What about a turbocharged P36?

[Shortround6]

Ok, guys, lets remember that Curtiss did build 14 turbocharged hawk 75 fighters, they just used Allisons instead of P & W or Wright radials

Slot around the fuselage over the wing was the exit for the intercooler.
The XP-37 was actually the original Hawk 75 prototype rebuilt and not a brand new airframe.

These 14 planes helped convinced the army that the turbo set up was still several years away from service use.
Not only turbo regulator problems but the turbines shedding buckets/blades.

P-36/Hawk 75 production was running down in the Spring of 1940 with the P-40 replacing it on the production lines ( simple to do, early P-40s kept a very large amount of the basic P-36/Hawk 75 structure) but this means that to bring the P-36 back with a turbo (or even without) you either need more production space (and jigs/fixtures) or cut back on P-40 production. Chances of a turbo P-36 really doing better than a P-40 is going to depend on cowl technology and better turbo installations that don't really exist until some point or points in 1942 which means production planes won't show up in numbers until some time in 1943 at which point you have a 1200-1350hp machine trying to fight what kind of opponents?

There is an interesting set of drawings of the Hawk demonstrator with two stage supercharger here:

The bulge in the bottom just behind the engine and in line with the front of the wing is the aux supercharger. The tunnel further back is the intercooler. The drawing seems to exaggerate the size of both items compared to photographs.
Again without 1943 production technology getting small intercoolers doesn't seem likely.

BTW using the engine coolant for a liquid inter-cooler is a non-starter. Trying to cool 150-200 degree air with 160-200 degree engine coolant (even after radiator before it is even higher, Mustang had max temp of 135 degrees centigrade/275 degrees Fahrenheit) doesn't seem like a good idea. Spitfire and Mustang both used separate liquid cooling circuits for their inter-coolers.

 

[GregP]

If you click Shortround's link, you can see the drawing of the 2-stage, supercharged P-36. Here is a pic:

Didn't find another one, but also didn't look too long.

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

 

[Shortround6]

Thank you Greg.

I have only seen one other photo of this plane, more of a side view.

I would note that the inter-cooler in the "pod" was meant to handle an engine rated at 1050hp max continuous (no military rating) at 2550rpm at 17,500ft. The engines used in production F4Fs (and some PBYs and some C-47s) were rated at 1000hp at 2550rpm at 19,000ft max continuous.
The turbo charged engines were rated at 1200hp military (2700rpm) and 1100hp max continuous (2550rpm) at 25,000ft.
Due to the thinner air and more compression needed by the aux stage/turbo an even bigger inter-cooler would be needed than the one used on this prototype.
It might be fast high up but performance at 15,000ft or under is going to take a real hit due the extra drag.

 

[kool kitty89]

The YP-37 is interesting, but it's not really a good example of a practical installation of a turbocharger (or V-1710 alone) in the airframe. (though on that note, a radial engine would at least make that turbo arrangement somewhat more reasonable with the much shorter length and lack of coolant radiators, but it'd still be a hose-nose, just maybe a tolerable one more like the F4U)

Do remember the long nose of the YP-37 wasn't primarily to accommodate the turbocharger, but to cram the radiators, main fuel tank and a bunch of other equipment all behind the engine.
Photo by jon farrelly

The radiator, intercooler, and aux fuel tank take up most of the space. (that nose-mounted turbo installation would be OK if the radiators had been moved to the wings, but not so good for a belly radiator due to obvious turbo exhaust injection -a P-39 style radiator arrangement might work, but require adding fuel tanks to the wings, and P-38 style embedded rear-fuselage radiators adds long coolant/oil plumbing AND intercooler ducting if it's moved rearward as well)

That said, a nose-mounted turbo might have been the better configuration for a radial engined Hawk 81 derivative too. (particularly given the added space in the nose already noted in the P&W Hawk 81 testbed -see below)

The 2-stage R-1830 still seems like an all-around more useful option for 1940-42. (and possibly an R-2000 after that ... it was never 2-stage supercharged, but did get a turbocharged installation in the XF5U, apparently with 1600 HP WEP)

Please, please, please !!! Forget about fuselage mounted .50 cal guns. The Big .50 took to synchronization about like cats take to water. British found that it often cycled under 500rpm when synchronized, maybe you could get it to 600rpm when the wing guns were doing 800-850rpm. With the weight of the .50 cal you want all the punch you can get for the installed weight, not crippled by synchronizing gear. There is a reason that only few planes used fuselage mounted .50s later in the war and they were legacy installations from the early part or even pre war design. (except for the P-75 and the less said about that the better)

The Japanese and Finns both preferred Browning-derived .50 cals in the nose over ones in the wings, or used both (but in the Finns case, crammed 4 .50 cal FN brownings in the nose of the Myrsky)

And while FN brownings were substantially faster than American brownings, the loss in performance from synchronization should be proportionally high (ie 1000~1200 RPM down to 650~800) so the concentration of fire and better accuracy were felt to be worthwhile.

The lack of demand for synchronized .50s (outside the P-39) may have contributed to little/no improvement in their rate of fire. There were attempts to speed the .50 up to 1200 RPM during the war (and likely had some common engineering solutions to the FN examples) but none met the Army's stringent requirements for mechanical integrity (ironic given the lacking quality of the American hispanos). However, the examples that failed requirements at 1100-1200 RPM may have passed at 700-800 RPM synchronized. (lower RoF also obviously improved barrel wear, particularly with the browning's tendency to overheat the barrel during long bursts and burn out the rifling)


Additionally, at some point in a previous thread, I was convinced that the better rate of fire of .30 cal M1919s would be more worthwhile synchronized than .50s (particularly if twice as many could be mounted), but after reading the article linked earlier in this thread on the studies in fuel tank and armor vulnerability to .50s vs .30s, I'm convinced even slow firing .50s are more useful, particularly early-war. (and that a P-40 or P-36/Hawk-75 with a pair of .50s at 500 RPM in the nose would often have been superior to the full 8 LMGs used by the Spitfire and Hurricane when employed against german bombers -perhaps not fighters- ... particularly tanking into account the limited usage of incendiary bullets in .303 loadings during the BoB and more so for aircraft with the guns intentionally calibrated for maximum spread rather than harmonized at maximum effective range -that said, the F4F-3 or F2A/B239/339 with four .50s -synchronized+wing or all wing- would obviously be suprior to either, as would have been 4x wing mounted .50s on the Spit or Hurricane ... particularly FN brownings)

Aside from the syncro issues, cowl mounted MGs take up a great deal of space that could be otherwise dedicated to engine components...

This depends on the installation, and it's usually the ammunition boxes and ejection chutes that pose more of a problem for engine compartment space than the guns themselves. (this would likely be the main limiting factor for adding 2 more .50s to the P-39's nose armament and why it only had a capacity of 200 rounds for each of the existing guns -compared to 350 on the P-40B/C)

The P-51 had no trouble including a pair of .50s in the lower nose.

However, for a radial engine installation, the added frontal area provides more leeway for potential increases in armament and ammunition loads. (in this respect, the 9-cylinder R-1820 might actually be more useful as the gun barrels fit between the cylinder gaps, but honestly, adding small cheek bulges for the barrels -and possibly breeches- of the guns in the cowling and fuselage sides with the R-1830 would still likely result in much lower drag, aside perhaps from a turbo+intercooler installation, in which case the larger frontal area might allow for more of that added drag to be hidden as well -aside from turbos, there was also the likes of the R-1820-40 and -56 which both had exceptional altitude performance only beaten by the 2-stage R-1830s, but those would be better matched to the F4F than Hawk-75/81 style airframe -and likely better matched to the F4F than the -40 was to the F2A-2/3's smaller, slimmer airframe)

Turbos got better and were allowed higher rpm in later models which helped airflow and pressure.
Please note that B-17s are sometimes listed as having different critical altitudes for the outboard engines and inboard engines due to differences in the intercooler ducting. And B-17s had a lot of room to arrange things compared to a single engine fighter.

