XP-65/F7F Development

[Zipper730]

I was thinking about a couple of things regarding the XP-65 and the later F7F: I'm not sure if my timeline is right.

From what I remember

1. The USN issued a requirement for an interceptor program: This produced the XF5F, XFL-1, and F4U, of which the latter one.
2. The USAAF procured a long-ranged escort fighter in the form of the XP-65 which was to use 2 x R2600 with either a twin-stage supercharger or turbocharging, a pressurized cockpit, and 37mm cannon; they also procured the XP-50, which was an XF5F with a nose-gear and turobcharging.
   
3. The USN issued specifications for a heavy twin-engined naval fighter using 2 x R2800's with superchargers, and cannon armament
4. The USN & USAAF requirements were joined together.

I know the designs were eventually so different that Grumman was unwilling to pursue both, and decided to just develop the USN design, which became the F7F Tigercat.

What I'm curious about is the following

• Why didn't they just use R-2800's with either 1 stage supercharging (F7F), or two-stage supercharging, or a turbocharger?
• Why did the USAAF want 37mm cannon: They were brutal when they hit, but fired slowly, had bad ballistics, and permited only 40-45 rounds; the 20mm hit greatly harder than the 50 calibur, had similar refire rate, ballistics were a little inferior but acceptable at 400-yards for most purposes
• Why did the USN just put 4 x 20mm in the nose and disregard the 0.50? That was more than enough...

 

[wuzak]

Regarding the engines, the project timing probably didn't allow consideration of the R-2800.

According to this, the project started in 1939. At which point the R-2800 was still relatively early in development, while the R-2600 was in production.

The XF6F also used an R-2600.

As for the guns, the aircraft was to shoot bombers down. So the more guns the merrier.

The USAAC/F did have a fascination with the 37mm cannon. Probably because of its destructive power.

 

[GregP]

You are asking questions that I doubt younger people can answer. There are a few in here who stubbornly think the U.S.A. should have bought foreign weapons, particularly the DH Mosquito.

But the attitudes of the time would preclude that from ever happening at the time when it might have been really useful. That being said, even if we could post the real-world reasons for the items above, very few people today would accept the reasoning behind them. Everyone wants to take apart the decisions and replace them with whatever decision seems best using the infallible power of hind-sight and knowledge of everything we know today after years of digging and changes in attitudes.

I believe the desire for large cannons was simple: they wanted any hits on a bomber to have maximum stopping power, and didn't really think much about fighter vs. fighter combat. That was a necessary evil for the planners.

Same with the four 20's: Having four 20 mm cannons "fixes" the slow firing cannon issue and allows more hits on bombers.

I seriously doubt anyone in here has detailed knowledge of the number ot turbine wheels that were avilable for use at what time in WWII, and the supply of metals required for the hot section was likely the simple deciding factor. If someone DOES know, maybe they'll share it.

The valves in an Allison were sodium-filled Stellite, and that required tungsten and molybdenum. The supplies weren't unlimited and ALL military piston engines needed valves, so they got first priority for the alloys. Turbine wheels came further down the food chain. Other makers also needed wear-resistant valves, and Allison was forced to share that with Rolls-Royce and, eventually, commerical competitors. That was NOT a welcome decision by Allison or GM, but helped the war effort overall.

I don't know why the engine makers didn't experiment more with 2-stage superchargers, but it most likely is due to their primary customers wanting turbochargers instead. Everyone was sort of waiting for the "breakthough" in turbocharging and it never did get to a stage of development that was as reliable as 2-stage superchargers. I think charge coolers were a big issue: inter / after coolers that could drop the etemperature of the compressed air and/or air/fuel mixture and delay detonation.

If you go look up the history of the R-2800, you will find a LOT of master-rod designs, a lot of failures before "getting it right," a lot of cooling fin designs, and a lot of hard work. Early R-2800s weren't necessarily great engines. The White book, "R-2800 Pratt and Whitney's Dependable Masterpiece" is a great source. The timeline doesn't exactly allow for early use in large numbers. "B series engines were built from 1943 and "C" series were from 1945. An "A" series engine has very few parts in common with an "E" series engine.

The R-2800 first flew in 1940, but the "B" series is what started them down the right path. That said, actual development times for the Grumman Cats wasn't bad at all.

