Advanced French Fighters vs 1942/1943 contemporaries

[Bretoal2]

Do you have any information about this 12B engine? I can't seem to find anything online about it other than that it existed.

Here - from Flight....

 

[TM06]

Here - from Flight....

About 1,750 horsepower at ~4,000 meters and 2,200 horsepower for takeoff, same for the Type 40 all the way until ~8,500 meters, and at only 900 kilos! That's quite the capable engine there! I'm salivating to imagine what that would do in the frame of a D.551 or VG.39/VG.60.
But that's well beyond the scope of this discussion - although I definitely wouldn't mind if there was any information regarding the planes that used it.

 

[Bretoal2]

About 1,750 horsepower at ~4,000 meters and 2,200 horsepower for takeoff, same for the Type 40 all the way until ~8,500 meters, and at only 900 kilos! That's quite the capable engine there! I'm salivating to imagine what that would do in the frame of a D.551 or VG.39/VG.60.
But that's well beyond the scope of this discussion - although I definitely wouldn't mind if there was any information regarding the planes that used it.

From Emmanuel Lage, five HS 12B prototypes were built, but the engine never flew. It was approved after type-tests in 1951.

 

[TM06]

This topic got a bit sidetracked into engine discussion, so I decided to add a few things to specify exactly what I meant in this post. Of course, the discussion is appreciated nonetheless!

 

[Bretoal2]

This topic got a bit sidetracked into engine discussion, so I decided to add a few things to specify exactly what I meant in this post. Of course, the discussion is appreciated nonetheless!

Another weakness of French fighters were the 7.5 mm machine guns, whose bullets were too light and lost their effectiveness at short distances.

From this point of view, very good machines were the Bloch 155 series (well developed in the spring of 1940) or 157 (released later): with their two HS 404 cannons which could be belt-fed. Their firepower could be remarkable.

And don't forget the "heavy fighter" Sud-Est 100, which carried nine HS 404s...

 

[TM06]

Another weakness of French fighters were the 7.5 mm machine guns, whose bullets were too light and lost their effectiveness at short distances.

The French 7.5 mm MAC was a solid gun for its calibre; compact, reliable, good muzzle velocity, blistering fire rate of about 1,000~1,200 rounds per minute and an absurd ammo capacity of 300~500 rounds per gun. But it did lack the hitting power of even other rifle calibre guns owing to its smaller cartridge, and this was around the time when they started to shift to heavier calibres.
I can see 3 paths that France ends up taking.
A) They ditch the MG's entirely and go for a full-cannon setup.
B) Hotchkiss produces the belt-fed version of their 13.2 mm gun and that gets accepted as an aircraft HMG.
C) M2 Browning's are supplied via lend-lease similar to how it was with Britain.
The M2 seems the most likely, as France had aircraft that were planned to be armed with them (SE.580, VG.60) or were armed with them (VB.10-02).

From this point of view, very good machines were the Bloch 155 series (well developed in the spring of 1940) or 157 (released later): with their two HS 404 cannons which could be belt-fed. Their firepower could be remarkable.

According to Replic'Air, the D.551 and SE.520Z were planned to have 3 cannons (one motorcannon and one in each wing) and 4 machine guns (2 per wing), so that would make a very solid armament platform to work with.

 

[Shortround6]

I can see 3 paths that France ends up taking.
A) They ditch the MG's entirely and go for a full-cannon setup.
B) Hotchkiss produces the belt-fed version of their 13.2 mm gun and that gets accepted as an aircraft HMG.
C) M2 Browning's are supplied via lend-lease similar to how it was with Britain.
The M2 seems the most likely, as France had aircraft that were planned to be armed with them (SE.580, VG.60) or were armed with them (VB.10-02).

1st path, keep the 7.5 machine guns, In combination with the 20mm guns.

France was going to have major production issues. What they could build was not always what they wanted.
which leads to the elimination of the Hotchkiss 13.2mm gun as an aircraft weapon. It is heavy and slow firing. Both things might have been able to be fixed but time was not on the side of the French. They needed stuff in 1940-41 in this scenario, not holding the line with 1940 aircraft and guns while they wait for better stuff in 1942/43. Light weight/faster firing 13.2mm guns are going to take time.
This also takes the M2 Browning off the table in 1940-41-42. Not enough of them to go around. Late 42 or early 43 maybe. But you have to design the new airplanes in 1940-41 when the gun/ammo supply is a lot iffier.

