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

[mig-31bm]

I'm late to this, but the XP-51F had the 1650-9 with ADI dropped in it in January 1945

XP-51F – Final Flight Testing with Packard 1650-9

Notes: This airplane had an alternate load of 4x20mm with 125rnds/gun.

If I understand correctly, this load is probably in conceptual stage and never tested on actual XP-51F airframe?.
I have looked at all prototype picture of all light weight Mustang from XP-51J, XP-51G to XP-51F, none of them have any holes in the wing, which indicate that they probably did not even carry the 0.5 cal M2.

The 20 mm hispano is quite a bit longer than Mustang wing width and probably have negative impact on top speed and acceleration if added.

 

[GregP]

The Spitefuls, the MkXIV and XVI were operational fighters ready for production, but jets were the future. The XP-72 reaching 540mph is pure fantasy

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.

When the first and only XP-72 came along, the engine was not fully developed and it made 3,450 hp. It is reasonable to assume some power development (I'd estimate 4,400 hp minimum after development) since ALL production piston engines to that point were developed into higher horsepower units during their production runs. The Merlin went from 840 hp to over 2,000 hp.

Suppose we only add that same 34 mph to the XP-72's top speed ... 490 +34 = 524 mph. That isn't too far from 540 mph and I don't think it is at all unreasonable to expect 540 mph from a developed XP-72. Of course, we never GOT a developed XP-72, but pure fantasy? I must disagree, although it didn't happen in real life, it isn't pure fantasy to think it could have.

That wouldn't only make it about all that much slower early jets ...

 

[drgondog]

6530 fpm of climb is enormous. Beats the best figure for the Bearcat of 6300 fpm.
How long did it take for the P-51F to get to 3000 m?
A (modified?) Bearcat needed 91 second iIrc.

I don't recall if I have seen 10,000 ft time to climb for XP-51F but the XP-51G performed 20,000 feet in 3.4 minutes with a fully functioning R-R RM 14 SM according to Gruenhagen. I have not seen that Report.

For that matter I haven't seen the test reports for the F8F which always brings takeoff weight comparisons with respect to combat loadings.

That said, I don't know what Gross Weight was at takeoff was for that test (XP-51G) and I suspect that it was at or below that for a Fighter Mission with only 105-gal fuel. The XP-51G was about 100 pounds lighter with the R-R than the XP-51F was with the 1650-9.

 

[drgondog]

If I understand correctly, this load is probably in conceptual stage and never tested on actual XP-51F airframe?.
I have looked at all prototype picture of all light weight Mustang from XP-51J, XP-51G to XP-51F, none of them have any holes in the wing, which indicate that they probably did not even carry the 0.5 cal M2.
View attachment 883278
View attachment 883279

The 20 mm hispano is quite a bit longer than Mustang wing width and probably have negative impact on top speed and acceleration if added.
View attachment 883280

The tests were documented in Chilton's January 1945 logbook as well as AAF tests in Feb-March 1945 at Wright Field. Chiltons values were higher for top speed with XP-51F at 493mph and 6550fpm - but no other notes such as GW or altitude data. But he did say he took the P-51F to 46,000 and was limited due to unpressurized cabin to go further.

As to armament, the fairings are smoothed over in the Wright Field images you show above. The NAA testing and comparisons flown between the P-51B and XP-51F were conducted with combat loads, including 1,000 rnds for the XP-51F ad 1,440 for the P-51B when the results of comparable engines and MP (1650-3) and 105-gal fuel.

FWIIW - the NA-133 P-51H when proposed to USN post WWII, in a performance Summary developed for the NA-133 P-51H navalized, was compared with comparable fuel and greater ammo loads and six gun vs 4-gun F8F and F2G. The BuAir values were less than NAA but

The H was lighter by 400 pounds with comparable loadout, was 20kts faster at SL (417 vs 391kts/hr), 60kts faster at 25,000 feet.
ROC at SL was 5720 at WEP for H vs 4650 for F-8F. The 4650fpm ROC were USN values for the above combat load.
The carrier take-off run was 299ft for H vs 267ft for the F8F
Notably the BHP at WEP was 1860HP at 19,000 feet for the P-51H vs 1860 at 14,000 feet for the F8F.

Both the F8F and P-51H out performed the F2G with the R-4360 in every category

 

[PAT303]

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.

When the first and only XP-72 came along, the engine was not fully developed and it made 3,450 hp. It is reasonable to assume some power development (I'd estimate 4,400 hp minimum after development) since ALL production piston engines to that point were developed into higher horsepower units during their production runs. The Merlin went from 840 hp to over 2,000 hp.

