeed, though the P-39 and P-40 themselves weren't really in a different class of design/engineering/airframe performance than the likes of the Spitfire or Bf 109. The difference being the RAF HAD no P-47 equivalent (or F4U equivalent -but that was more an engine than airframe issue given the R-2800 MADE the Corsair and Thunderbolt and might have made the Typhoon as well -or say Bristol Centaurus being serviceable at 2000 HP in 1941 with altitude performance on par with the F4U-1's engine).

The RAF might have had a P-38 if there'd been serious heavy support/interest in twin merlin engined heavy fighter/interceptors while Germany had the Fw 187 and later Fw 190 yet continued to pour resources into the Bf 109, 110, and Me 210/410.

In fact, compared to the Spitfire, Hurricane, and 109, the P-40 and P-39 both had some rather outstanding features like notably superior range and superior structural strength (at the expense of weight -as with all american fighters save maybe the F2A), lower drag in the P-39's case as well. (with any given engine performance, the P-39 seems to be the best interceptor of the bunch in terms of top speed and decent climb -higher weight being somewhat balanced by low drag) The P-39 also should have been a good candidate for the MK 108 and possibly even MK 103.

The P-39 is several years newer than the Spitfire or 109, and had NACA testing to help refine it, so drag improvements aren't that surprising. It's more of the same vintage as the Fw 190 though the latter is closer in size/weight to the P-36/P-40. The P-36/P-40 itself is a pretty well streamlined design for being the same age as the 109 and Hurricane, and slightly older than the Spitfire. Both the P-40 and P-39 also had better high-speed handling than the other aircraft and better all-around visibility (prior to the hooded spit) and the P-40 in particular had a slightly roomier cockpit and I believe a better heating system than the spitfire. (the P-36 also had a heater before the spitfire)

The fact of the matter is, though, that the USAAF had several superior options by 1942, so focusing on P-51, P-38, and P-47 production makes tons of sense. Without the P-51, a V-1650-3 powered P-40 might have become a staple of the USAAF. (particularly with Curtiss's attempted successors falling flat) The P-47 may have become the staple escort fighter for the ETO, and the reduced drag/weight and improved range/fuel efficiency of the P-47J may have put greater emphasis on getting that into service as well. (the lightened wing and streamlined cowling should have benefited the late P-47D production blocks too, ie even without the R-2800-57, the P-47J airframe should have supplanted P-47D production)

Honestly, even with the P-51 on the scene, the greater survivability, exceptional high-altitude performance, and comfortable cockpit (improved situational awareness) of the P-47D might have made it more worthwhile if it could be produced in enough numbers. (even aside from preserving the lives of american airmen, there's the cold logistical angle of having fewer aircraft downed, more ability to limp back to base on damage, and greater ability to develop highly skilled, experienced pilots before being captured or killed)

Both the P-47 and P-38 likely would have been of greater post-war value as well, particularly if the P-47J had entered production (or the P-47N had adopted the same nose) and displacing some of the need for the P-82 or P-51H to ever enter production. (including as a night fighter -build more P-38Ms and possibly finally introduce the P-38K's prop)

And yes, the P-47J and P-38K would both have been pushing closer to their critical mach limits than the P-51 ever did in level flight, but so long as they were fitted with dive recovery flaps this should have been acceptable. (aside from hypothetical wing modifications that never actually took place, namely wing root extensions akin to the Me 262 HG-I or slightly akin to the P-51's wing root -wing root extensions engineered to also include fuel tankage may have been a better alternative to the P-47N's wing as well, same goes for expanding P-38 fuel capacity) I'm also not suggesting they intentionally adopt swept/delta wing roots, but the standard streamlined inner wing extensions fillets tend to end up 'swept' by design and a straight taper is cheaper/easier to build than a more refined curve. (like on the XP-67) Thinner airfoil profiles increasing critical mach and high speed drag was more universally understood or at least more consistently inferred in the early 1940s.

I'd appreciate it if anyone with greater engineering knowledge could correct me on this, but modifying the leading (and possibly trailing) edges of the wing close to the root seems like one of the simpler modifications to improve critical mach number and reduce compressibility issues including turbulence ahead of the tailplane. (not including that caused by the tailplane's airfoil itself) If nothing else it seems like a more useful experiment than the laminar flow wing tested on the P-47




drgondog said:


> The timeline for the P-47, P-51 and P-38 was within months of entry date had the AAF killed the P-39 and P-40 before Pearl harbor which would not been a very good idea


Yes, and even in some alternate reality where the XP-38 never crashes and the entire program progresses a full year sooner (Lockheed tooling and production capacity expansion included), you at very least would want the P-40 in production though the C model until volume production of the P-38E/F could meet service demands (possibly along with supplemental un-turbocharged models -probably using F3R/F3L engines in place of similar engines being allocated to P-39D and P-40D/E production)

Changes less aerodynamic testing dependent aspects like a flight stick rather than yolk (potentially extended to aid with mechanical advantage for aileron operation), greater emphasis on cockpit heating/ventilation improvements (cool vent air for hot cockpit conditions would also be significant) and overall cockpit comfort improvements might have been possible to accomplish early on, independent of aerodynamic and structural issues. The same would apply to an increased twin-engine pilot training program. (especially efficient if the bulk of USAAF fighters are twins, let alone a single type)





All that said: with the historical P-38 development timeline, hypothetical P-38's using basically the same un-turbocharged engines couldn't displace the P-39 and P-40 prior to 1942. (or 1942 on the production line, perhaps 1943 in service)

I also forget for sure, but seem to recall that the direction of rotation on the V-1710 was easier to change on the production line (maybe in the field) than most contemporary designs, sort of going along with the modular nature of the design.


Merlins should still be allocated to P-51s as soon as they're available. (that includes the several thousand V-1650-1s that would otherwise go to P-40F/Ls -other V-1650-1s going to Hurricane/Lancaster production would be stuck with those unless British/Commonwealth allocation priorities changed)


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## kool kitty89 (May 31, 2015)

Shortround6 said:


> The weight of the turbo is not a big problem. The Problem is the size of the intercooler and the size of the ducts needed to carry the intake air and the cooling air for the intercooler. It just won't fit without some major surgery.
> The extra drag on the P-39 mock ups cost around 40mph below 15,000ft. A P-39 could outrun a Zero at low altitude rather easily. With the turbo the new P-39T or P-40T may be able to out run them at high altitude but they will loose the ability to out run them at low altitude.


The P-39 'saddle' mount turbos seem far from the attractive option compared to more embedded arrangements, especially with a liquid intercooler. (short of that, it's not going to be practical without excess drag) Same goes for the P-40: you could certainly do better than the configuration on the YP-37, but short of a compact water-to-air intercooler the increase in drag is simply going to be too much.

For that matter, an aux supercharger stage on the V-1710 (even a side-mounted one avoiding a length increase) probably isn't going to much out-perform the V-1650-1 until water injection or an intercooler is introduced. Likewise, the 2-stage merlin would have been pretty well useless and a waste of weight/bulk over the Merlin 20 series without its intercooler. (the Merlin 46 probably would have gained a good deal of performance at all altitudes with the improved charge density an intercooler could offer -or water injection but I suppose the same would be true for the charge heating on the 9.6:1 supercharged V-1710)

For that matter, there is one other option for a turbo P-39 or P40: no intercooler at all, and water injection used for combat power with strict limits to manifold pressure/temperature at normal/cruise power. That does require development and testing of a water injection system, but the engineering overhead there might be less than that required of an effective air-water intercooler. (and in any case, appears to be what Allison themselves favored)

Aside from that, might it also have been possible to adopt a smaller turbocharger in combination with the 8.8 supercharger gearing? (especially if the GE turbos heated the charge more than allison superchargers, but minimizing the size/weight of the turbo itself would have some benefit either way)


It's not just the auxiliary supercharger stage that Allison's funding/engineering resources limited, but things like solving the problems with the supercharger gearing to allow the 9.6:1 ratio, addressing the bottlenecked intake manifold, and possibly eliminating the backfire screens sooner. Even with that little 9.5" impeller (and overall supercharger dimension constraints -to avoid redesigning the entire accessories section), overall improvements may have been possible to remain directly competitive with the single-stage, single-speed merlin contemporaries, or possibly even do a bit better if water injection had been added. 2 speeds would be nice, but managing performance on par with the Merlin 45/50 series alone would be pretty good for the time. Allison did experiment with different impeller and diffuser arrangements, but I'm not sure how much changed on any production engines. (supercharger performance from the V-1710-33 of the P-40B to V-1710-73 of the P-40M seemed pretty much identical)


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## tomo pauk (Jun 1, 2015)

gjs238 said:


> OK, there's a lot here, let's pick one for now...
> 
> At this point, what if the Bell company is told - this thing bites...
> Make licensed P-40's, P-38's, P-51's, etc.



Bell producing the P-40 instead of P-39 does not add much IMO. The P-51 is too late, the 1st non-turbo XP-39B is delivered in Nov 1939; Bell producing P-51s instead the P-63s would've make plenty of sense, though. Bell produced 939 P-39s before 1942, vs. only 208 of P-38s by Lockheed in the same time, so we'd probably see a deficit of some 500+ US-produced fighters prior 1942 with Bell producing the P-38.



kool kitty89 said:


> ...
> A bunch of aerodynamic problems with the basic airframe got addressed due to the XP-39 tests, and it wasn't the turbocharger but the intercooler installation that was the problematic portion of the system. (too much drag, poor location and not enough cooling capacity) That issue probably wouldn't be effectively solved without developing a water-air intercooler to replace the bulky air-to-air arrangement.