Turbo controls early-war were also less than satisfactory, both confusing and far from foolproof with lack of features like automatic turbo RPM limiting. (no overrev protection, and this resulted in the majority of catastrophic failures or exploding turbochargers)

The P-47 seemed somewhat less dramatically impacted by such (aside from the turbo exhaust igniting the magnesium tailwheel strut/assembly and causing the prototype's tail control linkages to melt -and subsequent bailout and crash), but in any case it would probably be a better bet to avoid turbos if possible early-war. (the P-39 might have seen as many problems as the P-38 -compressibiltiy aside- had they pursued turbos more aggressively)

Multiple, insistent petitions from aircraft designers/manufacturers for improvements in V-1710 supercharging may have been more worthwhile. (and possibly specifying auxiliary supercharger installations that were more compatible with the single-stage dimensions and mounting requirements -ie thus making them useful on the P-40, P-51, and P-39 without substantial modification)

Another look at the P & W test Hack;


Note what seems to be exhaust stains on the side behind the cowling and the absence of normal exhaust stubs like on the P-36 or F4F. Maybe not the exhaust thrust of a V-12 but better than a number of radials? Also please note the even worse view over the nose compared to a P-36 or P-40.

That 'even worse' implies visibility in the P-40 and P-36 (and Hawk 75/81) was already poor, which it wasn't. The P&W test mule Hawk 81 actually appears to have maintained the dimensions of the P-40B/C (Tomahawk) rather closely, perhaps mainly for maintaining center of gravity. (the engine is definitely significantly farther forward than in the Hawk 75, and while part of that added length is likely also being used for the 2nd supercharger stage and intercooler+oil coolers, I suspect it also makes for some added compartment space between the engine and cockpit bulkhead for potential added weapons or fuel capacity)

In fact (synchronization argument aside) that added space for a forward reserve fuel tank might be more worthwhile than attempting to add outboard wing fuel tanks. (and instead modify the wings to accept 4 .50s like the P-40D did, but preferably avoided the added weight gain from wealth of other modifications the Hawk 87 saw over the 81, including perhaps avoiding the 20 mm requirement/provisions in the wings if that added any weight) Blast tubes in the wings might actually generate less drag than bulges in the cowling (at least with the twin-wasp ... Cyclone is another story entirely).

More space for fuel is also a bigger advantage over the turbocharger. Even the best turbo installation is going to compromise fuel space, and while turbo performance (with proper RPM, manifold pressure, and mixture settings) should improve fuel efficiency there's going to be a notable trade-off there. (the turbocharged R-1830 also didn't see the WEP ratings of the R-2800, so the 2-stage vs turbo P-47 discussion we had some time back wouldn't have the same weight here)


The P&W Hawk 81 (along with apparently all Hawk 75s) omitted a spinner from the propeller. This should reduce drag further, especially at high speed, but for some reason most American aircraft omitted spinners on radial engines.

This has already come up in this thread:
Drag of radial-engined fighters

but I haven't yet seen an explanation for so many aircraft omitting a rather obvious component of streamlining. (weight savings is the only explanation I've ever seen, and rarely even that is cited -mostly in a few F2A articles) A well designed spinner and cowling should be simpler to achieve pre-war than a tight cowling and cooling fan (or narrow ram-intake cowling and cooling cuffs on the propeller -a la P2V) and seems to be what both Brewster and most Japanese developers worked towards (as well as XF4U and XF4F-3) but for some reason the P-36 never got one and all American military aircraft from 1941 onward also lacked them.

Post-war air racers added them in many cases too, and did achieve performance boosts (sometimes with tight cowlings and added fans, sometimes without), so they weren't useless.

It can be but the only engines to use liquid to air coolers were the RR ones and they are more properly called after coolers. They cool the intake charge after all the compressing is done. The American engines did all of their cooling between the 1st and 2nd stag.
I will grant that there was some sort of coolant passage in the supercharger housing on the Merlins and Griffon but the vast majority of the cooling was done in the matrix in the box at the rear of the engine.
The Americans need to duct the air from the first compressor (either mechanical or turbo) to the cooler and then duct it back to the carb inlet with the carb mounted on the 2nd compressor housing on the turbo engines.
RR just ran the intake air from the inlet to the carb, from the carb to the inlet of the 1st compressor, then to the inlet of the 2nd compressor and then to the after cooler and then to the intake manifold.
The turbo system is a lot more stretched out to begin with and a small inter-cooler matrix isn't going to change a large part of the ducting.

I'd thought the Merlin intercooler was fed with air from the first supercharger stage and then fed into the 2nd stage ... in spite of seeing cut-away drawings to the contrary. Somehow I'd failed to realize those all depicted aftercoolers.

The 2-stage Allison (no intercooler/aftercooler at all) also moved the carburetor to behind the aux stage, unlike P&W configurations which seem to treat the Aux stage more like a turbocharger with the carb still mounted to the inlet of the integral stage. (which honestly is how I'd assumed the Allison configuration worked too)

https://encrypted-tbn0.gstatic.com/...CHYVU4V2dCtHQIiCEt8qBx5hwuFiYFdIl5jrcbjHa_tMo

reference pic of a 2-stage V-1710 from this interesting article:
http://www.enginehistory.org/Convention/2009/Presentations/SuperchargingAllison.pdf
(which also shows a V-1710 with merlin style aftercooler, including the installation in the XP-51J, plus neat notes on the 2-stage arrangements, including placing the carb between the supercharger stages improving altitude performance significantly ... though the added details makes it look even more like making the aux stage side-mounted would have been trivial, especially since the existing aux stage already used an articulated joint in the accessory drive extension shaft)

The Merlin's aftercooler arrangement would also explain why some post-war examples suffered from overcooling issues. (an intercooler arrangement should at least partially avoid this given the 2nd stage will be heating and re-mixing any fuel that's condensed in the intercooler rather than staying as condensed droplets in the aftercooler -granted, the P-38J had some overcooling issues as well)

Oddly, I've seen some references to a 'pre-cooler' for some of the Mikulin AM-35 or AM-37 articles. (this seems to either be a mistranslation, or reference to the supercharger outlet feeding into the carb intake, thus placing the aftercooler between the supercharger and carb and 'pre-cooling' the charge before entering the carb)

Have Curtiss manufacture the P-51 or even A-36 under license.

