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

[drgondog]

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

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

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

You (and they) are correct about P-38 = 23016 airfoil. Brain Fart on my part. The Root chord does relate to the actual starting Chord of the wing as a separate component. The P-51 bolted together at the CL whereas the P-38 had a contiguous spar 79.5 inches from CL to attach fittings so The Root Chord would have been at CL just like the 51. It was 16% of Chord for thickness ratio and tapered down to 12% at Tip Chord.

The leading edge sweep of the P-38 was 5 degrees to the Mustang 3 1/2 degrees but neither affected Mcrit which really must approach 15% to have a noticeable affect.

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




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

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

Note that the T/C of F4U and P-38 airfoils are very close at the wing/fuselage interface and in turn are about the same as the P-51B/D at the wing/fuselage interface as far as T/C. The huge difference is the laminar flow PROFILE with a gradual velocity gradient from nose to 45% Chord versus the more accelerated velocity gradient of the 23015 at 25% Chord.

Additionally the BL separation as the shock wave forms begins just aft of the Shock wave, which for the 23xxx begins at ~ 25% and exists immediately for 3/4 of the wing, whereas the same phenomena begins for the Mustang at 45-50% chord and takes more time to develop as a function of the free stream velocity


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

Not necessarily. With same engines and a prop designed with more area to improve climb, would more likely have a detrimental effect on drag at high speed.

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


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

We learn that we always design and produce extensions of what worked for us in the last war. Having said that, the P-51H and P-82 in fall of 1945 were superior long range escort choices for the soon to be SAC. Additionally they were cheaper to operate.


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

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

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

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

 

[tomo pauk]

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

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

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

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

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

 

[pbehn]

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

 

[drgondog]

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

I believe the manual draws an incorrect conclusion. It states that the dive flaps disrupt the flow under the outer wing and thereby creates more lift thereby relieving the nose down forces". Partly true but only if by deploying the dive brakes in a timely manner (BEFORE Mcrit Velocity is reached over center section).. otherwise the shock wave initiates creating the separation and decreased lift of the center section of the wing - doing three things:

1.) immersing the horizontal stab/elevator in the extreme turbulent flow and
2,) by decreasing lift, change the angle of incidence on the stab/elevator to nearly straight on, rather than downwash - creating a loss of pitch authority
3.) move the Center of Pressure from about 25% C to about 40-50%, thereby creating and SUSTAINING a nose down Moment (CMac) about the design a/c. It doesn't matter much about improvement of lift on the outer wing as the conditions that must be overcome are focused on the center wing span and horizontal stab.

http://www.ww2aircraft.net/forum/flight-test-data/ww2-fighter-critical-mach-speed-802-5.html is where I discussed this in more detail



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

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

 

[drgondog]

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

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

 

[tomo pauk]

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

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

 

[drgondog]

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

 

[kool kitty89]

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

[kool kitty89]

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

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

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

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

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

 

[tomo pauk]

If I may

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

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

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

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

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

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

 

[kool kitty89]

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

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

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

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

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

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

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

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

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

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

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

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

 

[wuzak]

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

Bi.

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

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

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

 

[Shortround6]

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

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

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

 

[kool kitty89]

Bi.

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

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

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

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

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

 

[wuzak]

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

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

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

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

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

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

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

 

[Koopernic]

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

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

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

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

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

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

 

[tomo pauk]

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

...

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

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

 

[tomo pauk]

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

The P-40 with turbo (P-40H) never progressed beyond paper, so the amount of misery there is zero. Bell, with XP-39, really tried to put one liter of beer into a half liter bottle, as SR6 nicely put it. The Prestone radiators were already a significant source of drag on the XP-39.
That is not to say that air-to-air intercoolers are that a good thing, but there need to be enough of 'elbow room' when one wants this or that feature on the new aircraft.

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

The increased size of radiators is indeed needed because of increased engine power, but we also need to add needs of the intercooler, that amounted to 1/2 of the Prestone radiator on the Merlin Mustang, for example. Basically - a 2-stage Merlin 61 will need twice as capacious radiator system as the 1-stage Merlin 45. Or, increase a bit the 'main' radiator and add a smaller radiator for the needs of intercooler (as historically).
The system you described was used on Mikulin's AM 37, the engine overheated a lot - it would've probably needed a cooling system (for coolant and inter-cooler together) of twice the capacity than what was used on the AM 35A.

The Germans successfully introduced an air to air intercooler on the otherwise liquid cooled Jumo 211J which added about 6% at takeoff power and well over 10% at altitude to the single stage two speed engine Jumo 211F.
Nevertheless when they wanted to use this engine on high speed aircraft such as the Ta 154 Moskito they used the non intercooler Jumo 211F for prototypes and were planning to use the more powerful non intercooled Jumo 211N for production. It seems possible that the slightly higher drag of the air to air intercooler Jumo 211J and its advanced version Jumo 211P were not thought worth it on high speed aircraft where drag is an issue. When intercooler versions of the Jumo 213 came out they used air liquid heat exchangers.

Looks the engine choice was a 'misery' of the Ta 154 - 3 prototypes were lost because of powerplant let them down (25% of the loss). The intercooled Jumo 211 will provide a bit better rate of climb than non-intercooled.

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

On the P-39 - I doubt it would be an easy thing to add it. On P-40 and P-51 - it would probably mean the power level of the 1-stage Merlin, with a bit more drag. Better to go with water/alcohol injection?

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

Depends on the current state of the art, experience, capability of the designer, engine choice? There won't be much (any?) intercooled radials around, BTW.

 

[kool kitty89]

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

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

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

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


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

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

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

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

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

 

[Shortround6]

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

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

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

View attachment 294279

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

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