R-2800 for fighters: how would've you done it?

[davparlr]

you have a couple of problems with the comparison. The F4U prototype, if it actually carried guns, had one .30 and and one .50 in cowl and a single .50 in each wing. It was also a whole light lighter than service planes. In fact it comes just about the numbers you want. 7505 lbs empty, 9375lbs normal gross and 10,500 max take-off. It's fuselage was also about 3 ft shorter than the F4U-1. This is part of the difference between a plane with the single stage engine and one with the two stage and inter coolers. (Granted they also moved most of the fuel from wing tanks into the fuselage). The Prototype had no armor or self sealing tanks.

I don't see this as an issue at all. True, it was a prototype, but it was a Navy prototype with a big wing and most likely carrier landing stress levels. It had four machine guns, probably .50s. I suspect that over a 1000 lbs could be removed from the prototype to make it AAF stress requirements and smaller wings. Contemporary Army aircraft had much lower empty weight, P-40C (which included armament, self sealing tanks and armor), 5812 lbs, P-36C, 4620 lbs, P-66, 5236 lbs. Adding 1500 lbs to the P-40C for engine and strengthening, and it comes to about 7300 lbs. True, these did have less armament, but that tended to be standard for the time. Also, the Fw-190A-3 had an empty weight of 6380 lbs, and the later F8F, with the R-2800 engine, had a empty weight of 7070 lbs. Now, unless the Germans had such advanced engineers that they could build a 6300 lb fighter with a big heavy radial where the American engineers could not, or it took Grumman engineers three years to figure out how to make a light weight fighter, the argument against a adequately protected and armed, fighter with an empty weight of around 7000 lbs seems unsupportable.

The Prototype, being a prototype, was fitted with a least two different R-2800s, there seems to be a fair amount of confusion as to the exact configuration at times.

Engines are problematic. I do know the following. The B-26, with the R-2800-5, generating 1850 hp, but this seemed to be turbocharged. The XR-2800-4 seemed very capable. I do not know the maturity of design but it was soon replaced by the R-2800-8.

I just don't have much data at all on the Fw engine performance vs. altitude nor for the non-turbo R-2800.

The guns and ammo for a P-40 (235rpg?) went 900lbs. The FW 190 guns went about 330lbs instead of 470 or so for six .50s and its ammo came up to about 430-330lbs or about the same as 1400 rounds of .50 cal ammo. Yjat is for 2000rounds of 7.9mm, 400 rounds of ammo for the Mg 151/20s and 120 rounds (+drums) for the MG/FFMs.

Ammo is not part of empty weight and I don't want to go there. Four .50s were pretty standard as used in the P-51B and F4F-3, or some mixture with .30s.

Neither the Spitfire or the Mustang were ever fitted with the 2 speed Merlin. It may make an interesting "what if". It does show that without a two stage supercharger the R-2800 fighter wasn't going to perform much better than what was already available.

I think it would be quite a bit faster than the Spitfire V and the Bf-109F, and roughly equal to the Fw-190A1/2 and later 3 after upgrade to the -8.

While a long range escort fighter may not have been needed at the time the Spitfires barely had the range for what they were trying to do with them. It is one thing to cross the channel, it is another to make it past Paris, and yet even more just to make to the Rhine. Forget Berlin.

I am sure the P-40C and Spitfire were not great at that, either. I think the Brits were busy trying to keep Fw-190s off their tail.

You also have the development problem, 6 two stage engines built in 1941, none in Dec 41 and Jan of 1942, and the 100th engine isn't delivered until some time in June. The "B" series single stage engine has 2 delivered in 1941 By P&W and and 2 by Ford, both are in the process of switching from "A" series to "B" series production, P&W delivers over 900 single stage "B"s in the first 3 months of 1942 and Ford delivers about 700 "A"s and 250 "B"s in those Months.

Without knowing what the power profile of the early R-2800, can't argue anything.

 

[Shortround6]

Radials had more trouble with jet thrust than V-12s. The V-12s had all their exhausts in a row which means they had 6 exhausts with the frontal area of one, all exhausts had identical (or nearly) exhaust lengths and paths. Radials either had short individual exhausts (like a B-25 or late model Zero) or pipes of varying lengths and shapes. First method gives better thrust but increases frontal area and drag. Second method gives much better streamlining but reduces thrust. Thrust is mass of the gas times velocity squared. the longer pipes give reduced gas velocity at the exit of the pipe.
Jet exhausts can only be optimized for one flight condition. That is one speed at one altitude, any other flight condition will be a compromise. US fighters units found that when they started using WER ratings they had to cut off part of the exhaust tips to get best performance. The size tip (opening) that performed best at 1150hp was too restrictive for 1400hp.

Many modern gas turbines are rated at ESHP to make their fuel consumption numbers look better. You only get jet thrust if the exhaust stream is directed rearwards or nearly so. This is going to mean that on many aircraft the exhaust pipe is going to need a 90 bend sticking out into the airflow to turn the exhaust stream rearward. See:

Fileiper PA48 Enforcer USAF.jpg - Wikipedia, the free encyclopedia

Exhaust thrust vector is at a considerable angle to the line of flight. Yes there will be some forward thrust but not full value.

