Advanced French Fighters vs 1942/1943 contemporaries (1 Viewer)

[Shortround6]

2. Nobody was touting that Yak-3 or it's other European counterparts were good for multi-hour 1500 mile missions over the enemy land.

You may want the ammo to last longer than 9 seconds even on short distance missions. Russians deliberately used lighter armament to keep performance up. They often tried to increase it, like using 23mm and 37mm cannon but those were special purpose or at least not main production types.
The French had a lot of flawed theories. Aircraft, tanks, AA guns, artillery in general. Which lead to flawed products. Designers were trying to do what the customer wanted. What the customer wanted was often not what the customer really needed.

French have the MB fighters to try in that role after all, should they feel the urge.

They felt the urge in 1940, what they didn't have was the doctrine, training, tactics, etc. Heck, they were trying to use Amoit 143s for close support at times (they didn't have anything else due to flaws in procurement). Now we can argue over well the Bloch 152s would have done

The HS 12Z might not be making 1600 HP at 6 km in 1942-43, but it might be making 1300-1400 at 5 km.

It might, but an unresolved problem is for how long. The Soviets had added about 100kg to the engine just to get it to the M-105PF stage. French were claiming they could hit the next level (1300-1400hp at 5km) for the same weight. In fact they were claiming they could hit the 1600hp at 6km (?)level for the same weight (600kg) as the M-105PF or only 20kg heavier? Swiss thought they needed 685kg to make 1410hp at 4.8km.

The 'altitude power' metric is showing how good was the engine ABC at the higher altitudes when compared with the competing engine XYZ or RPQ. Per that metric, V-1710 was not as good as the German competition (typically DB 601/605), or the British competition (Merlin); comparison vs. the BMW 801 or Griffon is ever worse for the V-1710. Even for the altitudes of 5-6 km, let alone for 8+ km
Unfortunately, the V-1710 badly needed the improvement of the S/C already before the time of Pearl Harbor attack.

What I was getting at was the basic engine. Much like the Merlin had shown it could last at 1600hp for hours in 1938 and allowed development to go other places (like the superchargers) the Allison was being improved during 1940-41 to where they had a very good basic engine. It did need some tweaks (new piston rings and valve springs and such) but in 1944 they were using 60in of MAP in P-38s and were approved for 70in on 44-1 fuel. P-63s were approved for 75in WEP (with water) and 1820. Now there were significant problems with the aux supercharger making power at altitude but the basic engine seemed to OK surviving those power levels. Allison needed better superchargers, not denying that.
But if you stick a really great supercharger on a crap engine you are stuck with a crap engine. And that is the problem with the French engines.

Allison knew they needed to improve the supercharger but the quick fix in Dec 1941 (9.60 gears) didn't work and had to wait almost a year.
Allison and the US were a little late to the game. Allison was building about 200-300 engine a month at the beginning of 1941 (it fluctuated a lot) but hit 500 engines in July and just over 1000 engines in Dec 1941. Better superchargers would have been nice in 1942, very nice, but for the US there wasn't a lot of choice. Allison made just over twice as many engines in 1942 as Packard and 2/3rds of Packards engines were contracted to the British.
Turbo P-40s, P-60s, etc were not built.

 

[tomo pauk]

You may want the ammo to last longer than 9 seconds even on short distance missions. Russians deliberately used lighter armament to keep performance up. They often tried to increase it, like using 23mm and 37mm cannon but those were special purpose or at least not main production types.

Russian 20mm cannon was firing the weakest shell of all 20mm cannons (bar the Ho-5), and there was often only one cannon per fighter. French 20mm cannon is much more powerful.

The French had a lot of flawed theories. Aircraft, tanks, AA guns, artillery in general. Which lead to flawed products. Designers were trying to do what the customer wanted. What the customer wanted was often not what the customer really needed.

A part of the guilt is also at the designer's feet. Like not being to make the aero engines to compete against the best engines of the world.
Granted, the theories were flawed.

It might, but an unresolved problem is for how long. The Soviets had added about 100kg to the engine just to get it to the M-105PF stage. French were claiming they could hit the next level (1300-1400hp at 5km) for the same weight. In fact they were claiming they could hit the 1600hp at 6km (?)level for the same weight (600kg) as the M-105PF or only 20kg heavier? Swiss thought they needed 685kg to make 1410hp at 4.8km.

Seems like the 1st HS 12Z (aka 12Y-89) was at 640 kg, for 1300 HP @2600 rpm at the (amazing) height of 7200 m - perhaps the sales brochure was a tad too optimistic?

Allison needed better superchargers, not denying that.
But if you stick a really great supercharger on a crap engine you are stuck with a crap engine. And that is the problem with the French engines.

If a supercharger improves the engine power down low by 10%, and at greater altitudes by 20+%, the engine is possibly now in 'decent' category, not in 'crap' category anymore. S-P supercharger did that, splitting the difference between the legacy HS 12Y engines and the DB 601A. Note that engine was still very light.

Allison knew they needed to improve the supercharger but the quick fix in Dec 1941 (9.60 gears) didn't work and had to wait almost a year.
Allison and the US were a little late to the game. Allison was building about 200-300 engine a month at the beginning of 1941 (it fluctuated a lot) but hit 500 engines in July and just over 1000 engines in Dec 1941. Better superchargers would have been nice in 1942, very nice, but for the US there wasn't a lot of choice. Allison made just over twice as many engines in 1942 as Packard and 2/3rds of Packards engines were contracted to the British.
Turbo P-40s, P-60s, etc were not built.

2-stage R-1830s were tested by the Army before 1941, while there were hundreds of aircraft with turboed engines manufactured in 1941 (inlcuding the fighters) - perhaps there were S/Cs, but having them implemented on 1-engined fighters was lagging?

 

[Shortround6]

Russian 20mm cannon was firing the weakest shell of all 20mm cannons (bar the Ho-5), and there was often only one cannon per fighter. French 20mm cannon is much more powerful.

Well, you get what you pay for, mostly. You want a powerful gun you pay for it in gun weight and ammo weight. Took the British a while to stick belt fed Hispanos in Spitfires but the slower firing drum Hispanos would last about 6 seconds, I don't think the pilots were happy.
Perhaps the French could get Belt fed guns 9-12 months sooner than the British but they have to pay the weight.

A part of the guilt is also at the designer's feet. Like not being to make the aero engines to compete against the best engines of the world.
Granted, the theories were flawed.

Well, two different sets of designers. Engine designers certainly dropped the ball, Government helped by not demanding more?
Air frame designers were stuck with what they could get? waited too long to "outside". (Buy Merlins or Allisons)

Seems like the 1st HS 12Z (aka 12Y-89) was at 640 kg, for 1300 HP @2600 rpm at the (amazing) height of 7200 m - perhaps the sales brochure was a tad too optimistic?

I think too optimistic is being kind, very kind.
Swedes in 1946-47 were building DB 605s (35.7 liters) at 725kg and rating them at 1475hp at 2800rpm for take-off using 6.3lbs (1080mm) on 91/96 octane fuel.
Normal rating was 1250hp/2600rpm at 19,000ft (5800m) MAP unknown.
RR Griffon 37 (single stage) (36.7 liters) was 875kg and offered normal (not military) 1365hp at 2600rpm at 16,500ft (5000 m)? Granted the Griffon was built heavier but that SP supercharger was amazing in the brochure. Even a two stage Griffon was only good for 1420hp at 2600rpm (normal) at 20,500ft ( 6200m). Using the normal ratings to equalize the rpm and possibly the boost ratings?