The P-51 didn't become an Army-qualified/priority fighter aircraft until after Pearl Harbor, so much more war time production interrupted for Curtiss by the time tooling could get started. (hell, with the P-39's Bell-specific technology in both the modular production optimized bulkhead design and -especially- massive amounts of electrical equipment, that also seems potentially problematic for Curtiss)

With the P-38 already being an Army favorite pre-war priority and earlier development timeframe combined with the simplicity of omitting the turbocharger (and relying on Curtiss's fairly good ram intake and ejector exhaust design -and the wing root fillets Lockheed had developed during the YP-38's testing), they should have been reasonably well suited to license-building a non-turbocharged P-38D derivative that outperformed the Lockheed equivalent below 15,000 ft and was somewhat cheaper. (and avoided the high-altitude engine, cockpit heating, and compressibility problems, thus being service-ready prior to the P-38F and more satisfactory in all-around operations) Comparing 9.6 supercharged (1480 hp WEP at ~10,000 ft) V-1710 powered P-38 vs the equivalent J model with no ejector exhaust and added intercooler drag (and weight of turbo installation) would have been interesting to see too. That aside from overreving and overboosting the 8.8 V-1710. (I'd imagine the P-38 could have come close to the P-51's speed on similar engines but with better climb and acceleration due to power/weight advantage)

 

[kool kitty89]

P-36/Hawk 75 production was running down in the Spring of 1940 with the P-40 replacing it on the production lines ( simple to do, early P-40s kept a very large amount of the basic P-36/Hawk 75 structure) but this means that to bring the P-36 back with a turbo (or even without) you either need more production space (and jigs/fixtures) or cut back on P-40 production. Chances of a turbo P-36 really doing better than a P-40 is going to depend on cowl technology and better turbo installations that don't really exist until some point or points in 1942 which means production planes won't show up in numbers until some time in 1943 at which point you have a 1200-1350hp machine trying to fight what kind of opponents?

There is an interesting set of drawings of the Hawk demonstrator with two stage supercharger here:
Untitled Document
The bulge in the bottom just behind the engine and in line with the front of the wing is the aux supercharger. The tunnel further back is the intercooler. The drawing seems to exaggerate the size of both items compared to photographs.
Again without 1943 production technology getting small intercoolers doesn't seem likely.

Missed this before.

The P&W Hawk 81 testbed for the (X)F4F-3's engine certainly had the cowl technology in play in 1940, and ahead of the game compared to Curtiss's own work on the XP-42.


I believe the P-36/Hawk 75 with the external intercooler was an earlier experiment using the less refined non-production version of the 2-stage supercharged twin-wasp and is quite obviously a converted Hawk 75 or early P-36 given the wheel fairing and lack of wing guns. (or taped-over blast tubes as with the Hawk 81 conversion)

Given the development timeline for the 2-stage R-1830 and V-1710-39, the P&W experimental 1940 installation should have been a serious consideration for displacing the P-40D/Hawk 87 of 1941 while not disrupting Tomahawk production. (and also better avoiding the initial shortage of 2-stage engines the Wildcat suffered)

Competing with F4F engines might have been an issue, but the various 2-speed single-stage R-1830s (and especially R-1820s) should have mostly compensated. (for some reason the R-1820-40 was only fitted experimentally to the F4F, perhaps due to higher volumes of 2-stage twin-wasps by the time a production installation would have been ready, but that engine should have had better altitude performance than the single-stage R-1830s or 1820s used on Wildcats and Martlets and close to that of the FM-2 -though without the low altitude water injection boost; I wish I had more detailed performance specifications for that engine, but all indications of its use in the F2A-2/3 point to exceptional altitude performance for the time)

A 2-speed R-2000 with (presumably) similar or better critical altitude to the 2-speed R-1830 examples of 1940/41 (around 14,000 ft in high gear for the Wildcat/Martlet) should be fairly useful as well, lacking 2-stage adaptations of that. (and probably better in most situations than the V1650-1 due to lower weight and similar or better power -and possibly increased fuel capacity in the space between the engine and cockpit)

The R-2600 on the Hawk airframe has been discussed to death, and even if shortening the Hawk 81's nose to correct CoG would have matched the R-2600 fairly well, the weight, diameter, and altitude performance combined with need of broader chord or 4-blade prop to avoid a larger diameter prop (and impractical landing gear extensions) all make it unattractive. (not to get into some of the development and reliability problems the R-2600 saw during the war -rusty cylinder liners and undersized carburetors among other things, at least in A-20 usage). The increase in drag and weight would also compromise range and all around usefulness for anything other than interception and short-range fighter-bomber duties. (might be a good competitor to the Typhoon, but that's about it ... maybe Fw 190A -but again, with similarly limited range) Besides, if Curtiss was going to make an R-2600 powered fighter, the XP-46 would have been the right time/place to do it. (and honestly, a new airframe optimized around the R-2600's strengths probably would've been better than the existing XP-46, though probably still shorter ranged than the P-40D/E, but certainly good for the interceptor capacity the XP-46 seemed to be targeting)

 

[Shortround6]

I was half joking with the P-37 pictures, but only half. A book on aircraft power plants published in 1943 estimated about 400lbs and about 10 cu.ft. of space were needed for a turbo installation on a 1000hp class engine. Book was written by a Packard test engineer. Granted it was not a restricted book and therefore wasn't giving any military secrets away during war time so it may have been a bit behind the state of the art.

There was nothing mystical or magical about the Hawk 75/P-36. It was a fighter of it's time (1936-38, prototype first flew on 6 May 1935) and had wing loading of about 24lbs per sq ft for a gross weight of 5700lbs (average of the A and C) and well coordinated/harmonized controls, in fact better than a good many of it's contemporaries.
The P-40 should have retained many of it's good characteristics except for the large increase in weight and perhaps minor CG shift.

Any attempts to fit more powerful or more sophisticated engine setups, more operational equipment (protection) and more armament are going to follow the path of the P-40 in the degradation of flying characteristics.
The Allison was NOT responsible for the increase of around 2000lb in gross weight. Any radial version equipped with the same fuel tanks, armor and bullet proof windscreen and the same armament is going to following very closely to the P-40 in weight and if you want "trick" engine installations (like 2 stage or turbos) then you are going to wind up within a few hundred pounds of the equivalent P-40 and suffer from having higher drag. P-36 had 22% higher drag than the XP-40 and the P & W test hack had 8% higher drag. A turbo plane would have had higher drag than the P & W Test Hack due to larger intercooler (let alone adding guns).

There seems to have been something a bit out of wack with the Supercharger on the R-1830 as power increased only slightly going from 2550rpm to 2700rpm and a number of altitudes during propeller tests. Please note that changing from a 7.15 gear to a 8.0 gear on the P-36 changed the engine from 1050hp at 6500ft to 950hp at 14,300ft but cost 100hp at take-off. Two speed superchargers were about take-off power as much as they were about trying for any real altitude rating. C-47s flying the hump got two stage superchargers like the F4F.

You can forget about the R-2000 as a fighter engine. P & W may have used the same supercharger as the R-1830. while take-off power jumped quite a bit, power at altitude didn't follow suit. Early versions with 1350hp for take-off were rated at 1000hp at 16,000ft (at best) and in 1942 that is nowhere near good enough. Later versions rated at 1450hp for take off got rated at 1100hp at 16,000ft (at 2700rpm) but that usually required a few modifications, like different bearings (plain instead of roller) and with a 2 speed R-2000 weighing about 1600lbs it pretty well cancels out the weight of the radiators and coolant of the Allison.
A two speed R-1830 could be good for 1200hp at take-off, 1200hp at 4900ft MIlitary in low gear and 1050hp at 13100ft in high gear. (all 2700rpm) while weighing 1495lbs. The R-2000s were good for either 1350hp at 2000ft low gear or 1450hp at 1000ft in low gear depending on take-off rating.

We have no data on the breakdown rate of the FN guns. There were well over a dozen attempts to develop high rate of fire US .50 guns and while the US requirements may well have been too stringent a few of the attempts failed miserably well before getting even 1/5 of the required rounds fired. Some failures were rather simple like broken extractor hook but I believe in at least one case the gun was wrecked. Placement of guns may have to do with other issues than rate of fire and "concentration" of fire. Like gun heating or design of the wings, adding gun bays that will hold .50 cal Brownings and ammo to existing structures may be more than some design teams wanted to deal with. Japaneses Navy however added big Brownings in the wing of the Zero.

