Pinnacle of Piston fighter: XP-72 vs Spiteful Mk XVI?

[GregP]

According to the internet, which is sometimes less than relaible, the R-4360 VDT (Variable Discgharge Turbine) produced 4,300 hp. Also, apparently, they built and ran 1 uinit.

A-Series (Early Military Variants): ~3,000 produced
These were the earliest operational engines (e.g., R-4360-4, -13, -20) rated at 3,000 hp. They primarily powered early bombers and transports like the Douglas C-74 Globemaster and early Convair B-36 variants. [1, 3, 4, 5]

B-Series (Mainstream Production): ~12,000+ produced
The backbone of the R-4360 lifecycle. These models stepped up performance to 3,250–3,500 hp and introduced better thermal cooling. They outfitted the mass-produced B-36 bombers, Boeing B-50s, C-97 Stratofreighters, and Fairchild C-119s. [, 2, 3]

C-Series / TS3G2 (Late-Stage Commercial & Military): ~3,500+ produced
The final evolution, maximizing mechanical refinement. This series featured the "Wasp Major" commercial variants (like the TSB3-G used in the Boeing 377 Stratocruiser) and late military variants rated up to 3,800 hp. [1, 2, 3]

• R-4360-41 VDT (1 Built): The single experimental 4,300 hp engine using a variable discharge turbine, canceled in favor of jet engines.
• R-4360-4A (Packaged Prototypes): A handful of early units custom-crafted for ground training tools and flight test beds.

 

[drgondog]

Why is that so?, isn't Hornet extremely fast? Its top speed is pretty much comparable to P-51H and much faster than P-51D, shouldn't dive limit also quite high?

If the Ultimate Load factors, historically less than US, 11G to 12G, the stresses applied at high Match velocities by the Q loading will have a lower safety margin for Brit design than US. I haven't researched but suspect Spit IX and Spit XIV should show similar dive limits as the Hornet vs the P-51D shown in Pilot Operating Manual. I know the Spit survived a 0.9M dive but recall the damage to the airplane.

The 0.75M limit dive for the Mustang was highest predicted (and tested) resulting in no damage.

Drag is the primary issue and it increases exponentially above ~ 0.55M.

 

[Shortround6]

Mission changed for the R-4360 in it's history. The work on the R-4360 started in 1940. By 1944-45 Jets were the front runners for interceptors and high altitude work. Front runners but not the only path, See Vought F4U-5 with the R-2800-32W "sidewinder" engine. Look at the air intakes on the F4U-5, they were trading some drag for higher altitude power.
Adding enough supercharger capacity to get the R-4360 engines to make high power at 30,000 involves weight, volume, and drag.

Speed is power vs drag. Race planes at low altitude (5000ft) use much higher powers than any military power settings. They also have lower drag,

Much better fit/finish, much smaller canopy, other modifications to profile? Spray bars assisting cooling of the radiators/oil coolers.

In 1947 the US flew 4 different jet bomber prototypes. The B-36 prototype flew in 1946. The R-4360 had a small window of opportunity as a high-altitude engine before jets took over.

 

[don4331]

According to the internet, which is sometimes less than relaible, the R-4360 VDT (Variable Discgharge Turbine) produced 4,300 hp. Also, apparently, they built and ran 1 uinit.

A-Series (Early Military Variants): ~3,000 produced
These were the earliest operational engines (e.g., R-4360-4, -13, -20) rated at 3,000 hp. They primarily powered early bombers and transports like the Douglas C-74 Globemaster and early Convair B-36 variants. [1, 3, 4, 5]

B-Series (Mainstream Production): ~12,000+ produced
The backbone of the R-4360 lifecycle. These models stepped up performance to 3,250–3,500 hp and introduced better thermal cooling. They outfitted the mass-produced B-36 bombers, Boeing B-50s, C-97 Stratofreighters, and Fairchild C-119s. [, 2, 3]

C-Series / TS3G2 (Late-Stage Commercial & Military): ~3,500+ produced
The final evolution, maximizing mechanical refinement. This series featured the "Wasp Major" commercial variants (like the TSB3-G used in the Boeing 377 Stratocruiser) and late military variants rated up to 3,800 hp. [1, 2, 3]

• R-4360-41 VDT (1 Built): The single experimental 4,300 hp engine using a variable discharge turbine, canceled in favor of jet engines.
• R-4360-4A (Packaged Prototypes): A handful of early units custom-crafted for ground training tools and flight test beds.