File:Turboprop P&W PT6A-67D.jpg - Wikipedia, the free encyclopedia

This exhaust would do much better for thrust but obviously would create more drag than this exhaust.

http://www.planefax.com/radar/T-34C-turbo-Mentor.jpg

The Early Corsair didn't seem to really use jet exhaust thrust.

http://topmdi.net/flagcarrier/wp-content/uploads/2010/07/corsair.jpg

Two exhaust outlets at 5 and 7 o'clock.

later versions add exhaust outlets at about 3 and 9 o'clock ( two outlets at each location, total 6).

 

[Shortround6]

I don't see this as an issue at all. True, it was a prototype, but it was a Navy prototype with a big wing and most likely carrier landing stress levels. It had four machine guns, probably .50s. I suspect that over a 1000 lbs could be removed from the prototype to make it AAF stress requirements and smaller wings. Contemporary Army aircraft had much lower empty weight, P-40C (which included armament, self sealing tanks and armor), 5812 lbs, P-36C, 4620 lbs, P-66, 5236 lbs. Adding 1500 lbs to the P-40C for engine and strengthening, and it comes to about 7300 lbs. True, these did have less armament, but that tended to be standard for the time. Also, the Fw-190A-3 had an empty weight of 6380 lbs, and the later F8F, with the R-2800 engine, had a empty weight of 7070 lbs. Now, unless the Germans had such advanced engineers that they could build a 6300 lb fighter with a big heavy radial where the American engineers could not, or it took Grumman engineers three years to figure out how to make a light weight fighter, the argument against a adequately protected and armed, fighter with an empty weight of around 7000 lbs seems unsupportable.

Not quite so unsupportable. It is 1250lbs for the engine swap alone on the P-40C, not including fuel systems. That is for the R-2800-8 engine with two stage supercharger. Of course we instantly run into a problem, swapping the 11 ft prop on the P-40 for the 13 ft 4in prop from the Corsair means we need a lot longer landing gear. Assuming you can solve that one with little weight increase (and lots of luck with that trick) We run into problem #2. The P-40C was rated at 12"G"s ultimate load factor at about 7500lbs. Changing to an 8750lb gross weight (allowing for the engine swap only) lowers the load factor to 10.28 "G"s which was not acceptable to US authorities at the time. Yes you allowed some extra weight for strengthening but but the strengthening is for the gross weight not empty. The "yardstick" F8F-1 went 9386lbs gross, and the P-40 might not have been rated with the aft tank full? even if it was, going from 7500lbs to 9386lbs means the ultimate load factor dropped to 9.6 which even the British and Germans would not have accepted. The F8F was designed with trick wings with break away tips to avoid overloading the entire wing allowing for lighter structure. The tips were deactivated, beefed up and the plane was restricted in the "G"s it could pull.
By the way, going from 7500lbs to over 9000lbs is going to require heavier landing gear of some sort (even if only heavier tires). The Bearcat at 9386lbs carried 300rpg, about 100lbs less ammo than P-51B and 120lbs less than an F4F-3.
One reason for the FW 190 light weight might be the fuel capacity 524 liters on the early models? 138.5 US gallons. Not bad for a defensive interceptor but not good for plane trying to take the fight to the enemy. The F8F-1 carried 185 gallons. 300lbs difference in fuel for the loaded airplanes.

Engines are problematic. I do know the following. The B-26, with the R-2800-5, generating 1850 hp, but this seemed to be turbocharged. The XR-2800-4 seemed very capable. I do not know the maturity of design but it was soon replaced by the R-2800-8.

I just don't have much data at all on the Fw engine performance vs. altitude nor for the non-turbo R-2800.

The -5 was not turboed. engine performance at altitude is not hard with a little work. Get some graph paper and make a chart, Hp up the side and altitude across the bottom. plot a point for the rated altitude, like 1600hp at 13,500ft for a R-2800-27 "B" series engine as used in later B-26s. Plot another point for zero hp at 55,000ft and draw a straight line between the two points. There is some argument as to the 55,000ft point but shifting it up or down a few thousand feet makes little difference to the 25,000-35,000 ft ratings. Plot and draw in your engines for comparison and there you are. Going lower than the rated altitude gets a bit trickier. Engine strength limits, cooling problems and such limit the power. A continuation of the line you already drew would show the potential power of the engine from an airflow perspective. (and in some cases shows were WER ratings came from and their limits.

Ammo is not part of empty weight and I don't want to go there. Four .50s were pretty standard as used in the P-51B and F4F-3, or some mixture with .30s.

As pointed out above, ammo is part of the gross weight and part of the load used to figure the strength limits of the aircraft and it's required airframe strength. Most WW II fighters, if measured "clean" actually will show a fairly small range of useful lad to either gross weight or empty weight. A Razor back P-47 at 12700lbs gross has about 28% of that weight as useful load, at 13,582lbs it has about 36% useful load. A P-40C at 7500lbs has a 30% useful load and most other single engine fighters are going to be around 30% give or take a few percent. The higher the percentage of useful load while keeping up strength the better the aircraft designer and engine designers did their job. Sacrificing useful load for performance means the plane will be lacking in some way. Light armament or short range or both.