Suppose we only add that same 34 mph to the XP-72's top speed ... 490 +34 = 524 mph. That isn't too far from 540 mph and I don't think it is at all unreasinable to expect 540 mph from a developed XP-72. Of course, we never GOT a developed XP-72, but pure fantasy? I must disagree, although it didn't happen in real life, it isn't pure fantasy to think it could have.

That wouldn't only make it about all that much slower early jets ...

So your estimating a top speed of 524mph based on the speed of a prototype not production fighter as well as assuming the engine will provide the rated HP, from what I have read the 540 mph claim is considered optimistic.

 

[drgondog]

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.

When the first and only XP-72 came along, the engine was not fully developed and it made 3,450 hp. It is reasonable to assume some power development (I'd estimate 4,400 hp minimum after development) since ALL production piston engines to that point were developed into higher horsepower units during their production runs. The Merlin went from 840 hp to over 2,000 hp.

Suppose we only add that same 34 mph to the XP-72's top speed ... 490 +34 = 524 mph. That isn't too far from 540 mph and I don't think it is at all unreasinable to expect 540 mph from a developed XP-72. Of course, we never GOT a developed XP-72, but pure fantasy? I must disagree, although it didn't happen in real life, it isn't pure fantasy to think it could have.

That wouldn't only make it about all that much slower early jets ...

Greg - all Performance Estimates were made assuming complete power development per specifications. I would assume Republic engineers were just as good at analysis as NAA. Actual performance achievement to predicted for the 7 Mustang Performance Analysis reports are Very accurate to real world test results, particularly for the P-51A thu H as the cooling systems improved.

The analogy of the delta between P-47B at 2000 Hp to P-47M/N at 2800HP isn't (IMO) analogous to increasing HP to 4,400Hp. Too may missing data elements to do a parasite and pressure drag build up.

Has anybody laid hands on the actual flight test reports?

Net - I haven't seen either the Republic analysis or the Flight Test but AAF protocols were to assume that the bench data from engine manufacturers were correct and that only when issues were encountered would the issues and questions about delivered power.

Not only is Thp not proportional to increase in HP, but the additional pressure drag on the airframe and inner wing also increases.

Additional and major issue for extrapolating the increase in top speed was the severe Parasite Drag rise beginning at ~ 0.55M but steeply increasing as it approached .6 to .7M. The XP-72 was just about at peak development for a quasi-stock conventional airplane.

That said, the 4360 on the B-36 was just about peak for development on that series. I don't recall what WEP rating was nor do I know if it had the same supercharger scheme.

 

[mig-31bm]

As to armament, the fairings are smoothed over in the Wright Field images you show above. The NAA testing and comparisons flown between the P-51B and XP-51F were conducted with combat loads, including 1,000 rnds for the XP-51F ad 1,440 for the P-51B when the results of comparable engines and MP (1650-3) and 105-gal fuel.

It seem that XP-51F s/n 43-433 still have the .50 caliber machine guns installed while it in USA

but the gun were removed When it were transported to UK

I think they probably removed all the gun rather than simply smoothed and cover the fairing. Because when they simply cover the fairing, you can still kind of see where the gun barrel are
For example:

FWIIW - the NA-133 P-51H when proposed to USN post WWII, in a performance Summary developed for the NA-133 P-51H navalized, was compared with comparable fuel and greater ammo loads and six gun vs 4-gun F8F and F2G. The BuAir values were less than NAA but

Both the F8F-1 and F2G used 0.5 cal so I think P-51H compared to them probably have two configuration, first with four 0.5 cal and second with six 0.5 cal.
As far as I know, none of the P-51H, XP-51G or XP-51J ever intended to use the 20 mm hispano

Both the F8F and P-51H out performed the F2G with the R-4360 in every category

If I recalled correctly, F2G didn't have supercharger like previous F4U so it kinda expected that P-51H would out perform it significantly.

 

[mig-31bm]

According to some estimation made by Warthunder forum member, it seem that XP-51G out perform Spiteful XVI by a decent margin in climb rate and turning

Unfortunately no XP-51G were ever fitted with weapons (note the smooth wing edge).

 

[GregP]

mothSo your estimating a top speed of 524mph based on the speed of a prototype not production fighter as well as assuming the engine will provide the rated HP, from what I have read the 540 mph claim is considered optimistic.

