Republic XP-72

[HoHun]

Hi,

Performance of the XP-72 attached.

Thanks a lot, that's quite fascinating!

It's surprising the XP-72 was supposed to achieve 460 mph @ 30000 ft on "only" 2200 HP ... that's faster than the P-47N on 2800 HP: http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg

It seems the R-4360 with turbo-supercharger was getting quite a bit of exhaust thrust, while my somewhat vague impression is that the R-2800 didn't really provide much in that regard. I could be wrong, though.

Regards,

Henning (HoHun)

 

[riggerrob]

Now you are starting to near a comparison with post-1945 Tupolev Bear bombers - the fastest prop driven production airplanes. Bear bombers can exceed 500 mph. but we wonder how efficient their props are at those airspeeds.
How close are Bear prop tips to super-sonic?
How much thrust do they enjoy from props at 500 mph.?
How much thrust do Bears get from turboprop engine exhaust?

In an alternate time-line both Whittle and von Ohain fail to complete their prototype jet engines, so engines can only evolve with ever more powerful turbo-superchargers ala. R-4360.
At what point does supercharger exhaust exceed prop thrust???????

 

[HoHun]

Hi,

How close are Bear prop tips to super-sonic?
How much thrust do they enjoy from props at 500 mph.?
How much thrust do Bears get from turboprop engine exhaust?

From the various numbers I've seen, I suspect that at top speed at high altitude, the tips are actually supersonic. (I don't really have a matching set of numbers, and the altitude at which the top speed is reached hasn't been stated in the sources I checked.)

From an article written by the engine design Brandner, published in the Schweizerische Bauzeitung from 10th of August, 1957, the NK-12 provides 8000 HP of shaft power and 1200 kp of thrust at 11 km altitude: https://www.flugzeugforum.de/thread...-tu-142-tu-114nk-12.91372/page-4#post-2586552

Regards,

Henning (HoHun)

 

[Metrallaroja]

Hi,



Thanks a lot, that's quite fascinating!

It's surprising the XP-72 was supposed to achieve 460 mph @ 30000 ft on "only" 2200 HP ... that's faster than the P-47N on 2800 HP: http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg

It seems the R-4360 with turbo-supercharger was getting quite a bit of exhaust thrust, while my somewhat vague impression is that the R-2800 didn't really provide much in that regard. I could be wrong, though.

Regards,

Henning (HoHun)

The XP-72 uses a tighter cowling and shorter wings than the redesigned P-47N ones, the XP-72 also has a contra rotating 6 bladed prop that increases the efficiency at high alt.
On top of that XP-72 does not use a turbo, it is using a two stage supercharger so the thrust out of the engine is considerable taking in count that it uses a R-4360 with 28 R-2800 cylinders and that it uses a fan in the cowling to decrease cowling flap area drag and optimize exhaust gases.

 

[sienar]

Hi,



Thanks a lot, that's quite fascinating!

It's surprising the XP-72 was supposed to achieve 460 mph @ 30000 ft on "only" 2200 HP ... that's faster than the P-47N on 2800 HP: http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg

It seems the R-4360 with turbo-supercharger was getting quite a bit of exhaust thrust, while my somewhat vague impression is that the R-2800 didn't really provide much in that regard. I could be wrong, though.

Regards,

Henning (HoHun)

The Rainbow was getting the equivalent of 362hp from the exhaust @ 3000hp after it went through the turbo.

https://www.secretprojects.co.uk/threads/understanding-and-calculating-exhaust-thrust.40891/#post-582001

 

[Metrallaroja]

The Rainbow was getting the equivalent of 362hp from the exhaust @ 3000hp after it went through the turbo.

https://www.secretprojects.co.uk/threads/understanding-and-calculating-exhaust-thrust.40891/#post-582001

Nice, reminds me of the similar configuration in Ki-83

 

[Basil]

Now you are starting to near a comparison with post-1945 Tupolev Bear bombers - the fastest prop driven production airplanes. Bear bombers can exceed 500 mph. but we wonder how efficient their props are at those airspeeds.
How close are Bear prop tips to super-sonic?
How much thrust do they enjoy from props at 500 mph.?
How much thrust do Bears get from turboprop engine exhaust?

