The improved D 520 (D520 amélioré) benefited above all from the tests carried out at the "Institut de mécanique des fluides de Banlèves".
Among the most significant elements, the fully faired landing gear housings and the oil radiator removed and replaced by a heat exchanger (on the engine's prestone circuit). And particular care given to the sealing of the intake air scoops - very important to recover a maximum of "ram effect". We can clearly see that the actual recovery altitude of the engine (6.900 m.) is well above that measured on bench.
The 12Y 49 differed from the -45 only by the S/C ratio, and many -45 engines were "retro-fitted" by simply changing the gears. The -49 was already in production at the end of 1939.
See here, note #9 on the left, on this 1939 document :
We note in these same lists that the 12Y51 is planned with the pitiful home superchager !. With which the published performances are given. I do not know if the performances of the -51 with a P/S compressor have actually been established.
In the relevant Docavia books, it is the D523 (150 planned) that was to get 12Y-51+ S-P and D525 (30 planned) with 12Y-51 + H-S "high altitude supercharger".
According to the same books, the D523 achieved around 570kph in its test flights and this was also the anticipated top speed.
I also heard of a 12Y-51 variant which used the same reductor and similar housing as the 12Z to use the "long nose" setup which reduced drag (seen on the HS-50 and VG 39Bis). Your document mentions 12Y-5X variants with different reductors, maybe there was indeed such a thing?
Without distracting from the main subject, the issue is that as France fell IRL, we cannot know for sure how things would have evolved past mid-1941 when the programs we know about were to be complete (Plan VI aircraft production program and the related fighters). Considering the documents lost in the Debacle and later the Liberation, we may not even have the full story on ongoing developments as of May-June 1940.
The uncertainties regarding the HS-12Z's and GR-14R's fates are good examples, but for example in the field of superchargers a lot could suddenly change between 1940 and 1942-43. IRL, many supercharger improvements for the Merlin (2-stage and then 2-stage intercooled) had barely started development as of mid 1940, and the trend towards very high altitude engagements was also not completely noticeable until the battle of Britain. France only had the single speed single stage S-P, Hispano-Suiza and Gnome Rhone superchargers for fighters (but we at least know what Gnome Rhone was cooking for the 14R), but NACA testing of S-P with 2-speed drives in October 1940 shows that it is a realistic development. Regarding the S-P specifically, we could at least see the following developments at some point in the war: 2-speed drives, 2 stages, intercooling, axial instead of centrifugal swirl throttle. While many of these changes will require a longer fuselage/nose, they are possible for the kinds of fighters we could see by 1942-43.
There is also the factor that in the event of the Entente holding off the German invasion, the French engineers would not be isolated like they were during the occupation: exchanges could continue with the British and Americans, noting that as of mid-1940 and even since the beginning of the war, the French were increasingly ramping up their economic, industrial and military cooperation with the British even if this was not as noticeable for the airforce as it was in other areas. There is even the prospect of cooperation with Switzerland through Saurer that had only been offered to Vichy after July, but may have been possible without the Fall. France also got to retrieve German engines prior to May 1940, but did not have the time to exploit any of their learnings and obviously never got to retrieve later engines. Finally, both occupied and postwar France lost a lot of scientific equipment and also missed the shipments of American or British tooling and scientific equipment to be delivered after May 1940 (I have a list of such things, but for the ballistic research laboratory at Satory). All of this conspired to further slow down any promising developments. Switzerland and the USSR were in a better situation IRL but it can also be questionned if they had access to the allies and scientific equipment that a post-May 40 France would have.
The synergy with the British and Americans also goes both ways. While it may be more valid for ground rather than air equipment, French engineers and developments, as well as continued economic cooperation with the British, the lack of a Blitz on Britain and a better supply situation (the Battle of the Atlantic is going worse for the Germans without French bases and with the French navy still operational), could all contribute to improvements for the British situation. Calum Douglas in the
Secret Horsepower Race noted in particular the merit of adding a swirl throttle like that of the S-P supercharger to the supercharger of the Merlin 45. It would have allowed the supercharger to be optimized for high altitude while the swirl throttle would considerably reduce the throttling losses at take off and lower altitudes. The HS-12Y-51 would probably not be able to use superchargers with extra boost, however it could still benefit from improved superchargers by being able to achieve its full 1000hp over a greater altitude range.
The planned force for mid 1941 is certainly at least a good start to see where France may be by 1942-43 as long as the situation on the ground doesn't deteriorate again in favor of teh Germans (considering Entente production and manpower increases relative to the Germans over late 1940 and 1941, this is relatively unlikely unless the Germans can truly massively increase industrial capacity like they did in 1943 and 44, which would have to face more severe shortages of personnel and ressources without control over France and all the neutral countries Germany invaded or allied after France was defeated):
Fighter force: 1770 in line, including 90 2-seat night fighters and 180 two-seat heavy fighters (the night fighters were instead replaced by additional P-38 orders, the heavy fighters were to be Breguet 700). Plus 120% of reserves to account for losses, making 3894 fighters to be built, none of which were of the old generation (MS-406, Bloch 151/152, H-75). This is about the same force as the Germans had in 1940 and presumably in 1941 if the Luftwaffe endured typical losses.
Production: by mid-1941 it should achieve a rate of 650 per month spread over 4 production lines: 2 D520 lines producing 350/month in total, with the D55X series progressively replacing the D520s here throughout 1941 and possibly late 1940; 1 Hispano-Suiza-powered VG33/36/39 line and 1 Allison-powered VG32 or 34 (don't recall the designation), both producing 150 each. On top of this there is meant to be a US production line for Allison-powered D522s, and a MB 155 production line with eventually 125/month, but MB 155 production was meant to end before Summer 1941 and any further Bloch fighter production would be dependent on the performance of the MB 157.