The turbocharger, along with 4 waste gates was a draggy affair, all sticking out in the slipstream. The intercoolers were without boundary layer separator, and without any means to control the cooling air passage - it means drag and low capacity for intercooling. 



kool kitty89 said:


> ...
> The P-39 is several years newer than the Spitfire or 109, and had NACA testing to help refine it, so drag improvements aren't that surprising. It's more of the same vintage as the Fw 190 though the latter is closer in size/weight to the P-36/P-40. The P-36/P-40 itself is a pretty well streamlined design for being the same age as the 109 and Hurricane, and slightly older than the Spitfire. Both the P-40 and P-39 also had better high-speed handling than the other aircraft and better all-around visibility (prior to the hooded spit) and the P-40 in particular had a slightly roomier cockpit and I believe a better heating system than the spitfire. (the P-36 also had a heater before the spitfire)



The Spitfire aged the best, when compared with P-40, P-39 and Bf 109. We can be sure that early Spitfire (I to V) would've been even better with earlier adoption of fully covered undercarriage, faired mirror, internal armored glass and early installation of 6 exhaust stacks per side. Not sure whether deletion of snow guard on the ram air intake would've be safe for all-weather operation, deleting it gives 8.5 mph on the Spit V.


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## drgondog (Jun 1, 2015)

Kool Kitty wrote:
_
And yes, the P-47J and P-38K would both have been pushing closer to their critical mach limits than the P-51 ever did in level flight, but so long as they were fitted with dive recovery flaps this should have been acceptable. (aside from hypothetical wing modifications that never actually took place, namely wing root extensions akin to the Me 262 HG-I or slightly akin to the P-51's wing root -wing root extensions engineered to also include fuel tankage may have been a better alternative to the P-47N's wing as well, same goes for expanding P-38 fuel capacity) I'm also not suggesting they intentionally adopt swept/delta wing roots, but the standard streamlined inner wing extensions fillets tend to end up 'swept' by design and a straight taper is cheaper/easier to build than a more refined curve. (like on the XP-67) Thinner airfoil profiles increasing critical mach and high speed drag was more universally understood or at least more consistently inferred in the early 1940s.

*There was not a clear understanding of boundary flow and separation behavior due to negative pressure gradients aft of a forming shock wave until very late in the war and far too late to alter existing designs.

The Only way to eliminate the earlier Mcrit for the P-38 would have been to replace the entire wing with a laminar flow wing similar to the P-51. That would have pushed the Mcr from ~.68 to perhaps .72 given a same T/C (the P-38 23018 was a couple of percent fatter and the a/c was at ~ .25C whereas the 45-100 was 16% and a/c was at ~.45C. The immediate nose down pitch phenomena would have been delayed until the full shock at ~45% Chord with complete separation behind it - which in turn would immerse the horizontal stab/elevator in the associated increase of turbulent wake and greatly diminish elevator authority. *

I'd appreciate it if anyone with greater engineering knowledge could correct me on this, but modifying the leading (and possibly trailing) edges of the wing close to the root seems like one of the simpler modifications to improve critical mach number and reduce compressibility issues including turbulence ahead of the tailplane. (not including that caused by the tailplane's airfoil itself) If nothing else it seems like a more useful experiment than the laminar flow wing tested on the P-47_

*two ways at that time (in historical knowledge context) to delay Mcrit:
1.) decrease the T/C ratio either by deepening the cord of the wing while maintain the original max Thickness, or reducing the Max Thickness while retaining the existing plan form (i.e. replace the existing 23018 to 23016 if that derived same basic internal volume required for fuel and did not require extraordinary beefing up of the wing structure (i.e. Spar weight due to requiring greater bending resistance with shallower beam heighth.
2.) replace existing conventional airfoil with Laminar Flow type with Max T/C pushed back from .25 to . 40%+ Chord.
*

Something that always gets lost in 'what if' discussions of P-38K and P-47N/J is the extraordinary requirements to modify from P-38J (in cost) by adding a Rolls 1650-7 or the change in the wing; and the GW and cost of the redesign of the P-47N over the D - is that it took ALL the evolutions of both airframes to nearly match or slightly exceed (P47N/J to P-51D) of either the P-51D or P-51H if we keep model introduction in parallel - but ALL P-47s were approximately 1.5X in delivery cost plus 1.4-1.5X in operating cost and ALL P-38s were approximately 2X in delivery and 2X+ on Operating cost.


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## kool kitty89 (Jun 1, 2015)

tomo pauk said:


> Bell producing the P-40 instead of P-39 does not add much IMO. The P-51 is too late, the 1st non-turbo XP-39B is delivered in Nov 1939; Bell producing P-51s instead the P-63s would've make plenty of sense, though. Bell produced 939 P-39s before 1942, vs. only 208 of P-38s by Lockheed in the same time, so we'd probably see a deficit of some 500+ US-produced fighters prior 1942 with Bell producing the P-38.


With Bell producing P-38s (and no production P-39s/P-400s), IF they could get the tooling patterns in a timely manner, expecting exactly half the production volume of individual fighters would seem realistic. Overall monetary expenditure may have been somewhat less (P-38 was not quite 2x the cost of the P-39 at similar volumes, and omitting the turbo/intercooler systems should help) but overall man-hours, material, and especially engines would be major limiting factors. If they couldn't get the tooling set up quickly, it may have been somewhat worse than that.

Still, with only half the number it may have still been more worthwhile given the myriad of performance advantages for the P-38 over the P-39 (even assuming similar engines). Bell's experimentation with ejector exhaust and ram intake on the redesigned P-39 might have helped accelerate redesign of the P-38's cowling and exhaust stacks as well. (the 12 individual exhaust stacks per side in particular seems interesting, and while not all P-39s used it, the return on the P-63 says something about its usefulness)



> The turbocharger, along with 4 waste gates was a draggy affair, all sticking out in the slipstream. The intercoolers were without boundary layer separator, and without any means to control the cooling air passage - it means drag and low capacity for intercooling.


You'd definitely want boundary layer separators, or similar overlapping ducting on the intercooler and any external radiators (like the P-38's tail radiators and the P-40's chin radiator) reducing turbulence and improving airflow over the radiator surfaces. (at least part of it relates to the venturi effect and has a good deal in common with NACA cowling and duct research, including those applied to thrust augmentation for rocket/jet exhaust)

The same could apply to the exposed hot section of the turbocharger, a duct shrouding that portion may have both aided cooling of the turbine/exhaust and reduced drag. (possibly improved jet thrust from the waste gates and turbine exhaust as well)

I'm actually surprised this wasn't tried on the P-38, including the P-38M where it would have hidden the exhaust glow at night. (or maybe it was tried and omitted from production for other reasons)



> The Spitfire aged the best, when compared with P-40, P-39 and Bf 109. We can be sure that early Spitfire (I to V) would've been even better with earlier adoption of fully covered undercarriage, faired mirror, internal armored glass and early installation of 6 exhaust stacks per side. Not sure whether deletion of snow guard on the ram air intake would've be safe for all-weather operation, deleting it gives 8.5 mph on the Spit V.


The spitfire and 109 had the best consistently improving engine upgrades too, and the Americans had better options than the P-40 and P-39, so direct comparisons are a bit more difficult. The P-39 was substantially faster than its similarly powered counterparts, in spite of being heavier. (the P-39N and Q performed better than the Spitfire V in a number of areas in spite of their engines still not quite matching the Merlin 45 ... granted, the Spit IX was in production by that point)

The P-39 really could have needed that 9.6:1 supercharger gearing sooner even with the 1941 manifold pressure limits, that and using the hispano cannon as standard, working on improving the 20 mm ammunition capacity, introducing a re-cocking system to clear jams (like the P-38 -something facilitated by nose mounted guns and not practical in wings until the post-war M3 variant), and seriously consider deleting the wing guns, possibly in favor of additional small fuel cells but if nothing else just to save weight on those 4 M1919s and (up to) 4000 rounds of .30 cal ammunition. (if the cutaway diagrams I've seen are accurate, the outer wing gun bays appear to have significant space for added fuel tankage)
http://img525.imageshack.us/img525/2236/bellairacobrai1939airen.jpg


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## kool kitty89 (Jun 1, 2015)

drgondog said:


> The Only way to eliminate the earlier Mcrit for the P-38 would have been to replace the entire wing with a laminar flow wing similar to the P-51. That would have pushed the Mcr from ~.68 to perhaps .72 given a same T/C (the P-38 23018 was a couple of percent fatter and the a/c was at ~ .25C whereas the 45-100 was 16% and a/c was at ~.45C. The immediate nose down pitch phenomena would have been delayed until the full shock at ~45% Chord with complete separation behind it - which in turn would immerse the horizontal stab/elevator in the associated increase of turbulent wake and greatly diminish elevator authority. [/B]


I was under the impression that the reason the P-38 suffered critical mach issues at lower speeds than contemporary aircraft with similar or thicker airfoils of similar types (including the Corsair) related to the pod/boom arrangement leading to accelerated airflow along the root section of the wings (on top of being the thickest section of the wing), so modifying the airfoil profile there alone could have dramatically impacted initial onset of compressibility problems.