 

[Siegfried]

Well, if you consider having 350 less hp at 22,500ft and 500 less hp at 26,000 ft to be barely suffering. 75% of the power at 26,000ft. and that is for P-47B that never saw combat. The "C"s could hold 2000hp to 27,000t. If I am reading the graph right the 2 stage mechanically driven supercharger was giving 1400hp at 27,000ft. 42% more power does tend to cover a few sins in the weight and drag departments.

I would argue two things need to be considered:
1 Jet thrust: the mechanically supercharged engine is robbing its output shaft to power is supercharger but unlike the turbo supercharged engine it is not robbing its exhaust of jet thrust.
2 It's not really a fair comparison. The two stage mechanically supercharged engines of the corsair don't seem to have had an inter-cooler whereas the two stage Merlin, Grifon and Jumo 213E (ta 152) did and of course the R-2800 on the P-47 did have an inter cooler.

 

[tomo pauk]

Corsair did have 2 (two) inter-coolers, one in each wing root.

 

[Shortround6]

I would argue two things need to be considered:
1 Jet thrust: the mechanically supercharged engine is robbing its output shaft to power is supercharger but unlike the turbo supercharged engine it is not robbing its exhaust of jet thrust.
2 It's not really a fair comparison. The two stage mechanically supercharged engines of the corsair don't seem to have had an inter-cooler whereas the two stage Merlin, Grifon and Jumo 213E (ta 152) did and of course the R-2800 on the P-47 did have an inter cooler.

As noted above the F4U-1 did not use exhaust jet thrust. The F4U-4 did. the F6F did use exhaust jet thrust, the F4F did not. The R-2600s in A-20s did not use exhaust jet thrust, the R-2600s in B-25s did not.

Something else to consider, Exhaust jet thrust works best the closer the velocity of the exhaust stream matches the forward speed of the vehicle. With exhaust stream speeds well over 1300fps (in fact in some cases at 1900fps) piston aircraft is not a good match, it also means that a Spitfire and a Hurricane using the same engine and throttle settings will get different exhaust jet thrust due to their different speeds. It also means that while the exhaust thrust works pretty good at high speed it can provide significantly less thrust when trying to climb, or accelerate. Exhaust thrust also depends on the outside atmospheric pressure. Test done by RR on a Merlin XX engine in a Hurricane at full throttle show exhaust horsepowers from 86.5 to 126.8 as the plane climbs from 15,000 to 20,000ft exhaust power declines to 65hp at 35,000ft were the engine is making 700hp to the propeller. In no case does the Merlin XX make more than 10% of it's shaft horsepower in exhaust hp. However exhaust hp is proportional to charge weight ( weight of air and fuel) so exhaust hp is proportional to the boost level. The air cooled engines seldom used the boost pressures of the liquid cooled engines although they did move more air.

2. The F6F also used intercoolers. No P&W R-2800 that used two stage superchargers did not also use an inter-cooler.

 

[tomo pauk]

...

It's worth considering what might have been possible had the P-47 been equipped with the two stage mechanically driven supercharger rather than a turbo.
Altitude performance would likely barely have suffered while a large amount of space for fuel carriage would be liberated.

A vice-versa proposal: F4U hull mated with P-47-style wings?

The BMW 801TJ is worth have a look at for the interesting way they packed the inter-coolers.

If anything, P-47 used a rather bulky inter-cooler layout

added: picture; the landing gear can now be sized styled as in F6F. Neat thing about landing gears of the F4U F6F is that they enabled carriage of multiple heavy loads near the centreline.

 

[wuzak]

It's not really a fair comparison. The two stage mechanically supercharged engines of the corsair don't seem to have had an inter-cooler whereas the two stage Merlin, Grifon and Jumo 213E (ta 152) did and of course the R-2800 on the P-47 did have an inter cooler.

The Merlin two stage engines had an intercooler and an aftercooler.

The intercooler was a coling water jacket around the supercharger volute. The aftercooler fitted between the supercharger outlet and the engine intake manifold. The aftercooler is the big box at the rear top of a Merlin two stage engine.

V-1710s used intercooling for their turbocharged applications. In their 2 stage mechanical supercharged versions most went without intercooling or aftercooling, though some early models were fitted with a Merlin style aftercooler.

 

[Shortround6]

If anything, P-47 used a rather bulky inter-cooler layout

Yes and no. the size of the inter-cooler is based off the amount of airflow needed by the engine (in mass or pounds/KG) and the amount of temperature reduction desired, this tells you how much cooling air is needed (in mass not volume) but the air passages and cooling surfaces have to arranged for volume. The air at 30,000ft is about 70% the weight of air per at 20,000ft per cubic foot. You need that much more air flow to get the same cooling effect. Unfortunately you may need an even bigger inter-cooler because the thinner air requires more compression in order to meet the required pressure at the carburetor intake which means the air is heated even more. The falling of the outside air temperature is not enough to make up all the differences
Remember that the P-38 was restricted in the power it's engines could put out in some versions because the inter-coolers were too small.

added: picture; the landing gear can now be sized styled as in F6F. Neat thing about landing gears of the F4U F6F is that they enabled carriage of multiple heavy loads near the centreline.