If a supercharger improves the engine power down low by 10%, and at greater altitudes by 20+%, the engine is possibly now in 'decent' category, not in 'crap' category anymore. S-P supercharger did that, splitting the difference between the legacy HS 12Y engines and the DB 601A. Note that engine was still very light.

What I mean by crap engine is one that is weak and/or unreliable and/or not cooling well. Putting a better supercharger on a crap engine does not fix the structural weakness or cooling problems. The 12Y-51 may very well have split the difference between the earlier 12Y engines and the DB 601. But the 12Y-51 didn't go into squadron service with the French.
It went into service with the Swiss and early on it had problems, including broken crankshafts, which limited deployment (manufacture) until they 'fixed' it. Quite probably the French might have been able to fix it faster but it took the Swiss somewhere between 1-2 years. Granted nobody was shooting at them but it also means they didn't have to use questionable engines and loose aircrew while they straightened things out.

2-stage R-1830s were tested by the Army before 1941, while there were hundreds of aircraft with turboed engines manufactured in 1941 (inlcuding the fighters) - perhaps there were S/Cs, but having them implemented on 1-engined fighters was lagging?

P&W did get the 2 stage supercharger into service before anybody else. Problem seems to be it wasn't all that good.
The two stage P&W R-1830 engine was giving 1100hp at 17,200ft but that was with 315mph of ram?
Or 1000hp at 19,000-19,400ft.
You can find power ratings using 2700rpm (or even 2900rpm at times) for take off. Military power in both neutral and low blower is usually give as 2700rpm. BUT in high gear the engine was just about always rated at 2550rpm, Military power was the same as max continuous.
Allison was rated at 1040hp in the low 14s without ram. Allison -39 was rated at 1150hp at 14,000ft with ram. P&W two stage supercharger was worth 3200ft ? or bit less?
Now for climbing things got a bit worse for the Allison.
I have no good guess as to what was happening with the P&W engine in high gear. Heating problems? Supercharger was taking most of the extra power at high rpm?
Turbo R-1830s in B-24s were allowed to run at 2700rpm and make 1200hp at 25,000ft. But B-24 engines were not burning up 100-200hp driving the aux supercharger and may have had bigger/better intercoolers helping with the heat load???

 

[TM06]

The French had a lot of flawed theories. Aircraft, tanks, AA guns, artillery in general. Which lead to flawed products. Designers were trying to do what the customer wanted. What the customer wanted was often not what the customer really needed.

Looking through some of the documents I've found, France seemed to understand what it needed as they creeped closer to 1940.
Regarding the aircraft in question, the C1 tender they created in September 1939 and launched in January 1940 was for much more capable aircraft - the D.55x, VG.39bis and M.B.157 were designed to meet the specifications required by this tender. There were more tenders launched like the B4 bomber tender, but that's not quite as relevant. Danel and Cuny described it in Dewoitine 520, Docavia n° 4, p. 184 which I've linked below.
The plan seemed to be bridging the gap between the existing aircraft from the A23 C1 program (D.520, M.B.152, VG.33) with their more advanced versions (D.523/D.524, M.B.155, VG.36) until they could get the previously mentioned new C1 fighters up and running around late 1941 to early 1942.

Well, you get what you pay for, mostly. You want a powerful gun you pay for it in gun weight and ammo weight. Took the British a while to stick belt fed Hispanos in Spitfires but the slower firing drum Hispanos would last about 6 seconds, I don't think the pilots were happy.
Perhaps the French could get Belt fed guns 9-12 months sooner than the British but they have to pay the weight.

The basic Hispano 404 only had a single 60 round drum on the fighters then in service, but there were larger ammunition capacities in circulation. The Br.693 and Potez 631 both had 90 round drums for their Hispanos, and the M.B.155 started production with the 120 round belt fed Hispano late into the Battle of France. There's an official document I linked below for the M.B.157 (called SO.157 due to Bloch being merged into SNCASO) which describes it as intending to fit 120 round belt fed Hispano 404's.
However I should note that in 1940, there was a version of the Hispano designated 405 undergoing testing that had a 200 round belt. The text (that I've also linked below) indicates it was supposed to be in production by 1942. If I'm reading the document right, it also claims that the 405 was only a remarkably light 28 kilograms without the ammo, which has interesting implications on the weight of the aircraft from 1942 onwards.

 

[Geoffrey Sinclair]

First 4 columns are War Production Board figures. V-1650 in MAP figures are those built for the US. MAP figures have 1 more than US figures in April 1943, 4 less in July 1943 and 2 less in November 1943, figures not available for December 1943.