The French had the design for the belt feed for the 20mm guns in the summer of 1940. It could have showed up much sooner than the British used it (they spent a lot time redesigning it only to return to near the original).
French, like some other countries, were more than a little optimistic about some the gun batteries in some of their proposed fighters. Climb and turn suffer more from heavy gun batteries more than speed. Also assumes you can build the specified guns.

Now maybe in this scenario the Germans flub things so bad that part of Belgium with the FN factory stays in allied hands. And survives bombing by the Luftwaffe.

French were already going in too many directions at once with weapons and aircraft and engines.

French also don't have to shoot down much in the way of 4 engine bombers.

French aircraft engines were not going to support three 20mm guns plus four 7.5mm machine guns. The Hispano gun weighs almost 50% more than the German MG151/20.
Trying to fit 50% heaver 20mm battery plus 100% heavier 7.5/7.9mm battery than a 109F or 109G "gunboat" using similar power engines doesn't seem like a good idea.

 

[Bretoal2]

1st path, keep the 7.5 machine guns, In combination with the 20mm guns.

France was going to have major production issues. What they could build was not always what they wanted.
which leads to the elimination of the Hotchkiss 13.2mm gun as an aircraft weapon. It is heavy and slow firing. Both things might have been able to be fixed but time was not on the side of the French. They needed stuff in 1940-41 in this scenario, not holding the line with 1940 aircraft and guns while they wait for better stuff in 1942/43. Light weight/faster firing 13.2mm guns are going to take time.
This also takes the M2 Browning off the table in 1940-41-42. Not enough of them to go around. Late 42 or early 43 maybe. But you have to design the new airplanes in 1940-41 when the gun/ammo supply is a lot iffier.

The French had the design for the belt feed for the 20mm guns in the summer of 1940. It could have showed up much sooner than the British used it (they spent a lot time redesigning it only to return to near the original).
French, like some other countries, were more than a little optimistic about some the gun batteries in some of their proposed fighters. Climb and turn suffer more from heavy gun batteries more than speed. Also assumes you can build the specified guns.

Now maybe in this scenario the Germans flub things so bad that part of Belgium with the FN factory stays in allied hands. And survives bombing by the Luftwaffe.

French were already going in too many directions at once with weapons and aircraft and engines.

French also don't have to shoot down much in the way of 4 engine bombers.

French aircraft engines were not going to support three 20mm guns plus four 7.5mm machine guns. The Hispano gun weighs almost 50% more than the German MG151/20.
Trying to fit 50% heaver 20mm battery plus 100% heavier 7.5/7.9mm battery than a 109F or 109G "gunboat" using similar power engines doesn't seem like a good idea.

Three points :

1) The French aeronautical industry sought in 1940 to solve their issues caused by the aberrant management of programs and orders (1931 - 1936) and political instability (1936 - 1938) and nationalizations. And this in all areas: airplanes, engines, weapons. Hence the impression of "dispersion". In fact, it is very difficult to "guess" what would have happened if the collapse of June 1940 had not occurred.

2) More technically : yes, it is not certain that the lightened wing of the Dewoitine 551 would support two HS 404s. On the other hand, no questions for the Bloch 157 owing to its 152 ancestry.

3) Finally, we must keep in mind that in 1939 - 1940, the Allies did not have a real technical cooperation program, each country thinking first 100 % "national". The purchase of American equipment raised a storm of protests in the Assemblée Nationale, and also ruined the French trade balance as US planes were very expensive - much more than french ones. Also, attempts to produce English engines (Merlin and Hercules) in France were more or less sneakily opposed by French manufacturers, Gnome-Rhône in particular.

 

[Frog]

Three points :

1) The French aeronautical industry sought in 1940 to solve their issues caused by the aberrant management of programs and orders (1931 - 1936) and political instability (1936 - 1938) and nationalizations. And this in all areas: airplanes, engines, weapons. Hence the impression of "dispersion". In fact, it is very difficult to "guess" what would have happened if the collapse of June 1940 had not occurred.