The Incomplete Guide to Airfoil Usage also lists the P-38 as using 23016 at the root, so is that another error on their part? (they seem to have a loose definition of 'root' as well, and seemed to start with the inboard wing section rather than the actual root on the F4U -listing 23015 rather than 23018 as it is at the oil cooler intakes)



> 1.) decrease the T/C ratio either by deepening the cord of the wing while maintain the original max Thickness, or reducing the Max Thickness while retaining the existing plan form (i.e. replace the existing 23018 to 23016 if that derived same basic internal volume required for fuel and did not require extraordinary beefing up of the wing structure (i.e. Spar weight due to requiring greater bending resistance with shallower beam heighth.
> 2.) replace existing conventional airfoil with Laminar Flow type with Max T/C pushed back from .25 to . 40%+ Chord.


#1 is mostly what I had in mind with the high degree of leading edge taper on the extension intended to avoid decreasing internal volume/strength without totally obliterating the pilot's forward-downward field of view. (the engine nacelles compromised that badly enough as it was) Any delta/sweep effect would have been incidental. (as would any wing fence effect of the nacelles)

That or combining 1 and 2, extending the chord and changing the airfoil shape, but minimize changes to the existing internal wing structure. (ie build out from the existing wing and manipulate the 23018 shape into something thinner and potentially smoother flow or lower lift/lower drag -aside from specific laminar flow designs, simpler symmetrical or near symmetrical NACA airfoils with little/no chamber should help as well -NACA 00xx series airfoils were fairly common pre-war with both the B-17 and P-39 using them -0018 and 0015 I believe)



> Something that always gets lost in 'what if' discussions of P-38K and P-47N/J is the extraordinary requirements to modify from P-38J (in cost) by adding a Rolls 1650-7 or the change in the wing; and the GW and cost of the redesign of the P-47N over the D - is that it took ALL the evolutions of both airframes to nearly match or slightly exceed (P47N/J to P-51D) of either the P-51D or P-51H if we keep model introduction in parallel - but ALL P-47s were approximately 1.5X in delivery cost plus 1.4-1.5X in operating cost and ALL P-38s were approximately 2X in delivery and 2X+ on Operating cost.


The P-38K is the P-38J/L with modified nacelles accepting larger, broader chord propellers with slightly larger spinners. Improvements in take-of performance, acceleration, turn, and climb would have been more significant than top speed (akin to the P-47 switching to paddle props) but some gain in speed and increase in ceiling were experienced as well.

And as to the P-47, delays in production for shifting from the D model is valid as well, and I was mostly suggesting it be adopted in place of the P-47M and N (or adopting the J's cowling on the N). Mentioning using the late model D's engines in the J airframe was more a comment on the off change that production capacity for the J model ramped up more rapidly than expected and outstripped R-2800-57 production.


I was also more seriously alluding to their post-war use compared to the P-51H and P-82.


Edit: in terms of sheer all around performance and dogfighting ability, the P-47J (not M or N) would be the one to actually challenge the P-51. It was lighter than the P-47D and made the compromise of carrying 6 rather than 8 guns (though I believe still higher ammo capacity than the P-51) while using the more streamlined cowling and more powerful engine. Any gains in range over the production D models would have been from weight/drag reduction and not fuel capacity increase. (by extension, the introduction of the 200 gallon belly tub tank would have been even more useful and made more serous reason for omitting the performance hampering wing pylons -all improving fuel efficiency as well, though obviously not close to Mustang levels)

It did predate the transition to the bubble canopy, and as far as I know was never modified to such a configuration so performance impact is up to conjecture.

And in any case, yes, the Mustang would remain the best place to invest in using V-1650s and modifications adapting them to the P-38 would be rather wasteful and pointless.


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## wuzak (Jun 1, 2015)

kool kitty89 said:


> The P-38K is the P-38J/L with modified nacelles accepting larger, broader chord propellers with slightly larger spinners. Improvements in take-of performance, acceleration, turn, and climb would have been more significant than top speed (akin to the P-47 switching to paddle props) but some gain in speed and increase in ceiling were experienced as well.



The P-38K also used a V-1710 with a different reduction gear ratio, which apparently changed the thrust line, requiring the changes to the nacelles.


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## Shortround6 (Jun 2, 2015)

kool kitty89 said:


> The same could apply to the exposed hot section of the turbocharger, a duct shrouding that portion may have both aided cooling of the turbine/exhaust and reduced drag. (possibly improved jet thrust from the waste gates and turbine exhaust as well)
> 
> I'm actually surprised this wasn't tried on the P-38, including the P-38M where it would have hidden the exhaust glow at night. (or maybe it was tried and omitted from production for other reasons)



The hot sections were exposed for a reason. Cooling of the turbine blades. It may not have been the most elegant way especially in way of streamlining put failed turbines tended to throw large (compared to bullets) around at high speed. Some American aircraft had steel scatter shields between the turbines and cockpits crew sections. 
http://static.rcgroups.net/forums/a...196332-209-P-38 Turbocharger.jpg?d=1312733318

You have the drag of the installation vs the drag of teh ducts needed to bring cooling air to the turbine and back away form it again if the trubo is buried. Since at full boost the waste gate is closed and all the exhaust gas is exiting the turbo through the exposed turbine blades you also need a duct to get rid of the exhaust gases. The P-47 used a buried turbo and that is _part_ of the reason it was so bulky, certainly not the only reason though.


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## drgondog (Jun 2, 2015)

kool kitty89 said:


> I was under the impression that the reason the P-38 suffered critical mach issues at lower speeds than contemporary aircraft with similar or thicker airfoils of similar types (including the Corsair) related to the pod/boom arrangement leading to accelerated airflow along the root section of the wings (on top of being the thickest section of the wing), so modifying the airfoil profile there alone could have dramatically impacted initial onset of compressibility problems.
> 
> *It is possible that the pod/boom may have contributed a very slight 'end plate' effect to the center wing section but the actual greater T/C from WS 15 to 79 versus ~ WS 119 to tip is more likely to introduce initial Mcrit but just a second or two in a dive. Otherwise, it begs the question "Why place the dive flap outboard of the engine nacelle"? *
> 
> ...



The P-38 was definitely the 'odd man out'. The P-47N had to be compared to the P-82B in September 1945 vis a vis Very Long Range Escort. A lot of fatigue on single pilots in 8 hour rides favored the P-82 conceptually over the P-47N and twin engine performance (with 1650-21/23) and potential survivability for both long trips and CAS.


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## tomo pauk (Jun 2, 2015)

The manual for the P-38 notes that venturi created by nacelle and pod is to be blamed for low critical mach (link). 
IMO - sure enough, a 'legacy' wing profile with a bit thicker TtC ratio didn't help either.



kool kitty89 said:


> ...
> The spitfire and 109 had the best consistently improving engine upgrades too, and the Americans had better options than the P-40 and P-39, so direct comparisons are a bit more difficult. The P-39 was substantially faster than its similarly powered counterparts, in spite of being heavier. (the P-39N and Q performed better than the Spitfire V in a number of areas in spite of their engines still not quite matching the Merlin 45 ... granted, the Spit IX was in production by that point)



The P-39 and P-40 have already shot the bolt re. suggested improvements - 6 exhaust stacks per side, fully retractable covered U/C (bar the exposed tires of the P-40), they don't have mirror nor ice/snow guard. There was maybe an option for the P-39 to position it's ram air intake a bit away from the canopy (to one or another side); that, perhaps with a longer ram air intake should provide better use of ram effect, hence improving altitude performance.



> The P-39 really could have needed that 9.6:1 supercharger gearing sooner even with the 1941 manifold pressure limits, that and using the hispano cannon as standard, working on improving the 20 mm ammunition capacity, introducing a re-cocking system to clear jams (like the P-38 -something facilitated by nose mounted guns and not practical in wings until the post-war M3 variant), and seriously consider deleting the wing guns, possibly in favor of additional small fuel cells but if nothing else just to save weight on those 4 M1919s and (up to) 4000 rounds of .30 cal ammunition. (if the cutaway diagrams I've seen are accurate, the outer wing gun bays appear to have significant space for added fuel tankage)



Agreed all the way. The engine with faster supercharger will wait until mid 1942, though.


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## pbehn (Jun 2, 2015)

I dont see how you could choose unless producing more 40s instead of P39s (or vice versa) Since the Russians were happy with the P39 things worked out pretty well.


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## drgondog (Jun 2, 2015)

tomo pauk said:


> The manual for the P-38 notes that venturi created by nacelle and pod is to be blamed for low critical mach (link).
> IMO - sure enough, a 'legacy' wing profile with a bit thicker TtC ratio didn't help either.
> 
> I believe the manual draws an incorrect conclusion. It states that the dive flaps disrupt the flow under the outer wing and thereby creates more lift thereby relieving the nose down forces". Partly true but only if by deploying the dive brakes in a timely manner (BEFORE Mcrit Velocity is reached over center section).. otherwise the shock wave initiates creating the separation and decreased lift of the center section of the wing - doing three things:
> ...



I only disagree in the context that the P-39 morphed into the P-63 which was arguably a better airframe - but there were many new design parts. The analogy is more like P-51D to P-51H.