Month

V-1710

V-1710

V-1650

V-1650

Ministry

of

Aircraft

Production

Figures

V-1650

V-1650

Month

1-stage

2-stage

1-stage

2-stage

M28

M29

M31

M33

M38

M68

1 stage

2 stage

Jan-40

3

Feb-40

7

Mar-40

6

Apr-40

7

May-40

12

Jun-40

29

Jul-40

60

Aug-40

70

Sep-40

230

Oct-40

285

Nov-40

176

Dec-40

256

Jan-41

134

Feb-41

300

Mar-41

311

Apr-41

237

May-41

403

Jun-41

404

Jul-41

513

Aug-41

709

4

2​

2​

Sep-41

725

4

3​

1​

Oct-41

765

5

3​

2​

Nov-41

845

10

7​

3​

Dec-41

1,101

26

17​

9​

Jan-42

1,101

109

74​

35​

Feb-42

1,041

149

22​

79​

48​

Mar-42

1,177

333

57​

160​

1​

115​

Apr-42

1,151

505

334​

1​

2​

168​

May-42

1,203

602

406​

2​

194​

Jun-42

1,252

702

458​

2​

242​

Jul-42

1,265

801

528​

36​

13​

224​

Aug-42

1,326

800

505​

82​

23​

190​

Sep-42

1,330

800

476​

71​

63​

190​

Oct-42

1,379

800

502​

20​

43​

235​

Nov-42

1,301

796

4

418​

28​

105​

245​

4​

Dec-42

1,378

1

849

1

430​

3​

135​

281​

1​

Jan-43

1,430

0

850

0

439​

128​

283​

Feb-43

1,345

1

864

0

519​

43​

302​

Mar-43

1,451

1

612

3

568​

44​

3​

Apr-43

1,548

3

605

1

119​

487​

1​

May-43

1,668

32

1,206

16

147​

1059​

16​

Jun-43

1,878

47

946

56

190​

756​

56​

Jul-43

1,987

33

962

184

384​

574​

184​

Aug-43

2,103

2

593

371

4​

589​

371​

Sep-43

2,083

1

728

475

728​

475​

Oct-43

1,894

42

840

450

36​

804​

450​

Nov-43

1,422

92

721

547

721​

2​

547​

Dec-43

1,740

260

576

689

1940​

1141​

0​

0​

0​

0​

0​

0​

0​

0​

0​

0​

0​

1941​

6,447​

0​

49​

0​

32​

0​

0​

0​

0​

0​

17​

0​

1942​

14,904​

1​

7,246​

5​

4,210​

480​

389​

0​

0​

0​

2,167​

5​

1943​

20,549​

514​

9,503​

2,792​

958​

0​

171​

880​

6,286​

2​

629​

2,103​

F4F-4/FM-1 as of 1 July 1943

Engine Maker	P & W
Engine Type	R-1830-86
Engine Gear Ratio	3 to 2
Propeller		Curtiss Elec. CS 3 Bl. 9 ft 9 in, Bl. Des. No. 512-ICL-5-15
Engine Rating Take-Off	BHP /RPM	1,200 / 2,900
Engine Rating Normal	BHP/RPM/Feet	1,100 / 2,550 / 0-3,300
Engine Rating Normal	BHP/RPM/Feet	1,080 / 2,550 / 3,800
Engine Rating Normal	BHP/RPM/Feet	1,090 / 2,550 / 11,300
Engine Rating Normal	BHP/RPM/Feet	1,030 / 2,550 / 13,000
Engine Rating Normal	BHP/RPM/Feet	1,040 / 2,550 / 18,400
Engine Rating Military	BHP/RPM/Feet	1,200 / 2,700 / 0-1,800
Engine Rating Military	BHP/RPM/Feet	1,135 / 2,700 / 3,400
Engine Rating Military	BHP/RPM/Feet	1,150 / 2,700 / 11,500
Engine Rating Military	BHP/RPM/Feet	1,030 / 2,550-2.700 / 15,000
Engine Rating Military	BHP/RPM/Feet	1,040 / 2,550 / 18,400

F4F-3 as of 14 August 1943.

Engine Rating	For Performance	Military	Normal
VM Sea Level	MPH	290​	280​
VM	MPH/Feet	295/1,800	288/2,500
VM	MPH/Feet	294/3,100	287/4,200
VM	MPH/Feet	316/11,500	310/12,000
VM	MPH/Feet	311/15,100	307/13,300
VM	MPH/Feet	323/19,000	323/19,000
VM (Critical Altitude)	MPH/Feet	329/21,100	329/21,100

 

[Shortround6]

Looking through some of the documents I've found, France seemed to understand what it needed as they creeped closer to 1940.
Regarding the aircraft in question, the C1 tender they created in September 1939 and launched in January 1940 was for much more capable aircraft - the D.55x, VG.39bis and M.B.157 were designed to meet the specifications required by this tender. There were more tenders launched like the B4 bomber tender, but that's not quite as relevant. Danel and Cuny described it in Dewoitine 520, Docavia n° 4, p. 184 which I've linked below.
The plan seemed to be bridging the gap between the existing aircraft from the A23 C1 program (D.520, M.B.152, VG.33) with their more advanced versions (D.523/D.524, M.B.155, VG.36) until they could get the previously mentioned new C1 fighters up and running around late 1941 to early 1942.

Thank you for the documents. Many other people in 1939 had planes for great fighters in late 1941/42. See Hawker Typhoon and Tornado for two

The basic Hispano 404 only had a single 60 round drum on the fighters then in service, but there were larger ammunition capacities in circulation. The Br.693 and Potez 631 both had 90 round drums for their Hispanos, and the M.B.155 started production with the 120 round belt fed Hispano late into the Battle of France. There's an official document I linked below for the M.B.157 (called SO.157 due to Bloch being merged into SNCASO) which describes it as intending to fit 120 round belt fed Hispano 404's.
However I should note that in 1940, there was a version of the Hispano designated 405 undergoing testing that had a 200 round belt. The text (that I've also linked below) indicates it was supposed to be in production by 1942. If I'm reading the document right, it also claims that the 405 was only a remarkably light 28 kilograms without the ammo, which has interesting implications on the weight of the aircraft from 1942 onwards.

There is a lot of confusion on drum sizes and feeds for the H-S 20mm guns. Not helped at all by the confusion between the Hispano HS-9 cannon (licensed Oerlikon FFS) and the HS 404, at least in many English language articles. H-S was advertising quite a selection of drums/magazines for the HS-9 but how many were adopted for service (and by who) is subject to question. The drums for the HS 404 are slightly different but I see no reason why drums of similar capacities could not be made for the HS 404 if somebody wanted them.
The intended Belt feed for the HS 404 is not a real problem, but intentions and actual service are not quite the same thing. A British officer escaped Paris in June 1940 with the drawings for the Belt feed. How far the French were in actual production and fitting to service aircraft I don't know but use in early 1941 does not seem to be a problem. British fumbled the whole thing as some of their armament experts didn't like that the French feed pushed on the nose of the round (on the fuse) to push the rounds out of the belt. British designed several belt feeds of their own (and at least one other feed system) that mostly tried to pull the rounds out from the rear. Less said about the pneumatic powered magazines in the experimental Whirlwind the better. After quite a number of months the British settled on a close copy of the original French feed. Once you have a belt feed the length of the belt is usually not a problem. Usually.
British slowed the firing rate of the HS 404 from about 700rpm to about 600rpm for reliability. Also did a few other things to it but nothing the French could not do themselves had they lasted longer in the war.
English sources claim the HS 405 used magazine or drum but once you have a belt feed I don't see why they couldn't have used it on the smaller gun. Since it never left the prototype stage there is no telling how it would have ended up. But smaller it was. It used a cartridge case about 39mm shorter for a loss of around 200m/s in velocity. Gun had the ballistics of a MG FF or just a bit better (or close to a MG 151/20) which helps put the weight in perspective. Shell weight is lacking in specs so assuming standard HS or Oerlikon projectiles.
Most English sources claim the HS 405 was designed for turret mounting. Doesn't mean it couldn't be used as fixed gun. The HS 404 was a beast of a gun to put in a turret/powered mount. In part because it needed a forward support part way out the barrel.

I will note that the French fell into the trap of putting too many rounds of 7.5mm ammo into planes. While we can argue about needing more than 9-12 seconds of cannon ammo the need for 40-50 seconds of 7.5-7.9mm machine gun ammo is a lot more dubious. Requirements of the Fairly Fulmar aside. Beaufighter could carry 1000rpg for the .303s but then the Beaufighter carried 240-283 rounds for each 20mm cannon. German 109Es and BF 110s carried very dissimilar amounts of ammo for their 20mm and 7.9mm guns.
Nobody says you have to fill up the ammo boxes but figuring out where to put 800-1000 rpg of 7.5mm ammo maybe more trouble than it is worth?

 

[tomo pauk]

Well, two different sets of designers. Engine designers certainly dropped the ball, Government helped by not demanding more?
Air frame designers were stuck with what they could get? waited too long to "outside". (Buy Merlins or Allisons)

Shambles that were in the French politics, startegy and procurement certainly deserve a few good books to be described in.
Buying Allisons is fraught with bad timing. Buying Merins always gets my vote - both timing and power are very competitive.

What I mean by crap engine is one that is weak and/or unreliable and/or not cooling well. Putting a better supercharger on a crap engine does not fix the structural weakness or cooling problems. The 12Y-51 may very well have split the difference between the earlier 12Y engines and the DB 601. But the 12Y-51 didn't go into squadron service with the French.

I was thinking about the HS 12Y-45 splitting the difference between the -31 and the DB 601A.
French will be lucky if they get the -51 in service by late 1940 (assuming no mishaps, like a lucky bomb hit on the factory etc.), and the -98 (aka 12Z, that is mostly likely to have the rated altitude of ~5000m, not of 7000+, 1300+- HP?) by Spring of 1941?

P&W did get the 2 stage supercharger into service before anybody else. Problem seems to be it wasn't all that good.
The two stage P&W R-1830 engine was giving 1100hp at 17,200ft but that was with 315mph of ram?
Or 1000hp at 19,000-19,400ft.