Thank you for bring up another problem with R-2800 powered ( or any big engine) fighters. Where do you stick the landing gear needed for propeller clearance? The big wing planes had room in the big wings to hide the landing gear but even the P-47 resorted to having the landing gear telescope 9 inches when retracted compared to extended in order to have room for the 4 guns and ammunition in each wing. for another example of landing gear trickery

Please notice the backwards (or 180 degree) hinge in the landing gear leg and that the a fair portion of the wheels are housed in the fuselage. guns in the wings have to located where there is room for them vertically and lengthwise. Receivers have to fit between spars or main spar and rear false spar/ aileron/flap attachment point. The Bearcat used a 12'4" prop instead of the 13'4" of the F4U. Any body want to add another 6 in to the Bearcats landing gear?

The lower powered Fw 190 used a smaller diameter propeller and used rather broad cord blades to get the needed blade area, something that US propeller makers took a while to do.

 

[tomo pauk]

US manufacturers have found the way to avoid the issue of U/C vs. prop clearance for R-2800, even if we exclude F4U 'trick' wing, so I see no issues there.

As for inter-cooler size on a P-47, one can see that I didn't even mentioned that property of the int.cooler

Now back to the wing size shape - when we read the numbers associated with wings of Typhoon Tempest, the later have even a tad (7-8%?) greater area (but the wing was thinner). So perhaps a wing of 280-300 sq ft would've been even better suited for 'my' bird? Or a 250 sq ft, if we go for single stage, 220-250 gal, 350 rpg machine of circa 7500 lbs empty weight?

 

[Shortround6]

The US manufacturers did find a way to avoid the issue of of undercarriage vs prop clearance but they didn't do it with 190-220 sq ft wings. The Bearcat used a 35'6" wing of 244 sq ft and put the wheels in the fuselage. It is not just the length of the landing gear legs but where do you put the wheels/tires. Just for numbers if you have a wing with 100" of cord that has a 16% section the max thickness of the wing is 16" you may have quite an area of the wing to chose from in where you put the wheel. A wing with 80" of cord and still 16% section has a max thickness of 12.9". the area to choose from just got a lot smaller. If you want a thin section wing to avoid compressability problems and a small wing area the area of the wing to stick landing gear and guns into gets rather small. If your nation happens to use somewhat large guns things can get a bit sticky. Use a small wing with a thick high drag section to get volume or a larger thin section wing to get the needed volume?

Think of a Mustang. roughly an 11 ft propeller. Now put a 13 ft prop in the same place. wheels are in the fuselage/wing root so no problem. But the hinge point needs to move out 1 foot in each wing or we need telescoping struts or that double hinge from the Bearcat or the guns and ammo have to move a foot further out each wing, assuming there is room it does nothing for the aircraft's roll response. The other solutions add weight and complexity (cost) but can be done. Or maybe part trick undercarriage and the guns ammo only move 4-6" out each wing.

Or maybe the 1850hp "A" series engine only needs a 12'4" three bladed prop? After all it only has a about 3/4 of the power of the Bearcat if the Bearcat is using water injection and with a single stage engine it has 75% of the power of the F4U-1 at 22,500ft so it doesn't need the big prop. But with 75% of the power you aren't going to get the same performance either.

I can also imagine the reaction of the air staff in 1940/early 1941 if this idea is pitched to them. A plane with 50% more power than an F4F using an engine about 1/2 ton heavier but carries the same number of guns (4) and less ammo? Is the reaction going to be "why didn't we think of that!!!" or a polite "don't call us, we'll call you.......Next please)

 

[wuzak]

I can't see what the problem woudl have been with a de-navalised version of the F4U-1.

Maybe the USAAF would have preferred the turbocharged XF4U-3, but overall I think that the F4U-1 was competitive with the Fw190A as it was.

btw the bent wing served two purposes - it allowed for a shorter landing gear and had the wing-fuselage interface at 90°, theoretically the best aerodynamically, to minimise the requirement for wing root fillets.

 

[Shortround6]

I don't think there is much of a problem with a "de-navalised version of the F4U-1" except that it doesn't seem to fall into the time line originally proposed.

However there is a bit of a problem with the time line as originally proposed. If you start work in Jan 1940 you need to practically tie some of the fastest development programs of WW II in order to get the proposed fighter into just one service squadron by Jan of 1942. It took until Oct/Nov 1942 to get 500 P-47s and many of the them were the "B" model which was never used for combat but then the "B" and "C" were ordered on the same day. It took months longer to get the F4U into production.

If the goal is to have a combat ready fighter in production and large scale squadron (not single squadron) service in, say, the summer of 1942, then production would have to start around Dec of 1941. at least of an "A" or MK I version.

Maybe the idea is to see what we would have done differently but the deployment of an R-2800 fighter wasn't going to come much sooner than it did unless you use the simplest R-2800,the "A" to start, followed by the "B" for most production planes, in the single stage two speed form, use an existing prop hub (3 blade) and build the simplest,least tricky airframe you can. Landing gear that folds up like a piece of origami may be trick but it sucks up a lot of design time. With dozens of men using drafting tables, T-squares, triangles and drafting pencils to make all the drawings, the fewest, simplest parts are going to be the way to get the airplane flying fastest.

 

[Siegfried]

Corsair did have 2 (two) inter-coolers, one in each wing root.