In an alternate time-line both Whittle and con Ohain fail to complete their prototype jet engines, so engines can only evolve with ever more powerful turbo-superchargers ala. R-4360.
At what point does supercharger exhaust exceed prop thrust???????

Although a little bit off topic but a few words regarding the Tu 95 with its NK 12: The propellers turn at constant 750 rpm which is well below Mach 1. But at high flight speeds the combined vector of forward and rotational speed let the propeller tips reach ca. Mach 1.1, so being slightly supersonic. In clean configuration the Tu 95 could go over 900 km/h in record flights which you can only reach with turboprops with a higher proportion of exhaust thrust compared to todays (civil) turboprops. The NK 12 was layed out as a high speed military turboprop from the beginning. As we know it was designed by ex-BMW and -Junkers engineers under the lead of Ferdinand Brandner; they previously worked on projects like the BMW 028 and Jumo 022 turboprops which would have had a split between propeller and exhaust thrust of about 50/50. Of course thrust proportion between propeller and exhaust varies with the flight speed and height. All in all the NK 12 is a highly interesting engine concept.

 

[Basil]

The Rainbow was getting the equivalent of 362hp from the exhaust @ 3000hp after it went through the turbo.

https://www.secretprojects.co.uk/threads/understanding-and-calculating-exhaust-thrust.40891/#post-582001

Looking at the graphs you posted earlier it seems that this additional thrust hp were not generated by the gases that flowed through the turbo but by the waste gate gases which were not decelerated by the flow through the turbine.

 

[riggerrob]

The XP-72 uses a tighter cowling and shorter wings than the redesigned P-47N ones, the XP-72 also has a contra rotating 6 bladed prop that increases the efficiency at high alt.
On top of that XP-72 does not use a turbo, it is using a two stage supercharger so the thrust out of the engine is considerable taking in count that it uses a R-4360 with 28 R-2800 cylinders and that it uses a fan in the cowling to decrease cowling flap area drag and optimize exhaust gases.

The eventual production version would probably have a much shorter drive shaft and mount the turbo-charger forward of the firewall. Yes, that long and heavy an engine/charger combination would create some serious balance problems best solved early in the process of designing an entirely new fuselage.

Note that P-47 was unlike most other WW2 fighters in that its firewall was a yard/meter forward of the wing's leading edge. Most other WW2 fighters had their firewall at the wings' main spar. So that gives Republic an extra yard/meter or 2 to play with when re-balancing an up-engined version.
Speculation ...........

 

[HoHun]

Hi,

The eventual/in-built production version would probably have a much shorter drive shaft and mount the turbo-charger forward of the firewall.

Hm, was that seriously considered? That would seem to change the entire forward fuselage both in arrangement and in aerodynamics ...

Regards,

Henning (HoHun)

 

[F119Doctor]

Hi,



Thanks a lot, that's quite fascinating!

It's surprising the XP-72 was supposed to achieve 460 mph @ 30000 ft on "only" 2200 HP ... that's faster than the P-47N on 2800 HP: http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg

It seems the R-4360 with turbo-supercharger was getting quite a bit of exhaust thrust, while my somewhat vague impression is that the R-2800 didn't really provide much in that regard. I could be wrong, though.

Regards,

Henning (HoHun)

While the P-47M/N R2800 was rated for 2800 WEP, this is only under high overboost with water injection. It is unclear if the turbocharger was able to supply that level of overboost to make that 2800 WEP rating at 32,000 ft. The R4325 with the large diameter remote mechanical 2nd stage supercharger in the P-72 may have had higher power output at 32K. In addition to the higher exhaust thrust and fan boosted cooling flow others have noted.