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## drgondog (Jun 2, 2015)

I don't have proof but I suspect that when the Dive flap design was complete in mid 1943 that Lockheed had no idea regarding shock wave movement phenomena, and thought that the primary result of drag divergences was to lose lift and immerse the elevator in turbulent flow and did not realize that the shock wave resulted in the change to Moment about a/c due to the Center of Pressure moving aft. 

You can certainly suspect that is the case because they focused on mods to the tail


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## tomo pauk (Jun 2, 2015)

drgondog said:


> I only disagree in the context that the P-39 morphed into the P-63 which was arguably a better airframe - but there were many new design parts. The analogy is more like P-51D to P-51H.



The P-63 was certainly a better airframe than P-39, however it was worse than P-51 in many aspects. Hence my previous suggestion that Bell produces the P-51 under license, rather than P-63.

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## drgondog (Jun 2, 2015)

Tomo - Bell was selling P-63 with full margins when they could Not sell P-63 to AAF. Have no ide what the financials would be to produce the P-51D under License


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## kool kitty89 (Jun 3, 2015)

Shortround6 said:


> The hot sections were exposed for a reason. Cooling of the turbine blades. It may not have been the most elegant way especially in way of streamlining put failed turbines tended to throw large (compared to bullets) around at high speed. Some American aircraft had steel scatter shields between the turbines and cockpits crew sections.


I wasn't suggesting covering the turbine section entirely, but providing a duct/cowling similar to those used on radiators or around radial engines, potentially both improving cooling and reducing drag. (with the red hot temperatures and exhaust gas flow, there'd be a lot more potential for induced ram thrust there too, unlike the lower temperatures of radiators and radial engines)

Of course, on the P-39 it would also obscure the belly shackle point. (more interesting for the P-38)




wuzak said:


> The P-38K also used a V-1710 with a different reduction gear ratio, which apparently changed the thrust line, requiring the changes to the nacelles.


Yes, there would have been some delay in retooling, but nowhere near as severe as the Merlin would take. (maybe it would have been a more attractive change for the P-39M given retooling required for the pod/canopy)







drgondog said:


> I don't have proof but I suspect that when the Dive flap design was complete in mid 1943 that Lockheed had no idea regarding shock wave movement phenomena, and thought that the primary result of drag divergences was to lose lift and immerse the elevator in turbulent flow and did not realize that the shock wave resulted in the change to Moment about a/c due to the Center of Pressure moving aft.
> 
> You can certainly suspect that is the case because they focused on mods to the tail


Given the elevator still worked at high speeds, it just became incredibly stiff, and using boost tabs or the normal trim tabs for control DID work but could easily rip off the tail (failure at the booms), it pointed to shift in center of lift, making the plane extremely nose heavy, enough to be able to rip the tail off if enough elevator force was applied.

At some point, tail blanking would occur for sure (be it from the tailplane itself or otherwise), but that was never the problem on the P-38, and any pilots that survived to tell the tale provided information corresponding to that behavior. (Me 262 was similar in a dive, and I think the P-47 was as well)






drgondog said:


> I only disagree in the context that the P-39 morphed into the P-63 which was arguably a better airframe - but there were many new design parts. The analogy is more like P-51D to P-51H.


I'm not sure the P-63 was all that much better. Laminar flow wing, but larger and thicker, lower fuel capacity in the wing (where the P-39 could have had more if the fuel tanks were installed in the outer wings in place of the guns/ammo) and not really any faster (indeed slower than the P-39Q below 15k feet) until major increases in engine power materialized. Had the P-63 increased fuel capacity and supported internal wing guns it would be another story.

Problems on the P-39 would be the length of the 2-stage Allison, nose gun ammunition capacity, and the spin characteristics. (the latter may have been worsened by lengthening the rear fuselage to accommodate the engine unless, worse still if a 20 mm hispano replaced the M4 in the nose -being tail heavy compromised stability) Given allison had catered to the extension shafts needed by Bell's aircraft, maybe they could also have catered to a side mounted configuration for the aux supercharger, akin to those of german engines. (the hydraulic coupling should have made that a relatively simple engineering undertaking, and also would have allowed a better positioned ram air intake)

The 2-stage allisons aren't really much more attractive than the 9.6:1 single stage units until water injection is available. (only slightly better -if any- performance than the V-1650-1 and hardly surprising given that engine was close to the practical limits of supercharging without intercooling or water injection -due to charge heating, hence the limited gains for the Merlin 46 over the 45, in fact the initial production aux stage allisons would likely be about the same as if the Merlin 46's impeller had been applied to the merlin XX's 2-speed gearing, maybe slightly better given the neutral speed on the allison improving take-off power)

Water injection added to the P-39Q would likely have allowed it to outperform the P-63A at most altitudes. (charge cooling helping somewhat at high alts as well)

2 synchronized M2s and 1 hispano with P-38 (or similar) cocking mechanism and a decent supply of belt-fed ammunition (compared to the P-400's 60 rounds) would have made for a pretty decent overall armament against other fighters, and with the wing guns replaced with more fuel, the P-39 might have even been usable as an escort. (granted that would have been possible on early models as well, but limited to low/medium altitude escort)

The spin problems might have required changes to the wings or CoG (ballast in the nose in the worst case).




tomo pauk said:


> The P-39 and P-40 have already shot the bolt re. suggested improvements - 6 exhaust stacks per side, fully retractable covered U/C (bar the exposed tires of the P-40), they don't have mirror nor ice/snow guard. There was maybe an option for the P-39 to position it's ram air intake a bit away from the canopy (to one or another side); that, perhaps with a longer ram air intake should provide better use of ram effect, hence improving altitude performance.


Perhaps dual ram air intakes to either side of the canopy top? And I thought some P-39 models did get 12 exhaust stacks eventually, and mirrors (or at least fitted with mirrors in the field). The P-39's canopy is suitable for an interior rear view mirror too, unlike the P-40. (the clam shell glazing might give some useful rear view to the P-40 too, but the P-40N's glazing would be more notable)



> Agreed all the way. The engine with faster supercharger will wait until mid 1942, though.


One thing that would require zero structural/manufacturing changes would be more aggressive testing of the engines, and perhaps looser standards for clearing service ratings based on those tests. Given what the British were doing with Merlins in the field (and sometimes V-1710s), introducing increased maximum boost limits for emergency power (officially, not unauthorized overboost). Raising limits to 48" Hg manifold pressure (+9 psi boost) would have given engine performance similar to the Merlin III on +12 psi or slightly better but limited to 8000 ft rather than 10k. (relevant even for the V-1710-33 of the P-40B/C/Tomohawk, though the reduction gearing might not have coped with that power)
Going up to +12 psi boost (54" Hg) would be what the early P-39's engines were rated for when finally cleared for WEP, resulting in 1490 hp at 4000 ft (with ram) I believe.


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## kool kitty89 (Jun 3, 2015)

drgondog said:


> It is possible that the pod/boom may have contributed a very slight 'end plate' effect to the center wing section but the actual greater T/C from WS 15 to 79 versus ~ WS 119 to tip is more likely to introduce initial Mcrit but just a second or two in a dive. Otherwise, it begs the question "Why place the dive flap outboard of the engine nacelle"?


Structural reasons perhaps? That and intentionally placing the flaps outboard of where Mcrit is being exceeded. The inboard wing sections have internal fuel tanks and (especially) external hard points complicating things. They were placed at the inboard section of the outer wings, directly beneath the spar, so pretty much the strongest point on the wing outboard of the booms.

Prop wash is also an issue apart from venturi effect and would be accelerating airflow around the nacelles and wing portions just inboard and outboard of those.



> The F4U had a complicated wing airfoil definition as the Root Chord was given at NACA 23018 but the actual airfoil starts with the Panel next to the fuselage where both wings are joined to a massive Carry Through structure bolted the fuel cell Bulkhead. It is a NACA 23015 at about ~ WS 25. The Tip Chord is 23008. The T/C ratio spanwise tapered more rapidly for the F4U but the F4U had just a couple of mph advantage over the P-38 per limit dive speed.


Shifting topic slightly, but this reminded me also of the F4U's boost tabs making the ailerons much more effective with low stick forces. Tabs were tried on the P-38's elevator (with disastrous results due to strain in terminal dives), but might not that have also been an earlier and simpler solution to the P-38's heavy ailerons prior to or in place of hydraulic boosting?


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## tomo pauk (Jun 3, 2015)

If I may 


kool kitty89 said:


> ...
> The 2-stage allisons aren't really much more attractive than the 9.6:1 single stage units until water injection is available. (only slightly better -if any- performance than the V-1650-1 and hardly surprising given that engine was close to the practical limits of supercharging without intercooling or water injection -due to charge heating, hence the limited gains for the Merlin 46 over the 45, in fact the initial production aux stage allisons would likely be about the same as if the Merlin 46's impeller had been applied to the merlin XX's 2-speed gearing, maybe slightly better given the neutral speed on the allison improving take-off power)



The worst of 2-stage V-1710s that entered service have had 7000 ft gain in rated height vs. the best 1-stage V-1710 - 22500 ft vs. 15500 ft for 1125 HP, no ram. The 2-stage V-1710 can make a good hi-alt fighter, the best service 1-stage V-1710 cannot. 
The gains of Merlin 46 vs. 45 were not that limited, some 3000 ft gain? There we can see the limits of 1-speed supercharging - low alt power is woefully low, so the proposal of the 2-speed gearing for big impeller on Merlin would be a good one.