Per Graham White's Major Miracle:

A-Series <10 produced (Prototypes w/ R-2800 cylinders, etc) ~2,800hp

B-Series ~15,000 produced R-4360-4, -13, -14, -19 are all listed as B series

3,000 hp 100/130 fuel​

3,250 - 3,500 hp 115/145 fuel​

Those with mechanical supercharger for 2nd stage, e.g. -13 (XP-72), -29 (XB-44) drop to 2,400hp @ 25k'; those with turbochargers, e.g. -35 (B-50A,B & D) kept 3,250hp to 30k'​

Note R-4360-14 is dual rotation*, remote supercharger version of -13​

*dual rotation is P&W's nomenclature for contra-rotating propellers​

​

C-Series ~3,500 produced

3,500 hp 115/145 fuel (dry)​

3,800 hp 115/145 fuel (wet) i.e. with water injection.​

​

There have always been some questions about where XP-72 had remote supercharger. IMNSHO, it had to for 2 reasons: First, without remote supercharger (and intercooler) the plane would have severe balance issues - why bolt in couple hundred pounds of lead instead. Second, without remote supercharger e.g. -4 engine, performance was down to 2,400hp @ 13.5k'. It wouldn't have been a very good show case of P&W's new engine.

 

[Howard Gibson]

If the Ultimate Load factors, historically less than US, 11G to 12G, the stresses applied at high Match velocities by the Q loading will have a lower safety margin for Brit design than US. I haven't researched but suspect Spit IX and Spit XIV should show similar dive limits as the Hornet vs the P-51D shown in Pilot Operating Manual. I know the Spit survived a 0.9M dive but recall the damage to the airplane.

The 0.75M limit dive for the Mustang was highest predicted (and tested) resulting in no damage.

Drag is the primary issue and it increases exponentially above ~ 0.55M.

The Mach_0.78 limit for the Mustang comes from Eric Brown. This was compared with 0.68 for the P38 Lightning, 0.71 for the P47 Thunderbolt, and 0.75 for Bf109s and Fw190s. Greg's Airplanes and Automobiles complained about this in the YouTube video US Fighter Dive Test By British Test Pilot Eric Brown. My best guess on this is that the numbers are comparable, as opposed to maximum possible. Brown probably used some force device to measure the force on the control sticks, and arbitrarily picked some measureable number as the maximum allowable. I, personally, own a 50lb spring scale. I am a big guy. I can pull more than 50lb, but I cannot measure it.

In his book Wings on my Sleeve, Brown describes diving Spitfires to Mach_0.83, using 60lb force, his limit, to pull out. This number almost certainly is not based on the protocols above, so it is not comparable. A large, powerful colleague was able to dive a Spitfire to Mach_0.92.

 

[drgondog]

The Mach_0.78 limit for the Mustang comes from Eric Brown. This was compared with 0.68 for the P38 Lightning, 0.71 for the P47 Thunderbolt, and 0.75 for Bf109s and Fw190s. Greg's Airplanes and Automobiles complained about this in the YouTube video US Fighter Dive Test By British Test Pilot Eric Brown. My best guess on this is that the numbers are comparable, as opposed to maximum possible. Brown probably used some force device to measure the force on the control sticks, and arbitrarily picked some measureable number as the maximum allowable. I, personally, own a 50lb spring scale. I am a big guy. I can pull more than 50lb, but I cannot measure it.

In his book Wings on my Sleeve, Brown describes diving Spitfires to Mach_0.83, using 60lb force, his limit, to pull out. This number almost certainly is not based on the protocols above, so it is not comparable. A large, powerful colleague was able to dive a Spitfire to Mach_0.92.

Eric Brown was incorrect. The Limit dive is 0.75M for both the P-51and P-47 per the Pilot Manuals (Which is developed by the Service divisions in concert with Engineering. Additionally, 0.80M is the Not to exceed Dive speed.

Was not oriented to stick force, was derived from Qmax as a structural limit stress before 'break' (i.e. Ultimate vs Limit)

 

[Howard Gibson]

Was not oriented to stick force, was derived from Qmax as a structural limit stress before 'break' (i.e. Ultimate vs Limit)

Would Eric Brown test for that? Eric Browns degree was in modern languages, specializing in German. He was not an engineer.