Yes, not that awe-inspiring, but very competitive for 1940, and still decent power for 1941, at least from the French perspective. Might've been good on the MB.150 series of the fighters.
Turboed American engines (R-1820, -1830, V-1710) were a bit better, but these will require a bit of foresight to be installed on the 1-engined fighters (ie. like what was done on P-43, and not on the XP-37 and XP-39).

 

[Shortround6]

Buying Allisons is fraught with bad timing. Buying Merins always gets my vote - both timing and power are very competitive.

You are correct on the Allison, I am thinking about licensing but the summer of 1940 not a good one in some respects for Allison. Somewhere in here is when the US Army decided they wanted 1090hp about 1000ft lower than the Allison was rating the engine at 1040hp. The engine would make the power, the problem was that the engine would not stand up to the power long term (US also used a 150 hour test and not 100 hours like many other nations). Didn't help that the test stand was too rigid and didn't absorb vibrations well. Allison spent a lot time beefing up/tweaking the engine to pass the test. ALL the early US engines were rebuilt at Allison's expense. Which helped later Allison engines durability. But in the summer/fall of 1940 which engine are the French getting? Early Tomahawks got the early version of the engine.
Now compared to 1939/40 Hispano engines the Allisons may have been just fine. But as a basis for a 1942-43 1400hp engine the early Allison was somewhat lacking (even if you change the reduction gear).
Merlin is the best bet for license, assuming they get the same same Merlin XX that the US was offered. But French production is only going to be in advance of Packard by a few months, depending on both factory space and tooling supply.

I was thinking about the HS 12Y-45 splitting the difference between the -31 and the DB 601A.

French will be lucky if they get the -51 in service by late 1940 (assuming no mishaps, like a lucky bomb hit on the factory etc.), and the -98 (aka 12Z, that is mostly likely to have the rated altitude of ~5000m, not of 7000+, 1300+- HP?) by Spring of 1941?

Might depend on what failure rate on the crankshafts the French are willing to put up with. Or if they can fix it faster than the Swiss did.
The -51 has the advantage of being built on the old tooling (mostly) but the 12Z of whatever number suffix needs new cylinder heads and valve/cam drives. Also needs new crankshaft and different than the -51 crank and crankcase.

Yes, not that awe-inspiring, but very competitive for 1940, and still decent power for 1941, at least from the French perspective. Might've been good on the MB.150 series of the fighters.
Turboed American engines (R-1820, -1830, V-1710) were a bit better, but these will require a bit of foresight to be installed on the 1-engined fighters (ie. like what was done on P-43, and not on the XP-37 and XP-39).

The Allison supercharger of 1939/early 40 was not bad compared to the rest of the world. The P&W setup was one of the best in the Spring/summer of 1940. However RR was going into production on the Merlin XX and that leaped far out in front.
Metric that I am using is getting around 1100hp at 18,000-20,000ft out of roughly 1500lb engine.
The Merlin could do it with a single stage, two speed supercharger. The R-1830 needed a two stage supercharger and intercools or a turbo with intercooler.
Allison could do it with an early P-38 but we are back to weight (?) cost and bulk.
P-43 is sort of a case in point. You could stuff a P&W R-1830 into that rather chubby fuselage and hide the turbo and intercooler in the back. It was NOT going to fit into a Bloch 155 even with a very good and large shoehorn. The R-1830 engine would fit, The turbo, intercooler and duct work was going to be a problem.

 

[TM06]

I will note that the French fell into the trap of putting too many rounds of 7.5mm ammo into planes. While we can argue about needing more than 9-12 seconds of cannon ammo the need for 40-50 seconds of 7.5-7.9mm machine gun ammo is a lot more dubious. Requirements of the Fairly Fulmar aside. Beaufighter could carry 1000rpg for the .303s but then the Beaufighter carried 240-283 rounds for each 20mm cannon. German 109Es and BF 110s carried very dissimilar amounts of ammo for their 20mm and 7.9mm guns.
Nobody says you have to fill up the ammo boxes but figuring out where to put 800-1000 rpg of 7.5mm ammo maybe more trouble than it is worth?

It's a position that makes sense when you consider the limited ammo capacity of the main cannons.
If you ask me, it's less of a trap and more of a considered trade off to allow what would otherwise be low-endurance fighters to stay in the battle for longer.
France, like basically everyone else at the time, lacked a proper high-calibre aircraft gun (a fault of their own making this time) and couldn't fit a large amount of rounds for their main cannons. IIRC, the D.520's 675 rpg was seen as a considerable advantage versus the 500 rpg of the M.B.152.
Given the advent of the production-ready 120 round HS.404 and the 200 round HS.405 on the way, it's likely that they'd ditch some of the light machine guns for increased cannon capacity post-1940.

What they'd do for a total armament layout by 1942 and beyond is a different story, since every other major player started converting to HMG's in place of the rifle calibre guns around then. Converting the Hotchkiss 13.2 is possible, but they already spurned that opportunity in the 20's. I've found mentions of an 11 mm conversion of the MAC 34 gun here and there, but I can't seem to find any information on it.

 

[tomo pauk]

You are correct on the Allison, I am thinking about licensing but the summer of 1940 not a good one in some respects for Allison. Somewhere in here is when the US Army decided they wanted 1090hp about 1000ft lower than the Allison was rating the engine at 1040hp. The engine would make the power, the problem was that the engine would not stand up to the power long term (US also used a 150 hour test and not 100 hours like many other nations). Didn't help that the test stand was too rigid and didn't absorb vibrations well. Allison spent a lot time beefing up/tweaking the engine to pass the test. ALL the early US engines were rebuilt at Allison's expense. Which helped later Allison engines durability. But in the summer/fall of 1940 which engine are the French getting? Early Tomahawks got the early version of the engine.
Now compared to 1939/40 Hispano engines the Allisons may have been just fine.

There was no 1939 V-1710s worth speaking about even for the needs of the USAAC service. So the HS 12Y-45 is a far better bet.
1940 for the V-1710s for the Fench service is iffy, they can consider themselves lucky if a few squadrons of P-40s are in time to take fight before bad weather sets in in the Autumn. French need something substantial, both in quantity and quality, to cover themselves between May and September of 1940; P-40/V-1710 is not the answer, with it's production getting in the triple digits by August. Heck, they will be very lucky if the latest P-36s can be accepted in service for the biggest air battles of 1940.
We also have a thing of the French needing to redesign their aircraft for the V-1710, that was not just heavier than the HS 12s, but also without the motor-cannon capacity: again, P-40 is probably the fastest way for them to have a V-1710-powered fighter in service.

But as a basis for a 1942-43 1400hp engine the early Allison was somewhat lacking (even if you change the reduction gear).
Merlin is the best bet for license, assuming they get the same same Merlin XX that the US was offered. But French production is only going to be in advance of Packard by a few months, depending on both factory space and tooling supply.

Me, I'd get them to make Merlin XII ASAP (=hopefully in 1940), and then the Merlin 45, and then later the Merlin 61.
Introduce 2-piece block engines when the pressure subsides, and hopefully a better carb.

The Allison supercharger of 1939/early 40 was not bad compared to the rest of the world.

We have a good S/C installed on an engine that is not available, at least not to the French.