 

[HoHun]

While the P-47M/N R2800 was rated for 2800 WEP, this is only under high overboost with water injection. It is unclear if the turbocharger was able to supply that level of overboost to make that 2800 WEP rating at 32,000 ft.

http://www.wwiiaircraftperformance.org/p-47/p-47n-republic-wep.jpg

 

[Nicknick]

Looking at the graphs you posted earlier it seems that this additional thrust hp were not generated by the gases that flowed through the turbo but by the waste gate gases which were not decelerated by the flow through the turbine.

This was a little bit more complex, but in each case, all the exhaust gases were used for thrust to some degree. I find it hard to read, but I guess in the first graph we have "recycling waste gate flow" and “utilizing waste gate flow”. I’m not sure what is ment with that, but one could be a separate nozzle for the waste gate and the other a combined nozzle. In both cases, the waste gate if fully closed a t maximum altitude and all the thrust is soley produced with the exhaust gases after the turbine.

The second graph is more clear, here a variable and a fixed nozzle is placed behind the turbine to optimize exhaust thrust. When the nozzle is minimized, the system will produce the most thrust, but also the most exhaust back pressure, so that a compromise has to be found. With the variable nozzle, boost pressure could be regulated by back pressure instead of a waste gate which is here the most efficient configuration. The table shows very likely the exhaust thrust of the variable system.

 

[F119Doctor]

http://www.wwiiaircraftperformance.org/p-47/p-47n-republic-wep.jpg

Interesting, always good to have real data.

Trying to interpret the performance graph, the climb data ends at 28K, and the speed data has a kink to a different trend around 26-28K. One interpretation would be that full WEP boost is available to that 26-28K altitude, decreasing above that altitude but still above normal rated power boost levels.

Another interpretation is that ram pressure at speed increases the critical altitude for full WEP boost from 28K to 32K, and the kink in the top speed line is beginning to reflect compressibility drag effects.

I suppose the right answer is to review the P-47M/N Dash-1 for Normal Rated and WEP critical altitude performance numbers.

 

[HoHun]

Hi,

I suppose the right answer is to review the P-47M/N Dash-1 for Normal Rated and WEP critical altitude performance numbers.

It does say "WAR EMERGENCY 2800/2800 S. L. TO CRITICAL" on the chart I linked.

It also says "HIGH SPEED CRIT. ALT. 32,000 FT."

(All caps in the original.)

Regards,

Henning (HoHun)

 

[BlackBat242]

The eventual/in-built production version would probably have a much shorter drive shaft and mount the turbo-charger forward of the firewall.

Hi,

Hm, was that seriously considered? That would seem to change the entire forward fuselage both in arrangement and in aerodynamics ...

Regards,

Henning (HoHun)

And significantly change its forward-aft center-of-gravity, requiring changes in tail surfaces and maybe aft fuselage length.

 

[Metrallaroja]

Interesting, always good to have real data.

Trying to interpret the performance graph, the climb data ends at 28K, and the speed data has a kink to a different trend around 26-28K. One interpretation would be that full WEP boost is available to that 26-28K altitude, decreasing above that altitude but still above normal rated power boost levels.

Another interpretation is that ram pressure at speed increases the critical altitude for full WEP boost from 28K to 32K, and the kink in the top speed line is beginning to reflect compressibility drag effects.

I suppose the right answer is to review the P-47M/N Dash-1 for Normal Rated and WEP critical altitude performance numbers.

No need to guess:

Wayback Machine

 

[Metrallaroja]

The eventual/in-built production version would probably have a much shorter drive shaft and mount the turbo-charger forward of the firewall.

Idk from where this wild guess comes from, it does not even use a turbo.

 

[HoHun]

Hi,

No need to guess:

Wayback Machine

View attachment 710464

Note that while this gives a slightly lower critical altitude than the document I referred to initially, it also provides a slightly lower top speed than said document.