 

[tomo pauk]

Yay, KJ Lesnick is here

You are asking questions that I doubt younger people can answer. There are a few in here who stubbornly think the U.S.A. should have bought foreign weapons, particularly the DH Mosquito.

But the attitudes of the time would preclude that from ever happening at the time when it might have been really useful. That being said, even if we could post the real-world reasons for the items above, very few people today would acceot the reasoning behind them. Everyone wants to take apart the decisions and replace them with whatever decision seems best using the infallible power of hind-sight and knowledge of everything we know today after years of digging and changes in attiudes.
....
The valves in an Allison were sodium-filled Stellite, and that required tungsten and molybdenum. The supplies weren't unlimited and ALL military piston engines needed valves, so they got first priority for the alloys. Turbine wheels came further down the food chain. Other makers also needed wear-resistant valves, and Allison was forced to share that with Rolls-Royce and, eventually, commerical competitors. That was NOT a welcome decision by Allison or GM, but helped the war effort overall.

Oh boy. Talk about mis-informations and off-topic drumming.

I don't know why the engine makers didn't experiment more with 2-stage superchargers, but it most likely is due to their primary customers wanting turbochargers instead. Everyone was sort of waiting for the "breakthough" in turbocharging and it never did get to a stage of development that was as reliable as 2-stage superchargers. I think charge coolers were a big issue: inter / after coolers that could drop the etemperature of the compressed air and/or air/fuel mixture and delay detonation.

Bristol, Junkers and P&W experimented with 2-stage superchargers before the war of 1939.

If you go look up the history of the R-2800, you will find a LOT of master-rod designs, a lot of failures before "getting it right," a lot of cooling fin designs, and a lot of hard work. Early R-2800s weren't necessarily great engines. The White book, "R-2800 Pratt and Whitney's Dependable Masterpiece" is a great source. The timeline doesn't exactly allow for early use in large numbers. "B series engines were built from 1943 and "C" series were from 1945. An "A" series engine has very few parts in common with an "E" series engine.
The R-2800 first flew in 1940, but the "B" series is what started them down the right path. That said, actual development times for the Grumman Cats wasn't bad at all.

January of 1942 saw 220 copies of R-2800 'B' engines produced by P&W, and 65 by Ford. In 1942, more than 11000 (eleven thousand) of R-2800 B series of engines produced.
The R-2800 C series were produced in numbers from mid-1944, with 3100+ delivered in 1944.

In 1941, more than 1700 R-2800 were produced.

 

[wuzak]

You are asking questions that I doubt younger people can answer. There are a few in here who stubbornly think the U.S.A. should have bought foreign weapons, particularly the DH Mosquito.

That has nothing to do with the topic at hand.

Not to mention the initial moves in procuring the XP-65/F7F predated the first flight of the Mosquito.

Or the fact that they were designed around a completely different requirement.

The valves in an Allison were sodium-filled Stellite, and that required tungsten and molybdenum. The supplies weren't unlimited and ALL military piston engines needed valves, so they got first priority for the alloys. Turbine wheels came further down the food chain. Other makers also needed wear-resistant valves, and Allison was forced to share that with Rolls-Royce and, eventually, commerical competitors. That was NOT a welcome decision by Allison or GM, but helped the war effort overall.

Oh no, even more technology Allison was forced to share with that dastardly British Rolls-Royce company. Oh, how badly were Allison screwed?

Except that Allison invented neither stellite steel or sodium filled valves. The former was invented by a steel company (go figure) and the latter by the USAAC Engineering Division (who would also develop the "hyper" cylinder) under Sam Heron - an Englishman.

So Allison would have had to buy licences to make their own, as would Rolls-Royce.

Assuming Rolls-Royce used stellite and hadn't developed their own high temperature wear resistant steel.

 

[tomo pauk]

...
From what I remember

1. The USN issued a requirement for an interceptor program: This produced the XF5F, XFL-1, and F4U, of which the latter one.
2. The USAAF procured a long-ranged escort fighter in the form of the XP-65 which was to use 2 x R2600 with either a twin-stage supercharger or turbocharging, a pressurized cockpit, and 37mm cannon; they also procured the XP-50, which was an XF5F with a nose-gear and turobcharging.

....

Item 2 is wrong re. USAAF wanting a long-range escort fighter with XP-65 - doctrine for long range escort did not existed in 1941 in the USA.