2) More technically : yes, it is not certain that the lightened wing of the Dewoitine 551 would support two HS 404s. On the other hand, no questions for the Bloch 157 owing to its 152 ancestry.

3) Finally, we must keep in mind that in 1939 - 1940, the Allies did not have a real technical cooperation program, each country thinking first 100 % "national". The purchase of American equipment raised a storm of protests in the Assemblée Nationale, and also ruined the French trade balance as US planes were very expensive - much more than french ones. Also, attempts to produce English engines (Merlin and Hercules) in France were more or less sneakily opposed by French manufacturers, Gnome-Rhône in particular.

In 1940 it was called the Chambre des Députés. Postponed in 1940, it became Assemblée Nationale in 1946.

 

[Shortround6]

In fact, it is very difficult to "guess" what would have happened if the collapse of June 1940 had not occurred.

Very true, for instance there would have been no BoB, or it would have been much reduced as the the Germans concentrated on trying to finish off France. And that cascades into What Italy does, or does not do in North Africa, and the invasion (or lack of) of Russia in 1941 and that flows into of Japan, faced with a less distracted Britain and America attempts their conquest of Asia. On a local level, inside of France, what the disruption of French Industry would have been depends on how far the Germans got and how successful the German bombers were at attacking French factories.

More technically : yes, it is not certain that the lightened wing of the Dewoitine 551 would support two HS 404s. On the other hand, no questions for the Bloch 157 owing to its 152 ancestry.

Not sure were this came from. I was referring to the German DB 601/605 engines not having the power to carry the 5 gun armament (3 three MG 151s and two mgs) of the 1941-43 Bf 109s. Beefing up wings may be (or many not?) fairly easy compared to getting more power from the engines. Having the wings not break is not the same as retaining maneuverability as armament loads increase.

3) Finally, we must keep in mind that in 1939 - 1940, the Allies did not have a real technical cooperation program, each country thinking first 100 % "national". The purchase of American equipment raised a storm of protests in the Assemblée Nationale, and also ruined the French trade balance as US planes were very expensive - much more than french ones. Also, attempts to produce English engines (Merlin and Hercules) in France were more or less sneakily opposed by French manufacturers, Gnome-Rhône in particular.

French were also playing games when it came to currency exchange rates. Made imports expensive and exports cheap for other countries. Also distorts actual French costs of domestic French products.
French may have lucked out on not getting the Hercules to manufacture in 1939/40. Trying to tool up a factory in France when Bristol could not figure out how the make the thing in quantity was not going to end well for the French. Bristol did figure it out, but not until 1941(?). Now considering the lack of cooperation between Bristol and Napier on sleeve valves one is inclined to wonder how forthcoming Bristol would have been to the French.

For there to be advanced French fighters of 1942-43 France not only has to not collapse in June of 1940, it has to survive the rest of 1940 and all of 1941. Germany is not going to say "oh, well" in the Spring of 1941 and dig a trench across France, man it with 2nd line troops, and march off to invade Russia (Stalin might attack Germany?)

 

[Shortround6]

As a follow on to the US .50 cal machine gun problems for use in Britain/France in 1940/41.

US Production of .50 cal machine guns of all types (water cooled and air cooled AA guns, etc)

1940.....................5155
1941..................49,479
1942................347,492
1943................641,638
1944................674,011
1945................239,891

It can take 1 to 2 years to get an existing gun into production in a new factory.
By the time of Pearl Harbor US Ordnance already had contracts in place for .30 cal and .50 cal MG production with 10 factories (some working for the British)

 

[yulzari]

Also, attempts to produce English engines (Merlin and Hercules) in France were more or less sneakily opposed by French manufacturers, Gnome-Rhône in particular.

British engines. The Rolls Royce Hillington shadow factory in Hillington near Glasgow was to be the model for the French Merlin factory as it was being built from scratch as a mass production facility.

 

[TM06]

1st path, keep the 7.5 machine guns, In combination with the 20mm guns.