Thanks, I was under the impression these were purely oil coolers. Were these air to air or was their some kind of interposing liquid circuit?

 

[Shortround6]

All US designed engines used air to air until the P-82? The Intakes on the F4U-1 in the wing roots provided combustion air, intercooler air and oil cooler airflow. Later F4Us added extra scoops and split up some of the functions.

 

[tomo pauk]

The US manufacturers did find a way to avoid the issue of of undercarriage vs prop clearance but they didn't do it with 190-220 sq ft wings. The Bearcat used a 35'6" wing of 244 sq ft and put the wheels in the fuselage. It is not just the length of the landing gear legs but where do you put the wheels/tires. Just for numbers if you have a wing with 100" of cord that has a 16% section the max thickness of the wing is 16" you may have quite an area of the wing to chose from in where you put the wheel. A wing with 80" of cord and still 16% section has a max thickness of 12.9". the area to choose from just got a lot smaller. If you want a thin section wing to avoid compressability problems and a small wing area the area of the wing to stick landing gear and guns into gets rather small. If your nation happens to use somewhat large guns things can get a bit sticky. Use a small wing with a thick high drag section to get volume or a larger thin section wing to get the needed volume?

My original proposal was a plane with 250 sq ft, not 190-220

Think of a Mustang. roughly an 11 ft propeller. Now put a 13 ft prop in the same place. wheels are in the fuselage/wing root so no problem. But the hinge point needs to move out 1 foot in each wing or we need telescoping struts or that double hinge from the Bearcat or the guns and ammo have to move a foot further out each wing, assuming there is room it does nothing for the aircraft's roll response. The other solutions add weight and complexity (cost) but can be done. Or maybe part trick undercarriage and the guns ammo only move 4-6" out each wing.

Or maybe the 1850hp "A" series engine only needs a 12'4" three bladed prop? After all it only has a about 3/4 of the power of the Bearcat if the Bearcat is using water injection and with a single stage engine it has 75% of the power of the F4U-1 at 22,500ft so it doesn't need the big prop. But with 75% of the power you aren't going to get the same performance either.

1850 HP version perhaps until the production of the two-stage machines ramps up (without wing tanks, only 250 gals in hull), but with 4-bladed prop from B-26?

I can also imagine the reaction of the air staff in 1940/early 1941 if this idea is pitched to them. A plane with 50% more power than an F4F using an engine about 1/2 ton heavier but carries the same number of guns (4) and less ammo? Is the reaction going to be "why didn't we think of that!!!" or a polite "don't call us, we'll call you.......Next please)

Now I'm scratching my head about where when I've proposed a 4 gun plane for this thread?

 

[davparlr]

Not quite so unsupportable. It is 1250lbs for the engine swap alone on the P-40C, not including fuel systems.

I come out with different numbers. The engine weight of the R-2800-10 (F6F-3 engine) is 2469 lbs and the engine weight of the P-40C with cooling is 1645 lbs or an 824 lbs difference. This would allow nearly 700 lbs for gear and other strengthening efforts including engine mounts, quite a bit. Remember, I am not recommending modifying the P-40, only using it for parameters in assessing risk in a AAF spec for a 7000 lb empty weight fighter using the new R-2800 engine.

That is for the R-2800-8 engine with two stage supercharger. Of course we instantly run into a problem, swapping the 11 ft prop on the P-40 for the 13 ft 4in prop from the Corsair means we need a lot longer landing gear.

This should be no problem. The F6F gear weighs 733 lbs, the P-40C's gear weighs 628 lbs, only 105 lbs more for a much stronger than necessary gear.

Assuming you can solve that one with little weight increase (and lots of luck with that trick) We run into problem #2

Why is that a problem when it is designed to the requirement? My American engineers are certainly as good as the German's; I guess your American engineers are not.
.

The P-40C was rated at 12"G"s ultimate load factor at about 7500lbs. Changing to an 8750lb gross weight (allowing for the engine swap only) lowers the load factor to 10.28 "G"s which was not acceptable to US authorities at the time. Yes you allowed some extra weight for strengthening but but the strengthening is for the gross weight not empty. The "yardstick" F8F-1 went 9386lbs gross, and the P-40 might not have been rated with the aft tank full? even if it was, going from 7500lbs to 9386lbs means the ultimate load factor dropped to 9.6 which even the British and Germans would not have accepted.

This actually doesn't appear to be a big problem. The P-51A, at fighter weight, grossed out at 8153, while the P-51D, at fighter weight, grossed out at 9600 lbs with no change in structure. I would accede to some weight increase but not much, especially since it would be designed in.

The F8F was designed with trick wings with break away tips to avoid overloading the entire wing allowing for lighter structure. The tips were deactivated, beefed up and the plane was restricted in the "G"s it could pull.

If designed from the ground up for land based operations, I have no doubt enough weight savings could be made to make the wings strong enough to meet AAF stress requirements. It is interesting that smart guys can come up with some really dumb ideas.

By the way, going from 7500lbs to over 9000lbs is going to require heavier landing gear of some sort (even if only heavier tires).

See previous comment above on F6F gear.

The Bearcat at 9386lbs carried 300rpg, about 100lbs less ammo than P-51B and 120lbs less than an F4F-3.