 

[Shortround6]

Missed this before.

The P&W Hawk 81 testbed for the (X)F4F-3's engine certainly had the cowl technology in play in 1940, and ahead of the game compared to Curtiss's own work on the XP-42.

The test hack set it's speed record in Sept of 1942 I believe. Along time away from 1940. How long it took P & W to get to that point I don't know by they may have had the plane/airframe for over two years.

 

[Shortround6]

Missed this before.

The P&W Hawk 81 testbed for the (X)F4F-3's engine certainly had the cowl technology in play in 1940, and ahead of the game compared to Curtiss's own work on the XP-42.

Addressed above.

believe the P-36/Hawk 75 with the external intercooler was an earlier experiment using the less refined non-production version of the 2-stage supercharged twin-wasp and is quite obviously a converted Hawk 75 or early P-36 given the wheel fairing and lack of wing guns. (or taped-over blast tubes as with the Hawk 81 conversion)

Quite right, the plane in question was at the 1939 fighter trials along with two experimental Seversky's one with the same mechanical two stage supercharger used in the Curtiss and and the other with a turbo R-1930 which lead to the P-43.

Given the development timeline for the 2-stage R-1830 and V-1710-39, the P&W experimental 1940 installation should have been a serious consideration for displacing the P-40D/Hawk 87 of 1941 while not disrupting Tomahawk production. (and also better avoiding the initial shortage of 2-stage engines the Wildcat suffered)

The installation was actually late 1938/early 1939 for the Curtiss and Seversky. While P & W got the Hawk 81 airframe in 1940 the picture above shows the plane in mid to late 1942. the engine was listed as the SSC7-G which was equivalent to the R-1830-86 used in the F4F-4 which doesn't show up until Dec of 1941, The earlier R-1830-76 had some operating troubles as the impellers/airflow had some miss matches that caused rumbling in the intake ducts (rapid pulses/shock waves as one impeller stalled or choked)

Competing with F4F engines might have been an issue, but the various 2-speed single-stage R-1830s (and especially R-1820s) should have mostly compensated. (for some reason the R-1820-40 was only fitted experimentally to the F4F, perhaps due to higher volumes of 2-stage twin-wasps by the time a production installation would have been ready, but that engine should have had better altitude performance than the single-stage R-1830s or 1820s used on Wildcats and Martlets and close to that of the FM-2 -though without the low altitude water injection boost; I wish I had more detailed performance specifications for that engine, but all indications of its use in the F2A-2/3 point to exceptional altitude performance for the time)

The "exceptional altitude performance" was actually pretty mediocre. Military power was 1200hp at a whopping 1800ft in low gear and 1000hp at 13,500ft in high gear. It was the standard engine in Martlet Is and IVs. Please compare to the two speed engine in the F4F-3A or Martlet II & III. That R-1830 was good for 1200hp at 4900ft and 1000hp at 14500ft.

Edit, Lets not confuse cause and effect. The F2A-2/3 was around 900/600lb lighter than an F4F-3, difference between a Cyclone powered Martlet and a F2A is less but ammunition and fuel loads need to be taken into account.

The Engine used in the FM-2 had about the same relation to the engine used in the Martlet I & IV as the R-2800 "C" did to the R-2800"B". New crankcase, new crank, new cylinders with a new system of making the fins and new cylinder heads. Quite possibly a new supercharger? 1300hp at 4000ft in low blower and 1000hp at 17000ft in high blower. Needed a stronger crankshaft in order to be rated at 1350hp for take-off.

A 2-speed R-2000 with (presumably) similar or better critical altitude to the 2-speed R-1830 examples of 1940/41 (around 14,000 ft in high gear for the Wildcat/Martlet) should be fairly useful as well, lacking 2-stage adaptations of that. (and probably better in most situations than the V1650-1 due to lower weight and similar or better power -and possibly increased fuel capacity in the space between the engine and cockpit)

Covered in earlier post the R-2000 is both late (production starts Dec 1941) and lacking in high altitude performance. Some of the two speed early (1350hp take-off) R-2000s were rated at 1100hp at 16,000ft Military and others at 1000hp at 14,000ft "Normal" just like some R-1830s only had a military rating in low blower. No Military rating in high blower.

 

[wuzak]

I'd thought the Merlin intercooler was fed with air from the first supercharger stage and then fed into the 2nd stage ... in spite of seeing cut-away drawings to the contrary. Somehow I'd failed to realize those all depicted aftercoolers.

Yes, the big heat exchanger you can see on the two stage Merlins and Griffons is an aftercooler. For referemce, the supercharger housing contained cooling water passages around the air flow path from 1st to 2nd stage compressors, so it had a small degree of intercooling.

The 2-stage Allison (no intercooler/aftercooler at all) also moved the carburetor to behind the aux stage, unlike P&W configurations which seem to treat the Aux stage more like a turbocharger with the carb still mounted to the inlet of the integral stage. (which honestly is how I'd assumed the Allison configuration worked too)

Most production 2 stage Allisons had the carburettor mounted on the engine stage supercharger.

The V-1710's auxiliary supercharger was driven via a fluid coupling which meant that it operated much more like a turbocharger than the P&W systems, which used distinct gear ratios - HI, LO and Neutral. P&W engines tended to have an intercooler between the stages.

The Merlin's aftercooler arrangement would also explain why some post-war examples suffered from overcooling issues. (an intercooler arrangement should at least partially avoid this given the 2nd stage will be heating and re-mixing any fuel that's condensed in the intercooler rather than staying as condensed droplets in the aftercooler -granted, the P-38J had some overcooling issues as well)

Over-cooling would most likely be a problem of the radiator airflow control.

 

[Shortround6]

Over cooling was a problem in low power cruise conditions, some post war Merlins (especially commercial) were fitted with charge heaters in the intake tract (if not in the heat exchanger box) to keep the mixture warm enough to prevent puddling or fuel condensation.
Not a P-38/Allison problem alone

 

[Shortround6]

The P&W Hawk 81 (along with apparently all Hawk 75s) omitted a spinner from the propeller. This should reduce drag further, especially at high speed, but for some reason most American aircraft omitted spinners on radial engines.

This has already come up in this thread:
Drag of radial-engined fighters

but I haven't yet seen an explanation for so many aircraft omitting a rather obvious component of streamlining. (weight savings is the only explanation I've ever seen, and rarely even that is cited -mostly in a few F2A articles) A well designed spinner and cowling should be simpler to achieve pre-war than a tight cowling and cooling fan (or narrow ram-intake cowling and cooling cuffs on the propeller -a la P2V) and seems to be what both Brewster and most Japanese developers worked towards (as well as XF4U and XF4F-3) but for some reason the P-36 never got one and all American military aircraft from 1941 onward also lacked them.

Post-war air racers added them in many cases too, and did achieve performance boosts (sometimes with tight cowlings and added fans, sometimes without), so they weren't useless.

Spinners were actually used for several different purposes.
one was to 'streamline" (shroud) the prop hub and blade roots which aren't very streamline as they rotate.

two was to sometimes assist in ground cooling. sounds counter intuitive but is true. Look at where the blades start to get an airfoil shape. that area of the cowl opening is going to have high pressure while in the inner blade root area and hub is going to be lower pressure. it was possible (depending on internal baffles and exit doors/ducts) for the airflow around the prop hub to reversed and air flowing forward around the prop hub while the plane was stationary. A few planes actually used a flat disc behind the prop to prevent this.

and yes the spinner was often used in an attempt to streamline the engine installation. However on many radials only about 10% of the airflow headed towards the engine was actually needed to cool it (depends on engine and speed) the other 90% has to routed around the cowling. At speed you will get a high pressure area in front of the engine that can extend in front of the propeller hub. a lot of the air was routed around the cowling by the shape of the cowling itself and this high pressure area.
I can't find any at the moment but there are photos of engine installations in wind tunnels using smoke that show this (a poor set can be found at Access forbidden!