> Water injection added to the P-39Q would likely have allowed it to outperform the P-63A at most altitudes. (charge cooling helping somewhat at high alts as well)



With both engines employing water injection, the P-63A would've still have the advantage. Let alone the P-51 with a 2-stage V-1710.



> One thing that would require zero structural/manufacturing changes would be more aggressive testing of the engines, and perhaps looser standards for clearing service ratings based on those tests. Given what the British were doing with Merlins in the field (and sometimes V-1710s), introducing increased maximum boost limits for emergency power (officially, not unauthorized overboost). Raising limits to 48" Hg manifold pressure (+9 psi boost) would have given engine performance similar to the Merlin III on +12 psi or slightly better but limited to 8000 ft rather than 10k. (relevant even for the V-1710-33 of the P-40B/C/Tomohawk, though the reduction gearing might not have coped with that power)
> Going up to +12 psi boost (54" Hg) would be what the early P-39's engines were rated for when finally cleared for WEP, resulting in 1490 hp at 4000 ft (with ram) I believe.



All fine proposals, but there won't be any gains over 15-17 kft, and 1-stage V-1710 was already a good low to mid-alt engine.


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## kool kitty89 (Jun 4, 2015)

tomo pauk said:


> The worst of 2-stage V-1710s that entered service have had 7000 ft gain in rated height vs. the best 1-stage V-1710 - 22500 ft vs. 15500 ft for 1125 HP, no ram. The 2-stage V-1710 can make a good hi-alt fighter, the best service 1-stage V-1710 cannot.


I believe I was thinking more of some engines only used in prototype testing, not in service. Particularly the XP-76 and XP-63, but looking again at P-63 Performance Tests I see I was mistaken, and the engine was producing significantly more power at all altitudes at WEP than the P-39Q or N. (and also better performance than the Merlin V-1650-1 on the P-40F tests on that site)

On that note, I do recall all the production 2-stage allison engines using larger impellers for the aux stage. Might it not have expedited development and eased production if they made the auxiliary supercharger as similar as possible to the integral one while also saving on bulk/weight? (using the 8.8:1 ratio for the integral stage seems fine so long as the aux stage has a neutral setting like P&W engines did, maintaining good take-off power and low-level WEP)

Possibly easier to mount a smaller aux stage in a DB/Jumo style side position as well.



> The gains of Merlin 46 vs. 45 were not that limited, some 3000 ft gain? There we can see the limits of 1-speed supercharging - low alt power is woefully low, so the proposal of the 2-speed gearing for big impeller on Merlin would be a good one.


Agreed, possibly closer to the performance of the DB-605AS as well.



> With both engines employing water injection, the P-63A would've still have the advantage. Let alone the P-51 with a 2-stage V-1710.


Agreed, with the possible exception at low level (9.6:1 single stage engine with 60" boost and water injection) or more likely, an 8.8 ratio engine overboosted at sea level. (granted, not good for much, but notable ... possibly a decent V-1 interceptor, but then the Mustang I with overboost would probably be even faster)



> All fine proposals, but there won't be any gains over 15-17 kft, and 1-stage V-1710 was already a good low to mid-alt engine.


Yes, and a 2-stage 3-speed (neutral) engine without water injection could be tuned for smoother power curves, plus any drag from the aux stage in neutral might be compensated (or exceeded?) by an improved side-mounted ram intake. (possibly better placed on the starboard side, avoiding having the cockpit door as a limiting factor for length/shape/placement)

And thinking on it further, it might even have been worth not bothering with the 9.6:1 single stage models (and manufacturing/engineering changes needed for the different gearing and stronger/broader pitch gear teeth) if they could have expedited development for at least some form of auxiliary stage that could be coupled to the existing models. That and more aggressive testing of the existing 8.8:1 engines. Even the 9.6:1 models wouldn't be worth too much earlier on if they had similar boost limits (and even less power) as the 8.8:1 engines were rated for. In fact, I'd say allowing WEP on the 8.8 engines would be more significant than getting the 9.6:1 engines into service. (especially in the Pacific and MTO -but also for low altitude intruder and fighter bomber missions in the ETO)

With the V-1710-39 cleared for 54" Hg by the time of the XP-51's first flight, performance figures may have made the USAAF even more interested (if initially dubious). It's obviously come in handy for the Mustang I as well, and the A-36's engine should have been cleared for 60" WEP. (some Allison documentation points to the -39 being allowed to run at 60" too, but I'm not sure this was adopted officially)


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## wuzak (Jun 4, 2015)

kool kitty89 said:


> On that note, I do recall all the production 2-stage allison engines using larger impellers for the aux stage. Might it not have expedited development and eased production if they made the auxiliary supercharger as similar as possible to the integral one while also saving on bulk/weight?



Bi.

Since the two impellers had to be matched for the same mass air flow. As the air was compressed after the first stage it took up less volume. So it required a smaller impeller.

Mpst Merlin 2 stage engines had a 12.0" first stage and 10.1" second stage. Others had 11.5" first stage and 10.1" second stage. The ultimate wartime development, the RM.17SM, had 12.7" and 10.7" impellers.

The V-1710 had a 12.1875" auxiliary supercharger impeller and a 9.5" main supercharger impeller.


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## Shortround6 (Jun 4, 2015)

The first Allison two stage engines (prototypes or experiments) did use 9.5 in impellers in the first stage. This was in the interest of using common parts (or what was available?) and as mentioned above, it didn't work and may have actually cost development time. 

As Wuzak has said you need to match airflows. Since the air at 25,000ft is 44.8% of the weight of air at sea level per cubic foot and you need pounds of air for combustion you need a supercharger than can turn the low density air into higher density for the next stage. A small impeller/supercharger can't do the job. 

Allison gave up on the fixed gear ratio auxiliary supercharger pretty quick and went to a variable hydraulic coupling much like the drive on a DB engine. This gave them about the smoothest power curve you are going to find on a 2 stage supercharger.

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## kool kitty89 (Jun 4, 2015)

wuzak said:


> Bi.
> 
> Since the two impellers had to be matched for the same mass air flow. As the air was compressed after the first stage it took up less volume. So it required a smaller impeller.


I know that's a problem from going directly from one stage to another, but if the ducting between the two stages was necked out into an expanded chamber before reaching the carburetor, it should have been able to defuse more and avoid that conflict ... at least in theory. That arrangement might still be too impractical to be efficient, though, or just to bulky. (with the side mounted arrangement, you'd have more space/length to the ducting anyway, so maybe more practical?)

A taller impeller (broader pitch vanes) would increase mass flow too, but then you're still needing a new impeller, diffuser, and housing.



Shortround6 said:


> Allison gave up on the fixed gear ratio auxiliary supercharger pretty quick and went to a variable hydraulic coupling much like the drive on a DB engine. This gave them about the smoothest power curve you are going to find on a 2 stage supercharger.


Didn't Pratt and Whitney's auxiliary superchargers work the same way? (in fact, encouraging a licensing arrangement between Allison and P&W might have been the best option for expediting development there -granted, the Army would actually have to CARE to intervene beyond private dealings in the matter, or actual interest from the Navy in the V-1710)


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## wuzak (Jun 4, 2015)

kool kitty89 said:


> I know that's a problem from going directly from one stage to another, but if the ducting between the two stages was necked out into an expanded chamber before reaching the carburetor, it should have been able to defuse more and avoid that conflict ... at least in theory. That arrangement might still be too impractical to be efficient, though, or just to bulky. (with the side mounted arrangement, you'd have more space/length to the ducting anyway, so maybe more practical?)
> 
> A taller impeller (broader pitch vanes) would increase mass flow too, but then you're still needing a new impeller, diffuser, and housing.



If you diffuse the compressed air so much that it can use the same compressor size for teh second stage then I would suggest there was no point having the first stage at all.




kool kitty89 said:


> Didn't Pratt and Whitney's auxiliary superchargers work the same way? (in fact, encouraging a licensing arrangement between Allison and P&W might have been the best option for expediting development there -granted, the Army would actually have to CARE to intervene beyond private dealings in the matter, or actual interest from the Navy in the V-1710)



The R-1830 certainly used fixed gear ratios - neutral, Lo and HI. 

Certainly the R-2800-32W ("sidewinder") used a fluid coupling for the first stage, but I would have to check for the R-2800-8 (as used in F4U) and -10 (as used in F6F). I believe they used the same system as teh R-1830 two stage units.

What was common between the P&W 2 stage engines and the V-1710 2 stage engines was that the engine supercharger and auxiliary supercharger were independent of each other (were not driven by the same shaft or at the same speed), unlike on the Merlin.


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## Koopernic (Jun 5, 2015)

Allison/GE/USAAAF added a huge amount of misery in airframe integration issues when they chose to use air to air inter coolers for the Turbo versions of the V-1710 intended for single engined aircraft such as the P40 and P39. 

Rolls-Royce used liquid air inter coolers for the Merlin 60 series, something which merely required an increase in radiator area proportional to the power increase. Considering that the liquid cooling was already available for primary engine cooling one can only think that funds were not made available.

The Germans successfully introduced an air to air intercooler on the otherwise liquid cooled Jumo 211J which added about 6% at takeoff power and well over 10% at altitude to the single stage two speed engine Jumo 211F.