Standard load for AAF in 1941 seems to be about 200 rounds per gun.

One reason for the FW 190 light weight might be the fuel capacity 524 liters on the early models? 138.5 US gallons. Not bad for a defensive interceptor but not good for plane trying to take the fight to the enemy. The F8F-1 carried 185 gallons. 300lbs difference in fuel for the loaded airplanes.

If you could find a place for it, you could add 100 gallons of fuel and it would still weigh less than the gross weight of the F8F.

Get some graph paper and make a chart, Hp up the side and altitude across the bottom.

Thanks. I did this comparing the B-26 engine against the BMW 801D and it is interesting that at 17,000 ft., the power lines are almost identical. When mapped with the -8 engine, a more time critical comparison, the -8 engine has about 400 hp advantage at 25k. That is, if my engine data is correct. I have no data on the BMW 801C, but it appears much less powerful at altitude since the top speed of the Fw-190A1/2 is only 388 mph.

As pointed out above, ammo is part of the gross weight and part of the load used to figure the strength limits of the aircraft and it's required airframe strength. Most WW II fighters, if measured "clean" actually will show a fairly small range of useful lad to either gross weight or empty weight. A Razor back P-47 at 12700lbs gross has about 28% of that weight as useful load, at 13,582lbs it has about 36% useful load. A P-40C at 7500lbs has a 30% useful load and most other single engine fighters are going to be around 30% give or take a few percent. The higher the percentage of useful load while keeping up strength the better the aircraft designer and engine designers did their job. Sacrificing useful load for performance means the plane will be lacking in some way. Light armament or short range or both.

I have no argument with this. I feel the AAF made the correct decision in building large powerful aircraft capable of taking the fight to the enemy even if it did mean there was a lack of capability in 1941-42 time frame. However, I am trying to fill in that shortage by specing an aircraft about the same time as the F4U was speced that would fill in the 1941-42 performance deficiency of US fighter aircraft. The Navy ordered a R2800 test aircraft in June '38. If the Army had done the same thing with a light weight fighter, this is what I would have built.

The US manufacturers did find a way to avoid the issue of of undercarriage vs prop clearance but they didn't do it with 190-220 sq ft wings.

I think this problem is overstated. The P-51H with a 2200 hp engine seemed to work pretty well with a standard type landing gear.

I can also imagine the reaction of the air staff in 1940/early 1941 if this idea is pitched to them. A plane with 50% more power than an F4F using an engine about 1/2 ton heavier but carries the same number of guns (4) and less ammo? Is the reaction going to be "why didn't we think of that!!!" or a polite "don't call us, we'll call you.......Next please)

Well, the question wasn't what the air staff would do, but what we would do.

I don't think there is much of a problem with a "de-navalised version of the F4U-1" except that it doesn't seem to fall into the time line originally proposed.

I think a simpler F4U design without the hindrance of Navy requirements could have been fielded much faster. There doesn't seem to be too much of a problem for the engine development. Also, a P-66 type should offer far less complexity than the P-47.

However there is a bit of a problem with the time line as originally proposed.

I agree. Work would have to have started in '39, even with this much simpler design.

Maybe the idea is to see what we would have done differently but the deployment of an R-2800 fighter wasn't going to come much sooner than it did unless you use the simplest R-2800,the "A" to start, followed by the "B" for most production planes, in the single stage two speed form,

Yes

use an existing prop hub (3 blade) and build the simplest,least tricky airframe you can. Landing gear that folds up like a piece of origami may be trick but it sucks up a lot of design time.

I disagree with this. I don't think a bigger prop is a particularly big deal.

 

[Shortround6]

My original proposal was a plane with 250 sq ft, not 190-220

My apologies, another poster has been talking about 190-220 sq ft wings.

1850 HP version perhaps until the production of the two-stage machines ramps up (without wing tanks, only 250 gals in hull), but with 4-bladed prop from B-26?

There only about 2,485 1850 "A" series engines ever built. Production stopped at East Hartford in Jan 1942 and at Ford in March of 1942. All of 1942 saw 1,085 of the two stage engines built, 740 of them in the last 4 months of the year. 1942 did see about 10,000 of the 2000hp "B" series single stage engines built though.

I had forgotten about the B-26 prop being four blade that early, although it might have been 13'6" in diameter. As long as you have a hub shorter blades aren't really a big problem.

A question I have about going from the single stage engine to the two stage is do you add a couple of feet of fuselage to the model that uses the two stage or does the single stage version just fly around with a few feet of empty fuselage behind the engine?

fuel tank has to be on or close to the CG. For fat planes like the P-47 and the Hellcat the fuel can go low with cockpit over part of the tank/s. In a skinny fuselage plane like the F4U the fuel (most of it) has to go between the engine and cockpit. The second stage and the intercoolers need a lot of volume but don't actually weigh that much. 300-400lbs? Now maybe you can trade some fuselage tank space for space to put the intercoolers and ducts. I am also thinking of a P-51 type wing. Landing gear has to go about where the P-51s landing gear went. Folding it backward puts it into the wing fuel tanks. Guns have to be moved out 6" per side to clear bigger prop and longer landing gear ( allowing for a 12'4" prop ?)