Link does seem to work, in any case it is NACA report no 662, "Design of N.A.C.A. cowlings for radial air cooled engines."

Spinner design without good wind tunnel work was mostly guess work (or worked only at certain speeds/conditions) and use of spinners was by no means a universal cure for either drag or cooling problems.

 

[kool kitty89]

Spinners were actually used for several different purposes.
one was to 'streamline" (shroud) the prop hub and blade roots which aren't very streamline as they rotate.

two was to sometimes assist in ground cooling. sounds counter intuitive but is true. Look at where the blades start to get an airfoil shape. that area of the cowl opening is going to have high pressure while in the inner blade root area and hub is going to be lower pressure. it was possible (depending on internal baffles and exit doors/ducts) for the airflow around the prop hub to reversed and air flowing forward around the prop hub while the plane was stationary. A few planes actually used a flat disc behind the prop to prevent this.

The introduction of cooling cuffs on some propellers (particularly some curtiss-electric ones) would make propeller blade roots being exposed more useful (especially with a cowling designed for it), but otherwise yes, spinners directing airflow outward and towards the hottest portion of the cylinders would be the most useful. (the P&W cowling actually looks like it would fit rather well with cooling cuffs OR a spinner, either way avoiding the drag and turbulence around the blade roots and hub and -in the spinner's case- potentially creating more of a ram airflow effect between the spinner and cowling)

Spinner design without good wind tunnel work was mostly guess work (or worked only at certain speeds/conditions) and use of spinners was by no means a universal cure for either drag or cooling problems.

Curtiss's work with the XP-42 trials certainly seemed to show this, among other things. (particularly when cuffs and tight cowlings come into play -in some cases more useful than spinners+fans+tight cowlings and usually mechanically simpler)

The same should actually be true for annular radiator design for inline engines ... the right cowling (especially with cooling cuffs) might merit a smaller (or no) spinner than German examples used during the war. (though I'd imagine the conical hub covers at least reduced turbulent flow slightly compared to the clunkier bare hubs used in some cases)

The test hack set it's speed record in Sept of 1942 I believe. Along time away from 1940. How long it took P & W to get to that point I don't know by they may have had the plane/airframe for over two years.

Ah thanks, that makes some sense. So it was more in line with the ongoing XP-42 program and potentially somewhat related to it.

The P&W example also features ram intakes for the carb and oil/intercooler that the P-36 never featured (though F2A did, and twin-wasp wildcats had ram carb inlets while Cyclone ones seem not to)

I was half joking with the P-37 pictures, but only half. A book on aircraft power plants published in 1943 estimated about 400lbs and about 10 cu.ft. of space were needed for a turbo installation on a 1000hp class engine. Book was written by a Packard test engineer. Granted it was not a restricted book and therefore wasn't giving any military secrets away during war time so it may have been a bit behind the state of the art.

There was nothing mystical or magical about the Hawk 75/P-36. It was a fighter of it's time (1936-38, prototype first flew on 6 May 1935) and had wing loading of about 24lbs per sq ft for a gross weight of 5700lbs (average of the A and C) and well coordinated/harmonized controls, in fact better than a good many of it's contemporaries.
The P-40 should have retained many of it's good characteristics except for the large increase in weight and perhaps minor CG shift.

There's also the one benefit from higher weight: dive acceleration (which was one of the P-36's weak points, particularly noted in British testing of the Hawk 75) which actually in some ways better fit the superior high-speed control surface response and roll rates compared to most others (particularly most non-American fighters).

Compromises other than just engine design would need to be made to keep weight down though, yes, particularly in the list of changes the Hawk 87 saw (the P-40E gained nearly 1000 lbs empty over a P-40B, though the P-40C had already gained some and the D/E radiator likely added weight and drag on its own, there were other structure changes, the heavier armament, increased self-sealing fuel tank capacity over the P-40C, among other things). If they could have taken the Hawk 81 airframe and introduced a new wing with 4 .50 cal guns without substantial changes in weight, it would have been much more competitive for the time. (either with the Allison or with the 2-stage R-1830).


On the general note of the P-37/40 though, I stumbled on these pages:
Curtiss P-40 Warhawk: One of WW II's Most Famous Fighters | HistoryNet
Military Aviation Archives - P-40 Warhawk Historical Development Photo Set
http://www.aerofiles.com/curtiss-yp37.jpg

I'd forgotten the original XP-40 conversion used an aft ventral radiator like that and ducted (non-ejector) exhaust ... and lacked a ram intake for the carburetor. It almost looks like portions of the conversion were for an alternate turbocharger installation compared to the YP-37. (or rather, alternate intercooler+radiator installation given the nose-mounted turbo didn't take up much space -though in the actual XP-40 the oil cooler appears to have taken the place of the turbo in a location rather similar to the eventual production Hawk 81 variants; regardless of there was any relation to an alternate turbo installation, the forward turbo and aft radiator arrangement would rather mirror the P-38's configuration, more so if wing leading edge intercooling had been attempted)

The lack of ejector exhausts, ram intake, and reduced power of the initial test engines obviously explain the modest performance of the initial XP-40 compared to the eventual V-1710-33 configuration.

Additionally, the XP-40K's belly radiator appears to be embedded between the landing gear pod protrusions and thus probably doesn't compromise the wing center section fuel capacity. (though at a glance, I'd think extending the radiator intakes further down the nose for better ram performance and possibly lower drag at high speeds would be better -which may have been what the XP-40Q eventually did with its small chin intake, though it moved the oil coolers to the wings whereas the XP-40K appears to leave the oil cooler between the two coolant radiators and retain its separate small ventral outlet used on all production P-40s) It also doesn't look like clearance in the belly is negatively impacted as far as the bomb/drop tank rack position is concerned.

You can forget about the R-2000 as a fighter engine. P & W may have used the same supercharger as the R-1830. while take-off power jumped quite a bit, power at altitude didn't follow suit. Early versions with 1350hp for take-off were rated at 1000hp at 16,000ft (at best) and in 1942 that is nowhere near good enough. Later versions rated at 1450hp for take off got rated at 1100hp at 16,000ft (at 2700rpm) but that usually required a few modifications, like different bearings (plain instead of roller) and with a 2 speed R-2000 weighing about 1600lbs it pretty well cancels out the weight of the radiators and coolant of the Allison.
A two speed R-1830 could be good for 1200hp at take-off, 1200hp at 4900ft MIlitary in low gear and 1050hp at 13100ft in high gear. (all 2700rpm) while weighing 1495lbs. The R-2000s were good for either 1350hp at 2000ft low gear or 1450hp at 1000ft in low gear depending on take-off rating.

