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

[tomo pauk]

After hard work, time for some what-if:
As the XR-2800 proved it's potential, the decision is made to build a fighter plane around it. What supercharger set-up would've you choose? Perhaps we could divide it in 3 groups:
-turbo-charger for the 2800 (a P-47-like design, with great all-altitude speed, but dictating a hefty airplane with poor mileage climb rate)
-two-stage supercharger (something along F4U, good performance, even better for a true land-based version)
-single stage supecharger (basically the engine from B-26, less power than the versions from above, high blower FTH at ca. 14kft (unless modified), but needing a smaller airframe - better mileage climb rate? - a pre-F8F?)

The time frame is 1940-42, from basic design 'till start of production.
Any takers?

 

[davebender]

Skip the turbo.

RR Merlin and DB601 engines performed adequately at high altitude using compact supercharger systems. Copy the best features of British and German supercharger systems for the new American fighter aircraft engine. If you want to get really clever copy the German fuel injection and hydraulic supercharger coupling also.

 

[post76]

After hard work, time for some what-if:
As the XR-2800 proved it's potential, the decision is made to build a fighter plane around it. What supercharger set-up would've you choose? Perhaps we could divide it in 3 groups:
-turbo-charger for the 2800 (a P-47-like design, with great all-altitude speed, but dictating a hefty airplane with poor mileage climb rate)
-two-stage supercharger (something along F4U, good performance, even better for a true land-based version)
-single stage supecharger (basically the engine from B-26, less power than the versions from above, high blower FTH at ca. 14kft (unless modified), but needing a smaller airframe - better mileage climb rate? - a pre-F8F?)

The time frame is 1940-42, from basic design 'till start of production.
Any takers?

The turbo would be ideal for consistency if it didn't require such a large airframe to house it.
Otherwise, the compromise for weight would be the two stage supercharged version, i don't see much point of the one-stage.
After reviewing other comparisons i sometimes question why the F4U wasn't also deployed in the ETO as a land based fighter.

 

[Shortround6]

Skip the turbo.

RR Merlin and DB601 engines performed adequately at high altitude using compact supercharger systems. Copy the best features of British and German supercharger systems for the new American fighter aircraft engine. If you want to get really clever copy the German fuel injection and hydraulic supercharger coupling also.

Neither engine performed at 30,000ft or above like a turbo engine. The Merlin set up was as compact as it was going to get and it needed that big intercooler radiator under the wing of the Spitfire. The Merlin two stage supercharger also doesn't show up until late 1941 (in the shops) or 1942 (in service) also making in hard to copy in 1940-41.
Coping the oh so clever German fuel injection system means lots of manufacturing expense. American carburetors did not suffer from the negative "G" problem that the British carburetors did so no advantage there. Using the the German fuel injection also means no cooling of the intake charge from fuel evaporation so you have to use less boost which means less power.

DB 601s performed like crap at high altitude, not a one had a full throttle altitude over 5,000meters. Allisons could get a 4400meter full throttle altitude once they used 9.60 gears. The DB 605s did better but the high altitude ones don't show up until when? 1944? Nice trick coping that in 1941-42.

P&W did use the hydraulic coupling on the R-2800 used in the Bearcat F8F-2. They also used a "C" series power section which doesn't go into production until 1944. Using 115/145 fuel AND water injection it was good for 1600hp at 22,000ft.
Earlier Bearcats were good for 1600hp at 16,000ft and they also used "C" series engines.

An "A" series engine was good for 1850hp take-off and 1500hp at 2600rpm at 14,000ft with a two speed single stage supercharger.
A "B" series engine was good for 2000hp take-off and 1600hp at 2700rpm at 13,500ft with a two speed single stage supercharger.
A "B" series engine was good for 2000hp take off, 1800hp at 2700rpm at 15,500ft and 1650hp at 2700rpm at 22,500ft with the two stage supercharger.
A "B" series engine was good for 2000hp take-off and 2000hp at 25,000ft with the turbo.