 

[GregP]

Per Graham White's Major Miracle:

A-Series <10 produced (Prototypes w/ R-2800 cylinders, etc) ~2,800hp

B-Series ~15,000 produced R-4360-4, -13, -14, -19 are all listed as B series

3,000 hp 100/130 fuel
3,250 - 3,500 hp 115/145 fuel
Those with mechanical supercharger for 2nd stage, e.g. -13 (XP-72), -29 (XB-44) drop to 2,400hp @ 25k'; those with turbochargers, e.g. -35 (B-50A,B & D) kept 3,250hp to 30k'
Note R-4360-14 is dual rotation*, remote supercharger version of -13
*dual rotation is P&W's nomenclature for contra-rotating propellers
​

C-Series ~3,500 produced

3,500 hp 115/145 fuel (dry)
3,800 hp 115/145 fuel (wet) i.e. with water injection.
​

There have always been some questions about where XP-72 had remote supercharger. IMNSHO, it had to for 2 reasons: First, without remote supercharger (and intercooler) the plane would have severe balance issues - why bolt in couple hundred pounds of lead instead. Second, without remote supercharger e.g. -4 engine, performance was down to 2,400hp @ 13.5k'. It wouldn't have been a very good show case of P&W's new engine.

Figures. My text came from an AI reply. I was interested only in the higher horsepower engine, which I KNEW ran, but did not know they built only one of.

In this particular case, I am only interested in what could have been achieved with the XP-72. As for Reno, I went many years and and spent time both in the pits and up close and personal on a few unlimited airplanes. The Reno airplanes are nothing to compare with a service airplane. Their engines would not likely last for very long and they have no military application whatsoever, being optimized only for speed.

But, if I have seen aiything from actual service development, it is that there IS a development path for most service airplanes ... Vickers Venom notwithstanding. So, I have very little trouble believing a 490 mph prototype could go faster with development since higher horsepower engines WERE existing and WERE possible in the same package, though were not proceeded with, the propeller fitted to a prototype is usually not optimum, and there ARE aerodynamic improvements to be had on a prototype airframe. So, faster is possible.

Can I quantize it? Not without a LOT of figuring. I'm not nearly interested enough to try, let alone quaified at this time since I last did much aeronautical engineering in school before transferring into electrical engineering was in the late 1960s.

The prototype Spitfire went 349 mph in March 1936.. A Spitfire XIV went 440 mph in Oct 1943. It gained 91 mph.
The protoype XP-51 went 382 mph in Mar 1941. A P-51D did 437 mph at best altitude for most of 1944 and 1945, easily. It gained 55 mph.
The XF8F-1 Bearcat went 424 mph in Aug 1944. The F8F-2 service airplane went 455 mph in Aug 1944. It gained 31 mph.

Methinks there is a trend in there, and it does not point to no speed gains being possible with development that the XP-72 never got. We already KNOW the engine that made 4,300 hp was built and it ran, at least in a test cell. We also know that the military realized development would not overtake the speeds being achieved with jet engines, so it doesn't take a genius to figure out why they didn't proceed with the XP-72 development.

So, I'll not believe 540 mph is totally impossible since I have seen Voodoo go that fast on less horsepower than the experimental R-4360 made during the war in an actual test cell. Is it likely? Can't say since they never developed it.

But the XP-47J went 505 mph and was likely a bit less aerodynamically clean and had less horsepower than a developed XP-72 might have been. Altogether, it isn't worth an argument either way, since there is no way to positively quantize the answer short of a lot of effort in analysis that doesn't seem to have much practical value beyond this thread.

I'll conclude that I like the XP-72 and it could have been a decent airplane if history at the time is any indicator.

 

[drgondog]

Would Eric Brown test for that? Eric Browns degree was in modern languages, specializing in German. He was not an engineer.

No, but he would have been familiar with published data 'back in the day'. The video you referred to had Brown 'mis-remembering' his role and timing regarding P-47 compressibility issues, as well as remarks such as being directed by Doolittle to test the P-47. You are correct about his technical engineering background. I had lengthy correspondence with him in the 70's. He was engaging and a complete gentleman but incorrect in his beliefs the FW 190 had a higher Mcr than the P-51, which BTW was one of the reasons he ranked the FW 190 slightly higher than the P-51.