I'm saying that Republic Aviation easily added 34 mph to the P-47 with some development. The XP-72 started out faster, but I have no problem thinking Republic could have made it faster by a decent amount. Every other fighter developed after the P-40 got faster with development, without exception.

If they didn't change ANY aerodynamics and went from 3450 hp to 4400 hp, the formulas say the new top speed would be about 531. The R-4360 was SUPPOSED to make 5,000 hp, and I have little doubt that development would have made more than 4400 hp. It isn't a stretch to think aerodynamics could add another 9 mph. They added MUCH more than that just with aerodynamics at Reno for P-51s, Bearcats, and Sea Furies. Voodoo can hit 540 mph at Reno in a straight line with 3,800 or so hp. The XP-72 would have had more power than that, and the top speed altitude would have been in the 20,000 to 35,000 foot range.

Seems entirely reasonable to me that it could have happened ay altitude. Not too sure it would have happened to squadron service airplanes but, then again, the XP-72 never served. Suffice to say if it HAD served, it would have been formidable indeed against any piston opponent it encountered.

Just one of the many interesting "what ifs" in aviation. A potentially great fighter rendered obsolete by the jet engine ... go figure. The XP-72 wasn't the only one to be affected by the pesky jet engine. That's for sure. The Ta-152 never got to strut it's stuff, either. Ditto the Spiteful and Seafang, to name but a few. They would all have been formidable.

 

[mig-31bm]

They added MUCH more than that just with aerodynamics at Reno for P-51s, Bearcats, and Sea Furies. Voodoo can hit 540 mph at Reno in a straight line with 3,800 or so hp. The XP-72 would have had more power than that

btw, weren't Voodoo so much smaller compared to XP-72 since it supposed to be a modified P-51?
It ugly as *** though

On a side note, XP-72 competitor, the XP-69 looked pretty nice, very sexy and smooth, but pretty mediocre for an aircraft of that size

 

[mig-31bm]

Found some clear photo of XP-51F where the gun fairing cover are more clearly seen

 

[don4331]

I'm saying that Republic Aviation easily added 34 mph to the P-47 with some development. The XP-72 started out faster, but I have no problem thinking Republic could have made it faster by a decent amount. Every other fighter developed after the P-40 got faster with development, without exception.

If they didn't change ANY aerodynamics and went from 3450 hp to 4400 hp, the formulas say the new top speed would be about 531. The R-4360 was SUPPOSED to make 5,000 hp, and I have little doubt that development would have made more than 4400 hp. It isn't a stretch to think aerodynamics could add another 9 mph. They added MUCH more than that just with aerodynamics at Reno for P-51s, Bearcats, and Sea Furies. Voodoo can hit 540 mph at Reno in a straight line with 3,800 or so hp. The XP-72 would have had more power than that, and the top speed altitude would have been in the 20,000 to 35,000 foot range.

Seems entirely reasonable to me that it could have happened ay altitude. Not too sure it would have happened to squadron service airplanes but, then again, the XP-72 never served. Suffice to say if it HAD served, it would have been formidable indeed against any piston opponent it encountered.

Just one of the many interesting "what ifs" in aviation. A potentially great fighter rendered obsolete by the jet engine ... go figure. The XP-72 wasn't the only one to be affected by the pesky jet engine. That's for sure. The Ta-152 never got to strut it's stuff, either. Ditto the Spiteful and Seafang, to name but a few. They would all have been formidable.

I don't see how the XP-72 is even as fast as a P-47M;

The engine in the XP-72 is either a R-4360-13, -14 or -19 (-13 has the remote supercharger, -14 is contra-rotating/remote supercharger and -19 is the semi-production model)​

From P & W Major Miracle by Graham White; the R-4360 in the XP-72 makes 3k hp @ S.L. and 2,400 hp at 25k' - which makes sense, it takes 600hp to compress the volume of air that the corncob needs at that altitude.​

But that would be less power than what the R-2800-57 (or -73) would be making at that altitude.​

​

Now, yes, you can redesign the XP-72 for 2 - turbos in the aft fuselage* (both turbos from a P-38 would allow ~3,250 hp @ 25k') but the fuselage is going to need significant modification. 1 hp/in^3 from the engine needs 2 turbochargers of C-23 size and intercooler to match.

The other issue I see with the Super T-bolt - where are you finding space for the very thirsty engine? Making 4K+ hp is burning a lot of fuel...