Mileage depends on speed and altitude flown.

P-47s got bad "mileage" because they were hauling twice the guns of an early Bearcat and were carrying almost twice the fuel depending on model. Prototype Bearcat had 150 gals of internal fuel, production models had 185gals. Exactly 1/2 the internal fuel of late P-47D's. While later P-47s may have gotten dirtier (more drag) a P-47B had less profile drag than a F4F-3 Wildcat. I kind of doubt the Bearcat was that much more streamlined than the F4F-3 that it was going to show any significant improvement over the P-47 in "mileage" if flown at the same speeds and near the same altitudes.

 

[jim]

I like the F4U-4/5 applications . I also find in turbo engines more negatives than positives.Not really nessecary. I also like the concept of F8F (low weight) but it appears that led to unsatisfactory airframe strength for useful combat weights.

 

[davebender]

During 1940 to 1942 nobody flew that high except recon aircraft. Nor is there a requirement for a heavy bomber escort as the U.S. Army Air Corps are confident the B-17 can defend itself. Initial models of the new fighter aircraft should be designed for good performance up to 20,000 feet.

During the second half of 1942 the need for a B-17 escort will become apparent. That's when a high altitude variant of the engine will be designed, to enter service during 1943.

 

[Shortround6]

During 1940 to 1942 nobody flew that high except recon aircraft. Nor is there a requirement for a heavy bomber escort as the U.S. Army Air Corps are confident the B-17 can defend itself. Initial models of the new fighter aircraft should be designed for good performance up to 20,000 feet.

Both Spitfires and 109s were flying at 30,000 or just under during the BoB. The P-47 was NOT designed as a bomber escort.

The Spitfire MK V and the Hurricane II were both intended to perform at 20,000-30,000ft. Both Spitfires and Hurricane I's had fought at well over 20,000ft during the BoB.

During the second half of 1942 the need for a B-17 escort will become apparent. That's when a high altitude variant of the engine will be designed, to enter service during 1943.

Waiting until the second half of 1942 to design (or modify an existing design) will GUARANTEE that it will not be available in large numbers until the summer or fall of 1944.

2 years from design to FIRST squadron service is doing quite well, How many more months from first squadron service to equipping multiple groups?

The P-47 was started by Sept of 1940 with 733 being ordered that month. The 56th Fighter group gets it's first few aircraft in June of 1942. The last of 171 production P-47Bs rolls out the factory door ( not issued to a military unit yet) in Sept 1942. The P-47Bs are retained in the US as conversion trainers and never serve overseas.

The F4U was ACCEPTED in Feb 1941, a letter of intent to enter into a production contract was issued Mar 3rd 1941. VMF-124 was declared "operational" in Dec 1942 and sailed from San Diego in Jan 1943.

The F6F was started in June of 1941, it benefited from some preliminary studies already done by Grumman. First flight ( by the Wright powered prototype) is in June of 1942 which is quite remarkable. Jan 16 1943 sees navy squadron VF-9 issued it's first F6Fs. This is one of the fastest development programs of WW II.

Even the Mustang, for all of the 117 day from start to roll out (not flight) timing, doesn't equip a British squadron until April 1942, 23 months after the British approved the design. First combat operation isn't May 10, 1942.

I cannot think of a single fighter that actually designed in 'response' to an enemy fighter when the actual timing is taken into account. If you are designing a plane to "beat" the Zero or 109 in 1941-42 you are 2-3 years behind the goal. The Japanese and German designers already had the Zero and 109 replacements on the drawing boards if not cutting metal on them. For various reasons the newer replacements didn't work out but aiming just to equal or just exceed the performance of a fighter already in service is setting a mighty low goal.

 

[Shortround6]

I like the F4U-4/5 applications . I also find in turbo engines more negatives than positives.Not really nessecary. I also like the concept of F8F (low weight) but it appears that led to unsatisfactory airframe strength for useful combat weights.