That actually makes the R-2000 sound worse than the R-2600, which obviously wasn't a stellar option either. (though I suppose somewhat makes sense given Wright's single-stage superchargers tended to fare a bit better than P&W's even with the R-2600's development progressing less smoothly than the R-1830's)

On the note of the R-2600: the issue of larger propellers and longer landing gear (or clearance issues on existing gear) has come up in the past and looking at the P&W H81 test 'hack' a bit more closely, its propeller does appear to be a fair bit closer to ground level than the P-40's or P-36's, particularly taking the longer nose (than P-36) into account. This also might have been a bit of a bonus for the higher thrust line of the F series Allison (and Merlin) as it raised ground clearance and allowed a slightly larger propeller without making take-off or landing clearance trickier. (the P&W example looks like it may have required near 3-point landing conditions somewhat like large-prop P-47s)

We have no data on the breakdown rate of the FN guns. There were well over a dozen attempts to develop high rate of fire US .50 guns and while the US requirements may well have been too stringent a few of the attempts failed miserably well before getting even 1/5 of the required rounds fired. Some failures were rather simple like broken extractor hook but I believe in at least one case the gun was wrecked. Placement of guns may have to do with other issues than rate of fire and "concentration" of fire. Like gun heating or design of the wings, adding gun bays that will hold .50 cal Brownings and ammo to existing structures may be more than some design teams wanted to deal with. Japaneses Navy however added big Brownings in the wing of the Zero.

The Zero had been designed to carry Oerlikon FFF or FFL type guns prior to that, so the structure was there to some extent, even if API blowback made for lower recoil (more so for the FFF).

I do wonder if the P40D and E wing redesign played a significant role in the weight gain over the P-40B and C. The P-40D had only been fitted with 4 guns but had wings designed in mind for 4 20 mm hispanos while the P-40E adapted that to 6 .50s. Designing more tightly around the more balanced (especially for 1941/42) 4x .50 armament might have saved some weight and design modification complexity over the wings they ended up using. (or it wasn't a major issue and weight increase was from the fuselage structure and equipment changes instead)

The synchronization mechanism itself added weight and complexity, so in theory, deleting them might actually reduce weight in a 4-gun wing arrangement or been very close to the overall weight of the 2x 50s and 4x .30s.

Deletion of the guns and ammunition boxes should have added enough space for a small reserve fuel tank to be added too, but the P-40D seems to have re-distributed space differently. (a radial engine installation would -as I noted with the P&W testbed- left more open space for internal storage there and at a location close to CoG, good for fuel, and even if the intercooler and oil cooler(s) comprised the lower portion of that compartment, there should be significant room for fuel)

The installation was actually late 1938/early 1939 for the Curtiss and Seversky. While P & W got the Hawk 81 airframe in 1940 the picture above shows the plane in mid to late 1942. the engine was listed as the SSC7-G which was equivalent to the R-1830-86 used in the F4F-4 which doesn't show up until Dec of 1941, The earlier R-1830-76 had some operating troubles as the impellers/airflow had some miss matches that caused rumbling in the intake ducts (rapid pulses/shock waves as one impeller stalled or choked)

Sounds somewhat like some of the difficulties Allison was dealing with early on in their auxiliary supercharger development (and some of the problems you mentioned with mix-matched impellers and diffusers, pressure ratio, mass flow, etc).

That actually starts making the R-2600 look better than the 2-stage R-1830 too (on top of the R-2000 as mentioned above). I forget how big the weight difference is from an R-2600-3 (or -5, I know the -8 was heavier, more powerful, and later to production) and R-1830-76 is though. (if we're comparing models in production in 1940 or earlier) No intercooler to deal with either, but added diameter and drag (both from frontal area and cooling work required -ie cowling might be similar in diameter to an R-1820 Wildcat or Hawk 75, but cooling capacity would need to be greater). It might have been an interesting test-fit on the Wildcat (probably requiring yet a larger tail though) but for Curtiss in 1940 they might have more luck with a more heavily modified airframe than a Hawk 81 derivative. (the H81 airframe might have been more suitable than some discussions have concluded, but I think the best case would have been designing the XP-46 project around the R-2600 rather than the V-1710 at all ... in parallel with the XP-40D/H87 development)

The R-2600 would look even worse if Allison development/testing had been more aggressive, though. Had the V-1710-39 seen WEP ratings at introduction with the P-40D or E (particularly if emergency power RPM was raised to 3200-3400 RPM -with correspondingly higher supercharger speed and altitude performance) it wouldn't be much of a contest at all. Though I suppose Allison WER usage could have been a bit more freely implemented (or operationally tested) had there been more engines to spare relative to aircraft demanding them (more spare engines in the field to work with -particularly important compared to British engine repair/replacement.manufacturing much closer to the front lines). So in that sense, manufacturing both R-2600 and V-1710 powered Curtiss fighters might have made at least some sense.

I list the 3400 RPM figure limit there given that tends to be the upper end of structural ratings for propellers coupled with 2:1 reduction gearing (1700 RPM prop rotation) with genuine risk of failure much beyond that. (though hitting supersonic tip speeds under the right altitude and forward velocity could make that speed unattractive short of clipping prop tips -as some air racers take to doing) 3400 RPM is also proportional to the overrev engine type test conditions for the E and F series V-1710 vs the older C series. (3600 vs 4100 is a 13.9% increase, and applying that to the 3000 RPM nominal take-off/military rating of the C series results in 3417 RPM ... so round down to 3400 RPM and there you go) At 3400 RPM and 8.8 supercharger ratio, the impeller is also turning faster than 9.6 ratio at 3000 engine RPM, so added precautions would need to be considered there as well. (it's equivalent to 9.97:1 supercharger ratio with the exception the cylinders will be pulling more air at 3400 RPM than 3000 RPM so manifold pressures and critical altitudes wouldn't quite relate to that alone either) It also might have been feasible to raise the maximum continuous rating to 3000 RPM (at correspondingly lower manifold pressure). I still suspect experiences with the C series engines (especially operationally) led to overly conservative official ratings though, more so than just lack of funding and considerations for spare engines. (the C series seemed far less conservatively rated relative to its absolute limits and could take far less abuse without failure or severely reduced longevity ... though likely still more tolerant to overspeeding than most or perhaps all other aircraft engines of 1940/41)

The "exceptional altitude performance" was actually pretty mediocre. Military power was 1200hp at a whopping 1800ft in low gear and 1000hp at 13,500ft in high gear. It was the standard engine in Martlet Is and IVs. Please compare to the two speed engine in the F4F-3A or Martlet II & III. That R-1830 was good for 1200hp at 4900ft and 1000hp at 14500ft.

Edit, Lets not confuse cause and effect. The F2A-2/3 was around 900/600lb lighter than an F4F-3, difference between a Cyclone powered Martlet and a F2A is less but ammunition and fuel loads need to be taken into account.

The Engine used in the FM-2 had about the same relation to the engine used in the Martlet I & IV as the R-2800 "C" did to the R-2800"B". New crankcase, new crank, new cylinders with a new system of making the fins and new cylinder heads. Quite possibly a new supercharger? 1300hp at 4000ft in low blower and 1000hp at 17000ft in high blower. Needed a stronger crankshaft in order to be rated at 1350hp for take-off.

I'm not sure the Martlet/Wildcat's R-1820s are totally comparable to the R-1820-40. With ram (at top speed) they hit their critical altitudes at only 14,500 ft which was 500 ft higher than the competing 2-speed R-1830s but 2,000 ft lower than the F2A-2 and F2A-3 made their best speed.

The F2A-3's empty weight was also almost identical to that of the F4F-3 from what I've seen, though loaded weights might vary more. (F2A-2 and even B.339 were lighter though -the self-sealing material added to the Buffalo Mk.I apparently not adding much compared to the F2A-3's fusealage extensions and new self-sealing tanks)

I suspect the R-1820-40's performance is closer to the R-1820-56's supercharger performance (but with engine RPM, manifold pressure, and power reduced to operating levels of the older design more like comparing a V-1710-33 to a V-1710-73). The comparison would be much easier if I had better data on the -40 to go on, particularly had there ever been a production F4F to use it.