Nevertheless when they wanted to use this engine on high speed aircraft such as the Ta 154 Moskito they used the non intercooler Jumo 211F for prototypes and were planning to use the more powerful non intercooled Jumo 211N for production. It seems possible that the slightly higher drag of the air to air intercooler Jumo 211J and its advanced version Jumo 211P were not thought worth it on high speed aircraft where drag is an issue. When intercooler versions of the Jumo 213 came out they used air liquid heat exchangers. 

It possible that an liquid air intercooler added to the V-1710 even with its single stage single speed supercharger might have made a worthwhile improvement. The V-1710 operated on relatively high boost levels compared to the Jumo 211J suggesting inter cooling even more worthwhile. It should have eased integration issues.

Air to air inter coolers only work on purpose designed airframes or twins due to restriction in placement of the intercooler.


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## tomo pauk (Jun 6, 2015)

kool kitty89 said:


> ...
> Didn't Pratt and Whitney's auxiliary superchargers work the same way? (in fact, encouraging a licensing arrangement between Allison and P&W might have been the best option for expediting development there -granted, the Army would actually have to CARE to intervene beyond private dealings in the matter, or actual interest from the Navy in the V-1710)





wuzak said:


> ...
> 
> Certainly the R-2800-32W ("sidewinder") used a fluid coupling for the first stage, but I would have to check for the R-2800-8 (as used in F4U) and -10 (as used in F6F). I believe they used the same system as teh R-1830 two stage units.
> ...



The R-2800-8, -10 (link) and -18 (link)used neutral/low/high gearing, as indeed the 2-stage R-1830. The 'hydraulically-driven' suprecharger was used on E series of engines, eg. the 1-stage supercharged -30 (on F8F-2, link) used it.


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## tomo pauk (Jun 6, 2015)

> Koopernic said:
> 
> 
> > Allison/GE/USAAAF added a huge amount of misery in airframe integration issues when they chose to use air to air inter coolers for the Turbo versions of the V-1710 intended for single engined aircraft such as the P40 and P39.
> ...


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## kool kitty89 (Jun 6, 2015)

wuzak said:


> If you diffuse the compressed air so much that it can use the same compressor size for teh second stage then I would suggest there was no point having the first stage at all.


I think I see the issue here. If I understand correctly, two identical superchargers in series, turning at the same RPM, would produce the same mass flow and pressure as a single one at the same speed. Thus, the aux stage would simply be putting drag on the integral stage at any speeds lower than the integral stage's. However, wouldn't there still be a net gain when the auxiliary stage turned faster than the integral one? (though that would also nix my suggestion for using the 8.8 integral blower ratio and make more sense to use the lowest of the integral blower ratios)

Or maybe there's something else I'm missing about the mechanics involved for why a smaller impeller running at a higher speed couldn't approximate the mass flow of a larger compressor running at a lower speed. (albeit with actual pressure depending more on diffuser arrangements)

Obviously, twin superchargers in parallel rather than series would be another matter, but that wouldn't really be relevant unless they could run ducting from the aux stage directly into the engine manifold rather than into the carb intake.


I suppose if nothing else, an auxiliary coupled with a standard 8.8 blower engine could effectively make the integral stage superfluous when the aux stage is engaged. (say neutral, 9.6, and possibly something closer to 10.5~10.6 -tip speeds similar to that 10.5" impeller running at 9.6) That of course, assuming the engineering for a secondary 2-speed gearing arrangement would be simpler/faster to engineer than redesigning the accessories section for an integral multi-speed arrangement. (it would also mean not disrupting production of existing models, with the aux stage expressly designed to be added on)
Even neutral and 9.6 speeds for the aux stage would be useful.





Koopernic said:


> It possible that an liquid air intercooler added to the V-1710 even with its single stage single speed supercharger might have made a worthwhile improvement. The V-1710 operated on relatively high boost levels compared to the Jumo 211J suggesting inter cooling even more worthwhile. It should have eased integration issues.


I believe Lockheed resorted to developing their own liquid-to-air intercooler radiators for the P-38J, abandoning the air to air surface cooled intercoolers previously embedded in the wing leading edges.



> Air to air inter coolers only work on purpose designed airframes or twins due to restriction in placement of the intercooler.


Also easier on large, multi-engine aircraft and/or radial engined examples, in both cases where the intercoolers take up a smaller portion of overall drag as well as have more engineering freedom to integrate into the airframe without resorting to bulky external radiator installations. (same would go for embedding a turbocharger)


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## Shortround6 (Jun 6, 2015)

kool kitty89 said:


> I think I see the issue here. If I understand correctly, two identical superchargers in series, turning at the same RPM, would produce the same mass flow and pressure as a single one at the same speed. Thus, the aux stage would simply be putting drag on the integral stage at any speeds lower than the integral stage's.



It doesn't work that way either. ALL of my practical experience with centrifugal "compressors" was actually with centrifugal water pumps. Since water is not compressible (although we could get well over 200PSI on it) somethings don't come out quite the same. We did have old fire trucks with dual impellers that could be run in either parallel (capacity) or in series (pressure) which may help in understanding some of what is going on. In capacity/parallel the intake split and 1/2 went to each impeller after which the piping brought the flows back together. It could allow us to get (depending on the truck) 1000gpm at 150PSI. More pressure usually meant less volume (GPM) and without a good enough supply you could "cavitate" the pump. You could exceed 150psi in capacity by using more engine RPM but you were going to run up against the engine governor and the setup would shoe lower efficiency pretty quick. and The low pressure at the intake would allow very local areas of the impeller to fall below vapor pressure and the water would form vapor bubbles. In an impeller operating in air this same situation may be referred to as stalling. The impeller is trying to pull a vacuum and flow breaks down and re-starts in rapid succession. Causes vibration and weird noises. Back to the fire pump, if we need more pressure (pumping to upper floors or long hose layout from pump to nozzle) we could flip the change over valve/s (the intake and exit valves were controlled by the same lever/crank on the panel) to pressure and put the impellers in series, the pump was now good for 500gpm at around 250PSI _at the same engine RPM as before._ Pumps were "rated" at draft (sucking water from a pond or reservoir) at pretty much sea level (factories or test facilities were usually under 1000 ft). 
Getting water from a hydrant allowed for a significant increase in flow. Think RAM on an airplane. Instead of working at "negative pressure" negative being less than 14.7lb/sq in (and operating and more than 5-10lbs negative[still 5-10lb absolute positive] was a big NO-NO)while drafting the hydrant might give you a 20-60lb boost depending on flow (size of pipes in the street and pressure in them).
However for us water doesn't change much. A gallon of water is pretty much 8.3lbs per gallon and 62.4 lbs per cubic foot. which made things simple, suck in a cubic foot of water and put pressure on it and a cubic ft of water was going to come out the hose nozzle 500ft away (assuming you had enough water to fill the hose). 
AN engine wants pounds of air per minute for a given amount of power. Unlike water the air density is all over the place as the plane climbs and dives. The size impeller and housing that works just fine at 5,000ft just won't flow the same number of pounds of air at 25,000ft even if it is flowing the same or a bit more cubic ft of air. It will be flowing around 1/2 the pounds of air needed. You need the bigger first stage to 'compress' the low density air to higher level of pounds per cubic ft so the second stage can do it's job. 
Centrifugal pumps/impellers are not positive displacement. There is slippage, we could adjust to the pump to be holding 150psi for example with no water flow. out of the pump. Not a good idea for long as the spinning impeller heated the water. Do it too long and you could get the several hundred pound pump housing too hot to touch. 
With the air pump (Supercharger) you can only get so much pressure rise in one stage, since the air does compress and get smaller you can't get anywhere near the pressures we could get with water but you are still going to run into a limit on the output volume (choking) and there will be a limit on the upper pressure at which point the air is churning in the supercharging housing rather than flowing through it. 

This a a compressor map for a modern turbo from the Garrett company website. doesn't matter how the compressor is driven though, it will act the same. 

View attachment 294279


Please notice on this particular compressor increasing the impeller rpm by 16% only increases the mass flow (Lb per minute) a very small amount once 45lb a minute was reached and in fact increasing the rpm from just under 103000rpm to just over 140000rpm (36% rounded up) only increases the mass flow from 40lb/min to about 48lb/m or 20%. The unit has reached it's choke point. 

Also notice the "surge" line. Airflow _below_ the lb/min at the rpm of the impeller will cause the impeller to stall (a bit like an airplane wing stalls) as it is trying to pull a vacuum and the air flow will break down and then re-establish it self a number of times per second. You want the impeller to be operating near the middle of the chart as much as possible so you don't run into the problems near the edges. You also want the highest efficiency you can get, which is found in the middle. Say a supercharger needs 70hp to do the work of compressing the air. A 70% efficient supercharger will need about 100-102 hp at the input shaft (0.1-2.0 hp used up in the supercharger drive), the other 30hp goes into needlessly heating the intake charge over and above the heat created by the simple compression. A 65% efficient supercharger is going to need 108hp after the losses in the drive. the extra 8hp aren't that important to the propeller but they are important in that they ALL go directly into heating the intake charge, a 26% increase in _extra_ heat going into the intake charge pushing that much closer to the detonation limit in addition to the hotter charge being less dense and therefor not making as much power. 
Trying to go cheap on the supercharger is going to come back and bite you all too hard.