Cass Hough, Chief VIII FC Air Tech Services, tested the brand new P-47C-2 in ETO, before 56FG even flew them, in Jan 1943. He discovered the compressibility issues immediately in a near fatal terminal dive. He then proceeded to develop recovery processes for documentation to the new FGs destined to fly them in combat. Brown was still fleet RN operations in this time frame, but the RAF Did perform night flying tests, discovered the radio interference from engine harness, etc that kept the P-47 on the bench in Feb/March until BAD1 installed the fixes.

Doolittle was not even in ETO until mid January 1944 - when Brown was in Testing, but the issues with both the P-38 and P-47 were well understood and dive flap mods were in full test prior to introduction in production P-38J-10, P-47D-30. By early February, Doolittle had already settled on the P-51 and ordered the conversions to begin with veteran VIII FC P-47 groups. The 56FG was first, but turned them down. He traded the 358FG to 9thAF for the Mustang equipped 357FG, then in rapid order the 4th, 355th, 352nd.

 

[mig-31bm]

I'm today year old when I realized that there is actually a recorded performance trial for the Spiteful.

https://discovery.nationalarchives.gov.uk/details/r/C4076013

Unfortunately, it not available in PDF format

 

[mig-31bm]

the XP-47J went 505 mph and was likely a bit less aerodynamically clean and had less horsepower than a developed XP-72 might have been.

The XP-47J's widely cited 505 mph speed record, reported by Republic, was likely the result of faulty measuring equipment, as the aircraft was never able to reproduce that figure in subsequent tests. When the USAAF later took over testing, the highest speed they recorded was approximately 484 mph (779 km/h).

Even so, I believe the XP-47J deserves an honorable mention.

XP-47J had an empty weight of 4,383 kg and a gross weight of 5,625 kg (that around the same different between an empty and a loaded Spiteful), giving it a wing loading of 202 kg/m2. That roughly similar to an average Spitfire and much better than both Spiteful (at 231 kg/m2) and XP-72 (at 238 kg/m2)

Powered by a 2,800 hp Pratt & Whitney R-2800 engine, it achieved a power to weight ratio of 0.498 hp/kg (0.226 hp/lb) which is only 6% worse than Spiteful

Its initial climb rate at sea level was approximately 4,900 ft/min (24.9 m/s)

P/S: XP-47J have a turbo charger so it probably better at high altitude compared to XP-72

 

[GregP]

They only made 130 P-47Ms. The P-47Ns also had the 2800 hp in them, but were operationally limited to very liitle time at that pwoer setting, according to old P-47 pilots we heard talk in the past.

Nobody wantred to find out how durable the 2800 hp engire was when they were 800 miles over water away from home in the Pacific Theater.

 

[SaparotRob]

They only made 130 P-47Ms. The P-47Ns also had the 2800 hp in them, but were operationally limited toi very liitle time at that pwoer setting, according to old P-47 pilots we headr talk in the past.

Nobody wantred to find out how durable the 2800 hp engire was when they were 800 miles over water away from home in the Pacific Theater.

That makes perfect sense to me.

 

[Juha3]

I think it's best to forget what's new in the latest expanded editions of Brown's Wings on My Sleeve. My 2006 edition has a few oddities. Age may have played tricks on Brown in his later years.
During the RAE High Speed tests they were very interested in wing profiles anyway, so the main objects of interest were the Spitfire, Mustang, Gloster E 28/39 jet and Westland Welkin. Also both the Gloster Meteor I with standard engine nacelles "short nacelles"and extended engine nacelles, "long nacelles" as later on Mk IVs. The P-38 and P-47 were included only because of their dive recovery flaps. These and their operation and the optimal size of the dive recovery flaps were the objects of interest for the British in the Thunderbolt and Lighting. The highest Mach number achieved was 0.70 for the P-38 and 0.78 for the Thunderbolt. The latter was noted for "Very high stick forces". So much on Brown's idea, that recovery flaps were installed because of RAE recommendation.

 

[GregP]

Interestingly enough, there are two museums currently at Chino airport. One of them is the Planes of Fame, where I volunteer. The other is Yanks Air Museum, where I have volunteered in the past. Yanks has a Photo Recce P-38 and a P-47M and a P-47D.

