Made up/fictional "light" fighter (equal to an advanced Ki-61) for a made up/fictional Allied country.

[BarnOwlLover]

This is kind of what I've been working towards and thinking of for a while now. Let's say there's a fictional country that's aligned with the Allies, and is able to design and make their own fighter aircraft. Also, the country is relatively isolated from Axis attacks and is heavily defended against the attacks they can make.

Now that those conditions are out of the way, this country wants to make its own fighter aircraft and such to be self-sufficient in terms of defense. I'm thinking that this aircraft would be along the lines of an advanced version of the Ki-61 with advanced features. Or perhaps a lightweight version of the NA-73X (P-51 Mustang prototype) or even a Spitfire if R.J. Mitchell and especially Joe Smith could do it over again.

Basically, this aircraft was envisioned back in the late 1930s, with the expectation that it'd be 3-4 or so years before it'd be ready. Said country also has obtained a license to build Rolls-Royce Merlin engines with options of building other later designs (notably the Griffon), and it was envisioned to use a 1500+ derivative of the Merlin (namely the two-stage supercharged versions), which is why the 3-4 year wait period (getting 1500+ hp, and either good turbo-supercharging or two stage supercharging).

And this aircraft should have an advanced but relatively simple structure in terms of aerodynamics and build, similar to the aircraft that were mentioned. As a light fighter, it's sort of biased towards being an interceptor and air superiority fighter (short and medium range), but can do long range missions with some modifications.

Also, armament will be sort of a compromise in general purpose roles along the lines of later Italian fighers, namely 2 .50 MGs (400 rpg) and 2 20mm cannons (200 rpg), though this can be scaled up to 4 20mm cannons (150 or 200 rpg depending on role or mission).

Main performance specs are a top speed of over 400 mph, fast climb, excellent maneuverability, and ability for light/medium ground attack.

By say 1942 or shortly afterwards, would such a fighter be possible to make given what I've laid out?

 

[GrauGeist]

Italy built their own version of the P-35, the Reggiane Re.2000 series, which was compact yet well performing. Something along these lines, perhaps?

The KI-61 or perhaps He100 would be a good platform, but the cowling and framing are built around an inverted "V" inline.
Putting an upright "V" inline like a Merlin would create a bulged cowling like the Merlin powered Spanish Bf109s.

 

[z42]

By say 1942 or shortly afterwards, would such a fighter be possible to make given what I've laid out?

Technically, in a vacuum, sure, why not?

But I'm going to be a bore and criticize the setup of your alternative history scenario and why I don't find it very plausible. In the late 1930'ies, high end fighters were the rocket technology (or deep submicron semiconductor manufacturing, or whatever analogy you want to use) of the day. Making a world beating fighter is something banana republic dictators dream about, but it's just not realistic. You need an established aircraft design and manufacturing sector with decades of hard-won experience. And hence why all these WWII planes we remember (the 'good' ones, that is) came from countries where these preconditions where met.

So I think the best such a fictional Allied-aligned country could do would be to license produce an existing design; if they're already license producing the Merlin engine why not go all the way and license produce the rest of the plane as well (and what about the accessories like guns, radios, instruments etc.)? In fact, this would be an excellent way of establishing a domestic aircraft industry capable of producing high end aircraft.

As for which plane to license produce, I'd think the choice in the end would be based on which country and which company the fictional country manages to make a good deal with.

 

[BarnOwlLover]

Well, this country had been (though a small country) been making their own aircraft for years, and had been making high performance road and racing cars, but not aero engines of their own entire design or aircraft armament. The latter two were judged to be easier to license and sometimes develop from there than start 100% from scratch.

You do have to remember that the Soviets and Japanese licensed a lot of their engine technology and developed their own evolutions of it as well.

And this country is also no "banana republic". They were a respected democratic nation, though isolated from a lot of the political goings on in Europe (probably for the better all around, though their leaders and people highly disliked Nazi-ism, Fascism or Communism).

 

[Shortround6]

By say 1942 or shortly afterwards, would such a fighter be possible to make given what I've laid out?

No.

Let's say there's a fictional country that's aligned with the Allies, and is able to design and make their own fighter aircraft. Also, the country is relatively isolated from Axis attacks and is heavily defended against the attacks they can make.