It depends on your goal and your existing technology.

The R-2800 that powered the F4U-5 was a remarkable engine. It was also not even a gleam in the engine designers eye in 1941-42. The only things it had in common with the R-2800 that powered the F4U-1 was the dimensions of the of the bore and stroke and perhaps the starter dog. There were no common or interchangeable major parts or even minor ones. Not even valve springs or piston pins.

Saying the turbo wasn't needed in 1943 and offering up a 1947 engine as proof doesn't really prove much.

The F8F was a great plane, but it was designed for specific missions and wasn't capable (or intended) of performing all missions.

 

[wuzak]

Neither engine performed at 30,000ft or above like a turbo engine. The Merlin set up was as compact as it was going to get and it needed that big intercooler radiator under the wing of the Spitfire. The Merlin two stage supercharger also doesn't show up until late 1941 (in the shops) or 1942 (in service) also making in hard to copy in 1940-41.

Pratt Whitney already had a two stage engine by 1941 - the R-1830 for the F4F-3 had two stage two speed supercharging from 1939.

The intercooler radiator for the Spitfire was still smaller than the air-air intercoolers used in American two stage and turbocharged aircraft. In fact, if you look at the Mosquito with two stage engines the intercooler is fed by the small chin intake.

The size of the P-47 was all down to the ducting - some for the exhasut to the turbo, and some for the intake air from the turbo compressor, but the bulk was for the intercooler air.

 

[wuzak]

Coping the oh so clever German fuel injection system means lots of manufacturing expense. American carburetors did not suffer from the negative "G" problem that the British carburetors did so no advantage there. Using the the German fuel injection also means no cooling of the intake charge from fuel evaporation so you have to use less boost which means less power.

Not all American carbies coped with negative G. The prototype XP-55 fitted with an Allison V-1710 crashed when it stalled and flipped on its back, at which point the carby cut out due to being inverted (ie negative g). The aircraft had no forward speed, and with the engine cut out could not get any, so the control surfaces were of no use in changing the attitude. It remained stable, inverted and flat as it fell around 20,000ft to the ground (the test pilot got out at about 4-5000ft).

 

[wuzak]

I like the F4U-4/5 applications . I also find in turbo engines more negatives than positives.Not really nessecary. I also like the concept of F8F (low weight) but it appears that led to unsatisfactory airframe strength for useful combat weights.

The F4U-5 with its huge sidewinder auxiliary stage superchargers show what could have been done with turbochargier packaging - perhaps.

Instead of using a single C-series turbo it may have bene possible to use two B-series superchagers, mounted in a similar position to the sidewinder's mounting.

The compressor inlet would have to be pointed inwards, instead of outwards. That would leave the turbine towards the outside and allow the exhaust to directly exit.

 

[Shortround6]

The intercooler radiator on the P-51 was in the ventral duct with the radiator. Part of the size of the P-47 was due to the 305 gal fuel tank. Part of it was due to the eight gun armament requirement. And part was due to the ducting.

There is a difference between a carburetor cutting out for a few seconds or less due to negative "G" while maneuvering and a fuel system that will allow for inverted flight, as in supplying fuel for a number of seconds while inverted. There are a number of modern (1950s and newer) fuel injected light plane engines that are NOT rated for inverted flight. The fuel injection pump may supply fuel to the cylinders but the fuel systems will not supply fuel to the to the fuel injection pump. Some Jet trainers have a special small fuel tank for inverted flying and they are hardly using carburetors.

While it might be possible to place the turbos where the sidewinder superchargers are placed in a physical sense, it might not be a good idea to put them there from a mechanical sense. Putting a few more feet between the engine and the turbo allowed for a bit of cooling of the exhaust gases before they hit the turbo. The turbos were running very close to their temperature limits in many installations as it was. As the war went on later model turbos may have allowed both higher rpm and higher inlet temperatures.
You still have a number of cubic feet to devote to intercoolers and their ducting.