For comparison:

XP-72: 460 mph @ 30000 ft on 2200 HP
P-47N: 475 mph @ 32000 ft on 2800 HP according to http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg
F-47N: 444 mph @ 33500 ft according to Standard Aircraft Characteristics graph (]https://web.archive.org/web/2022100...m/SAC/F-47N_Thunderbolt_SAC_-_17_May_1950.pdf )

(Note that the graph and the tabulated figures don't match in the Standard Aircraft Characteristics chart (SAC). In my experience, the SACs for piston-engined fighters aren't always accurate, consistent, or even based on real-world test data. The screenshot you posted does indeed show that the SAC data is derived from performance calculations based on the listed power figures.)

Regards,

Henning (HoHun)

 

[F119Doctor]

No need to guess:

Wayback Machine

View attachment 710464

Thanks for the that data. It is similar to what I found for Military and Normal rated performance on a P-47N report at http://www.wwiiaircraftperformance.org/p-47/p-47n-88406.html - see below.

It shows turbo critical altitude at approx 40K, but power is dropping from 2200 HP at 30K to 1850 at 40K, despite maintaining 53” of boost. I put this down to lowering induction air density as the carburetor inlet temperature increases from 4 C to 50C as the turbo speed increases to the maximum 22,000 rpm to maintain boost.

Under WEP boost, the same carb air temp heating is occurring at lower altitude, as shown by your 36,500 altitude rating numbers. The water injection does offset some of the density loss due to turbo compression heating.

But for this discussion, it appear that the P-47M/N would have 2700-2800 HP available at 32K at WEP.

Altitude Ft.​	Rate of Climb Ft/Min.​	Time to Climb Min.​	BHP​	Turbo RPM​	Fuel Flow Gal/Hr​	Carb. Air Temp. °C​	Condition​
S. L.	1660​	0​	2040​	4100​	254​	24​	Military Power Climb
10,000	1740​	6.0​	2180​	9200​	272​	9​	2800 RPM 53" Hg
20,000	1680​	11.6​	2220​	12800​	280​	4​	Auto Rich​
25,000	1560​	15.0​	2220​	14400​	278​	4​	175 MPH IAS​
30,000	1300​	18.0​	2200​	16200​	274​	7​	16,400 lbs at T O
35,000	900​	22.6​	2140​	18600​	263​	20​	Clean (Wing Racks Only)
S/C 40/400 ft	100​	34.0​	1840​	22000​	230​	50​
A/C 40,600	0​	36.0​	1820​	22000​	226​	50​

 

[Metrallaroja]

Hi,



Note that while this gives a slightly lower critical altitude than the document I referred to initially, it also provides a slightly lower top speed than said document.

For comparison:

XP-72: 460 mph @ 30000 ft on 2200 HP
P-47N: 475 mph @ 32000 ft on 2800 HP according to http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg
F-47N: 444 mph @ 33500 ft according to Standard Aircraft Characteristics graph (]https://web.archive.org/web/2022100...m/SAC/F-47N_Thunderbolt_SAC_-_17_May_1950.pdf )

(Note that the graph and the tabulated figures don't match in the Standard Aircraft Characteristics chart (SAC). In my experience, the SACs for piston-engined fighters aren't always accurate, consistent, or even based on real-world test data. The screenshot you posted does indeed show that the SAC data is derived from performance calculations based on the listed power figures.)

Regards,

Henning (HoHun)

If anything I would trust the SAC more on this case.
Your document is from October 1944, the SAC is from 1950 and is based in flight tests.


Of course there might also be differences in terms of SAC using bomb racks as a standard and not a clean configuration, this is for all SAC really.
But SACs are pretty accurate and usually have ultimate performance numbers, sometimes is just a mater of knowing how to read the small details.
SAC references this document, TSFTE-2012 : http://www.wwiiaircraftperformance.org/p-47/p-47n-88406.html

And as you can see military power matches with the SAC, speed also closely matches:

 

[Metrallaroja]

Thanks for the that data. It is similar to what I found for Military and Normal rated performance on a P-47N report at http://www.wwiiaircraftperformance.org/p-47/p-47n-88406.html - see below.