Look at both Switzerland and Sweden and see what they were able to accomplish. You are going to need a much larger industrial base than either of them.

Basically, this aircraft was envisioned back in the late 1930s, with the expectation that it'd be 3-4 or so years before it'd be ready. Said country also has obtained a license to build Rolls-Royce Merlin engines with options of building other later designs (notably the Griffon), and it was envisioned to use a 1500+ derivative of the Merlin (namely the two-stage supercharged versions), which is why the 3-4 year wait period (getting 1500+ hp, and either good turbo-supercharging or two stage supercharging)

Here is where we need Dr. Who and the Tardis. The time line needs a lot of tweaking. You are planning of everything happening the way it did historically but there was both good planning and "luck" involved. RR knew that the basic Merlin could stand up to around 1800hp developed in the cylinders from the work done on the Speed Spitfire. But it ran on racing fuel, not avgas and it could use a supercharger set up for max power at sea level or darn close to it. To make 1500hp even in the mid/high teens you need avgas that didn't exist in 1938. again 100 octane was NOT 100/130 fuel. If your country gets it's 1938-39 sample fuel from the US and not the British you are going to be late getting the engine development where you want it.
The concept of a two stage supercharger is simple. The execution is not, The Merlin supercharger in 1938 was about the best in the world. RR hires Hooker and they just have him looking around to see what interests him. They did not assign him to supercharger work (P & W was working on two stage superchargers for several years before Hooker shows up at RR) right away. Hooker figures out that everybody was using faulty formulas to design superchargers. Just be redesigning the outer housing/inlet/carb mount he improves the Merlin supercharge by close to 30%. Trying to use two of the old style superchargers in series would not have worked well at all. The US ran into this with the Allison and the GE turbos. The GE turbos actually used compressors that weren't very good but the sheer power of the exhaust turbine overshadowed the poor compressor efficiency. Allison did pretty good with their own engine compressor but since everybody in the US had been using GE designed compressors (without the turbos) on their engines all the US makers were starting in a hole and had to dig themselves out. Using two 60% efficient impellers in series means you need a lot more intercooler (or water injection or.....) than if you use two 70% efficient compressors. A big limit on boost is the temperature of the intake charge.

It is sort of a chicken and egg thing. You don't need a good supercharger if you don't have good fuel and you won't develop good fuel unless you are using engines that use high boost. Hispano derived engines in three countries gained 150kg of weight (or more) when they tried to turn them into 1500-1600hp engines.
I would also note that having good race cars in the 1930s may not translate into good aircraft engines. Mercedes (DB) was running two stage superchargers on Grand Prix cars in 1939 and using 2.31 Atm of inlet pressure. The superchargers were Roots blowers and they were not using gasoline but a fuel that was about 86 methanol with about 3/4 other things mixed in. It may not hurt but there isn't much you can actually transfer over except creativity/problem solving.

armament will be sort of a compromise in general purpose roles along the lines of later Italian fighers, namely 2 .50 MGs (400 rpg) and 2 20mm cannons (200 rpg), though this can be scaled up to 4 20mm cannons (150 or 200 rpg depending on role or mission).

This is a problem with "what ifs", The goals are kind of fuzzy. It sounds reasonable but................
Italian fighter with two Italian 12.7mm mgs and two MG 151/20s.................................................142kg just for guns.
Japanese Army fighter with two 12.7mm mgs and two MG 151/20s.............................................132kg just for guns.
Japanese Army fighter with two 12.7mm mgs and two Ho-5............................................................120kg just for guns.
British/American fighter with two 12.7mm M2s and two 20mm Hispano....................................158kg just for guns (feeds not included)
British/American fighter with four 20mm Hispanos and 120rpg......................................................403kg.

The Italian/Japanese 12.7mm ammo was over 30% lighter than the US 12.7mm. The US case was heavier than the comparison of the projectile weights. US ammo was about 13.6kg per hundred rounds.
20mm Hispano ammo was about 28kg per hundred, or about 56kg for that "extra"50rpg.
German Mg 151/20 ammo was bit lighter around 21.5kg per hundred without links.
Japanese 20mm Ho-5 ammo was about the lightest 20mm ammo going. the cartridge case was sort of between the two (Mg 151 and the Hispano) but the projectile was only about 60% as heavy as a Hispano projectile.
I would note that the Japanese used 250rpg for the 12.7mm machine guns.