The F4U-5 may have been 10in longer than an F4U-4 and with the new cheek airscoops at 4 and 8 oclock the engine cowl was 8in wider at the maximum dimension.

 

[wuzak]

The intercooler radiator on the P-51 was in the ventral duct with the radiator.

Yes, I remembered that, and I corrected my post.

Part of the size of the P-47 was due to the 305 gal fuel tank. Part of it was due to the eight gun armament requirement. And part was due to the ducting.

I think the fuel tank fits entirely behind the profile of the R-2800. The rear fuselage is dominated by the turbocharger, its ducting and the intercooler. The 8 gun requirement doesn't really impinge on the fuselage because they are all mounted in the wings. The size and weight of the fuselage does have an affect on the wing area required.

The use of air:air intercoolers in the XP-37/YP-37 and XP-39 are part of the reason why they weren't very successful. The cockpit had to be relocated aft on the XP-37 to allow space for the intercooler, while the scoop/duct for the intercooler in the XP-39 was poor and contributed significantly to drag, and was one of the reasons why NACA recommended the removal of the turbo.

While it might be possible to place the turbos where the sidewinder superchargers are placed in a physical sense, it might not be a good idea to put them there from a mechanical sense. Putting a few more feet between the engine and the turbo allowed for a bit of cooling of the exhaust gases before they hit the turbo. The turbos were running very close to their temperature limits in many installations as it was. As the war went on later model turbos may have allowed both higher rpm and higher inlet temperatures.
You still have a number of cubic feet to devote to intercoolers and their ducting.

The F4U-5 may have been 10in longer than an F4U-4 and with the new cheek airscoops at 4 and 8 oclock the engine cowl was 8in wider at the maximum dimension.

A water to air intercooler like that on the Merlin could allow a more flexible installation - it would need less space for the radiator and could be placed remotely.

Mounting the turbos on the side like that and close to the exhaust may have caused temperature problems. Most turbo installations had the turbo closer to the exhaust outlets than did the P-47. Many installations had the turbine outlet exposed to the air - like n the P-38 and the B-17.

Mounting th eturbos in the sidewinder position would not have the intakes in the same place as on the F4U-5. The exhausts would be on the outside, with the intakes towards the centre of the plane.

 

[tomo pauk]

...

An "A" series engine was good for 1850hp take-off and 1500hp at 2600rpm at 14,000ft with a two speed single stage supercharger.
A "B" series engine was good for 2000hp take-off and 1600hp at 2700rpm at 13,500ft with a two speed single stage supercharger.
A "B" series engine was good for 2000hp take off, 1800hp at 2700rpm at 15,500ft and 1650hp at 2700rpm at 22,500ft with the two stage supercharger.
A "B" series engine was good for 2000hp take-off and 2000hp at 25,000ft with the turbo.

Thanks for the data
Could we have some data about the weight penalty of single stage, two stage turbo R-2800s?

Mileage depends on speed and altitude flown.

P-47s got bad "mileage" because they were hauling twice the guns of an early Bearcat and were carrying almost twice the fuel depending on model. Prototype Bearcat had 150 gals of internal fuel, production models had 185gals. Exactly 1/2 the internal fuel of late P-47D's. While later P-47s may have gotten dirtier (more drag) a P-47B had less profile drag than a F4F-3 Wildcat. I kind of doubt the Bearcat was that much more streamlined than the F4F-3 that it was going to show any significant improvement over the P-47 in "mileage" if flown at the same speeds and near the same altitudes.

The weight plays the part too - heavier plane would consume more fuel to climb at cruise altitude.

 

[Shortround6]

Thanks for the data
Could we have some data about the weight penalty of single stage, two stage turbo R-2800s?

AN "A" series R-2800 went about 2270lbs with the single stage two speed supercharger. A "B" series engine with the same supercharger went about 2300lbs. A "B" series engine with two stage supercharger went about 2480lbs and a "B" series with single stage single speed supercharger (for use with turbo) went about 2265lbs. I have no data on weights of turbochargers or the weights of intercoolers for the turbo and two stage engines.