Yep you are dead on, because SAC references that document as I said

 

[HoHun]

Hi,

But SACs are pretty accurate and usually have ultimate performance numbers, sometimes is just a mater of knowing how to read the small details.

No, absolutely not. I'm not going to go into detail here in order not to derail the thread, but I'd warn anyone against taking SAC data for the last word on anything if it can't be verified from other sources.

The F-47N SAC might be an example for a higher quality sheet, but note that even in the test report you reference, there's only a single data point for War Emergency Power level-speed performance, and it's for a 15000 ft data point, so it's really doubtful how well it supports the high-altitude data provided by the SAC report.

For the purpose of the P-47N, we all seem to agree that it needed considerable more propeller shaft power to achieve a markedly lower top speed than the XP-72, so really no reason to start a side quest here ;-)

Regards,

Henning (HoHun)

 

[Metrallaroja]

Hi,



Note that while this gives a slightly lower critical altitude than the document I referred to initially, it also provides a slightly lower top speed than said document.

For comparison:

XP-72: 460 mph @ 30000 ft on 2200 HP
P-47N: 475 mph @ 32000 ft on 2800 HP according to http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg
F-47N: 444 mph @ 33500 ft according to Standard Aircraft Characteristics graph (]https://web.archive.org/web/2022100...m/SAC/F-47N_Thunderbolt_SAC_-_17_May_1950.pdf )

(Note that the graph and the tabulated figures don't match in the Standard Aircraft Characteristics chart (SAC). In my experience, the SACs for piston-engined fighters aren't always accurate, consistent, or even based on real-world test data. The screenshot you posted does indeed show that the SAC data is derived from performance calculations based on the listed power figures.)

Regards,

Henning (HoHun)

Revisiting this thread, I think you got confused with the data:

P-47N: 475 mph @ 32000 ft on 2800 HP according to http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg

The document says 457mph not 475mph:

F-47N: 444 mph @ 33500 ft according to Standard Aircraft Characteristics graph (https://web.archive.org/web/2022100...m/SAC/F-47N_Thunderbolt_SAC_-_17_May_1950.pdf )

444mph is at 17228lb

As you can see it shows 397knots/457mph at 15123lb which matches dead on.

SAC references this document, TSFTE-2012 : http://www.wwiiaircraftperformance.org/p-47/p-47n-88406.html

As you can see on this document that matches on speed with the SAC the configuration includes wing racks.

http://www.wwiiaircraftperformance.org/p-47/p-47n-republic-wep.jpg

This one is at an even lower weight so it is faster and it specifies ''Fighter Offensive'' configuration so it most probably do not include wing racks which also makes it lighter and faster.


So SAC, P47 Comparison and TSFTE-2012 use wing racks while Fighter Offensive does not use wing racks, with this in mind all sources match.

This is what I refer with that SAC are usually correct but you have to look into the details to compare performance numbers.
In this case the SAC seems very good and matches many other sources.

 

[TomcatViP]

Performance of the XP-72 attached.
405mph at SL
490mph at 25000 feet
3.5 minutes to 15000 feet
Impressive engine horsepower:
3450hp at SL
3000hp at 25000 feet!ll

The attached doc (see original post) specifically states that HP under War Emergency power are estimations. Hence we have to understand that speeds given in the table just below are also calculated, as "W.E" is clearly indicated.



And we know how unreliable (over optimistic) calculated speed were in WWII due to unreliable models for drag at high speed (among others).

 

[HoHun]

Hi,

Revisiting this thread, I think you got confused with the data:


The document says 457mph not 475mph:
View attachment 725495

Oh, I think you're right, I had screwed up the comparison. Here's the corrected version:

XP-72: 460 mph @ 30000 ft on 2200 HP
P-47N: 457 mph @ 32000 ft on 2800 HP according to http://www.wwiiaircraftperformance.org/p-47/comp-p47dmn.jpg
F-47N: 444 mph @ 33500 ft according to Standard Aircraft Characteristics graph (]https://web.archive.org/web/2022100...m/SAC/F-47N_Thunderbolt_SAC_-_17_May_1950.pdf )

That looks decidedly more consistent than my previous version with the typo, thanks for pointing it out!