 

[mig-31bm]

I'm late to this, but the XP-51F had the 1650-9 with ADI dropped in it in January 1945

XP-51F – Final Flight Testing with Packard 1650-9

With Packard 1650-9, 90" Manifold Pressure, 150 Octane fuel, Water Injection.

2280 Brake HP at Full Throttle Height (FTH) of 3500 feet, Low Blower; 1855 Brake HP at FTH 17000 feet, High Blower.

1790 BHP at 21,500 feet.

• Gross Weight – 7,553 pounds at takeoff; 7,340 pounds for speed performance testing.
• Top Speed – 491mph at 21,500 feet with 1790 BHP at 90" MP.
• Rate of Climb – 6530 feet per minute at Sea Level @2200 BHP, 5,730 feet per minute at 20,000 feet@1725 BHP at 90" MP
• Absolute Ceiling – 42,600 feet.

Report "Memo Report TSCEF5ER-1883", dated May 7, 1945.

Notes: This airplane had an alternate load of 4x20mm with 125rnds/gun.

In Interceptor/Basic load out with 105gal fuel and Take-off weight of 6990 pounds, Chilton stated 493mph and 7300 fpm at SL, Ceiling 46,000 - only limited by non-pressurization of cockpit - Jan 21, 1945.

IMHO - this airplane was not bought in 1943 (Oct-43 ATSC requested 25 from Materiel Command but rejected by Echols) for any of three reasons. 1.) First use of 75ST (home grown by NAA as Alcoa had not started production when P-51F was beginning design phase, and AAF -MC still leery of 7.33 Limit Load approach, 2. Range - although with 2x75gal externals and 180gal Internal fuel the P-51F had a straight line Fighter Mission combat radius of 702mi, it was less than the P-51B and had no capability of jamming an 85gal fuse tank, or a 1000 pound external fuel tank, and 3.) Oliver Echols still had a hard on regarding North American and still fighting for the XP-75.

It was still very much in consideration for the Interceptor role - but low on priority for AAF.

The P-51H was the result of 'fixing' 1, 2. Arnold fixed 3 by basically moving Echols out of the R&D responsibility of Wright Field.

I devote a lot of time on these topics in my new book.

Ok now that I think about it, in term of pure performance, XP-51F is probably the best piston fighter ever made since it got the second highest top speed, best climb rate, best turn rate. The only weakness is the pathetic amarment it carry ( only four 0.5 cal ) and probabaly no armor.

 

[Howard Gibson]

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.

When the first and only XP-72 came along, the engine was not fully developed and it made 3,450 hp. It is reasonable to assume some power development (I'd estimate 4,400 hp minimum after development) since ALL production piston engines to that point were developed into higher horsepower units during their production runs. The Merlin went from 840 hp to over 2,000 hp.

Suppose we only add that same 34 mph to the XP-72's top speed ... 490 +34 = 524 mph. That isn't too far from 540 mph and I don't think it is at all unreasonable to expect 540 mph from a developed XP-72. Of course, we never GOT a developed XP-72, but pure fantasy? I must disagree, although it didn't happen in real life, it isn't pure fantasy to think it could have.

That wouldn't only make it about all that much slower early jets ...

The absolute speed record for propeller driven piston engined aircraft is a tad shy of 530mph. This speed is limited by propeller tips exceeding the speed of sound.

If speed is limited by aerodynamic drag, engine power is a function of velocity cubed. For example, early Spitfires had 1000hp and they did 350mph. Late Spitfires had 2000hp and they did 450mph. This a very crude example, but crunch the numbers. As you approach the maximum Mach numbers of the propellers, this assumption should not be reliable. As we approach 500mph, I would be cautious about estimating aircraft top speeds.

 

[drgondog]

I'm saying that Republic Aviation easily added 34 mph to the P-47 with some development. The XP-72 started out faster, but I have no problem thinking Republic could have made it faster by a decent amount. Every other fighter developed after the P-40 got faster with development, without exception.

Hardly 'easily', the M required a 40% increase to push it to near ~ 0.67M for 460mph at 30,000. with 2000hp the P-47C with WI achieved ~430mph at 30,000 ft. That is ~ 0.63M.
To make sense of your 'estimate', you would have to have at Least the CDp vs M for the Parasite Drag multiplier, the prop tip Mcr, the pressure drag increase due to the prop vortex and prop efficiency, and take into account the Thrust Hp is decreasing proportional to the inverse of Velocity.

The XP-72 was right at 0.75M with 505 and 3450Hp.

Yes to Voodo0 which achieved 0.75M with 3800Hp.