The weight plays the part too - heavier plane would consume more fuel to climb at cruise altitude.

While that is true once at cruising altitude the difference is only a few percent. Be aware that many "benchmark" ranges are at totally useless speeds and altitudes for some combat uses. Many ranges for Navy planes are at speeds like 163 to 185 mph and at altitudes of 5,000ft were the auxiliary stage of the supercharger is not being driven and so using no power, unlike trying to cruise at 20,000ft or higher.

Why do some modern (60s and up) light planes like Beechcraft's and Cessna's use trubo charged engines to allow them to cruise at 12,000-18,000ft instead of 5,000-8,000ft? Less drag in thinner air means more speed for the same fuel burn.

 

[davparlr]

I think I agree with all the Shortround6 has said. The P-47 was a tour-de-force of high altitude performance. Its flat rated power up to 33k was unmatched from any WW2 fighter I know, except maybe the P-38. It would have been unwise to modify this program.

In 1940, 41, and 42, the US did not have the fighter maturity as it did the bombers and transports, and did not field competitive aircraft against the Axis when we entered the war, especially in the low to mid altitudes. I think I would have left the P-47 program alone and would have encouraged the development of a smaller and lighter R-2800 vehicle. This could have been a plane based on the F4U, but not the F4U. It would have smaller, straight, wings, and less fuel, 180-220 gal. Empty weight would be around 7000 lbs, wing area 200-220 sqft. Initially, this plane would have the B-26, xF4U-1 type engine.

Another alternative would be to have Vultee redesign the P-66 to include the R-2800, which would include adjusting to the increased weight (the first Vanguard already had a heavier and longer aero front end) and size. Wing area, fuel quantity, and weight would increase. The plane was reported to be favored by pilots for its flying characteristics.

This aircraft would be available in late 1941 and should be faster than the XF4U (lighter and smaller wing) so it would be in 400-415 mph speed area. It should have outperformed the Fw-190A-1/2 and with the upgrade of the R-2800 would have soon over powered the Axis fighter. If this type of performance was available to the Brits in 1941, they would have bought lots. A drawback would be that if the Brits had this aircraft, would they have continued to pursue the P-51?

By the way, the stats look very close to the Fw-190, not all by accident. Think of the Fw-190 with the more powerful R2800 engine.

 

[post76]

It appears maybe the turbo would out due the supercharged version then.
Despite these facts, i've found that the performance benefit of a turbo is somewhat circumvented by aircraft limitations such as compressibility.
Although in 1942, this would only be an issue in dive, later model P-47s were reaching 470-80 in level flight which would only leave a 20-40mph speed margin before reaching compressibility (30kft).
If we are sticking with the 1940-42 data and what ifs, with hindsight in mind, i would've like to have seen the development of a P-47 wing with a similar naca curve to that of the P-51 putting the thicker part of the wing toward the middle and reducing the amount of drag on the leading edge of the airframe. This might also have changed the center of lift so the wing might actually need to be brought forward.
When teething out the P-47B model, they moved the ailerons six inches outward to help reduce aileron snatch. I wonder what such a minor adjustment in wing shape would do for the compressibility issue. Thats not really on the topic of the engine, but it seems evident that the power was there and more consistent with the turbo in place. Having a better drag profile would only increase performance.

It would be great to see the actual power chart of the engine using the different turbos and superchargers.
What i found was a bit more revealing about "on the bench" data used to find the limits of the engine but i doubt the turbo or supercharger could reproduce similar manifold pressure.

 

[Shortround6]

It appears maybe the turbo would out due the supercharged version then.

