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

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



 
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
Click to expand...
Link doesn't seem to work.




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.
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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.
Click to expand...
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)




 
kool kitty89 said:
...The P-40 was discontinued due to the P-51 being superior in nearly every aspect while sharing similar engine resources.
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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.
 
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.
 
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.
 

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.
 
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.
 
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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