Republic XP-72

Pioneer · 20 October 2012

I don't know if this is common knowledge, and if it is I apologize!

I've just finished readying the wonderful book - World's Fastest Four-Engine Piston-Powered Aircraft: Story of the Republic XR-12 Rainbow, by Mike Machat.

I found an interesting abstract in the book pertaining to:
"Minus its wings and tail section, the one-of-a-kind [Republic] XP-72 was, in essence, one engine nacelle [including the same Pratt & Whitney R-4360 radial engine] and contra-rotating propeller of the original [Republic] XF-12 [Rainbow]."

I was taken aback by this insight, which makes complete sense to me when you take a look at the XP-72's excellent clean aerodynamics!!

Regards
Pioneer

Justo Miranda · 20 October 2012

Beauty or beast?

JFC Fuller · 20 October 2012

That is fascinating, the 100 P-72 fighters that were ordered were to have been fitted with the R-4360-19 with a 6 bladed GM Aeroproducts contra-rotating propeller and a remote fluid drive super charger. The second X-72 prototype did fly with the contraprop but apparently not with the supercharger. See the document at the below address:

http://www.enginehistory.org/phpbb/viewtopic.php?t=650&sid=b726628cae767f7dc6f3930a0df410ef

I would say beauty AND Beast! Does anybody have any other photos of the second prototype with the contra-prop?

As if a supercharged R4360 driving a 6 bladed contra-prop from within a very sleek fuselage was not awesome enough the alternative armament (prototypes had 6 .50 cals with 1,600 rounds) was to be four 37mm cannon as well as underwing hard-points for fuel tanks or 1,000lb bombs.

The tails carry 36598 and 36599. What I have never been able to establish is exactly when the 100 production aircraft were cancelled?

Justo Miranda · 20 October 2012

Stargazer · 20 October 2012

Very interesting piece of information! Here are more images of the XP-72 (whose quoted name "Ultrabolt" is completely apocryphal).

Steve Pace · 20 October 2012

Are the XP-72 serial numbers 43-6598/-6599 or 43-36598/-36599? -SP

Antonio · 20 October 2012

One of my all time favourite

In June 1943, Republic started a study to match the most powerful engine with the P-47 airframe. The resulting beast was designated XP-72. The XP-72 was selected to replace the Republic XP-69 as the P-47 replacement in the fighter escort role.

Finally, the P-51 revealed as an excellent bomber escort and Republic effort to replace the P-47 in the fighter bomber role ended in the XP-84

From an unconfirmed source I found the name "Ultrabolt" for the XP-72

Stargazer · 20 October 2012

Steve Pace said:
Are the XP-72 serial numbers 43-6598/-6599 or 43-36598/-36599? -SP

USAAF and USAF aircraft never carry the year prefix in full, only the second figure. Therefore 43-6598 becomes 36598. Simple as that.

Steve Pace · 20 October 2012

Stargazer2006 said:
Steve Pace said:

Are the XP-72 serial numbers 43-6598/-6599 or 43-36598/-36599? -SP

Click to expand...

USAAF and USAF aircraft never carry the year prefix in full, only the second figure. Therefore 43-6598 becomes 36598. Simple as that.

But there's usually a hyphen which in this case there isn't which led me to my query. -SP

Stargazer · 20 October 2012

Steve Pace said:
Stargazer2006 said:

Steve Pace said:

Are the XP-72 serial numbers 43-6598/-6599 or 43-36598/-36599? -SP

Click to expand...

USAAF and USAF aircraft never carry the year prefix in full, only the second figure. Therefore 43-6598 becomes 36598. Simple as that.

Click to expand...

But there's usually a hyphen which in this case there isn't which led me to my query. -SP

No hyphens. There may have been at some point but that has never been the norm as far as I know.

Bill Walker · 20 October 2012

According to Joesph Baugher's web site, 43-6598 was a P-51B,and the XP-72s were 43-36598 and 43-36599.