On the note of the poorer (but still decent) performing single-stage 2-speed R-1830. It probably wouldn't have been a great option for the Hawk (at least to displace the existing Tomahawk) but might have better matched the F2A than it did the F4F given the smaller size and lower drag of the Buffalo. (the R-1830-76 or -86's added weight, length, and intercooling requirement might have compromised too much by comparison, particularly given Brewester's degrading management situation)

 

[Shortround6]

For cooling a "ram" intake may not be what is wanted.
Airflow over the engine cooling fins or through radiator/oil cooler cores is subject to the same drag considerations as the outside of the airframe. Drag goes up with the square of the speed. IF you can slow the speed of the air through the cooling fins or radiator/oil cooler you can get a substantial reduction in cooling drag. Hi speed air also doesn't allow for much heat transfer. 300mph airflow is 440fps and the time the air is in contact with the cooling fins is measured in thousands of a second. Most radiators/oil cooler cores were much longer than the fins on an air cooled cylinder. air flowed through tubes in the cooler.

Granted smooth airflow is better than turbulent airflow which is why some aircraft used airflow splitters to get around using boundary air.
P-51 had a low drag set up NOT because it used a ram intake or airflow splitter but because it used a whacking big radiator.

incoming air was allowed to expand in the duct slowing it down before it hit the radiator/intercooler matrix. The large radiator allowed for a small pressure drop as the air went through it and then the converging exit duct speed the air back up. There is a lot of argument about wither or not they actually got thrust out of it but it was at the very least one of the lowest drag radiator arrangements around. A smaller radiator core and higher air speed through the core would have a much higher pressure drop (drag).
AN air cooled engine should follow the same principle. Take in the amount of air needed ( and amount of air taken in was actually governed by the the amount of air allowed to leave through the exit doors/flaps) slow it down before it hits the engine cooling fins and baffles and then allow it to leave with the least turbulence and changes in air pressure.

I would also note that "ram" air is NOT free. RR figured that the air intake on a Hurricane II had over 25hp worth of drag anywhere from 15,000 to 25,000ft ( peak was 32.8hp at 20,000ft at 340mph true.) This was what it took to compress the air before it hit the carb and provide the ram effect and the drag in the internal duct.

Ram air intakes for combustion air make sense as you are trading drag for extra pressure at the carb (or engine intake) for higher altitude performance. Designing intakes for radiators/oil coolers/engine cooling fins that deliver higher than "normal" pressure in flight seems to be counter productive.

Cooling on the ground can be a whole different thing. Most if not all of the B-29 cooling problems were while taxing, taking off or climbing while heavily loaded. In any case when cooling loads are the most severe the plane has the least amount of airflow due to the forward motion of the plane. Ram inlets are going to nothing (or darn little) in these conditions just like many planes show differences of thousands of feet in critical altitude between level flight and climb as at climbing speed the RAM effect is not that great.

 

[Shortround6]

Compromises other than just engine design would need to be made to keep weight down though, yes, particularly in the list of changes the Hawk 87 saw (the P-40E gained nearly 1000 lbs empty over a P-40B, though the P-40C had already gained some and the D/E radiator likely added weight and drag on its own, there were other structure changes, the heavier armament, increased self-sealing fuel tank capacity over the P-40C, among other things). If they could have taken the Hawk 81 airframe and introduced a new wing with 4 .50 cal guns without substantial changes in weight, it would have been much more competitive for the time. (either with the Allison or with the 2-stage R-1830).


I do wonder if the P40D and E wing redesign played a significant role in the weight gain over the P-40B and C. The P-40D had only been fitted with 4 guns but had wings designed in mind for 4 20 mm hispanos while the P-40E adapted that to 6 .50s. Designing more tightly around the more balanced (especially for 1941/42) 4x .50 armament might have saved some weight and design modification complexity over the wings they ended up using. (or it wasn't a major issue and weight increase was from the fuselage structure and equipment changes instead)

From "America's Hundred Thousand" the empty weight of the P-40E (no guns and some other stuff) was about 450lbs more than a P-40B. of which about 100lbs was in the wing structure. Difference between P-40C and P-40E was about 300lbs. Empty equipped was about 710lbs between the "B" and "E". Of course that is in part due to the 480lbs worth of .50 cal guns in the "E" (but not including ammo, that was another 423lbs)
The "D" was designed to hold a single 20mm cannon under the wing. I don't know if it was supposed to keep both .50s in the wing or only one if the 20mm was fitted.
We have a slight problem with the "more balanced (especially for 1941/42) 4x .50 armament might have saved...." as the P-40Ds were ordered in May of 1940. I don't know when in 1940 the US got the rate of fire of un-synchronized .50 cal guns up from 600rpm to 800-850rpm. Please note that this is about 11 months before the first P-40C flies at the Curtiss factory.
A few dates to show how much over lap was going on ( and how little time there was for other intermediate solutions).
June 10th 1940, sees the XP-46 canceled in favor of the H87A
Last P-40B delivered April 24th 1941.
First P-40C delivered April 30th 1941.
First flight of XP-40F with Merlin engine was June 30th 1941.
First production P-40D delivered to US army July 11th of 1941.
Curtiss starts delivering P-40Es Aug 29th, 1941.

Granted leaving a pair of the .50s out in order to combat the weight escalation might have been a good idea but designing the "new" wing around only 4 guns bays might not have given the firepower desired at the start of the project.

The synchronization mechanism itself added weight and complexity, so in theory, deleting them might actually reduce weight in a 4-gun wing arrangement or been very close to the overall weight of the 2x 50s and 4x .30s.

Deletion of the guns and ammunition boxes should have added enough space for a small reserve fuel tank to be added too, but the P-40D seems to have re-distributed space differently. (a radial engine installation would -as I noted with the P&W testbed- left more open space for internal storage there and at a location close to CoG, good for fuel, and even if the intercooler and oil cooler(s) comprised the lower portion of that compartment, there should be significant room for fuel)

The Synchronizers are rather negligible. About 3-4 lbs each on the engine. Balanced out pretty well by the hydraulic charging systems needed for the wing guns.
weight of two .50s and four .30s was 245lbs. weight of four .50s was 314lbs. problem was in part due to ammo. P-40C carried a ridiculous 380 rounds for each fuselage .50 cal. Weight was 228lbs. Weight of the 2000 rounds in the wings for the .30s was 127.4lbs. weight of 235 rounds per gun for 4 wing guns was 282lbs.

I would also note that you have to be a bit careful as to what you want to put where, The long nosed P-40s had the oil tank behind the pilot while the D,E,K,M,N had the oil tank in cowl ahead of the instrument panel. Perhaps the Merlin versions had it there too.
As oil was a consumable (13 gallons was normal while 18 gallons were carried when drop tanks were used) some consideration as the CG should be taken into consideration.

That actually starts making the R-2600 look better than the 2-stage R-1830 too (on top of the R-2000 as mentioned above). I forget how big the weight difference is from an R-2600-3 (or -5, I know the -8 was heavier, more powerful, and later to production) and R-1830-76 is though. (if we're comparing models in production in 1940 or earlier) No intercooler to deal with either, but added diameter and drag (both from frontal area and cooling work required -ie cowling might be similar in diameter to an R-1820 Wildcat or Hawk 75, but cooling capacity would need to be greater).

R-2600 is roughly 350-400lbs heavier than the two stage R-1830 and that is for the bare engine. you have 1400hp Military at 12,700ft. By the time you fit the bigger prop and all the bits and pieces it its going to be 5-600lbs heavier. A P-40's cooling system for the V-12s weighed about 300lbs. Allison + radiators and coolant was 300lbs lighter than an R-2600.