When you have pencils on paper the best fighter with 200kg of armament may be a bit different than the best fighter with 400kg of armament. The lower armament "ideal" fighter can use a bit smaller/lighter wing, perhaps lighter landing gear (even a few kg) or even a smaller tire size and so on. In peace time you can't do things that can you do in war time (use longer airfields, put ruts in the grass runways) relax "G" loading requirements by putting in the heavier guns and accepting a 7.6 G loading instead of the specified 8.0 for example.

 

[Shortround6]

making high performance road and racing cars, but not aero engines

Aircraft engines were the manufacturing equivalent of race car engines except you are trying make them, even for a small air force, at over a dozen month, maybe a dozen a week. Not two or 3 engines a month. You also don't build 27 liter V 12s on machinery that made 3-6 liters V-12s.

 

[BarnOwlLover]

Well, then what would be a good armament and such for a Merlin-powered GP fighter that can be an air superiority fighter and interceptor, and can handle ground attack missions? I don't really want it to be armed with rifle caliber weaponry, and at least 2 20mm cannons (4 if possible depending on mission) and have at least 650-700 miles range on internal fuel (can be boosted by adding fuel tanks like the Mustang or a diagram I saw on here for the Fw 190D).

Also, am I underestimating/overestimating firepower, namely in terms of ammo capacity? I know that planes like the Hellcat, Corsair and P-47 carried a lot of .50 ammo (at least 400 rpg for 6 or 8 MGs) and the Tempest V carried 200 rpg for it's 4 20mm cannons, but those were big planes powered by 2000+hp engines. Even most late war IJA fighters carried four guns (usually 2 12.7mm and 2 20mm, usually with 250-350 and 120-150 rpg respectively).

Also, the Fw-190 A8 carried 400 rpg for it's 13mm machine guns and 250 rounds for it's wing root cannons (I assume similar for the Fw-190 D?), but again, relatively big aircraft with a fairly powerful engine.

I'm sort of again turning to a fighter that combines attributes of the proposed Alf Faddy recon/fighter aircraft that he conceived at Supermarine, and a GP version of a proposed Miles high altitude interceptor.

I'm also thinking of coming up with a "heavy" fighter along the lines of the actual F.6/42, but with emphasis on being a "light" heavy fighter powered by a Rolls-Royce Griffon (or similar) engine.

 

[Shortround6]

For some of the gun questions see this picture.

The German 13mm (and few Japanese flexible guns) were the smallest, least powerful heavy machine guns. It meant you could carry more ammo
The 12.7 x 81SR was not only Japanese but Italian and without the rim it was the British .5 in machine gun used for the Navy AA guns and a few other small roles.
Note that the smallest 20mm rounds use about the same shell as the 20 x 110's (the RB was used in the Oerlikon guns), they just went a lot slower.
The Japanese 20 x 94 and the Russian 20 x 99 used significantly lighter shells.
The exception is the German 20mm "mine" shell used in the 20 x 80RB and the 20 X 82 ammo. The shell was light because of the thin wall construction that made more room for HE.
However to get the gun/s to function properly they had to use lighter than normal "regular" shell mixed with the mine shells. The "mine" shells could not use tracers.

The reason the US and British used big fighters to carry heavy armament is simple.
The Corsair carried just under 400lbs of .50 cal guns and "accessories". Careful, many sites list the weight of the bare gun. Wing guns need (most of the time) a charging system.
Now if you filled the ammunition boxes the Corsair was carrying just over 700lbs of ammo.
And you don't have the weight of the ammunition boxes/chutes or the brackets/supports that hold the guns or any reinforcement needed to handle the recoil.
The P-51 carried the guns, it left 140lbs of ammo on the truck because it wouldn't fit. That was worth 23 gallons of fuel.
P-40 carried the guns, it left another 140lbs of ammo on the truck.
P-40L strippers left out 2 guns and cut the ammo by 150lbs. (282lbs of ammo for four guns).

If you want performance with heavy armament you need big engines. Not just for speed but for climb and climb is not just pointing the nose up. At anything less than max speed it translates into excess power that can be used for maneuver.

There Ain't No Such Thing As A Free Lunch.

 

[BarnOwlLover]

Would having say 6 .50s but cutting the ammo cap per gun from 400 to say 300 or 270 rounds be a worth while weight savings?