What I'm curious about is the following

• Why didn't they just use R-2800's with either 1 stage supercharging (F7F), or two-stage supercharging, or a turbocharger?
• Why did the USAAF want 37mm cannon: They were brutal when they hit, but fired slowly, had bad ballistics, and permited only 40-45 rounds; the 20mm hit greatly harder than the 50 calibur, had similar refire rate, ballistics were a little inferior but acceptable at 400-yards for most purposes
• Why did the USN just put 4 x 20mm in the nose and disregard the 0.50? That was more than enough...

The US 20mm was not a done deal by 1941, while the 37mm was, and it had a role for the primary purpose of USAAC fighters of 1941 - killing the enemy bombers close and above the USA and it's territories.
Re. USN request - having just 4 x 20 mm on two big, powerful, heavy and expensive R-2800 probably lookes as a vaste, only one R-2800 was more than capable to haul those 4 cannons. USN was never sold on the 37mm M4 cannon.

 

[GregP]

It is right on topic guys. He was asking why some things happened the way they did and what I wrote is exactly on-topic. I'm amused that Tomo is ttelling me it's mis-information. That's funny. It's like me telling him why things happened the way they did in Poland. Thanks for a good one, Tomo.

And I didn't say nobody experimented with 2-stage superchargers, I said I didn't known why they didn't pursue it MORE, and still don't. I was taking about U.S. manufacturers, not British manufacturers. And nobody in here can tell me unless they were there. Perhaps the ones that were developed were pretty good for the radials, but I've never been happy with the auxiliary supercharger that Allison used. It worked, but the unit on the Merlin is much better designed. There was considerable room for improvement.

Allison's main beef was sharing bearings and we've had threads on it before. Valves that wear longer were important, and made their way around to everyone. It was inevitable, and even Allison knew it. I'm glad they did, and I bet everyone else is glad they lasted longer than WWI valves, too. I still don't see anyone posting the amount of metals that were avilable and when. It's is possible they could have made more of anything, but the supply of tungsten and molybdenum were limiting factors and the bombers were allocated the major supply of turbos in the U.S.A. You surely weren't going to make them from mild steel or magnesium!

I'm not too sure the very early R-2800s were all that reliable or long-lasting. Not too many are still flying around as with later series engines. We fly at least 3 on a regular basis and a few more occasionally and none are "A" series engines. I can say the same for most of the R-2800s that come around, and that's more than just a smattering. Most people who get one rebuilt want late master rods. It's sort of like getting an Allison rebuilt as an early dash number ... they still want 100-series internals, including the 12-counterweight crankshaft.

The actualy development timelines weren't bad at all, looking at when things were ordered and the priority places on them. We essentially declared was at the start of 1942, and many new things were coming into service not all that much later. I'm not sure it was possible to make things happen any faster and the planes we had on hind were required to carry the fight until new ones were ready. The only real way to speed it up would have been to order the new planes before we were in the war. Then development COULD have been moved back in time enough to matter. I'm doubting this one was realistically ever going to happen because of the ecomomic situation at the time in the late 1930s.

 

[fastmongrel]

Other makers also needed wear-resistant valves, and Allison was forced to share that with Rolls-Royce and, eventually, commerical competitors. That was NOT a welcome decision by Allison or GM, but helped the war effort overall.

Rolls Royce were using valves that were sodium cooled and with Cobalt alloy friction surfaces on the R racing engines in 1931. Stellite was old news by the 1920s and Dr Heron was experimenting with Sodium cooled valves at the Royal Aircraft Establishment in 1917.

Any engine company that didnt know about the above by 1939 was staffed by morons.

 

[Zipper730]

The USAAC/F did have a fascination with the 37mm cannon. Probably because of its destructive power.

True, but shooting is only one part of the equation... anybody can shoot a gun, not everybody can make it hit something.

The US 20mm was not a done deal by 1941, while the 37mm was, and it had a role for the primary purpose of USAAC fighters of 1941 - killing the enemy bombers close and above the USA and it's territories.

I guess there was the P-67 too...