Quoting from the same site (the last sentence appears to apply here):

"..on October 28, 1941, shortly after the USAAF had been formed, an order was given that numbers of no less that 4 digits would be painted on the tail fin of all Army aircraft (where feasible) in a size large enough to be seen from at least 150 yards away. This was officially called the radio call number, but was almost universally known as the tail number. Since military aircraft were at that time not expected to last more than ten years, the first digit of the fiscal year number was omitted in the tail number as was the AC prefix and the hyphen. For example, Curtiss P-40B serial number 41-5205 had the tail number 15205 painted on its tail fin, Curtiss P-40K serial number 42-11125 had the tail number 211125 painted on the fin... Unfortunately, there were many deviations from these rules--there are examples in which only the last 4 or 5 digits were painted on the tail, which makes identification of the aircraft particularly difficult. "

Stargazer · 20 October 2012

Thanks Bill for reminding us that rules are not always applied, and for giving the correct serial numbers for the XP-72 prototypes.

sienar · 21 October 2012

Ray Higginbotham, chief power plant engineer at Republic, was responsible for designing the propulsion systems of the XP-47J, XP-47M, XP-72 and XF-12. So it should come as no surprise that the XF-12 and XP-72 are siblings of sorts.

The XP-72 as tested had a R-4360-13 engine equipped with a two-stage variable speed mechanical supercharger. However, the production version was intended to have a -19 engine with a variable-speed remote centrifugal supercharger, engine driven through a fluid coupling via an extension shaft. This unique blower was to have a diameter of nearly 5 feet, and was designed to survive the strain of a 4000+ HP engine. The photo below is of this odd blower.

This information as well as the enclosed photo, comes from Thunderbolt; from seversky to victory by Warren M. Bodie. I highly recommend it to any p-47 fans (and you should be a p-47 fan

)

This book also states that the XP-72 could have been mated with an enlarged wing and dorsal fin ala the P-47N along with the gain in range and roll rates. This is a purely hypothetical scenario, but it does give an idea of how the 72 may have been developed had it seen production.

In addition this book states that the XP-72 may have been reverted a subtype of the P-47 line in production Another source I have discusses this issue in a bit more detail, but its name currently escapes me.

Pioneer · 21 October 2012

Thank you gentleman for your reply and input!
I was concerned that this information would have been like preaching to the converted

So it was the XP-72

Ultrabolt

.......................... thanks pometablava

Regards
Pioneer

Stargazer · 21 October 2012

I still see no justification in any official source for the "Ultrabolt" name, however tempting it may be to use it...

Steve Pace · 21 October 2012

According to my USAF document entitled: INDEX of AF Serial Numbers Assigned to Aircraft 1958 and Prior there were 100 Republic P-72s ordered 43-36498 to 43-36599 and that they were all cancelled. But we now know that the last two airplanes (43-36598/-36599) were completed as the two XP-72-RE aircraft. -SP

GTX · 21 October 2012

Stargazer2006 said:
I still see no justification in any official source for the "Ultrabolt" name, however tempting it may be to use it...

Names aren't always official: case in point being "Aardvark" or "Pig" for the F-111... :

Bill Walker · 21 October 2012

Steve Pace said:
According to my USAF document entitled: INDEX of AF Serial Numbers Assigned to Aircraft 1958 and Prior there were 100 Republic P-72s ordered 43-36498 to 43-36599 and that they were all cancelled. But we now know that the last two airplanes (43-36598/-36599) were completed as the two XP-72-RE aircraft. -SP

Slightly off topic, but where can one find this book? (Yes, I'm a serial number nut.)

Stargazer · 21 October 2012

GTX said:
Names aren't always official: case in point being "Aardvark" or "Pig" for the F-111... :

Ah, but the F-111 ended its career with Aardvark being officially assigned to at least one variant!

Steve Pace · 21 October 2012

Bill Walker said:
Steve Pace said:

According to my USAF document entitled: INDEX of AF Serial Numbers Assigned to Aircraft 1958 and Prior there were 100 Republic P-72s ordered 43-36498 to 43-36599 and that they were all cancelled. But we now know that the last two airplanes (43-36598/-36599) were completed as the two XP-72-RE aircraft. -SP

Click to expand...

Slightly off topic, but where can one find this book? (Yes, I'm a serial number nut.)

It's not a book - it's an official USAF document that I acquired back in the 1970s from the History Office at Edwards AFB, CA. -SP

Antonio · 21 October 2012

I still see no justification in any official source for the "Ultrabolt" name

I toke a superficial look at my sources but I can't find the name "Ultrabolt" gain. Possibly it ended in my notes from an internet source. So, by the moment, treat it with skepticism please.