The P&W setup was one of the best in the Spring/summer of 1940. However RR was going into production on the Merlin XX and that leaped far out in front.
Metric that I am using is getting around 1100hp at 18,000-20,000ft out of roughly 1500lb engine.
The Merlin could do it with a single stage, two speed supercharger. The R-1830 needed a two stage supercharger and intercools or a turbo with intercooler.
Allison could do it with an early P-38 but we are back to weight (?) cost and bulk.

Merlin was making that kind of power even with a 1-speed 1-stage S/C, as installed on the Merlin 45. Yes, not something from 1940, but still.

Weight of the turbo set-up was probably negligible when compared to the take-off weight of an A/C. Unfortunately, the V-1710-pilot-turbo layout didn't dawned on the US designers of A/C until the P-60A, ie. too late to matter. That left, from 1941 on, a small engine outfitted with a small S/C whose impeller was spinning slow to compete against the bigger German engines with ever better superchargers that have had faster-spinning impellers - no bueno. Even the 9.60:1 S/C drive improves the V-1710, that - again unfortunately - arrived too late; the big S/C never happened.

P-43 is sort of a case in point. You could stuff a P&W R-1830 into that rather chubby fuselage and hide the turbo and intercooler in the back. It was NOT going to fit into a Bloch 155 even with a very good and large shoehorn. The R-1830 engine would fit, The turbo, intercooler and duct work was going to be a problem.

My suggestion was that Bloch fighters receive the 2-stage R-1830. However, even with the very cooperative USA and P&W, that will have to wait well until 1941 to materialize, by what time Marcel might've took the page from the Italians and install the Merlin 45 on it's fighters.

BTW - the big elephant in the room is that with France being still alive and well into 1941 and on, Lend Lease might've never happened.

 

[Shortround6]

It's a position that makes sense when you consider the limited ammo capacity of the main cannons.
If you ask me, it's less of a trap and more of a considered trade off to allow what would otherwise be low-endurance fighters to stay in the battle for longer.
France, like basically everyone else at the time, lacked a proper high-calibre aircraft gun (a fault of their own making this time) and couldn't fit a large amount of rounds for their main cannons. IIRC, the D.520's 675 rpg was seen as a considerable advantage versus the 500 rpg of the M.B.152.

It made sense to desk jockeys (specification writers who had no idea of actual combat). They were not the only ones, Some Bozo thought that the P-40 didn't carry enough ammo in the early versions, four rcmg (Rifle Caliber Machine Guns) in the wing at 500rpg and some how the P-39 got 1000rpg, or boxes that would hold 1000rpg.
France was trying to build light fighters and then specifies ammo load outs that require a small truck for each plane? OK an exaggeration but...........

It is easy to do the math. Even if the MAC 34 actually fired at 1500rpm (25rps) 500 rounds gives you 20 seconds of firing time. Spitfire I about 15 seconds (guns fired at 20rps). Not saying the Spitfire was the goal, it wasn't. Spitfire II and later could shoot their MGs for just over 17 seconds. P-40 Tomahawk could manage 25 seconds with their wing guns. American Ealy P-40s went the other way and were carting around way too much .50 cal ammo for the cowl guns.

The question is how much trigger time are most of you pilots going to get? The D.520s were good for 27 seconds. The cannon ran out after about 5.2 seconds. Even a 120 round belt was going to run out in 10.4 seconds. British cannon fired a bit slower and a Spitfire VC was going to run out of cannon ammo in 12 seconds, leaving the pilot with the 4 mgs for another 5 + seconds. Maybe this was a good choice and maybe it wasn't.

Back to the P-39. It could carry 260lbs of ammo for it's four .30 cal guns, it carried 129lbs of ammo for the two .50 cal guns in the cowl and in the D model with a 20mm cannon 32.4lbs of ammo. Now some Bozo thought that having 6 seconds of firing time for the cannon was good idea, while the .50s would run out after about 24 seconds (synchronized US .50s were slow) but the intrepid US pilots would be able to keep shooting for 50 seconds with their four .30 cal guns. Sanity prevailed (eventually) and the weight and loading charts for the P-39K & L show 1200 rounds total (300rpg) for the wing guns. TOTAL weight of ammo for 30 rounds of 37mm, 400 rounds of .50 cal and 1200 rounds of .30 cal was 262lbs. The ammo trays/boxes were still there, they just blocked them off with pieces of wood.

Now for our poor overworked aircraft designers we have some Bozo (or group of them) insisting that the designers load up relatively small planes with low powered engines with about 160kg + of 7.5mm ammo for the just the wing guns. AND all this ammo is running out into the outer wings doing wonders (sarcasm) for the roll response/maneuverability, for the very, very few times a pilot is going to get over 30 seconds of gun time without running out of fuel in combat. Most people figure the average length of burst of fire in combat was between 2 and 3 seconds. The American Corsairs and Hellcats could fire for about 30 seconds or just under.

You got payload for 160kg of ammo? cut the ammo to 700rpg and add in 2 more guns.

Now maybe, maybe, the ammo specification was insurance on the plane coming it a bit overweight and by cutting the ammo load by 100-200rpg they could still hit the target weight?

Given the advent of the production-ready 120 round HS.404 and the 200 round HS.405 on the way, it's likely that they'd ditch some of the light machine guns for increased cannon capacity post-1940.

You are holding out a lot of hope for the HS.405 in role it was not intended for and may not have done what you wanted anyway. Germans were notorious for jamming guns of different ballistics together regardless of the problems that might give the pilots. If your firing at anything but close range against a turning target (even a bomber) the HS.404 and HS.405 are not going to have the same time of flight and will not hit to the same place. The HS.405 has about the same ballistics (or a little bit better) than the Japanese type 99-1 cannon used in the early Zeros. It will shoot faster and for a longer period of time. The Germans went to lighter shells to get the MV up to reduce the flight time at close range.
French could do that but that means a different production line for the projectiles.

Converting the Hotchkiss 13.2 is possible, but they already spurned that opportunity in the 20's. I've found mentions of an 11 mm conversion of the MAC 34 gun here and there, but I can't seem to find any information on it.

Trying to convert the Hotchkiss may be a lost cause. It fired a lot slower than even a 1930s .50 Browning and you have to come up with a belt fed system.

The 11mm idea should be dead on arrival. The French and British had used an 11mm cartridge in WW I for balloon busting but it was based on the old 11 x 59R Gras cartridge from about 1874. It gave a bigger bullet carrying a lot more incendiary material. They did use smokeless powder. However the velocity was not great but since the balloons and Zeppelins weren't moving very fast (and were huge) that didn't matter that much. I am not sure you can neck up a 7.5mm French case to 11mm and have it work (case is 11.3mm at the shoulder) the older more bottle necked rimed cases were a better bet. Maybe there is a little room left in the MAC 34 action for a slightly bigger cartridge but that is what you are going to get, a slightly bigger rifle case with a big bullet which means low muzzle velocity.
Maybe some bright boy remembered the WW I Balloon guns and suggested the 11mm idea? Maybe someone even tried it?

 

[Shortround6]

There was no 1939 V-1710s worth speaking about even for the needs of the USAAC. So the HS 12Y-45 is a far better bet.
1940 for the V-1710s for the Fench service is iffy, they can consider themselves lucky if a few squadrons of P-40s are in time to take fight before bad weather sets in in the Autumn. French need something substantial, both in quantity and quality, to cover themselves between May and September of 1940; P-40/V-1710 is not the answer, with it's production getting in the triple digits by August. Heck, they will be very lucky if the latest P-36s can be accepted in service for the biggest air battles of 1940.