 

[GregP]

Stellite was invented by Elwood Haynes up in Canaada in Deloro in 1909. He became a millionaire in 1912 and moved back to Indiana and took the Haynes Stellite Company with him, just up the road from Allison Engineering. The other Stellite company with Haynes original partner, Michael O-Brien was eventuallly sold and still exists in Germany today as the Kennametal Stellite Company. Haynes' museum is still in Indiana, with some of his original inventions and developments. Haynes was primarily using it for cutting teeth in saws and cutlery, but the wear resistance wasn't tried in poppet valves until the move back to Indiana. Coincidentally, Elwood Haynes also had an automotive interest and worked in Indiana to help devlelop them. Allison Engineering was among his first customers for them, and helped develop these. They propagated, but Allison was never happy about sharing developments. Later they were forced to share also bearing patents, which was their primary source of WW2 displeasure. But all along, developments they made were shared by others. Allison didn't develop these alone, they partnered with Haynes. Allison kept theirs in-house and Haynes shared with the industry, as had always been his intent. Allison didn't develop in partnership after that ever agian while James Allison was alive.

You can make out of that whatever you want.

The questions above were mostly concerned with why things developed the way they did. In point of fact, I tried to address why the Navy wanted 37 mm cannons, why they went to four 20s, and why 2-stage superchargers weren't better-developed in the USA, which was and IS because the main U.S. engine customers in the U.S. miltary were favoring turbochargers. I also said I didn't see how it was possible to field the R-2800 any earlier, and still don't. It basically got into service when it could get there.

Had the U.S.A. or even US companies been able to cooperate with Rolls-Royce on a standard 2-stage supercharger for the Allies, we might have had some better aviation engines sooner. I'm not sure either side would have been willing to do that without the war being on. It' s like one of those what-ifs about whether or not the Germans could have really gotten jets into the fray sooner.

There some unlikely circumstances which might have allowed it, but not realistically. We got stuck with the superchargers we had in the real world, and that's as good as they got. So we went with what we had when we had it.

Guys, nobody writes things exactly like someone else does, and this has gotten to the point of not being fun anymore.

 

[wuzak]

Greg, I have requested some documents form the UK National Archives, the title of which is:

Secrecy agreements re. manufacture of Allison bearings at aero factory, between Austin Co. & various employees

Which certainly sounds like they were made under licence agreement.

 

[wuzak]

Unfortunately those documents are not available at the Nation Archive, but rather the British Motor Industry Heritage Trust. So I can't get copies of the files.

 

[wuzak]

wuzak

I didn't realize the R-2800 and R-3350 were concurrent designs. I thought (for some reason) that the R-2800 and R-2600 were concurrent.

The R-2800 was slightly behind the R-2600 in timeline. The R-3350 was slightly behind the R-2800.

 

[wuzak]

The RAF early on had 8 x 0.303's in the Spitfire and Hurricane, followed by 12 x 0.303's for the Hurricane and Typhoon, followed by 4 x 20mm for the Hurricane, Typhoon, and Tempest.

And? So? Therefore?

 

[Zipper730]

Wuzak,

The point I'm getting at was that 4 x 20mm was fine

 

[Shortround6]

A few observations for now.

1. On the subject of guns. The US Army was enamored of the 37mm gun for a number of reason, part of it was because it was home grown, although they certainly did at least test several other options including Danish Madison cannon. The actual truth is that a LOT of the pre-war cannon promised a lot more than they delivered in regards to reliability. Also most of the cannon on offer had rather low rates of fire in the mid to late 30s. It was only in the very late 30s that the Hispano got to 600rpm. So.... a 37mm at 140rpm or a 20mm at 400-450rpm?
Another reason was the size of anticipated targets. The USAAC having built the XB-15 and XB-19 and with America's isolationist policies one theory was that any air attacks would be from similarly sized aircraft. There were several design studies between the XB-19 and the B-29 and as another indicator here is a drawing of the unbuilt Martin XB-16;

six Allison engines. And don't forget, the Army started initial design talks that would lead to the B-36 in the Spring of 1941. against such aircraft bigger than 20mm guns have an obvious attraction.
As a further note on the Army and Navy on guns, they did not always play well together. During the 1930s when the need for light automatic cannon for AA use was becoming glaring apparent the Army proceed with their own 37mm AA gun while the Navy designed and built the infamous 1.1 in (28mm) machine cannon. However the Navy changed direction sometime after summer of 1940 when they got their hands on a twin mount 40mm Bofors and started plans for production even before getting a licence, I don't know when the Army joined in but by 1943/44 the 40mm Bofors was in widespread Army use and the Army 37 AA gun was on the way out.

Please remember that if you are buying weapons to respond to what your enemy is actually using in combat you are 2-4 years too late. The designers and procurement people are trying to anticipate needs 2-5 years into the future.