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## wuzak (Jun 6, 2015)

kool kitty89 said:


> I think I see the issue here. If I understand correctly, two identical superchargers in series, turning at the same RPM, would produce the same mass flow and pressure as a single one at the same speed. Thus, the aux stage would simply be putting drag on the integral stage at any speeds lower than the integral stage's. However, wouldn't there still be a net gain when the auxiliary stage turned faster than the integral one? (though that would also nix my suggestion for using the 8.8 integral blower ratio and make more sense to use the lowest of the integral blower ratios)
> 
> Or maybe there's something else I'm missing about the mechanics involved for why a smaller impeller running at a higher speed couldn't approximate the mass flow of a larger compressor running at a lower speed. (albeit with actual pressure depending more on diffuser arrangements)
> 
> ...



I've probably not explained myself very well.

Two compressors operating in series will have the same mass flow. That is because the second compressor receives its air from the first.

And most compressor maps use mass flow in the horizontal axis. But that is for air at standard temperature and pressure. 

Compressors really work with volumetric flow rate. But the volumetric flow rate varies with temperature and pressure, and is different between the inlet and outlet - making defining what the flow rate is potentially confusing. (Most industrial compressors specify volumetric flow rate as Free Air Delivery - the volumetric flow rate at inlet conditions.)

You can use two compressors of the same diameter but they will need to rotate at different speeds. The problem is, if one is in the sweet, high efficiency area the other will be in a much lower efficiency area.







You can see that for a given pressure ratio and inlet flow rate the compressor will be operating at a certain efficiency.

If we use inlet air of 10m3/s and use a 2.2 pressure ratio we are operating at ~70% efficiency. The outlet air, however, will be at 4.54m3/s (10/2.2) because it has been compressed - reduced in volume.

As you can see in the graph, the area the second compressor can operate becomes quite small. You can possibly get ~1.8 PR, which would give an overall PR of around 4. Which is not a huge deal better than a single stage compressor can do.

Worse, you are operating near the surge line of the compressor. This is a breakdown of flow and will lead to loss of compression.




kool kitty89 said:


> I believe Lockheed resorted to developing their own liquid-to-air intercooler radiators for the P-38J, abandoning the air to air surface cooled intercoolers previously embedded in the wing leading edges.



The Lockheed P-38J/L, etc, used an air to air intercooler similar to what was used in other American aircraft.

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## wuzak (Jun 6, 2015)

To illustrate further, because I don't think I am explaining things very well, consider the fixed speed ratio (single or multiple) supercharger, such as on the "altitude" rated V-1710s and the Merlins.

To maintain the correct mass flow rate the intake ahead of the compressor is throttled. That is, the volumetric flow is restricted by a throttle plate. The restriction in flow and the lower volumetric flow rate put the compressor into a lower efficiency part of the map.

The result is that even though boost (pressure above sea level standard pressure) is maintained in that supercharger gear ratio from 0ft to the rated altitude, the power is lower than at rated altitude. 

As can be seen by this Merlin 46/47 power chart:
http://www.wwiiaircraftperformance.org/Merlin_46_47_Power_Chart.jpg

Interestingly the engine has more power @ 2850rpm than @ 3000rpm until the former meets its FTH. That is because the mass flow is less and the throttling is, therefore, less for a given boost pressure.


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## Shortround6 (Jun 6, 2015)

Keep trying Wusak, it _is_ a difficult subject to explain but between a number of us we just might get there, assuming we don't confuse each other in the meantime  

because of the difference in intake temp, pressure and density it might best be illustrated with a 3D chart of graph. A bit like playing 3D chess  

Stanley Hooker said _after_ he moved from RR to Bristol to work on Jet engines that he didn't think the engineers at Bristol really understood airflow so it is certainly not an easy thing to grasp on an intuitive level.


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## wuzak (Jun 6, 2015)

A couple of other examples might help illustrate the issue for multi-stage compressors.

Firstly, a 2 stage piston compressor.






There are two first stage piston compressors, and a sole second stage piston. The latter is noticeably smaller, even though it is fed by two of teh larger first stage pistons.

And an axial stage compressor

http://www.ichmt.org/abstracts/CHT-97/Image753.gif

Each stage has a smaller annular area than the last. As the pressure ratios in an axial compressor is small, the difference in size between the adjacent stages is less pronounced. 

The axial turbine at the back works opposite, getting larger as the air flow expands.

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## tomo pauk (Jun 7, 2015)

Great work, many thanks


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## Koopernic (Jun 7, 2015)

tomo pauk said:


> > The P-40 with turbo (P-40H) never progressed beyond paper, so the amount of misery there is zero.
> 
> 
> 
> ...


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## Shortround6 (Jun 7, 2015)

Koopernic said:


> tomo pauk said:
> 
> 
> > The misery is considerable considering the aircraft under performed in service at altitude, was shifted out of production,due to altitude related lack of performance.
> ...


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## tomo pauk (Jun 7, 2015)

Koopernic said:


> ...
> The misery is considerable considering the aircraft under performed in service at altitude, was shifted out of production,due to altitude related lack of performance.



The loss of P-40's performance had nothing to do with the way the turbo V-1710 (where installed) was intercooled.



> The PW R-2800 of the Corsair and Hellcat both had intercoolers: chin for the hellcat, wing root for the corsair. BMW801R also would have been intercooled and used on a Ta 152C variant, cancelled due to effects of bombing.



Indeed, the 2-stage R-2800 was intercooled, the air-to-air intercoolers were used. As SR6 noted, the intercoolers were behind the engine on the Hellcat, same for the Corsair. Plus Wildcat (for the variants with 2-stage engine), air feed was like the ram air intakes for the BMW 801.
The BMW 801R was to have independent 2 speeds per each impeller, employing both inter- and after-cooler, ie. the compressed air was to be cooled after each stage. The power was to be 1400 PS at 11 km on 2700 rpm, 2000 PS at SL.



> The increase in radiators size, a doubling in the case for provision of the extra cooling flow for intercoolers seems a little high as the coolant is still available for engine cooling after passing through the heat exchanger.



The temperature of the 'unified' coolant will rise after it cooled the charge, meaning more of it is needed to cool the engine now. More coolant to be cooled means greater radiators, plus there is an increase of power with 2-stage version of the 1-stage engine - 30-50% at altitude, where the air is thin? AT the end, we need 30-50% increase of coolant radiato capacity, with another 25-35% added for the needs of intercooler - makes 150-200% of the size of the presvious radiator. Or, go with what RR did - a bit increased 'main' radiator, plus a smaller extra radiator for the needs of intercooler. The 1st prototypes of the Merlin Mustang were with old radiator, it was quickly deemed as too small, so the new 3 cooling systems were designed for it. 



> I think the increase size comes from the fact that at high altitude an inter cooled engine still has the same charge density as at sea level and thus similar cooling requirements yet the air density for cooling the radiator is less.



Yep, the thinner air makes cooling and inter-cooling problematic, despite the lower temp of that air.


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## Shortround6 (Jun 7, 2015)

You also have the fact that the engine "may" be making more power in the cylinders. As an indication of this the R-2800 made 2000hp at 2000ft at 2700rpm at 52.5in with the aux blower in neutral. At 16,000ft it made 1800hp at 2700rpm at 53in with the aux blower in low. At 21000ft it made 1650hp at 2700rpm and 53 in with the aux blower in high. Aux blower took 350hp to run in high gear? 

Allison with two stage blower is making 1150hp at 25,000ft at 3000rpm and 50in. The Allison in a late model P-40 was making 1125hp at 15,000ft using 44in (or bit under) at 3000rpm. The extra power from the additional 6in of MAP goes into driving the aux stage of the supercharger. It still needs to be cooled however so the engine needs a bigger radiator even if the 'nominal power' is the same. If you add an inter cooler the cumstion temp won't be quite so hot but you need to cool teh charge air and you stillneed to cool the engine for the power it is making in the cylinders, not the power making it to the propshaft.


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## wuzak (Jun 7, 2015)

Tomo mentioned the "3 cooling system" for the P-51B.

Basically there were three bays to the cooling radiator - two were for the engine radiator and one was for the intercooler. 

Though they used the same fluid and were in the same radiator structure, engine cooling and intercooling were two separate circuits.

The F4U intercoolers were in the fuselage, one either side, fed by the leading edge ducts.


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## kool kitty89 (Jun 9, 2015)

Shortround6 said:


> This a a compressor map for a modern turbo from the Garrett company website. doesn't matter how the compressor is driven though, it will act the same.
> 
> View attachment 294279


Link doesn't seem to work.






wuzak said:


> If we use inlet air of 10m3/s and use a 2.2 pressure ratio we are operating at ~70% efficiency. The outlet air, however, will be at 4.54m3/s (10/2.2) because it has been compressed - reduced in volume.
> 
> As you can see in the graph, the area the second compressor can operate becomes quite small. You can possibly get ~1.8 PR, which would give an overall PR of around 4. Which is not a huge deal better than a single stage compressor can do.
> 
> Worse, you are operating near the surge line of the compressor. This is a breakdown of flow and will lead to loss of compression.