I'll also edit the first post to give performance specs.

 

[BarnOwlLover]

One thought after reading about the Rolls-Royce FTB (that was intended to use parts of the P-51 Mustang as its basis, though it was also supposed to incorporate a Hawker Tempest tail unit ultimately). There was a version with the Griffon 60 series engine that was supposed to have a max take off weight (if built) of about 9450 lbs. But there was also supposed to be a Merlin powered version (using normal 60 series, boosted developmental 60 series and 100 series engines) that had a max take off weight (again, if built) of 8950 lbs. I believe based on the Key Aero article about it (which wasn't paywalled earlier but is now again) and info from Secret Projects, these were MTO weights for fighter variants.

That, of course, does line up very well with the XP-51F and XP-51G MTO's of 8800-9100 lbs. So would a Merlin 60 series engine be of worthwhile power for such a MTO (knowing that the actual normal take off weight is significantly lower)?

 

[Shortround6]

I have found some information on the Italian/Japanese 12.7mm ammo.
It is about 82 grams per round without links.
US .50 cal was about 117 Grams per round without links (US links are going to be a bit heavier as they had to fit around a bigger case).
Basically 250 rounds of Italian/Japanese ammo weighed about as much as 175 rounds of US ammo did and 375 rounds of the 12.7mm will weigh as much as 263 rounds of .50 cal.

Also some interesting progression of the Merlin engine, depending on fuel/engine life.
As mentioned earlier the Merlin was giving over 1800hp when being test for the Speed Spitfire in racing fuel (10 hours at 1600hp? or better)
Merlin III would give 1440hp at 5500ft at 16lb boost in Sea Hurricanes ( they figured the engines weren't going to last anyway)
The Merlin 45 would give 1515hp at 11,000ft at 16lbs of boost.
Merlin 50 would give 1470hp at 9500ft at 16lbs of boost (different carb)
Merlin 45/46 would give 1415hp at 14,000ft at 16lbs of boost (modified supercharger)
Merlin XX would give 1485hp at 6000ft at 14lbs and 1490hp at 12,500ft at 16lbs boost.
Merlin 24s have more power at lower altitudes using 18lbs (they don't flow more air at higher altitudes)
The Merlin 61 gave 1565hp at 11,500ft and 1390hp at 23,500ft both at 16lbs of boost.
You could get 1400-1500hp out of the Merlin once you had good fuel and you were willing to accept lower engine life and lower altitude.
Most of the changes to Merlin were in better superchargers and tweaking it for longer life at the higher powers.
In some cases the newer superchargers came along before the boost limits were changed or before some of the final changes to parts were made. They beefed up the supercharger drive to handle more power to get to the higher boost limits on the single stage engines. Remember that it took hundreds of HP to drive the supercharger and the drive shaft intended for the Merlin III and other early engines of 6-10lbs boost had trouble with loads that 15-18lb boost needed.

 

[Howard Gibson]

This is kind of what I've been working towards and thinking of for a while now. Let's say there's a fictional country that's aligned with the Allies, and is able to design and make their own fighter aircraft. Also, the country is relatively isolated from Axis attacks and is heavily defended against the attacks they can make.

Now that those conditions are out of the way, this country wants to make its own fighter aircraft and such to be self-sufficient in terms of defense. I'm thinking that this aircraft would be along the lines of an advanced version of the Ki-61 with advanced features. Or perhaps a lightweight version of the NA-73X (P-51 Mustang prototype) or even a Spitfire if R.J. Mitchell and especially Joe Smith could do it over again.

Basically, this aircraft was envisioned back in the late 1930s, with the expectation that it'd be 3-4 or so years before it'd be ready. Said country also has obtained a license to build Rolls-Royce Merlin engines with options of building other later designs (notably the Griffon), and it was envisioned to use a 1500+ derivative of the Merlin (namely the two-stage supercharged versions), which is why the 3-4 year wait period (getting 1500+ hp, and either good turbo-supercharging or two stage supercharging).

And this aircraft should have an advanced but relatively simple structure in terms of aerodynamics and build, similar to the aircraft that were mentioned. As a light fighter, it's sort of biased towards being an interceptor and air superiority fighter (short and medium range), but can do long range missions with some modifications.