The engine question R-2600 vs R-2800 vs R-3350 gets really convoluted but lets try to sort through things.
1. None of these engines were worth much at high altitude without an auxilary or two stage supercharger. Even with a two speed supercharger FTH was around 12-14,000ft and engine power was several hundred HP below take-off power. A Series R-2800 had 1850hp for take off but only 1500hp at 14,000ft. The R-2600 was proportionally worse. If you want a low altitude plane you are all set, if you want to escort turbocharged bombers then single stage supercharged radials won't work.
2. The Wright engines have a bit of a twisted path. The R-2600A did predate the R-2800A and at some point they did design and build a handful of R-3350 engines. However to get to 1700hp Takeoff with the R-2600 required a total redesign including changing the crankcase from aluminium to steel. This R-2600B slots in between the R-2800A and B in timing. The R-3350 was placed on hold while the R-2600 was sorted out (and while Wright went down the R-2160 Tornado rathole) and was taken back out, dusted off and redone using a lot of the knowledge gained in working on the R-2600B.
Perhaps a list of dates on when the 5th engine was accepted will help put things in perspective.
R-2600A............March 1938
R-3350BA..........Oct 1939
R-2800A............March 1940
R-2600BA.........June 1941
R-2800B............Oct 1941
R-3350BB..........Aug 1942
Please note that for the R-3350BA it took from 1938 to the end of 1942 to build 77 engines. Lack of demand or lots of teething troubles?

Sodium cooled exhaust valves were pretty much a world wide industry standard by 1941. at least according to Wilkinson's "aircraft Engines of the World" granted it may have mistakes but about the ONLY high powered aircraft engines NOT using sodium filled exhaust valves are the Bristol sleeve valve engines. Even such US powerhouses as the Continental R-670 and Lycoming R-680 used sodium cooled exhaust valves and some sources say they were used on the Ranger inline 6 and V-12.

 

[Shortround6]

What I'm curious about is the following

• Why didn't they just use R-2800's with either 1 stage supercharging (F7F), or two-stage supercharging, or a turbocharger?
• Why did the USAAF want 37mm cannon: They were brutal when they hit, but fired slowly, had bad ballistics, and permited only 40-45 rounds; the 20mm hit greatly harder than the 50 calibur, had similar refire rate, ballistics were a little inferior but acceptable at 400-yards for most purposes
• Why did the USN just put 4 x 20mm in the nose and disregard the 0.50? That was more than enough...

I think we have answered most of the questions. I would add in regards to the engine question that it was not so simple to ADD two stage supercharging to an existing fighter design. One text book claimed about 10 cubic feet of volume was needed for a 1000hp engine for the supercharger AND intercooler AND ducts. Since horsepower is pretty much a function of airflow a 2000hp engine is going to need nearly double the volume, there is some savings in scaling up but no, you are not going to cram 2000hp worth of airflow into 12-14 cubic feet. On bombers with large wings and engine nacelles there was a lot of room to put things. On fighters you pretty much designed for one type of engine from the start. Please note that the Spitfire and P-51 and Mosquito all used substantially larger radiator sections to fit in the intercooler radiators than the single stage engined planes.

Finding information on the thinking of the Navy on the armament is hard. However sticking four 20mm cannon in the nose requires a bit of thought. The F7F as designed carried the 20mm guns and ammo pretty close to the center of gravity. 800 rounds of 20mm ammo weighs around 560lbs so you have to be careful where you put it or the flying characteristics of the plane change as the ammo is used.
You also have the ongoing saga of the high rate of fire .50 cal gun. The Belgians (The FN company) claimed a rate of fire for their 13.2 mm version in 1938/39 of close to 1200rpm. The US Army and Navy were getting 600rpm at the time and that with short belts. the .50 was boosted to about 800rpm fairly easily but then followed a succession of contracts/projects by several different companies to boost the rate of fire to 1200rpm or above. Some companies had several different projects all with the pretty much the same goal. After 15-20 projects and a number of broken and blown up guns and 2 1/2 to 3 years they finally got the M3 machinegun.
Now in the planning for future fighters to fly 2-3 years down the road which .50 cal machine gun were they planning on using? they one they already had or the one they were hoping to have after pouring all that time and money into it? Four of the fast firing guns almost equal the rate of fire of 6 of the standard 800rpm guns so a fighter with four 20mm guns and four .50s had a very formidable armament.
Also please note that on the F7F the 50.cal guns were located pretty much under the pilot and the nose gun ports required long blast tubes to reach the actual guns. Again they had to keep the ammo close to the center of gravity.