RyanC · 2 December 2016

Steve Pace said:
According to my USAF document entitled: INDEX of AF Serial Numbers Assigned to Aircraft 1958 and Prior there were 100 Republic P-72s ordered 43-36498 to 43-36599 and that they were all cancelled. But we now know that the last two airplanes (43-36598/-36599) were completed as the two XP-72-RE aircraft. -SP

From my visit to NARA, these 100~ P-72s were cancelled in favor of the AP-23 (P-84).

windswords · 4 December 2016

Top speed was supposed to be 490 mph, and that's with a 3500 hp engine. Later versions of the R-4360 Wasp Major made 3800 and 4300 hp. If it had reached production maybe the P-72 would have been the first piston engine fighter to exceed 500 mph.

Nick Sumner · 5 December 2016

Wasn't there a variant of the P47 that exceeded 500mph in level flight?

Deltafan · 5 December 2016

Nick Sumner said:
Wasn't there a variant of the P47 that exceeded 500mph in level flight?

The speedest propellers plane in level flight of the WW2 time was the XP-47J prototype with 505 mph : https://oldmachinepress.com/2013/12/17/republic-xp-47j-superbolt/

windswords · 5 December 2016

Deltafan said:
Nick Sumner said:

Wasn't there a variant of the P47 that exceeded 500mph in level flight?

Click to expand...

The speedest propellers plane in level flight of the WW2 time was the XP-47J prototype with 505 mph : https://oldmachinepress.com/2013/12/17/republic-xp-47j-superbolt/

According to the article linked above, the P-47J was a lightened version of the P-47. It was canceled in favor of the R-4360-powered XP-72 as it was felt that it had more future potential as an air-frame. So their was no chance of the J reaching production status, whereas the XP-72 actually reached the stage of getting an initial production order. But, it is true that the XP-47J was the first test piston aircraft to go faster than 500 mph.

Sundog · 5 December 2016

windswords said:
According to the article linked above, the P-47J was a lightened version of the P-47. It was canceled in favor of the R-4360-powered XP-72 as it was felt that it had more future potential as an air-frame. So their was no chance of the J reaching production status, whereas the XP-72 actually reached the stage of getting an initial production order. But, it is true that the XP-47J was the first test piston aircraft to go faster than 500 mph.

I think they would have been better off with the J, as it was somewhat "proven" technology. They also would have had it before the end of the war. Whereas, the XP-71 with it's contra-rotating prop had trouble with the gearbox. By the time they would have had those issues worked out jets would have surpassed it anyway. That's the problem with always looking over the horizon; sometimes you can't see the good enough right in front of you.

Deltafan · 5 December 2016

windswords said:
Top speed was supposed to be 490 mph, and that's with a 3500 hp engine. Later versions of the R-4360 Wasp Major made 3800 and 4300 hp. If it had reached production maybe the P-72 would have been the first piston engine fighter to exceed 500 mph.

Sundog said:
windswords said:

According to the article linked above, the P-47J was a lightened version of the P-47. It was canceled in favor of the R-4360-powered XP-72 as it was felt that it had more future potential as an air-frame. So their was no chance of the J reaching production status, whereas the XP-72 actually reached the stage of getting an initial production order. But, it is true that the XP-47J was the first test piston aircraft to go faster than 500 mph.

Click to expand...

I think they would have been better off with the J, as it was somewhat "proven" technology. They also would have had it before the end of the war. Whereas, the XP-71 with it's contra-rotating prop had trouble with the gearbox. By the time they would have had those issues worked out jets would have surpassed it anyway. That's the problem with always looking over the horizon; sometimes you can't see the good enough right in front of you.

As far as I know, there were two prototypes of the XP-72. Only the second had contra-rotating props and crashed during the flight tests. dit it crash because of the gearbox ?
which one of the two XP-72 prototypes reached 490 mph, the first one, without contra-rotating props or the second one, with contra-rotating props ? Or the two prototypes reached 490 mph ?

windswords · 5 December 2016

According to Wikipedia the top speed was 490 mph (789 km/h) / 387mph (623 km/h) at sea level. It does not specify which aircraft. The reference given for the top speed is from "The Whine of the Jug" by Warren Bodie, Wings Magazine (Vol. 4, No. 4), August 1974, pp. 33–39. The specific speed reference is on page 33.