France was going to have major production issues. What they could build was not always what they wanted.
which leads to the elimination of the Hotchkiss 13.2mm gun as an aircraft weapon. It is heavy and slow firing. Both things might have been able to be fixed but time was not on the side of the French. They needed stuff in 1940-41 in this scenario, not holding the line with 1940 aircraft and guns while they wait for better stuff in 1942/43. Light weight/faster firing 13.2mm guns are going to take time.
This also takes the M2 Browning off the table in 1940-41-42. Not enough of them to go around. Late 42 or early 43 maybe. But you have to design the new airplanes in 1940-41 when the gun/ammo supply is a lot iffier.

The French had the design for the belt feed for the 20mm guns in the summer of 1940. It could have showed up much sooner than the British used it (they spent a lot time redesigning it only to return to near the original).
French, like some other countries, were more than a little optimistic about some the gun batteries in some of their proposed fighters. Climb and turn suffer more from heavy gun batteries more than speed. Also assumes you can build the specified guns.

Now maybe in this scenario the Germans flub things so bad that part of Belgium with the FN factory stays in allied hands. And survives bombing by the Luftwaffe.

French were already going in too many directions at once with weapons and aircraft and engines.

French also don't have to shoot down much in the way of 4 engine bombers.

French aircraft engines were not going to support three 20mm guns plus four 7.5mm machine guns. The Hispano gun weighs almost 50% more than the German MG151/20.
Trying to fit 50% heaver 20mm battery plus 100% heavier 7.5/7.9mm battery than a 109F or 109G "gunboat" using similar power engines doesn't seem like a good idea.

I think you're overestimating the weight and size of the MAC M39 guns. A single MAC M39 was only 10.5~10.8 kilograms and a full ammo belt of 500 rounds would be around 4.5 kilograms, for a total of 15 kilograms per gun. The MG 17 was its closest comparison at 10.2 kilograms with a full ammo belt of 500 rounds being around 5 kilograms, which is almost exactly the same weight as the MAC at 15.2 kilograms. Four of those is only 60.8 kilograms, about 10 kilograms more than a single Hispano 404.
The length is also much shorter despite its longer barrel, 1.02 metres versus 1.175 metres.
The D.551's loaded weight was reportedly ~2,150 kg with one cannon and 6 machine guns. Drop two of the machine guns (-30 kilograms) and add two cannons and their 60 round drums (+100 kilograms per gun, +30.84 kilograms) and you get effectively a net increased weight of 100.84 kilograms to a grand total of 2,250 kilograms.
This is still lighter than the loaded Bf 109 G-6 (an aircraft designed for lightness) with only its basic layout (single 20 mm cannon and 2 13 mm guns) at 3,150 kilograms - a difference of 900 kilograms.

 

[Shortround6]

The D.551's loaded weight was reportedly ~2,150 kg with one cannon and 6 machine guns.

D-520 was 2677kg, with one cannon, 4 MGs and a lighter engine.

D-551 was constructed of unobtainium? Painted with anti-gravity paint?

They made the wing smaller? The entire wing of a P-40C doesn't weigh 500kg.
They didn't put in as much fuel? The D-520 only held 400L (?) in the main tank.

109 had pilot armor, BP glass in the windscreen (small) and some sort of self-sealing tank.
The D-551 had?????

The reported weight for the D.551 is about the weight of a 109B-2 with no protection, a Jumo 210 engine and a two blade prop and 270 liters of fuel.
Were Willy and boys that bad or were the French that good or somebody made a typo?

Potez 230. 1800kg, (no armament?) 118 sqft of wing. Hispano X engine. (670hp ?) 380 kg instead of the 480kg of a late model 12Y engine.

You want a 1942-43 fighter with standard "stuff" of the day (protection, radio equipment, etc) and roughly 400 liters of fuel (300kg) you pay the price.

Trying to turn race planes into working fighters usually doesn't end well.

 

[TM06]

D-520 was 2677kg, with one cannon, 4 MGs and a lighter engine.

D-551 was constructed of unobtainium? Painted with anti-gravity paint?