I seriously doubt that any WWII airframe with even unlimited increase in HP is going to punch 0.75M due to drag rise, much less Qmax structurally.

If they didn't change ANY aerodynamics and went from 3450 hp to 4400 hp, the formulas say the new top speed would be about 531. The R-4360 was SUPPOSED to make 5,000 hp, and I have little doubt that development would have made more than 4400 hp. It isn't a stretch to think aerodynamics could add another 9 mph. They added MUCH more than that just with aerodynamics at Reno for P-51s, Bearcats, and Sea Furies. Voodoo can hit 540 mph at Reno in a straight line with 3,800 or so hp. The XP-72 would have had more power than that, and the top speed altitude would have been in the 20,000 to 35,000 foot range.

Seems entirely reasonable to me that it could have happened ay altitude. Not too sure it would have happened to squadron service airplanes but, then again, the XP-72 never served. Suffice to say if it HAD served, it would have been formidable indeed against any piston opponent it encountered.

Just one of the many interesting "what ifs" in aviation. A potentially great fighter rendered obsolete by the jet engine ... go figure. The XP-72 wasn't the only one to be affected by the pesky jet engine. That's for sure. The Ta-152 never got to strut it's stuff, either. Ditto the Spiteful and Seafang, to name but a few. They would all have been formidable.

Yes, but highly unlikely to ever exceed 0.75M.

 

[drgondog]

The absolute speed record for propeller driven piston engined aircraft is a tad shy of 530mph. This speed is limited by propeller tips exceeding the speed of sound.

If speed is limited by aerodynamic drag, engine power is a function of velocity cubed. For example, early Spitfires had 1000hp and they did 350mph. Late Spitfires had 2000hp and they did 450mph. This a very crude example, but crunch the numbers. As you approach the maximum Mach numbers of the propellers, this assumption should not be reliable. As we approach 500mph, I would be cautious about estimating aircraft top speeds.

Howard, what was the altitude at which 530mph was achieved? And which aircraft?

 

[drgondog]

Ok now that I think about it, in term of pure performance, XP-51F is probably the best piston fighter ever made since it got the second highest top speed, best climb rate, best turn rate. The only weakness is the pathetic amarment it carry ( only four 0.5 cal ) and probabaly no armor.

The NA-117 P-51H was the XP-51F with 1650-9 and proposed with 4x20mm for interceptor. The P-51H was completely redesigned to NA-126 because of the fuel fraction requirements.

 

[GregP]

btw, weren't Voodoo so much smaller compared to XP-72 since it supposed to be a modified P-51?
It ugly as *** though
View attachment 883412

On a side note, XP-72 competitor, the XP-69 looked pretty nice, very sexy and smooth, but pretty mediocre for an aircraft of that size

View attachment 883413

You and I have a very different view of what is pretty One man's junk is another man's treasure and all that stuff.

 

[GregP]

Hardly 'easily', the M required a 40% increase to push it to near ~ 0.67M for 460mph at 30,000. with 2000hp the P-47C with WI achieved ~430mph at 30,000 ft. That is ~ 0.63M.
To make sense of your 'estimate', you would have to have at Least the CDp vs M for the Parasite Drag multiplier, the prop tip Mcr, the pressure drag increase due to the prop vortex and prop efficiency, and take into account the Thrust Hp is decreasing proportional to the inverse of Velocity.

The XP-72 was right at 0.75M with 505 and 3450Hp.

Yes to Voodo0 which achieved 0.75M with 3800Hp.

I seriously doubt that any WWII airframe with even unlimited increase in HP is going to punch 0.75M due to drag rise, much less Qmax structurally.


Yes, but highly unlikely to ever exceed 0.75M.

Let's say we disagree on this one, but it's completely unimportant since the XP-72 was never procured and pistons became extinct rapidly after the war except for the Skyraider.

I would not expect the XP-72 to be fast down low, but would expect it up at critical height. I believe the R-4360 could have made very good hp with development that it never got in real life.

 

[Shortround6]

I would not expect the XP-72 to be fast down low, but would expect it up at critical height. I believe the R-4360 could have made very good hp with development that it never got in real life.

According to Wiki (correction more than welcome) P&W built 18,697 of the R-4360s. Production didn't stop until 1955.
Granted P&W did not pursue fighter applications for very long but it was a major bomber and large transport engine while the jets were still too thirsty for intercontinental range.
P&W lists at least 57 (?) different military models.
Peak power without turbo assist seems to be 3800hp at sea level with 115/145 fuel and water injection.