So, if assuming there's at least some compatible combination of speeds to use, the options would be narrow. Pressure and flow moderation related to the throttle plate would also complicate matters. It seems like getting 2 useful speeds on a second stage would be even more difficult, or rather, finding a wide enough range of effective flow/pressure characteristics avoiding the stall and surge ranges at their respective altitudes. More likely would seem to be having only a single speed along with a neutral setting for low altitude/cruise. (though perhaps both the 8.8 and 7.48 gear ratios, and any lower speeds already used on early/mid-war allison engines would each have corresponding well-matched aux stage speeds for performance at higher altitudes)

It's more complex than just using a larger single stage supercharger with better airflow characteristics, but even if that was fully understood there remains the production problem: producing an accessory for use with existing engines in mass production and avoiding disrupting said production. (a single, fixed gear speed also seems like it should be the simplest arrangement to implement, but allowing a neutral setting with clutch mechanism would be pretty useful for improving takeoff power -maybe more so as a bomber engine)

Given the manifold pressure figures at given altitudes, the 8.8:1 supercharger seems to have managed somewhere around 1.9:1 overall pressure ratio without no ram, possibly less (with ram on the P-40E it's 2.15:1 going by 42" Hg at 12,000 ft -approx 19.53" Hg atmospheric pressure at that altitude). The 9.6:1 blower seems to have managed just under 2.5:1 compression without ram and over 2.75:1 overall pressure with ram on the P-40N. (and assuming the flow characteristics were acceptable, putting a 9.6:1 blower as the aux stage for an 8.8 integral supercharger should result in an OPR of around 4.5:1)

Except then you'd end up with a high alt power curve similar to the low alt one of the 9.6:1 engines (lower power but similar slope) and possibly more charge heating than really useful. (without intercooling or water injection at least)

The Merlin 46/47 seems to have managed closer to 3.5:1 (3.2:1 without ram) on a single stage, so approximating that for the allison via aux stage (without altering the base engine+integral stage) might be more practical. I don't have precise altitude or pressure figures for the 7.48 supercharger, but at a guess it seems like 8.8+7.48 would make a reasonable match.




> The Lockheed P-38J/L, etc, used an air to air intercooler similar to what was used in other American aircraft.


Ah thanks, though it does seem to be a fairly compact and streamlined affair. (the likes of which the P-39 or P-40 would have benefited from -though all the ducting for the turbo installation itself would still be problematic. (and those intercoolers did add significant drag to the P-38, not that that would have been such a bad thing with the diving issues on earlier models -ie reduced dive acceleration + more power for level flight and climb might have helped more than it hurt) 




wuzak said:


> Interestingly the engine has more power @ 2850rpm than @ 3000rpm until the former meets its FTH. That is because the mass flow is less and the throttling is, therefore, less for a given boost pressure.


Indeed, this is something I've wondered about the V-1710 as well, particularly the 9.6:1 supercharger. (running it at closer to 2800 RPM at low altitudes for better maximum power due to reduced charge heating and reduced supercharger power consumption/drag)






Koopernic said:


> tomo pauk said:
> 
> 
> > The misery is considerable considering the aircraft under performed in service at altitude, was shifted out of production,due to altitude related lack of performance.
> ...


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## GrauGeist (Jun 9, 2015)

kool kitty89 said:


> ...The P-40 was discontinued due to the P-51 being superior in nearly every aspect while sharing similar engine resources.


The P-40 was manufactured from 1939 until 1944 and remained in combat service in some areas, until war's end.

As far as engine conflict, only the P-51, P-51A and A-36 used the Allison...it could be said that the P-38 would have been more of a draw on the V-1710 than the P-40.


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## GregP (Jun 9, 2015)

After the A-36 and Allison P-51s, the P-51 didn't use similar engine resources to the P-40. The Allison was never a limiting factor in P-51 production after the P-51B.

The XP-40Q didn't fall short of the P-51D except in all-out top speed, and not much there. 422 mph versus 437 at best altitudes respectively. The XP-40Q rolled better, turned better, and climbed better than the P-51D.

I think it wasn't adopted because the P-51D was winning the war in the ETO already. Though I really like the XP-40Q personally, I can't make a claim that they chose wrongly since the P-51 did an excellent job overall in WWII. 

The XP-40Q wasn't the only potentially very good prototype airframe that failed to garner a production order, but we COULD have been flying them instead of all the P-40s that came after about March 1944 or so. Taken together, that makes up about 1,000 P-40s out of some 13,143 built, so the effect wouldn't have been "huge" anyway. If they had switched all P-40 production to the Q when they could have, the interruption wouldn't have been too great ... but the guys in charge thought otherwise. If I had all the information they had, I might have made the same choice ... I can't say with any certainty because I don't know what they were looking at to make the choice at the time.


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## Shortround6 (Jun 10, 2015)

Part of the choice was that by late 1943 ALL new P-40 production was either going to Allies as lend lease or to Advanced Fighter training schools. Perhaps a trickle to US units still flying P-40s as replacements but since P-40 equipped units in the Field were being re-equipped with P-47s and P-51s the demand for a "new" P-40 type was pretty small. They were looking to simplify the logistics. 3 Basic fighter types not 5 (P-39s also being phased out)This took quite a while to achieve. But units in the Field that transitioned to new equipment either handed their old fighters over to nearby units or to an in theater replacement pool. It could take months from when a fighter rolled out the factory door to when it arrived at an overseas combat unit. Planning and allocations were being worked on months before the fighter rolled out the factory door. 
To reverse some of these planning decisions at point 1/2 through the program required a major change in circumstance, not just that the latest fighter XXX was better than previous models.


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## drgondog (Jun 10, 2015)

GregP said:


> After the A-36 and Allison P-51s, the P-51 didn't use similar engine resources to the P-40. The Allison was never a limiting factor in P-51 production after the P-51B.
> 
> The XP-40Q didn't fall short of the P-51D except in all-out top speed, and not much there. 422 mph versus 437 at best altitudes respectively. The XP-40Q rolled better, turned better, and climbed better than the P-51D.
> 
> ...



I have never seen any hard data on the ECO's required for both the changes in tooling and or changes in parts or sub assemblies to change over while Curtiss set up a separate line at one Curtis facility to divert P-40N sub assemblies to the P-40Q. A bigger issue for Curtis is how do they get paid while re-tooling for the P-40Q if they shut down deliveries of the contracted P-40N's?

For the AAF, why invest in a maxed out dead end airframe that is slower and shorter ranged than the airframe that is in serial production and a proven commodity and there is an even better one on the boards?


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## Bad-Karma (Jun 10, 2015)

Is there any hard info available for the turn radius or rate of turn for P40? Something similar to this http://www.wwiiaircraftperformance.org/wade-turning.jpg?


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## kool kitty89 (Jun 10, 2015)

drgondog said:


> I have never seen any hard data on the ECO's required for both the changes in tooling and or changes in parts or sub assemblies to change over while Curtiss set up a separate line at one Curtis facility to divert P-40N sub assemblies to the P-40Q. A bigger issue for Curtis is how do they get paid while re-tooling for the P-40Q if they shut down deliveries of the contracted P-40N's?
> 
> For the AAF, why invest in a maxed out dead end airframe that is slower and shorter ranged than the airframe that is in serial production and a proven commodity and there is an even better one on the boards?


Aside from raw performance, there's also the huge internal fuel capacity and range advantages (due both to low drag and high fuel load) of the Mustang.

Given the timing of the P-40Q and the 2-stage allison engine used, the P-63 would be the more direct competitor there. It also lacked internal fuel load and had lower top speeds at most altitudes than the contemporary P-51, but should have been a better dogfighter and interceptor given the wing loading, power loading (depending on model -once water injection was standard with the P-63C it was more straightforward), acceleration at low/mid combat speeds, climb, and 3 hard points to the P-51's two. The M4 and M10 cannons weren't great for fighter combat but better for interception of heavier aircraft or heavy soft ground targets. (where machine gun strafing would be less effective) 

The M2 Hispano should have been straightforward to adapt to the airframe, possibly with similar ammunition capacity to the P-38. It should be a much simpler modification/revision than changing the wings to include more fuel cells. (though that was still odd and shortsighted not to implement more internal, especially modular wing space more like the P-39 -both for allowing machine guns in the wings rather than gondolas and for allowing more fuel, quite possibly allowing switching between either depending on general mission profile requirements of a squadron -or airforce needs in general -and any need for short/medium range interceptor could omit both some of the fuel cells and the wing guns in favor of the heavy nose armament)

If not for the odd fuel capacity (or internal wing space limit in general) issues with the P-63, it very well may have been in more direct competition with the P-38 for V-1710 allocation.


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## Shortround6 (Jun 10, 2015)

Not all two stage Allisons were created equal either. The P-40Qs were re-engined, sometimes more than once and with only 3 different airframes they also used 3 different model number engines. The testsdonein March of 1944 were with the 3rd version of the engine using the 12 counter weight crank and 3200rpm, a different reduction gear that kept the prop tip speed down at 3200rpm engine speed and different auxiliary supercharger gear than the first two stage engine fitted in the spring/summer of 1943 among other changes. No more than 4 of any of these versions of engines were built so production was still some months off.


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## wuzak (Jun 10, 2015)

The XP-51J, using the 2 stage V-1710, was being built at the same time as the XP-51F. But it was decided that the engine was not sufficiently ready, so teh airframe was sent to Allison as an engine test bed.


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