They made the wing smaller? The entire wing of a P-40C doesn't weigh 500kg.
They didn't put in as much fuel? The D-520 only held 400L (?) in the main tank.

109 had pilot armor, BP glass in the windscreen (small) and some sort of self-sealing tank.
The D-551 had?????

The reported weight for the D.551 is about the weight of a 109B-2 with no protection, a Jumo 210 engine and a two blade prop and 270 liters of fuel.
Were Willy and boys that bad or were the French that good or somebody made a typo?

View attachment 779257
Potez 230. 1800kg, (no armament?) 118 sqft of wing. Hispano X engine. (670hp ?) 380 kg instead of the 480kg of a late model 12Y engine.

You want a 1942-43 fighter with standard "stuff" of the day (protection, radio equipment, etc) and roughly 400 liters of fuel (300kg) you pay the price.

Trying to turn race planes into working fighters usually doesn't end well.

I'm simply repeating what I've read from Danel, Cuny, and Marcel Doret (test pilot of Dewoitine) along with Replic'Air, I do not know all the specifications or reasoning behind its light weight. Again, I'm relatively new when it comes to this aspect of historical aircraft and lack all the knowledge when it comes to building planes. There were a few fighters around that did weigh similarly, chiefly the Yak-1 and Yak-3, the latter of which also had a 3 x 20 cannon layout in its Yak-3P variant.
As far as I know, the D.551 was fitted with the same fuel storage as the D.520; a ~400 gallon fuel tank between the engine and cockpit along with the two wing fuel tanks combining to ~240 gallons.
But I also am aware that there was no armour or military equipment beyond the armament that was fitted to the D.551. A fully kitted D.551 with armour, radio and the like would've weighed a great deal more.
However I do stand by my statement that the 3 x 20 mm + 4 x 7.5 mm loadout is likely feasible for the aircraft, especially the D.554 that was to be fitted with the 12Z.

 

[Shortround6]

A few points. The Yak-3P used the Berezin B-20 20 cannon which was enough lighter that 3 of them weighed about the same as two ShVAK cannon. But those cannon didn't show up until 1944 in small numbers, a lot more later. In fact most Yak-3s used the B-20 cannon which saved about 17kg over the ShVAK cannon and about 35kg over the Hispano gun. Three of the Light soviet cannon are about 75kg, single French Hispano was 60kg.
A standard Yak-3 weighed about 2700 kg. Weight did fluctuate a little bit, There were several hundred early ones built with one light 20mm gun and a single 12.7mm MG (25kg) so watch out for any really light ones.
Yak-3s often carried about 370 liters of fuel. Granted their beefed up Hispano engines were heavier than French Hispano engines.

Small wings helped speed, they were not so good for wing loading and this is not just turning, it also has impact on take-off and landing runs.

I have real doubts about 12Z engine. It just falls outside of the norms for power to weight and for power at altitude in some sources. The Swiss had real problems with the 12Y-51 and the resulting YS-2 was beefed up a lot. Changing from 2400rpm to 2500rpm was a problem and going to 2600-2700rpm was harder. Getting to 2800rpm was more of the same.

RR cheated. Once they had 2750 rpm with the Griffon they stayed there and just kept raising the boost. You might have to make parts stronger but you don't change the vibration patterns of the crankshaft like changing rpm does. changing the Cylinder pressure is pretty much linier. Changing the Pressure by 10% means 10% more force acting on things/parts.
Changing RPM changes forces by the square of the speed. 10% change in rpm changes the forces by 21%. also increases the Friction by 21%. Not saying you can't do it, but it takes work. It also usually requires changes in weight more in line with the forces involved rather than the percentage of the speed of the rpm.
Of course it also depends on what engine life you are willing to accept. Semi-disposable engines let you get away with a lot, assuming you have a large supply of spare engines.