I haven't looked up the time line for the orders but it was the French who originally ordered the 'Tomahawks', British got them when they couldn't be delivered. Then there was the whole Lockheed 322 Lightning fiasco. Allison had to "forgive" the USAAC's $900,000 debt for permission to export the Allison engines, so the export deals were certainly in the works. They just didn't happen in time. France may have thought that buying or licensing Allisons gave them a 2nd source of V-12 engines. There were plans to stick them into the VG 30 series as the VG 32 and the D.522, They actually built the VG 32 but not quite in time.

We also have a thing of the French needing to redesign their aircraft for the V-1710, that was not just heavier than the HS 12s, but also without the motor-cannon capacity: again, P-40 is probably the fastest way for them to have a V-1710-powered fighter in service.

Well, without the weight of the cannon maybe you can keep the CG in check?

Me, I'd get them to make Merlin XII ASAP (=hopefully in 1940), and then the Merlin 45, and then later the Merlin 61.
Introduce 2-piece block engines when the pressure subsides, and hopefully a better carb.

Long term best plan.

Merlin was making that kind of power even with a 1-speed 1-stage S/C, as installed on the Merlin 45. Yes, not something from 1940, but still.

Well, the Merlin XX used the same supercharger as the 45, but yes, not something the French can build in 1940.

Weight of the turbo set-up was probably negligible when compared to the take-off weight of an A/C. Unfortunately, the V-1710-pilot-turbo layout didn't dawned on the US designers of A/C until the P-60A, ie. too late to matter.

The problem is all the "stuff". The turbo itself was around 120-130lbs (?) and the intercooler (air to air) was not all that heavy but it needed around 10-12 cu ft of space inside the airplane. That is a rule of thumb of needing 1 cu ft of space per 100hp for a turbo installation. Next problem is that you can't stuff everything in a compact box/compartment but have to spread it out, You need a certain distance from the exhaust ports on the engine to the turbo to let the gases cool a little bit so you don't melt the turbine blades. You also need a low drag airflow for the intercooler which may be flowing 2-3 times the combustion air the engine needs. Abrupt turns in the ducts are going to cause drag and/or poor airflow and cooling.

My suggestion was that Bloch fighters receive the 2-stage R-1830. However, even with the very cooperative USA and P&W, that will have to wait well until 1941 to materialize, by what time Marcel might've took the page from the Italians and install the Merlin 45 on it's fighters.

BTW - the big elephant in the room is that with France being still alive and well into 1941 and on, Lend Lease might've never happened.

Could go either way, The French had ordered a lot more than the British did in 1939, early 1940.
French ties to the US were at least as strong in 1939/early 1940. France collapsed faster than the publicity machine in the US could gear up?

 

[TM06]

You are holding out a lot of hope for the HS.405 in role it was not intended for and may not have done what you wanted anyway. Germans were notorious for jamming guns of different ballistics together regardless of the problems that might give the pilots. If your firing at anything but close range against a turning target (even a bomber) the HS.404 and HS.405 are not going to have the same time of flight and will not hit to the same place. The HS.405 has about the same ballistics (or a little bit better) than the Japanese type 99-1 cannon used in the early Zeros. It will shoot faster and for a longer period of time. The Germans went to lighter shells to get the MV up to reduce the flight time at close range.
French could do that but that means a different production line for the projectiles.

I'm holding out for a hero 200 round belt-fed Hispano rather than the 405 specifically. The existence of the 405 just proves that it's possible. I should note that even though it's a significantly slower round, 640~680 m/s is still a workable muzzle-velocity, especially for a gun that light.
The 405 could be fitted, but mating the 200 round belt to the 120 round belt 404 seems more reasonable.

Trying to convert the Hotchkiss may be a lost cause. It fired a lot slower than even a 1930s .50 Browning and you have to come up with a belt fed system.

The 11mm idea should be dead on arrival. The French and British had used an 11mm cartridge in WW I for balloon busting but it was based on the old 11 x 59R Gras cartridge from about 1874. It gave a bigger bullet carrying a lot more incendiary material. They did use smokeless powder. However the velocity was not great but since the balloons and Zeppelins weren't moving very fast (and were huge) that didn't matter that much. I am not sure you can neck up a 7.5mm French case to 11mm and have it work (case is 11.3mm at the shoulder) the older more bottle necked rimed cases were a better bet. Maybe there is a little room left in the MAC 34 action for a slightly bigger cartridge but that is what you are going to get, a slightly bigger rifle case with a big bullet which means low muzzle velocity.
Maybe some bright boy remembered the WW I Balloon guns and suggested the 11mm idea? Maybe someone even tried it?

Those are really the only indigenous options that France has for HMG's. However under the Vichy regime they have access to the Axis equipment.
The MG 131 and Breda-SAFAT 12.7 are there, but the most attractive option would definitely be the 13.2 FN Browning from Belgium. Not only is it an incredibly capable gun, France already had a massive amount of 13.2x99 mm Hotchkiss in circulation, they have experience with the Browning design due to the H-75 Hawk and Belgium is right there. Maybe they could negotiate a license for it?

 

[tomo pauk]

Well, the Merlin XX used the same supercharger as the 45, but yes, not something the French can build in 1940.

Yes, 1940 will still be fought with the lesser superchargers. As well as the many months of 1941.
Unfortunately, French didn't made a military-grade follow up on the 2-stage superchargers that was used on many record aircraft in the 1930s, including one of their own (by Farman).

The problem is all the "stuff". The turbo itself was around 120-130lbs (?) and the intercooler (air to air) was not all that heavy but it needed around 10-12 cu ft of space inside the airplane. That is a rule of thumb of needing 1 cu ft of space per 100hp for a turbo installation. Next problem is that you can't stuff everything in a compact box/compartment but have to spread it out, You need a certain distance from the exhaust ports on the engine to the turbo to let the gases cool a little bit so you don't melt the turbine blades. You also need a low drag airflow for the intercooler which may be flowing 2-3 times the combustion air the engine needs. Abrupt turns in the ducts are going to cause drag and/or poor airflow and cooling.

Agreed.
Basically, if the 1-engined fighter is not designed around the turbo, it will be one clumsy fighter once people start adding the turbo.

 

[Shortround6]

The French had ordered 230 Hawk 81A (P-40) models on Oct 9th 1939. French instruments and radios, reverse throttle movement and provisions for French armament.
These seem to slot into/between the Hawk 75 orders. The Hawk 75A-1 and A-2 being quite a bit earlier. A-2s being ordered March 8th 1939.
I have no date for the 75 A-3 order but it was "late" 1939 after the war started, 135 planes. Not all even left the US.
The 75 A-4 order was for 285 planes (date unknown?) but first acceptance at factory was April 8th 1940, These had the Wright engines instead of P&W and it seems there was overlap.
Meanwhile the Anglo-French purchasing commission had been established and a letter of intention for 500 Hawk 81As had been placed, 285 to Britain and 215 to France. As we know, made it to France.

As to the the US, they only took 200 out of their order for 560 (524 complete planes and 36 as spare parts) planes placed in April 1939. They allowed France and then England to take their place in the production que and deferred their orders to later versions (P-40B and P-40C). That is some of the context for French interest in the Allison engine. None of the French 81As made it to France, first flight at the factory was June 6th 1940.