Also included is a quote from the above article:

"Test pilot Tom Bellinger stated flatly that no flights ever exceeded 500 mph. The dash 13 engine was not supercharged. With the planned but never installed dash 19 engine (with a remote supercharger) rated at 3,650 HP at 25,000 ft. (3,000 HP at sea level) a top speed of 504 mph at approximately 25,000 feet was expected. Planned further development of the dash 19 engine was expected to yield approximately 4,000 hp and a speed of 540 mph at 25,000 ft".

source Wikipedia https://en.wikipedia.org/wiki/Republic_XP-72

So it seems likely that if put into production further developments would have produced a fighter capable of 500+ mph speeds.

sienar · 6 December 2016

Was the 72 based on the lightened J or the regular 47? Comparing empty weights doesn't give a clear answer.

windswords · 6 December 2016

It was based off the regular P-47 airframe, though like the XP-47J it had 6 .50 caliber guns. The J was based off a razorback Thunderbolt with a canopy that included an extended glazed area behind the pilot. The XP-72 had a bubble canopy and cut down rear fuselage of the later P-47 models.

Deltafan · 6 December 2016

The XP-47J second prototype was foreseen with a bubble canopy and contra-rotating props. But because of problems with the contra-rotating gear box it was not ready before the cancellation of the XP-47J program (in favor of the XP-72 program).

An artist view of the XP-47J 2nd prototype (but without the contra-rotating props) :

https://www.suggest-keywords.com/eHAtIDQ3ag/

sienar · 6 December 2016

windswords said:
It was based off the regular P-47 airframe, though like the XP-47J it had 6 .50 caliber guns. The J was based off a razorback Thunderbolt with a canopy that included an extended glazed area behind the pilot. The XP-72 had a bubble canopy and cut down rear fuselage of the later P-47 models.

No. The J was essentially an entirely brand new lighter airframe build to a lower max G requirement from the regular 47.

This is what I want to know, is the 72 using the lighter airframe from the J or the stronger regular 47 airframe.

windswords · 7 December 2016

"Based off of" does not mean the same as taken directly from. You don't have to be an aviation expert to see that they used the same form factor of the wing, tail and aft fuselage of the P-47 for the J. The XP-72 was a direct progression of the P-47: "As an expediency, the Xp-72 made use of the proven fuselage and wings of the P-47 before it".
http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=419

I hope this answers your original question.

sienar · 7 December 2016

windswords said:
"Based off of" does not mean the same as taken directly from. You don't have to be an aviation expert to see that they used the same form factor of the wing, tail and aft fuselage of the P-47 for the J. The XP-72 was a direct progression of the P-47: "As an expediency, the Xp-72 made use of the proven fuselage and wings of the P-47 before it".
http://www.militaryfactory.com/aircraft/detail.asp?aircraft_id=419

I hope this answers your original question.

Well of course it was the same form factor, but the structure was substantially lightened to the point that the J shared very little tooling with the regular 47, which was the main factor that prevented its production.

That link doesnt really answer the question either. What is needed is the max g factor of the 72.

windswords · 7 December 2016

"Well of course it was the same form factor, but the structure was substantially lightened to the point that the J shared very little tooling with the regular 47, which was the main factor that prevented its production."
sienar

In my reply "based off of" was in relation to form factor as a matter of design, as in based off the design of the P-47. I was not specific enough.

"On 31 July 1943, Republic issued a report comparing the estimated performance of the XP-47J with the P&W R-4360-powered XP-72 that was under development. The report concluded that the Republic XP-72 had more potential and recommended resources be focused on that aircraft."
This according to oldmachinepress.com was the reason the the J was shelved in favor of the XP-72.

"That link doesnt really answer the question either. What is needed is the max g factor of the 72."
sienar

In earlier repsonses you asked:
"Was the 72 based on the lightened J or the regular 47? Comparing empty weights doesn't give a clear answer."
and:
"This is what I want to know, is the 72 using the lighter airframe from the J or the stronger regular 47 airframe."

These were your last question, which was answered. I have no information on max G rating of the XP-72.

Johnbr · 29 January 2019

Republic XP-72
https://forums.spacebattles.com/threads/left-behind-by-the-jet-age-republics-xp-72-super-thunderbolt.629410/

Metrallaroja · 20 October 2023

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!

Colonial-Marine · 20 October 2023

I believe the planned production variant was supposed to use the new wing of the P-47N which contained some fuel unlike the earlier wing and had squared-off wingtips. I wonder how this would have effected performance?

Foo Fighter · 21 October 2023

There is a decent photograph depicting the new wing here..... https://hushkit.net/2023/02/21/the-republic-p-47-thunderbolt-mafia-family/

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