P-40s had trouble with stability as engine power went up requiring a 20in extension in the rear fuselage (which still wasn't a full cure) sticking a 1600hp engine for take-off and a bigger prop is really going give a beast to handle without extensive rear fuselage modifications.

The R-2600 would look even worse if Allison development/testing had been more aggressive, though. Had the V-1710-39 seen WEP ratings at introduction with the P-40D or E (particularly if emergency power RPM was raised to 3200-3400 RPM -with correspondingly higher supercharger speed and altitude performance) it wouldn't be much of a contest at all. Though I suppose Allison WER usage could have been a bit more freely implemented (or operationally tested) had there been more engines to spare relative to aircraft demanding them (more spare engines in the field to work with -particularly important compared to British engine repair/replacement.manufacturing much closer to the front lines). So in that sense, manufacturing both R-2600 and V-1710 powered Curtiss fighters might have made at least some sense.

While the P-40 carried a decent (but not great) amount of fuel for it's size sticking an R-2600 on it is really going to create problems. A R-2600 could suck down 2 1/2 gallons a minute at max continuous let alone at full power. Through in the fact that due to the higher drag it is going to use more fuel even at cruising speeds than the Allison. A P-36 needed more power to fly the same speeds than an early P40.

The comparison would be much easier if I had better data on the -40 to go on, particularly had there ever been a production F4F to use it

.
There was a production F4F that used it, it was called either a Martlet I or Martlet IV (both used Cyclone R-1820-G205 engines, and there wasn't that much to choose between the R-1820-G205 versions)

 

[Shortround6]

Some data from Mike Williams site concerning the P-36, the P-40B and the P-40D that may be of interest.

At 5000ft using max power
Plane..........power..................speed
P-36A...........1045...................280
P-40B...........1085...................319
P-40D...........1150...................326

At 15,000ft at about the same speed
Plane.............power...............speed
P-36A.............845..................292
P-40B.............600..................286
P-40D.............635..................290

At 15,000ft using max power
Plane.............power.............speed
P-36A.............845................292
P-40B............1090...............352
P-40D............1085...............354

Weights were normal gross (?? full gas and oil, no mention of ammo) so 5650lbs ??? for the P-36A and 6,835lbs for the P-40B and 7740lbs for the P-40D.

From this we can see the much higher drag of the radial engine at the time in question.
We can also see that the chin radiator on the P-40D and later had so little difference from the pointy nose P-40s as to be insignificant.
In fact the P-40D might be just a shade cleaner than the P-40B but that could be due to the gun installation or some other factor. Or using two different P-40Bs and Ds the actual difference in speed might be reversed?

Using the cube root rule it would take a plane with the same drag as the P-36A 1648hp to equal the speed of the P-40D at 5,000ft.

I will grant you that the P-40B and D were getting a boost from exhaust thrust that the P-36 was not but it is going take one tricky radial engine installation to come close to the standard P-40s. Of course they were also much heavier creating more induced drag.

Adding 1000-2000lbs to the P-36 in fancier radial (two stage/turbo/R-2600) protection, armament and beefed up structure (a 7300lb plane which is splitting the difference between a P-40B and D) would have an ultimate load factor of 9.36 if not beefed up from a standard P-36 structure, is going to leave you with a plane that is not really going to perform much different than a regular P-40.
Or perhaps it trades worse performance down low for better performance at altitude depending on exact engine.

edit: I would add that a test of a P-43 (plane had 250lb of armament which means???) gave speeds at 15,000ft of 339mph using 1200hp, 328 using 1100hp and 300mph using 840hp.

 

[tomo pauk]

Could we arrive at some rate of climb estimates for different plausible powerplants, feturing P-36/P-40 as base?
Eg. the R-2600 powered P-40 (8100-8200 lbs?) vs. Fw 190A-1 (3780 kg clean, or ~8330 lbs) or A-2 (3855 - 3978 kg, depending whether MG FFM is installed or not, clean; 8500-8770 lbs)? Granted, the Fw 190 has less drag, but also greater wing loading, power is 1380 PS at 4.6 km (1341 HP at 15100 ft, 3 min, vs. 1300 HP at 15100 ft, 5 min).
Also compared with historical P-40 in 1941/42?

 

[rinkol]

Turbo controls early-war were also less than satisfactory, both confusing and far from foolproof with lack of features like automatic turbo RPM limiting. (no overrev protection, and this resulted in the majority of catastrophic failures or exploding turbochargers)

Note that a multi-engine plane, such as the B-17, had a crew that was large enough to allow one of the crew members to carefully monitor the operation of the turbosuperchargers. Also, the engines in such a plane would operate under relatively constant conditions, thus simplifying the control issues.

It might also be noted that the B-17 had priority for the turbochargers - this was clearly an application where the performance of the plane was dramatically increased, even if, as was found later, it was still insufficient to provide immunity to interceptors.

Robert

 

[Shortround6]

Could we arrive at some rate of climb estimates for different plausible powerplants, feturing P-36/P-40 as base?
Eg. the R-2600 powered P-40 (8100-8200 lbs?) vs. Fw 190A-1 (3780 kg clean, or ~8330 lbs) or A-2 (3855 - 3978 kg, depending whether MG FFM is installed or not, clean; 8500-8770 lbs)? Granted, the Fw 190 has less drag, but also greater wing loading, power is 1380 PS at 4.6 km (1341 HP at 15100 ft, 3 min, vs. 1300 HP at 15100 ft, 5 min).
Also compared with historical P-40 in 1941/42?

A few things make this much more complicated.
One is figuring out the surplus power available for climb after figuring out the power needed for flying level at best climb speed and then figuring the extra drag after the plane points it's nose high for the climb, flight path is NOT in line with the wing but the wing is operating at several degrees higher angle of attack (or more than several) than flying level at the same speed. A simple comparison of wing loading takes none of this into account and in fact sometimes a higher wing loading plane with less drag can out climb a low wing loading (but high drag) airplane.

Getting a 8100-8200 lbs R-2600 powered P-40 airframe is going to be quite a trick also. The R-2600 weighed around 300lbs more than an Allison engine plus radiators and coolant. I dare say you might need a larger oil system for the R-2600 and of course the larger prop.

What can you leave out of the P-40 to get the weight back down to 81-8200lbs?

BTW the gross weights for the P-40 without center drop tank were 8250-60lb for an E, 8570lbs for an F, 8520lbs for a K, 8120lbs for an L and 8350lbs for a late model N. The L was a stripper model with only 4 guns, a fuel tank taken out (saved about 100lbs just in tank weight + 168lbs of fuel) and a few other things. Production planes could vary some what (government would take planes about over 100lbs overweight but charge penalties).

Finding performance numbers for the L version would be a good start.

 

[tomo pauk]

Indeed, I've took a peek in The Book, and listed P-40s with 6 HMGs are heavy beasts. So we'd probaby have the R-2600 powered P-40, 6 HMGs, at 8500-8600 lbs, or thereabout as with Fw 190A-2/A-3.

 

[Shortround6]

And then you have a higher weight plane, with more drag than the Fw-190 and with arguably less range. Range is going to be real problem as while the P-40 was no long range escort it did have around 25% more internal capacity than a Spitfire or Hurricane (and around 20% more than a P-39) which may have been important in the Med or in the Pacific. Sticking the high drag radial on it is going use more fuel at cruising speeds.

Just what mission/s is this modified P-40 supposed to fulfill?
Most people criticize the P-40 for lack of altitude performance and the R-2600 conversion isn't going to solve that.