It rather depends on the altitude. For a non-turboed plane consider the F8F. the prototype was supposed to be good for 393 at sea level with 2100hp. The -1 production version was about 600lbs heavier and probably had more drag, it was good for 382mph at sea level. The -2 version added another 860lbs (and drag?) managed 387 at sea level using 2250 hp. It is what happens at altitude that gets interesting. The Prototype was supposed to do 424 at 17,300ft using a -22W engine good for 1600hp at 16,000ft. The -1 using a -34W engine (1700hp at 16,000ft) was good for 421 at 19,700ft while the -2 was good for 447mph at 28,000ft. It's -30W engine could provide 1600hp at 22,000ft. At a rough guess the prototypes engine was down to around 1400hp at 22,000ft. and was going to stay 200-150hp below the later engine at any reasonable altitude.
The turbo offers nothing but weight and drag at sea level. The question is at what altitude does it cross over to being a plus and how much time is the plane going to spend at which altitudes.

It would be great to see the actual power chart of the engine using the different turbos and superchargers.
What i found was a bit more revealing about "on the bench" data used to find the limits of the engine but i doubt the turbo or supercharger could reproduce similar manifold pressure.

I am not sure what you mean by this. Major US engine manufacturers had test chambers that could simulate almost any desired altitude (air in the chamber would be the density/pressure and temperature desired for the test) while the engine was running a dyno. Many later P&W engines also had torque meters built into the gear box so torque readings could be taken in flight and compared to the pressure readings for the manifold. They KNEW what they were getting for power and at what pressure.

 

[gjs238]

It would have been unwise to modify this program.

Since the Spanish American War, the US was fighting expeditionary wars.
It's not unreasonable to suggest that the P-47 could have/should have been designed with increased range/endurance.

In 1940, 41, and 42, the US did not have the fighter maturity as it did the bombers and transports, and did not field competitive aircraft against the Axis when we entered the war, especially in the low to mid altitudes.

There have been other threads discussing how effective and underrated the P-40 was, particularly at those altitudes.

 

[tomo pauk]

I think I agree with all the Shortround6 has said. The P-47 was a tour-de-force of high altitude performance. Its flat rated power up to 33k was unmatched from any WW2 fighter I know, except maybe the P-38. It would have been unwise to modify this program.

It took the USAAC almost half of war to throw the P-47 in combat, or a year after P-38 made it's mark on WW2. So I doubt that P-47 was such a necessity for USAAC.

In 1940, 41, and 42, the US did not have the fighter maturity as it did the bombers and transports, and did not field competitive aircraft against the Axis when we entered the war, especially in the low to mid altitudes. I think I would have left the P-47 program alone and would have encouraged the development of a smaller and lighter R-2800 vehicle. This could have been a plane based on the F4U, but not the F4U. It would have smaller, straight, wings, and less fuel, 180-220 gal. Empty weight would be around 7000 lbs, wing area 200-220 sqft. Initially, this plane would have the B-26, xF4U-1 type engine.

I like the light F4U part
My idea would be a hull of F4U, with slightly smaller thiner wing (tailored for high speed, not for good CV performance), ca. 250 sqft, 250 gal of fuel.

Another alternative would be to have Vultee redesign the P-66 to include the R-2800, which would include adjusting to the increased weight (the first Vanguard already had a heavier and longer aero front end) and size. Wing area, fuel quantity, and weight would increase. The plane was reported to be favored by pilots for its flying characteristics.

IIRC Vanguard already have had 200 (220?) gals of fuel.

This aircraft would be available in late 1941 and should be faster than the XF4U (lighter and smaller wing) so it would be in 400-415 mph speed area. It should have outperformed the Fw-190A-1/2 and with the upgrade of the R-2800 would have soon over powered the Axis fighter. If this type of performance was available to the Brits in 1941, they would have bought lots. A drawback would be that if the Brits had this aircraft, would they have continued to pursue the P-51?

In such a scenario, P-51 would not be needed (of course, NAA could've produced 'our' new fighter).

By the way, the stats look very close to the Fw-190, not all by accident. Think of the Fw-190 with the more powerful R2800 engine.

A toss up between F4U, F8F Fw-190?