Both the P-38 and the P-47D at Yanks have the dive recovery flaps installed from the factory or in the field, I can't say for sure either way ... but they have them. Both dive recovery flaps are relatively small and are located at and inboard of the gear wells, in front of the center of gravity or center of lift. I am assuming they are located relative to center of lift since their function is to impart a nose-up pitching moment once critical Mach has been exceeeded.

Pretty interesting, if you are technically interested in these things, Many people are not and never notice these items. Heck, many aren't erven sure if they are Allied or Axis airplanes without looking at the national markings! I'm sure there a few who notice the dive flaps since airplane junkies tend to go look at airplanes. I do, anyway.

 

[wuzak]

Let's see. The Republic XP-47A was estimated at 415 mph. The actual P-47B made 433 mph, and the developed P-47N made 467 mph, an increase of 34 mph over the early P-47B.

I'm a little late in replying to this, but the XP-47A was never built, or never completed, and did not fly.

It was to be a "lightweight" fighter with an Allison V-1710 without turbocharger (IIRC).

The XP-47B was a completely different, larger and heavier, aircraft with a lot more power. And, crucially, a turbocharger to help maintain power at altitude.

 

[wuzak]

According to the internet, which is sometimes less than relaible, the R-4360 VDT (Variable Discgharge Turbine) produced 4,300 hp. Also, apparently, they built and ran 1 uinit.

Didn't the VDT require a flight engineer to constantly monitor the engine and adjust the exhaust outlet?

 

[GregP]

Don't know since it was an experimental test stand engine. Had it been developed, I think not.

No airliner with a flight engineer needs to cruise about WER power ratings, not if they wasnt to keep schedules and make money, anyway.

Had it been developed, I think it would have been a 5-munite WER rating, with maybe 3850 - 4000 hpw available normally. But, when they built one and didn't develop it due to the arrival of jet engines, we may never really know. Had it been an airliner engine, it would have made 3,300 hp for takeoff and 2,500 or so at cruise.

Your guess is likely as good as anyone's about a developed powerplant. I've seen R-4360s run, even reasoinably frequently considering their rareity, but I have nbever seen one operated outside of Reno on 140+ PN fuel and at speed.

 

[wuzak]

The prototype Spitfire went 349 mph in March 1936.. A Spitfire XIV went 440 mph in Oct 1943. It gained 91 mph.
The protoype XP-51 went 382 mph in Mar 1941. A P-51D did 437 mph at best altitude for most of 1944 and 1945, easily. It gained 55 mph.
The XF8F-1 Bearcat went 424 mph in Aug 1944. The F8F-2 service airplane went 455 mph in Aug 1944. It gained 31 mph.

The Spitfire prototype had ~ a single stage Merlin with 1,000hp and a fixed position prop.
The XIV was considerably refined aerodynamically, had a 2 stage Grifffon with ~ 2,000hp and constant speed propeller.

The XP-51 had an Allison V-1710 with a (relatively) low critical altitude single speed, single stage supercharger.
The P-51D had a 2stage, 2 speed supercharged Merlin with a lot more power and a much higher critical altitude.

The XF8F-1 Bearcat had a 2 speed single stage supercharger.
The F8F-2 had a variable speed single stage supercharger.
I suspect that the F8F-2 had a higher critical altitude than the F8F-1.

The point is that many factors differ between these aircraft. Not least of which is the improved altitude capability of the engines.

 

[wuzak]

Don't know since it was an experimental test stand engine. Had it been developed, I think not.

No airliner with a flight engineer needs to cruise about WER power ratings, not if they wasnt to keep schedules and make money, anyway.

Had it been developed, I think it would have been a 5-munite WER rating, with maybe 3850 - 4000 hpw available normally. But, when they built one and didn't develop it due to the arrival of jet engines, we may never really know. Had it been an airliner engine, it would have made 3,300 hp for takeoff and 2,500 or so at cruise.

Your guess is likely as good as anyone's about a developed powerplant. I've seen R-4360s run, even reasoinably frequently considering their rareity, but I have nbever seen one operated outside of Reno on 140+ PN fuel and at speed.

The VDT ditched the engine supercharger in favour of a pair of turbochargers. The turbochargers were controlled using a variable exhaust nozzle.

It is the nozzle that required monitoring and adjustment from an engineer. They had not yet developed a control system that woudl work.

From memory, Graham White in "R-4360 : Pratt & Whitney's major miracle" says that the prototype flew in a B-50 in place of one of the engines.