Performance:
D520: probably the one meant to receive the most changes. The 12Y-49 engine seems to increase top speed by 20 kph and raises the rated altitude by 1000m at the cost of performance below the rated altitude compared to D520/12Y-45, but it was hoped to eventually improve the supercharger to raise the rated altitude by 1400m in total and top speed by 30
kph. 12Y-51 + the normal S-P supercharger seems to increase top speed by around 30kph but this time at a lower altitude. I'm not sure what the actual gains are for the early 12Z considering that the D524 which was supposed to carry it was meant to include aerodynamic improvements. Allegedly estimated at 616kph tops.
The D52X were also meant to receive eventually various aerodynamic and other improvements and thus bridge the gap between the basic D520 we know and the D55X series:
- new "VG-33"-type propeller with larger blades to gain 20kph as the existing propeller was not fully adapted to the aircraft and engine
- new pneumatic instead of electric propeller controls (Chauviere 3981 to replace the Ratier 1606M)
- new wing with Mercier ailerons to increase the surface of the curvature flaps, basically to improve landing speed characteristics (landing speed gain of 8kph at the same weight)
- 6 7.5mm machineguns instead of 4...someday
We were supposed to see 350 original D520s, 175 D520/-45 with Chauviere 3981 propeller, 75 with 12Y-31 and high altitude H-S supercharger, 150 D523/-51 with S-P, 30 D525/-51 with H-S, and finally 12Z and new automatic Chauvière 10105 propeller after the 780th unit (imo quite optimistic as this would require the 12Z to be available in August 1940, when it's more likely to be in prototype/preproduction stage by then).
I wouldn't be surprised if we saw a combination of 12Y-51 and high altitude S-P supercharger of the 12Y-49 at some point.
Other aerodynamic/misc changes tied to D524 or Vichy era programs (total 100 Cx is 3.52, and it's 22.2 at +3° angle):
- improved radiator housing and increased water temp from 80 to 90°C, 0.25 gain on the 100 Cx
- fully faired landing gear housings. 0.20 gain.
- the oil radiator replacement and upscaled water radiator basically show no gain on drag but it must have been deemed simpler or easier to maintain
- removed grates and ventilation holes for the engine compartment. 0.10 gain. Really doable?
- Finer aircraft surface (they stuck canvas on most joints and gun orifices)
0.1 Cx. Really doable?
All of this was calculated to increase top speed by 37 kph.
New exhaust pipe setup increased speed by 5kph.
A D520 with all that plus the 12Y-49 engine achieved 570 kph at 6900m owing to slightly lower propeller efficiency and other issues.
Then there was a 2nd batch of changes:
- Long nose reductor. 0.10 gain on the 100 Cx
- new more rounded glass for the cockpit, 0.10 gain.
- more adapted propeller (similar to the VG-33 type intended before th fall?)
- The final improved Szydlowski supercharger with an extra 1000m to the rated height.
They estimated that a normal D520 with all this could reach 615-620kph at 7500m. I suppose a 12Y-51 could add another 20-30kph. Might be too optimistic overall but we're closing in on D55X aerodynamics, just with a 12Y-51 with really high altitude supercharger and without the small and light body of the 551 which played with a worse supercharger.
The Vichy projects with all these upgrades and the 12Z engine (1200- 1300hp at 7500-8500m with the improved S-P) with associated reinforcements were expected to reach around 650-670 kph at their rated altitude at a take-off weight of nearly 3200 kg, so that should give the upper end for such monstrosities. The HS-50 is also analogous to those.
D55X: meant to be even more aerodynamic and lighter than D520s, at the cost of reduced wing surface and less fuel until the 12Z-powered versions (after the 200th D551).
Realistically, it is D55X that we would see by 1942, not Vichy-style ultra buffed D520s, but that would be at the sacrifice of comfort/range/armament but with reduced weight and higher potential speeds.
VG series: no aerodynamic improvements of note other than the long nose and VG36-style radiator/intake housing. It is meant to switch to 12Y-51 and 12Z at some point. It seems to be more aerodynamic than the basic D520 owing to its reported top speed (about same as D520 but with a plain 12Y-31 engine), but overall potential at the moment was lower than the D55X series.
For the armament, it's worth noting that the Anglo-French were supposed to jointly produce 12.7 or 13.2mm FN Brownings from late 1940 onwards and Colt was supposed to supply some as well, and there was a French factory made to receive the evacuated FN personnel although tooling and personnel didn't manage to be evacuated completely. The improved Hotchkiss aircraft 13.2 was a thing and featured a greater rate of fire but was waiting for belt feed, we don't have much info on its fate.
Overall I suspect that for 1942-43 the French would probably be in a good position in terms of numbers, and in reality they would fight alongside the British which already competed on even/favourable terms with the Luftwaffe, so it wouldn't be such a disaster regardless of likely French technical inferiority for most of the air war. I'd still expect the Allied air game in general to be much better off in such a scenario than IRL when the RAF fought completely alone from June 1940 to June 1941, and then alongside the Soviets and finally with limited US involvement in 1942. The AdA may not look good compared to the RAF and late war USAAF, but over a 1940-43 period it has high odds of being more useful than the Soviet airforce (even more mediocre aircrafts at times, a fucked logistical situation and possibly less well-trained pilots) and the USAAF in 1942 (too small).
French fighters of 1942-43 are unlikely to outmatch the Germans in every way owing to the uncertainties regarding their future engines, but at least they can maintain fair terms against the likes of the Bf 109E and Bf 109F with their 1940/41 improvements (even without the 12Z) and later on are still probably closer on average than in 1940 or even 1939 when the bulk of the force was MS 406es and H-75s against 109Es.