It's even more evident with the corrected values that the XP-72 data shows an aircraft that is as fast as or even faster than the P-47N at just 80% of the P-47N's brake power.

This is what I refer with that SAC are usually correct but you have to look into the details to compare performance numbers.
In this case the SAC seems very good and matches many other sources.

Usually, but not always. I think we're in agreement that everything needs to be checked against other available sources

Regards,

Henning (HoHun)

 

[Metrallaroja]

Made a post in a videogame forum.
''History'' section might be interesting to some, directly from National Archives.

Republic XP-72 - The 28-Cylinder Thunderbolt

The XP-72 is the ultimate version of the P-47, sharing wings and parts of the fuselage but now having a 3450hp R-4360 Wasp Major and many other modifications that elevated the performance of the airplane to one of the best propellers to ever exist. Some modifications include a cooling fan...

forum.warthunder.com

 

[Justo Miranda]

Additional information for modellers

 

[Metrallaroja]

Additional information for modellers

ooh nice, can I use your images on my post?
Quoting you of course.

 

[Justo Miranda]

ooh nice, can I use your images on my post?
Quoting you of course.

OK

 

[MihoshiK]

Wonder what it would have looked like with the N wing and the four 37 mm cannons. Must have been a sight.

 

[Ronny]

The XP-72 uses a tighter cowling and shorter wings than the redesigned P-47N ones, the XP-72 also has a contra rotating 6 bladed prop that increases the efficiency at high alt.
On top of that XP-72 does not use a turbo, it is using a two stage supercharger so the thrust out of the engine is considerable taking in count that it uses a R-4360 with 28 R-2800 cylinders and that it uses a fan in the cowling to decrease cowling flap area drag and optimize exhaust gases.

anyone know why this two row have different value:

 

[F119Doctor]

anyone know why this two row have different value:

Looks like two different reduction ratios. Slower prop speed had higher airspeed but lower climb rate.

 

[riggerrob]

Additional information for modellers

That long drive shaft looks like its primary function is to balance all those heavy components when installed in a near-stock P-47 fuselage.
I suspect that the production version would have had a short shaft turning a super-charger that was mounted close behind the engine.

 

[sienar]

There is no room to stick the supercharger and all its accessories behind the engine unless you want to relocate the pilot to the tail.

 

[TomcatViP]

anyone know why this two row have different value:

https://www.secretprojects.co.uk/threads/pinnacle-of-piston-fighter-last-us-piston-fighters-vs-spiteful-mk-xvi.44242/post-707528

 

[blackkite]

Hi! Good video.

View: https://www.youtube.com/watch?v=fgIz5N11TZ8

 

[ayaromenok]

Is it any idea about gearbox ratio on this plane? (can be higher than 0.375:1 which was used for B-36)

Although a little bit off topic but a few words regarding the Tu 95 with its NK 12: The propellers turn at constant 750 rpm which is well below Mach 1.

Sorry for continue off topic - constant RPM of propellers is aprox 732 rpm (turbine 8300+/-50 x 0.0882 gear ratio). This constant mode is exist above (0.38+/0.02) of full throttle - but it's also a low throttle mode(0/-0.02) at 6600+/-200 rpm turbine (and ~582 rpm for prop) - data from ground level (I believe it's from engine tests itself - source (in RU) https://www.airwar.ru/enc/engines/nk12mv.html)

 

[PappyVF477]

anyone know why this two row have different value:

Yes, Several sources on contra rotating propellers of that era list the front prop at a lower speed than the rear prop to reduce harmonic vibration. The source on the Spiteful is in fact I think comparing the over all engine rotation on a single disk vs 2 disk Griffon engine.

Note that your report lists .425 ratio for the single prop... and .425 and .381 for the Contra prop... .425 is the BACK disk and .318 is the front disk.