XF-12 Data

[Zipper730]

I'm curious about a couple of things regarding the Republic XF-12

Firstly: I'm curious if anybody has any diagrams of the propulsion system: It was extraordinarily well-designed and seems like something that would be interesting to see from a historical and technical standpoint.

Secondly: While I have little interest in the airliner version of the aircraft, what I found interesting was that it was said to have a load-factor that was lower than the XF-12. That makes me wonder what the load-factor of the XF-12 was. While large aircraft tend not to be terribly nimble or agile, there have been some large aircraft that have been surprisingly agile (Ju 88: Ultimate load: 8.1g; He 177: Ultimate Load: 7.2g; Mosquito: Ultimate Load: 8g) and agility does help increase the survivability of a reconnaissance aircraft.



Airframes , fubar57 , Jimbob , Micdrow , M MIflyer , MiTasol , S Shortround6 ,
swampyankee tomo pauk , VERSUCH , W wuzak

 

[wuzak]

The engines were R-4360s with two turbos (C-type IIRC) with exhausts exiting the rear of the nacelle.

The nacelles were nearly as long as the P-47's fuselage.

Cooling for the engine was controlled by a sliding ring system, rather than the traditional cowl gills used in US air-cooled aircraft during WW2. Like the system used on some BMW 801 powered aircraft.

 

[AMCKen]

The engines were R-4360-VDT, the 'VDT' standing for variable discharge turbine. Apparently the engines were throttled by varying the size of the outlet of the turbochargers, not by the usual stuff in the intake tracts.

 

[GTX]

Highly recommend this book if you are interested in this aircraft:

 

[Tom Fey]

R-4360-37 engines on the XF-12 were not VDT but had two GE turbochargers (BM 4 and BM 5) in tandem per engine. Like the -37's in the XF-11, each engine could be selected on none (both waste gates open) , one, or two of the turbochargers depending on demand. See "R-4360: Pratt & Whitney's Major Miracle" by Graham White.

 

[ddwhitney]

I'm curious about a couple of things regarding the Republic XF-12

Firstly: I'm curious if anybody has any diagrams of the propulsion system: It was extraordinarily well-designed and seems like something that would be interesting to see from a historical and technical standpoint.

Secondly: While I have little interest in the airliner version of the aircraft, what I found interesting was that it was said to have a load-factor that was lower than the XF-12. That makes me wonder what the load-factor of the XF-12 was. While large aircraft tend not to be terribly nimble or agile, there have been some large aircraft that have been surprisingly agile (Ju 88: Ultimate load: 8.1g; He 177: Ultimate Load: 7.2g; Mosquito: Ultimate Load: 8g) and agility does help increase the survivability of a reconnaissance aircraft.



Airframes , fubar57 , Jimbob , Micdrow , M MIflyer , MiTasol , S Shortround6 ,
swampyankee tomo pauk , VERSUCH , W wuzak

I'm attaching a drawing from Graham White's masterpiece, R-4360-Pratt & Whitney's Major Miracle, page 474. It gives the ducting layout and shows each engine with a single turbosupercharger, type not given. A couple of pages later he shows the lubrication system for the XF-12, which shows tandem turbos, a General Electric BM-4 and BM-5.
GE identified their different "Types" of turbos as A/B/C/D/E/I, each for fitting to a larger horsepower rated engine. If memory serves, Type B was typically 1000 to 1800 hp, Type C 1,500 to 3,000 hp, etc.
There were also different developments within each range, such as BH and BM. These referred to improved components, the "BH" able to operate at higher RPM and efficiency, with the "BM" differing by being "multi-stage", actually two stages. Obviously, these were intended for different aircraft missions. It is clear that the single unit shown on the attached figure was likely a "CHM", which was used on some advanced programs. Whether a single CHM or two BM units were used in the actual XF-12 aircraft we do not yet know.
The XF-12 Rainbow was rated for a maximum ceiling of 40,000 feet, where the ambient air pressure is 5.558 inHgA. Engines identified for the Rainbow were R-4360-31 or -37 engines rated for 3,000 bhp at 2,700 RPM and were fitted with single-stage mechanical superchargers driven through 6.06:1 and 6.08:1 gearing respectively. Without turbos, critical altitude was 1,500 feet. Do not know its rated manifold pressure, but it was likely in the 45-50 inHgA range as the engine was rated on Grade 100/130 fuel.
Assuming the engine stage supercharger delivered a very good 3.0:1 pressure ratio, means that the two-stage turbosupercharger, at 40,000 feet, has to have an overall pressure ratio of 6.0:1, which is very aggressive. Reno air racers, with their two-stage Merlins running at overspeed, develop a PR of about 5.2:1. All of this is to say that the Rainbow definitely had to use two-stage turbosuperchargers, and to make 3,000 bhp, the single Type CHM would have been lighter and easier to package.
Some people describe the XF-12 as having been powered by the "VDT", variable discharge turbine supercharger. This was proposed, but never installed/developed. The first airplane had 3,500 bhp R-4360-35 engines and the second the 4,300 bhp R-4360-43.

 

[wuzak]

The engines were R-4360-VDT, the 'VDT' standing for variable discharge turbine. Apparently the engines were throttled by varying the size of the outlet of the turbochargers, not by the usual stuff in the intake tracts.

The VDT engines did not really work. The control systems of the day were not up to the task.

 

[Zipper730]

All I can say is I'm glad P&W didn't continue the development of the H-3730: I don't think there would have been an R-4360 without it.