Also, armament will be sort of a compromise in general purpose roles along the lines of later Italian fighers, namely 2 .50 MGs (400 rpg) and 2 20mm cannons (200 rpg), though this can be scaled up to 4 20mm cannons (150 or 200 rpg depending on role or mission).

Main performance specs are a top speed of over 400 mph, fast climb, excellent maneuverability, and ability for light/medium ground attack.

By say 1942 or shortly afterwards, would such a fighter be possible to make given what I've laid out?

Would Great Britain allow license production of Merlin engines? Would they tell mysterious, shifty damn foreigners all about their latest supercharger developments? Was Rolls Royce even working on two stage superchargers in 1938?

Read up on the Ambrosini SAI.403. This is supposed to be a cheap, lightweight fighter aircraft. It is credited with exceeding 400mph, but I wonder if it had armament and armour. It sounds a lot like a Spitfire_I.

Lightweight fighters did not do well in WWII. Bigger, faster, better armed and more rugged fighters used hit and run tactics against smaller, more maneuverable adversaries.

 

[Shortround6]

Read up on the Ambrosini SAI.403. This is supposed to be a cheap, lightweight fighter aircraft. It is credited with exceeding 400mph, but I wonder if it had armament and armour. It sounds a lot like a Spitfire_I.

• Guns:
• 2× fuselage-mounted 12.7 mm (0.500 in) Breda-SAFAT machine guns - (Dardo-A) light defense interceptor, 5459lbs, 350rpg 300 liters of fuel (OK it was a 750 hp engine)
• 2× fuselage-mounted 12.7 mm (0.500 in) Breda-SAFAT machine guns and 2× wing-mounted 15 mm (0.591 in) MG 151/15 cannon or 20 mm (0.787 in) MG 151/20 cannon - (Dardo-B), General purpose fighter, 5820lbs, 200 rpg of 12.7mm ammo and 200 rpg for the wing cannon
• 2× wing-mounted 20 mm (0.787 in) MG 151/20 cannon - (Dardo-C), long range fighter with the internal fuel raised to 90 imp gal (410 L) and two 150 liter drop tanks. 200rpg of cannon ammo.

One does wonder what they did to get the engine to make the rated power so much higher than older versions of the engine.

Ambrosini SAI.207 - Wikipedia

en.wikipedia.org

The early version had some serious flaws.

 

[tomo pauk]

Read up on the Ambrosini SAI.403. This is supposed to be a cheap, lightweight fighter aircraft. It is credited with exceeding 400mph, but I wonder if it had armament and armour. It sounds a lot like a Spitfire_I.

Whoever was crediting it, he was probably drinking too much of Kool-aid. Italians themselves back in ww2 said that 575 km/h (~360 mph) is the top speed.
see here

One does wonder what they did to get the engine to make the rated power so much higher than older versions of the engine.

Better supercharger?

 

[Shortround6]

Better supercharger?

So far no details, it seems to a a two speed engine (at least it has two altitudes in the engine designation.) But it is making as much power at 6000 meter as the old ones made at 4,000 meters? S.A.I. 403s top speed is usually given at 23,620ft (which is also looking like very good RAM effect) while the S.A.I. 207 was at 14,765ft which is pretty good for a 4,000 meter FTH engine.

 

[tomo pauk]

So far no details, it seems to a a two speed engine (at least it has two altitudes in the engine designation.) But it is making as much power at 6000 meter as the old ones made at 4,000 meters?

There was a host of engines that upped the rated height by 2000m during the life time, be it by employing more refined S/Cs, bigger impellers, faster-turning impellers better air flow towards the S/C, or a combination. Increase of engine RPM by a few % also helps.

S.A.I. 403s top speed is usually given at 23,620ft (which is also looking like very good RAM effect) while the S.A.I. 207 was at 14,765ft which is pretty good for a 4,000 meter FTH engine.

Italian doc that I've posted link above talks about the 207 (not 403, as I've claimed above - my bad). It gives 4000 m as no-ram rated altitude, and max speed of 575 km/h attained at 5500m (ie. rated height with ram) by the prototype. 1500m of difference is a lot between non-ram and with_ram, but not unheard of.

At any rate, I still don't believe the ~400 mph figure for the 403, not just because the books Wikipedia uses as source credit also the 207 with ~40 mph more than the wartime docs are stating.