I'm holding out for a hero 200 round belt-fed Hispano rather than the 405 specifically. The existence of the 405 just proves that it's possible. I should note that even though it's a significantly slower round, 640~680 m/s is still a workable muzzle-velocity, especially for a gun that light.
The 405 could be fitted, but mating the 200 round belt to the 120 round belt 404 seems more reasonable.

There is no 'mating' of the belt feed. You either have a belt feed gun (with infinite belt length) or you don't. British used feed lengths of about 90 rounds on the Hurricane IIC, 120 rounds on the Spitfire VC and later, 150 rounds on the Mosquitos, and other lengths up to 283 rounds on the Beaufighters. It was possible to convert a British drum feed gun to a belt
by taking off the muzzle brake and fitting one of several belt feed devices to the gun, the feed device exerted the pull on the belt, de-linked the ammo and feed the rounds into the same feedway the drum used (in the top of the gun).
Once you have a belt fed 404 the 405 is redundant except for special applications. Only problem with putting 200 rounds in a plane like the 406 or 520 is if you have enough space for the ammo.

Those are really the only indigenous options that France has for HMG's. However under the Vichy regime they have access to the Axis equipment.
The MG 131 and Breda-SAFAT 12.7 are there, but the most attractive option would definitely be the 13.2 FN Browning from Belgium. Not only is it an incredibly capable gun, France already had a massive amount of 13.2x99 mm Hotchkiss in circulation, they have experience with the Browning design due to the H-75 Hawk and Belgium is right there. Maybe they could negotiate a license for it?

The Italian Breda gun was reliable but it was heavy, about as heavy as the US Browning, except it used a shorter, lower powered cartridge, id did fire faster, right up until 1940 when the US changed the firing rate on the M2 gun. The MG 131 is the lightest of these guns but it is also the lowest powered. It does have the best rate of fire, aside from the Belgian Browning.
Belgian gun's capability is subject to question. The Finns and the Swedes used it. Maybe they were happy with it. But they didn't have a lot of choice. The US may have hurt themselves by demanding too much durability/reliability which meant greater weight and lower cycle rate. Finns and Swedes may have been happy with a faster cycle rate and more breakages.
More is relative, there is also a difference between breakages and jams. In 1940-41 the US guns jammed a lot, an awful lot. But while they stopped firing they weren't actually breaking that many parts. Things got better but it took a while.
And there is a barrel wear problem, The US .50 can burn out a barrel in fairly short order. Using a faster cycle rate can really burn out barrels. US fought that in a number of ways.
One was supposed to be fire discipline, rules on how many rounds could be fired at a time and what the cool down periods should be, pretty much thrown out the window in combat.
Two was the weight of the barrel, a heavy barrel takes longer to heat up. The heavier US barrel was supposed to be good for 75 rounds from a cold barrel and we are talking about 10-15,000ft cold with a 200mph wind chill
Three was improved materials like chrome plated bores and/or stellite linings in part of the barrel. Same stuff they made valve seats out off or coated valves and valve seats with. This didn't show up until late.
Four was US logistics. The US simply shipped a lot of spare barrels and changed them as needed (mostly).

I don't know what the Finns and the Swedes did. The soviets used a lot of spare barrels, they may have used chrome plating. They also treated the entire gun as somewhat disposable.
They would junk a gun rather than do extensive overhaul/rebuild with about 1/4 the amount of rounds through the gun as the US would.
Barrels were always an expendable item, but the Belgian gun is putting about 25% more heat into the barrel per second than the US gun was when the US gun was firing at about 800rpm. The Belgian gun will give you performance, but you are going to pay for it.

 

[Geoffrey Sinclair]

The 135 French H75A-3 were built February to April 1940, according to the French the final 2 shipments were 13 aircraft on the Ile de France Bill of Lading Date 1 May and 1 on Pierre Louis Dreyfus on 10 May, both ships also had H75A-4 on board, Contract F-273 according to the USAAF. French serials 13671 to 13805.

The first of the 285 French H75A-4 were built in April 1940, serials allocated were 13806 to 14090, the USAAF says 81 accepted for France, 204 for Britain, again under contract F-273, the French report the first 58 were on the Indo Chinois (25 May) or earlier ships, then
Bearn (15 June) serials 13864 to 69, 88, 90, 91, 93 to 95, 313997 to 99, 13901, 13902 (17 a/c)
Jeanne d'Arc (15 June) serials 138884 to 87, 89, 13900 (6 a/c)
To Britain serials 13870 to 83, 13896, then 13903 on (204 a/c)

The USAAF report 100 HK-87A accepted for Britain June to September 1940 off contract F-273 yet again, the French say 100 H81A added to the H75A-4 order, the French also cancelled the last 130 H75A-4 on order, replacing them with H81A-2, which was combined with the British P-40 order, result H81A/Tomahawk production contract F-273 Curtiss c/n 14446 to 545, RAF AH741 to 840 built first, contract A-84 Curtiss c/n 14091 to 220, RAF AH841 to 970 built second, contract A-84 Curtiss c/n 14582 on, RAF AH971 on built third, 1,080 aircraft on contract A-84, all up 140 Tomahawk I, 110 IIA and 930 IIB.

Meantime the USAAF contract AC-12414 26 April 1939, 524 a/c built as P-40 and P-40D/E/XF/G, contract AC-15802 13 September 1940, plus supplement, 1,842 a/c, P-40B/C/E/F

 

[TM06]

Belgian gun's capability is subject to question. The Finns and the Swedes used it. Maybe they were happy with it. But they didn't have a lot of choice. The US may have hurt themselves by demanding too much durability/reliability which meant greater weight and lower cycle rate. Finns and Swedes may have been happy with a faster cycle rate and more breakages.
More is relative, there is also a difference between breakages and jams. In 1940-41 the US guns jammed a lot, an awful lot. But while they stopped firing they weren't actually breaking that many parts. Things got better but it took a while.

From what I know about the Belgian Browning, it was an exceptional weapon. The Romanians considered it to be equal with the MG 151/20 which is extremely high praise for a heavy machine gun. It was much lighter than the AN/M2 (24.3 kg to 28 kg) and the rate of fire was amazing, however the main thing is the HE round developed for it that allowed it to punch well above its weight. Even if you were to crank down the fire rate to about 700~800 rpm, it'd still be a great weapon on its own. It also pairs quite well with the Hispano due to the muzzle velocities being similar.
13.2x99 Hotchkiss was everywhere in Europe, and France was already making the round in droves. Even if it was a merely "OK" weapon, It's still probably the most realistic choice due to the previously mentioned proximity of Belgium and lack of disruption it'd cause the production lines. If you ask me, the FN Browning is likely the best possible solution for France's HMG woes and Sweden proved (particularly with the tiny J 22) that it can be fitted to aircraft with relative ease.

 

[Shortround6]

From what I know about the Belgian Browning, it was an exceptional weapon. The Romanians considered it to be equal with the MG 151/20 which is extremely high praise for a heavy machine gun. It was much lighter than the AN/M2 (24.3 kg to 28 kg) and the rate of fire was amazing, however the main thing is the HE round developed for it that allowed it to punch well above its weight. Even if you were to crank down the fire rate to about 700~800 rpm, it'd still be a great weapon on its own. It also pairs quite well with the Hispano due to the muzzle velocities being similar.