 

[Shortround6]

As far as Stellite goes, James Allison died in Aug of 1928. What the Allison company did or did not do with stellite's development before then seems to have little bearing on it's use in the 30s and 40s.

A pair of books that shed considerable light on the state of the art of aircraft engine construction of the beginning of WW II are "Aircraft Engines" By A.W.Judge. Volumes I and II. first published in 1941. there are later editions.
The 1941 edition has a 14 page section just on valves and valve gear. And for all of us arm chair engineers (who think we know better than real engineers) it is full of details like alloy mixes, Brinell hardness, yield strengths and so on.
Stellite was used for a facing on the valves and valve seats although sometimes an alloy called "Brightray" was used. Stellite buttons were sometimes welded on to valve stems to increase wear resistance. There were quite a number of alloys used for valve seats but many were faced with Stellite or Brightray (or perhaps other propitiatory alloys). Some alloys were developed to mimic the expansion rate of aluminium alloy cylinder heads so there was less chance of the insert dropping out at high temperature.

 

[fastmongrel]

Rolls Royce developed quite a few Ferrous and Non Ferrous alloys for their own use and worked closely with companies like Firth Brown steel, High Duty Alloys Ltd, Henry Wiggin and Co (Brightray for exhaust valves similar to Stellite) and Reynolds Tubes Ltd.

I imagine the US aero engine builders did the same working closely with specialist companies. Allison were fortunate to be part of the giant GM corporation who had laboratories working on alloys for cars.

 

[Zipper730]

A few observations for now.

1. On the subject of guns. The US Army was enamored of the 37mm gun for a number of reason, part of it was because it was home grown, although they certainly did at least test several other options including Danish Madison cannon.

The Madsen was a 23mm cannon, if I recall correctly

The actual truth is that a LOT of the pre-war cannon promised a lot more than they delivered in regards to reliability. Also most of the cannon on offer had rather low rates of fire in the mid to late 30s. It was only in the very late 30s that the Hispano got to 600rpm. So.... a 37mm at 140rpm or a 20mm at 400-450rpm?

I see your point

Another reason was the size of anticipated targets. The USAAC having built the XB-15 and XB-19 and with America's isolationist policies one theory was that any air attacks would be from similarly sized aircraft.

By 1937 or 1938 weren't we aware what the Germans were doing at all?

As a further note on the Army and Navy on guns, they did not always play well together. During the 1930s when the need for light automatic cannon for AA use was becoming glaring apparent the Army proceed with their own 37mm AA gun while the Navy designed and built the infamous 1.1 in (28mm) machine cannon.

Infamous?

However the Navy changed direction sometime after summer of 1940 when they got their hands on a twin mount 40mm Bofors and started plans for production even before getting a licence, I don't know when the Army joined in but by 1943/44 the 40mm Bofors was in widespread Army use and the Army 37 AA gun was on the way out.

40mm would have been better as I think it had a higher muzzle velocity...

The engine question R-2600 vs R-2800 vs R-3350 gets really convoluted but lets try to sort through things.
1. None of these engines were worth much at high altitude without an auxilary or two stage supercharger. Even with a two speed supercharger FTH was around 12-14,000ft and engine power was several hundred HP below take-off power.

Wait, I can't speak for the R-2600 but the R-2800 could do around 23,800...

If you want a low altitude plane you are all set, if you want to escort turbocharged bombers then single stage supercharged radials won't work.

Okay

Please note that for the R-3350BA it took from 1938 to the end of 1942 to build 77 engines. Lack of demand or lots of teething troubles?

That was teething trouble...

I would add in regards to the engine question that it was not so simple to ADD two stage supercharging to an existing fighter design. One text book claimed about 10 cubic feet of volume was needed for a 1000hp engine for the supercharger AND intercooler AND ducts.

You would want that there from the start

Finding information on the thinking of the Navy on the armament is hard. However sticking four 20mm cannon in the nose requires a bit of thought. The F7F as designed carried the 20mm guns and ammo pretty close to the center of gravity. 800 rounds of 20mm ammo weighs around 560lbs so you have to be careful where you put it or the flying characteristics of the plane change as the ammo is used.

I think I understand, you could make an airplane with a wildly varying CG...