 

[TM06]

A few points. The Yak-3P used the Berezin B-20 20 cannon which was enough lighter that 3 of them weighed about the same as two ShVAK cannon. But those cannon didn't show up until 1944 in small numbers, a lot more later. In fact most Yak-3s used the B-20 cannon which saved about 17kg over the ShVAK cannon and about 35kg over the Hispano gun. Three of the Light soviet cannon are about 75kg, single French Hispano was 60kg.
A standard Yak-3 weighed about 2700 kg. Weight did fluctuate a little bit, There were several hundred early ones built with one light 20mm gun and a single 12.7mm MG (25kg) so watch out for any really light ones.
Yak-3s often carried about 370 liters of fuel. Granted their beefed up Hispano engines were heavier than French Hispano engines.

Small wings helped speed, they were not so good for wing loading and this is not just turning, it also has impact on take-off and landing runs.

The Yak-3 and its variants are the closest analogues to the D.55x series due to how similar they are, and can likely be used as a benchmark for performance.
The Yak-3 used a ~575 kg VK-105PF3 engine making about 1,200~1,300 horsepower nominally, the D.551 used a ~490 kg Hispano 12Y-51 making ~1,100 horsepower nominally, the former being a derivative of the base design of the latter, both of which are noted for relative unreliability.
Their wings were somewhat comparable in length, thickness and size - as far as I can tell comparing blueprints.
A clean base Yak-3 could reach a top speed of ~650 km/h (403 mp/h) at 4,000 m with a climb rate of 5,000 m in 4:30 (~21.3 m/s), a clean D.551 could reach a top speed of 662 km/h (411 mp/h) at 6,000 m with a climb rate of 6,000 m in 5:08 (19.5 m/s). Slightly higher top speed but slightly lower climb rate.
The base Yak-3 used an armament layout of 1 x 20 mm cannon and 2 x 12.7 mm heavy machine guns, the D.551 used an armament layout of 1 x 20 mm cannon and 6 x 7.5 mm machine guns which are similar in terms of burst mass (1 12.7 machine gun was worth 3 rifle calibre machine guns according to US testing).
As for fuel capacity, the two are still similar when the wing fuel tanks on the D.551 are removed, with the D.551 having a 400 litre fuel tank and the Yak-3 having a similarly sized tank. However with its wing fuel tanks giving it an extra 240 litres for capacity, the D.551 has the edge in endurance and range. Neither of which entirely need to be particularly long ranged due to their proximity to Germany - their main target.
Given how similar these aircraft are, I'm going to take a stab and say that a fully combat loaded D.551 would be a similar weight to a fully combat loaded Yak-3, likely in the 2,500~2,700 kg range.
The D.554 variant was going to use the 1,400 hp (theoretical max of 1,800 hp) 12Z engine which is very similar in design to the 1,750 hp VK-107 used on the Yak-3M. Given the similarities between the two aircraft in their base forms, the D.554 and Yak-3M would likely be comparable in performance as well.

have real doubts about 12Z engine. It just falls outside of the norms for power to weight and for power at altitude in some sources. The Swiss had real problems with the 12Y-51 and the resulting YS-2 was beefed up a lot. Changing from 2400rpm to 2500rpm was a problem and going to 2600-2700rpm was harder. Getting to 2800rpm was more of the same.

Your apprehension towards the 12Z is understandable given that its life was cut short due to the circumstances around it, however I should note again that the VK-107 engine is highly similar to the 12Z engine, meaning it can be used for comparison.
The VK-107A engine that lacked water-injection made around 1,500 hp - only a little bit more than the 12ZTer at 1,400 hp - and both of these engines ran at 2,800 rpm. Had it been allowed to mature, I'm very confident in saying that the 12Z could've been similar in power to the VK-107B or even the VK-108.
In fact, the Hispano 12B proved that in this very thread by being capable of 1,750 hp at 4,000 m/8,100 m. This is purely speculation on my part, but had France been allowed to continue developing its engines and aircraft, I could see the 12B or an engine like it seeing service in 1944~1945 as the war delayed France's engine development by about 4~5 years.
They probably still wouldn't be the most reliable engines out there, but every engine has its flaws.

