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)