Now I'm scratching my head about where when I've proposed a 4 gun plane for this thread?

Again my apologies, you haven't but another poster has, in order to keep the weight down. Even with the "B" series 2000hp single stage engine it looks like around only 100hp (or less) advantage over a 109G with a DB 605A engine at 19,000-20,000ft and we know the R-2800 fighter is bigger, heavier and will have more drag.

It will be like a super P-40. Great at low altitude but rather out of it's element at 20,000ft and above. How close it gets to 20,000ft and is still good is the question. Once it gets the two stage things get a lot better but then it is a 1943 airplane.

 

[Shortround6]

I come out with different numbers. The engine weight of the R-2800-10 (F6F-3 engine) is 2469 lbs and the engine weight of the P-40C with cooling is 1645 lbs or an 824 lbs difference. This would allow nearly 700 lbs for gear and other strengthening efforts including engine mounts, quite a bit. Remember, I am not recommending modifying the P-40, only using it for parameters in assessing risk in a AAF spec for a 7000 lb empty weight fighter using the new R-2800 engine.

An engine swap is not just the weight of the engine and radiators, the R-2800 needs bigger oil tanks, bigger oil coolers, a heavier exhaust system, a bigger starting system and a bigger propeller. Prop on an a F4U weighed 494lbs compared to the 337lb prop on a P-40C.

This should be no problem. The F6F gear weighs 733 lbs, the P-40C's gear weighs 628 lbs, only 105 lbs more for a much stronger than necessary gear.

Fine, a 16% increase in landing gear weight.

Why is that a problem when it is designed to the requirement? My American engineers are certainly as good as the German's; I guess your American engineers are not.

Was the Fw 190 designed to the same "G" limits as the American planes? The 109 was not. "My" American engineers are just as good, they just can't change the laws of physics.

This actually doesn't appear to be a big problem. The P-51A, at fighter weight, grossed out at 8153, while the P-51D, at fighter weight, grossed out at 9600 lbs with no change in structure. I would accede to some weight increase but not much, especially since it would be designed in.

Actually the P-51D was NOT stressed to 12 Gs at 9600lbs. It was stress for 8 Gs at 8000lbs with a 1.5 safety factor which is were the 12Gs comes from. All flight maneuvers required the actual weight of the plane to divided into 64,000 (8,000lbs X 8 Gs) to get the actual "G" limit. At 9600lbs a P-51D was rated for 6.66Gs plus the 1.5 safety factor or 10 "G"s. By the way, if you can call splicing in a 7in high horizontal section for a good part of the length of the fuselage "no Change in structure" what else are you glossing over? Do you have the weight of the wing of a P-51A?

A Hawk 75a (P-36 had wing that weighed 842lbs, the P-40C had wing that weighed 1002 and the P-40E-N had a wing that weighed about 1120lbs.

Standard load for AAF in 1941 seems to be about 200 rounds per gun.

Standard load in what? the P-39 with CG problems when it fired off the ammo?
Standard load in an early P-40, soon changed to 380rpg. Not the brightest move, the sychro-ed guns fired at under 500rpm. wing guns fired at about 800rpg giving you 15 seconds of firing time. Navy pilots preferred 4 guns with 400rpg to 6 guns with 240 for the longer firing time.
P-40s with 6 guns were often loaded with 200-235rpg in an effort to save weight for performance even though the ammo spaces would hold more. But 6 .50cal P-40s are pretty thin in 1941 aren't they?

If you could find a place for it, you could add 100 gallons of fuel and it would still weigh less than the gross weight of the F8F.

Well that is the trick isn't it? finding space on the center of gravity (or tanks space for and aft 9or side to side) to balance on the center of gravity, plus the weight of the tanks, plus the piping (fuel piping on a F4U-1 was over 100lbs) without making the fuselage or wing any bigger and adding weight that way.

I have no argument with this. I feel the AAF made the correct decision in building large powerful aircraft capable of taking the fight to the enemy even if it did mean there was a lack of capability in 1941-42 time frame. However, I am trying to fill in that shortage by specing an aircraft about the same time as the F4U was speced that would fill in the 1941-42 performance deficiency of US fighter aircraft. The Navy ordered a R2800 test aircraft in June '38. If the Army had done the same thing with a light weight fighter, this is what I would have built.

The Navy ordered the F4U on trust. The R-2800 doesn't make it's first flight until June 12, 1939 in the nose of a Northrop Y-19 test mule. Not that the Army couldn't have gone on trust but only 2 R-2800s were built in 1939 and only 17 more in 1940. Ford was given a contract for $14.3 million to build a factory for the production of R-2800s at a point in time when P&W had completed a total of EIGHT R-2800s. this contract was just for the factory, contracts for engines were separate. This was several months AFTER design work was stared on both the "B" and "C" series engines.
There just aren't many R-2800s of any type available until the fall of 1941, and I as noted in the Post to Tomo the "A" series goes out of production rather quickly, being replace by the "B" series single stage engine.

I think this problem is overstated. The P-51H with a 2200 hp engine seemed to work pretty well with a standard type landing gear.