RR cheated. Once they had 2750 rpm with the Griffon they stayed there and just kept raising the boost. You might have to make parts stronger but you don't change the vibration patterns of the crankshaft like changing rpm does. changing the Cylinder pressure is pretty much linier. Changing the Pressure by 10% means 10% more force acting on things/parts.
Changing RPM changes forces by the square of the speed. 10% change in rpm changes the forces by 21%. also increases the Friction by 21%. Not saying you can't do it, but it takes work. It also usually requires changes in weight more in line with the forces involved rather than the percentage of the speed of the rpm.
Of course it also depends on what engine life you are willing to accept. Semi-disposable engines let you get away with a lot, assuming you have a large supply of spare engines.

Given the close proximity of its primary target, France would likely take a similar path as Russia again here, taking engines with short service lives and making a large amount of them. Although France's industrial base was still recovering from a major reshuffle and nationalization, it was still the 4th largest industrial base in the world at the time. If Britain could ramp up its production to the point it did in the time it did, France could very likely do the same by 1942, especially regarding Hispano-Suiza engines due to the multitude of factories in Spain.

Trying to turn race planes into working fighters usually doesn't end well.

Didn't get around to this last time, but that's not exactly a fair description of the D.55x series.
While yes, the D.550 was a speed-record plane, it was based on the airframe of the D.520 - a dedicated fighter aircraft. If anything, the D.55x series is a return to form and the D.550 is the odd-one-out.

 

[tomo pauk]

As far as I know, the D.551 was fitted with the same fuel storage as the D.520; a ~400 gallon fuel tank between the engine and cockpit along with the two wing fuel tanks combining to ~240 gallons.

You probably mean 'liters', not 'gallons'
The 551 was limited to 400 (410?) L of fuel, while the 520 carried more. 551 was also with a smaller wing, that was also lighter by ?? kg.

link

But I also am aware that there was no armour or military equipment beyond the armament that was fitted to the D.551. A fully kitted D.551 with armour, radio and the like would've weighed a great deal more.

Armament fitted was 5 LMGs.

However I do stand by my statement that the 3 x 20 mm + 4 x 7.5 mm loadout is likely feasible for the aircraft, especially the D.554 that was to be fitted with the 12Z.

French don't need to bother with LMGs if 3 cannons are fitted, especially the belt-fed versions (that we can expect with France still in the fight) with it's heavy ammo loadout. Doubly so considering the high wing loading that D.551 will feature as a combat-worthy aircraft, ie. featuring protection, radios and (very) heavy firepower.

 

[TM06]

You probably mean 'liters', not 'gallons'
The 551 was limited to 400 (410?) L of fuel, while the 520 carried more. 551 was also with a smaller wing, that was also lighter by ?? kg.

link

My mistake, liquid measurements are definitely not my strong suit.
As for the wing itself, I do not have specifics on it but I do know that the D.551 was lighter than the D.520 by around 300~400 kg.

French don't need to bother with LMGs if 3 cannons are fitted, especially the belt-fed versions (that we can expect with France still in the fight) with it's heavy ammo loadout. Doubly so considering the high wing loading that D.551 will feature as a combat-worthy aircraft, ie. featuring protection, radios and (very) heavy firepower.

I was actually getting around to that but the discussion derailed. The 3 x 20 mm and 4 x 7.5 mm was the heaviest layout studied for the D.55x series, but it was not the only one they planned. The layouts I learned of via Replic'Air were as follows:
1 x 7.5 mm machine gun in the nose + 6 x 7.5 mm machine guns in the wings
1 x 20 mm cannon in the nose + 4 x 7.5 mm machine guns in the wings
1 x 20 mm cannon in the nose + 6 x 7.5 mm machine guns in the wings
1 x 20 mm cannon in the nose + 2 x 20 mm cannons in the wings
1 x 20 mm cannon in the nose + 2 x 20 mm cannons in the wings + 4 x 7.5 mm machine guns in the wings.
If I recall correctly, the triple cannon setup was the most desired but the 1 x 20 + 6 x 7.5 layout was the first one planned to be fitted.

 

[tomo pauk]

As for the wing itself, I do not have specifics on it but I do know that the D.551 was lighter than the D.520 by around 300~400 kg.

300-400 kg? That was probably the weight of the whole wing?
Small wing of the D.550, that also had no support for the tanks and weapons, was lighter by 140 kg than the bigger wing of the D.520 that supported also the tanks and firepower.