It may have been good or better than most of the alternatives. Some of the attributes may be a bit exaggerated or best case. Now none of the heavy machine guns were perfect. They all traded certain attributes for other attributes. For a good guide see the Japanese type 3 13.2mm machine gun. Also basically an M2 Browning clone it was 28kg and fired at 800rpm.
It is also rated at about 790m/s velocity and used the slightly longer barrel length of the M2 instead of the Belgian/Swedish version. The 790m/s may be for the 52gram ball round and other/lighter bullets may be faster. With round being used in the Hotchkiss machine guns there was certainly a variety of rounds around the world.
I am rather dubious about about the HE round 'punching well above it's weight' as I have no good details about the round/s. One Swedish HE round didn't show up until 1944?
Romanian experience would have to be viewed with what ammo they were comparing it to from the MG 151/20 (how many Mine shells the Romanians got) and what they were expecting it to do. Some Soviet pilots preferred the 12.7mm machine guns to the 20mm ShVAK as the 12.7mm penetrated better. A problem with 12.7-15mm HE shells compared to 'normal' 20mm shells is that they contained around 10-30% of the explosive a 'normal' 20mm HE shell. and a lot of the small caliber HE shells tended to explode on the surface (so did most of the early 20mm shells) and hits were somewhat repairable on metal airplanes (fabric covered suffered worse).
The problem with evaluating the 12.7-13.2 mm guns is there are two classes. The British/Italian/Japanese 12.7 and German/Japanese 13mm in one class and the American .50cal. Soviet 12.7mm and Hochkiss 13.2mm gun in the other class.
The first class had muzzle energies of 9,600-10,600 Joules depending on cartridge, projectile and who loaded it.
The 2nd class had muzzle energies of 15,000-19,200 Joules depending on cartridge, projectile and who loaded it.
The 2nd class could depend on doing more damage with kinetic energy given a good location hit. Poor location hits with 0.8-2.0 grams of HE aren't go do a whole lot except make work for the repair crews.
There is also a difference in effect of HE and Incendiary.

13.2x99 Hotchkiss was everywhere in Europe, and France was already making the round in droves. Even if it was a merely "OK" weapon, It's still probably the most realistic choice due to the previously mentioned proximity of Belgium and lack of disruption it'd cause the production lines. If you ask me, the FN Browning is likely the best possible solution for France's HMG woes

The Belgian gun may have been as good or even a bit better than the US .50 cal. just a bit different. It was going to be the best bet for the French, much quicker and easier than trying to do anything with the Hotchkiss gun. D.520 (Q ?) with a belt fed 20mm and a 13.2mm gun in each wing?
The Belgian gun has got some problems with barrel life and perhaps with reliability/durability. It is up to the French and other users if it was a deal breaker or not. A number of other guns were used with not great reliably/durability.

 

[TM06]

I am rather dubious about about the HE round 'punching well above it's weight' as I have no good details about the round/s. One Swedish HE round didn't show up until 1944?
A problem with 12.7-15mm HE shells compared to 'normal' 20mm shells is that they contained around 10-30% of the explosive a 'normal' 20mm HE shell. and a lot of the small caliber HE shells tended to explode on the surface (so did most of the early 20mm shells) and hits were somewhat repairable on metal airplanes (fabric covered suffered worse).
The problem with evaluating the 12.7-13.2 mm guns is there are two classes. The British/Italian/Japanese 12.7 and German/Japanese 13mm in one class and the American .50cal. Soviet 12.7mm and Hochkiss 13.2mm gun in the other class.
The first class had muzzle energies of 9,600-10,600 Joules depending on cartridge, projectile and who loaded it.
The 2nd class had muzzle energies of 15,000-19,200 Joules depending on cartridge, projectile and who loaded it.
The 2nd class could depend on doing more damage with kinetic energy given a good location hit. Poor location hits with 0.8-2.0 grams of HE aren't go do a whole lot except make work for the repair crews.
There is also a difference in effect of HE and Incendiary.

I'm not entirely sure about the amount of filler the HE (HEF-T?) had, I was able to find some documents around the internet but my Swedish isn't good enough to make sense of them. The most I could glean was a cartridge weight of 42 grams and 1~2 grams of explosive filler seems like a reasonable estimate for that.
However it is worth noting that the opponents Vichy France would be primarily facing such as Russia had quite structurally weak planes around 1942~1943. Britain also had some weaker planes, IIRC the Spitfire had fabric control surfaces for a long while and the Hurricane was highly susceptible to bullet damage. Not sure about the hit-taking abilities of the Tornado and Typhoon though, nor the FAA aircraft like the Barracuda or Firefly.
Regardless, it is still a very powerful gun and the HE would likely help at tearing through wings or tails with a well-aimed burst.

Romanian experience would have to be viewed with what ammo they were comparing it to from the MG 151/20 (how many Mine shells the Romanians got) and what they were expecting it to do. Some Soviet pilots preferred the 12.7mm machine guns to the 20mm ShVAK as the 12.7mm penetrated better.

I suppose the Romanian rating is different depending on how highly one rates the MG 151/20 as a cannon both with and without Minengeschoß. The basics of the cannon (muzzle velocity, accuracy, weight, rate of fire) are still solid but not exceptional and well below the Hispano in most regards, Minengeschoß seems to be what pushed it over the edge.
But even still, for a heavy machine gun to be even remotely comparable to a 20 mm cannon is quite telling.

The Belgian gun may have been as good or even a bit better than the US .50 cal. just a bit different. It was going to be the best bet for the French, much quicker and easier than trying to do anything with the Hotchkiss gun. D.520 (Q ?) with a belt fed 20mm and a 13.2mm gun in each wing?
The Belgian gun has got some problems with barrel life and perhaps with reliability/durability. It is up to the French and other users if it was a deal breaker or not. A number of other guns were used with not great reliably/durability.

Personally, I'd go with 2 x 13.2 mm per wing for any fighter with less than 3 cannons.
The D.520 and VG.33 along with their derivatives have the advantage of the motorcannon, so they could afford to carry more guns in the wings (especially the D.520 with its planned reinforced wings). The M.B.157 is an overall more robust design so 4 x 13.2 mm guns isn't unreasonable either - although 2 x 13.2 mm would likely work fine with its double cannon setup. But when you get to the planned 3 x 20 mm cannon setups, dropping the MG count to 1 per wing or even removing them altogether would suffice due to the increased offensive capabilities of the triple cannon loadout. Italy followed a similar method with their fighters, notably with the G.55 (Sottoserie had 1 x 20 mm and 4 x 12.7 mm, Serie 1 had 3 x 20 mm and 2 x 12.7 mm, G.56 had 3 x 20 mm with no Breda-SAFATs).
Weight might be an issue, but if they could lighten the Hispanos (which seems highly likely due to the very light weight of the 405 - weight saving methods could likely be transferred over to the 404 or a hypothetical 406) it probably wouldn't be too heavy.

 

[bf109xxl]

Some Soviet pilots preferred the 12.7mm machine guns to the 20mm ShVAK as the 12.7mm penetrated better.

Any reference on primary sources would be appreciated - for both statements ("some pilots preferred" and "12.7 penetrated better").
After reading many memoirs and interviews of Soviet fighter pilots, I got the impression that they were quite satisfied with the ShVAK, and only very rarely was there an opinion that it would be better to put another UB instead of the ShVAK, but the reason was not the worse penetration, but the ammunition. At the same time, many pilots stated that they rarely used the entire ammunition of the ShVAK in aerial combat.