Yep, but then it had the advantage of a 1944/45 propeller design. Hamilton Standard delivered it's first 2 bladed, two pitch propeller in 1935. Just like engines, propellers made tremendous strides in just a few years but expecting 1944 performance from a 1941 propeller may be asking to much. Try sticking an 11 ft diam 3 bladed 'tooth pick' prop on the P-51H and see how well it does.

I think a simpler F4U design without the hindrance of Navy requirements could have been fielded much faster. There doesn't seem to be too much of a problem for the engine development. Also, a P-66 type should offer far less complexity than the P-47.

A P-66 would be much simpler than a P-47 but then sicking an 1800-2000hp engine in/on a jumped up basic trainer is going to lead to all sorts of problems. I know, you are only using the P-66 as an indicator of size. But with that big R-2800 you are going to need bigger tail surfaces or move them back (longer fuselage). We are back to the landing gear problem, you can't use the same diameter prop as the R-1830 engine. The armament of the P-66 was barely adequate for 1941 and not at all what the customers wanted if given any choice at all. And 220-240 gallon capacity seems rather suspect given the empty and loaded weights quoted for the plane. Maybe the tanks would hold that much but what else had to be left out of the plane in order to fill them? You can find weights for US Navy planes in "ferry" condition. Lots of fuel but they actually pulled the guns out let alone the ammo in order to get to that weight.

I disagree with this. I don't think a bigger prop is a particularly big deal.

It is if you don't want the prop plowing ground. Remember you are designing in 1939-40, what kind of propeller do you KNOW you can get in Jan of 1942, not what kind of propeller do you HOPE to get.

 

[wuzak]

I don't think there is much of a problem with a "de-navalised version of the F4U-1" except that it doesn't seem to fall into the time line originally proposed.

According to Graham White, R-2800 - Pratt Whitney's Dependable Masterpiece, the first F4U-1 was delivered on June 30 1941, the first P-47B was accepted in December 1941. 171 P-47Bs were built and 688 F4U-1s. Also, the XF4U flew a year before the XF-47B (May 1940 vs May 1941).

Vought F4U Corsair - Wikipedia, the free encyclopedia
Republic P-47 Thunderbolt - Wikipedia, the free encyclopedia

The operational introduction for the F4U is shown as December 1942, the P-47 is just noted as 1942.

Joe Baugher gives the P-47 delivery date as mid 1942.

P-47Bs were first issued in mid-1942 to the 56th Fighter Group. This group was chosen to be the first recipient of the P-47B because it was based near New York City and hence located near the Farmingdale plant where Republic engineers could be easily called upon to help in ironing out problems as they arose. The P-47Bs of the 56th Fighter Group were used largely for stateside testing and operational training, and very few ever went overseas.

The 56th Fighter Group found the process of working up to its new mounts rather difficult--13 pilots and 41 aircraft were lost in accidents. By the end of June, the 56th FG had damaged or wrecked half of its aircraft. Many of the crashes were the result of pilot inexperience, but a significant number were caused by loss of control during high-speed dives. After a rudder was ripped from a P-47B in flight, an order was issued on August 1, 1942, restricting the speeds to 300 mph or lest, forbidding violent maneuvers, and stipulating that fuel be carried in the rear tank.

Republic P-47B Thunderbolt

But by late 1942 the British responses to the Fw190A were already in production. The Typhoon, while experiencing technical difficulties, was operational in mid 1941. The Spitfire Mk IX was introduced in the first half of 1942, while the Griffon powered Spitfire XII started deliveries in October 1942.

On the Spitfire XII:

At low altitude it was one of the fastest aircraft in the world; in one speed trial, held at Farnborough in July 1942 DP845 (now referred to as the Mk XII) piloted by Jeffrey Quill raced ahead of a Hawker Typhoon and a captured Focke-Wulf Fw 190, to the amazement of the dignitaries present.

On reflection the general scheme became clear. The Spitfire was to be a sort of datum pacemaker - 'Mr Average Contemporary Fighter' - and its job would be to come in last, the real excitement of the proceedings being by how much it would be beaten by the FW 190 and the Typhoon
...Outside on the tarmac at Worthy Down stood the in-offensive looking but highly potent DP485... All went according to plan until, when we were about halfway between Odiham and Farnborough and going flat out, I was beginning to overhaul the FW 190 and the Typhoon. Suddenly I saw sparks and black smoke coming from the FW 190's exhaust...and I shot past him and never saw him again. I was also easily leaving the Typhoon behind and the eventual finishing order was, first the Spitfire, second the Typhoon, third the FW 190...It certainly put the cat among the pigeons and among the VIPs.


However pilots found it difficult to exploit this advantage in combat as German pilots were reluctant to be drawn into dogfights with Spitfires of any type below 20,000 feet (6,100 m). When the Mk XII was able to engage in combat it was a formidable fighter and several Fw 190s and Bf 109-Gs fell victim to it. The Mk XII's speed advantage at lower altitudes again became useful near the end of its front line service in Summer 1944, in which it shot down a respectable number of V-1 Flying Bombs, 82.5. The Mk XII variant was retired in September 1944.

Supermarine Spitfire (Griffon powered variants) - Wikipedia, the free encyclopedia

The F6F, which was simpler, at least in appearence, than the F4U, didn't enter service until 1943.

Grumman F6F Hellcat - Wikipedia, the free encyclopedia