Airborne Awesomosity, Military-Grade Awesome

The Last Gasp For Large Piston Engines

Pratt and Whitney R-4360 VDT

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Starting in the mid nineteen forties there was a move to this newfangled jet turbine technology. Still it was mostly untested and the good old internal combustion engine was better known.  Into this small fragment of time was an interesting innovation of Pratt & Whitney’s.



Pratt and Whitney R-4360 VDT

[image credit –]



By 1944 the end of the line for piston engine technology was in sight.  On the flip side jet engines were still to new and used too much fuel for long range aircraft.  So an intermediate option was explored by engine manufacturer Pratt and Whitney.  Their plan was to utilize the exhaust from their two largest engines, the R-3350 and R-4360, to be the first fitted with this technology.



Mitsubishi Zero Showing Rear Facing Exhaust

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The idea of using the exhaust to provide additional forward trust wasn’t new to the industry. As you can see clearly here the Japanese laid out the exhaust for their relatively small and low powered radial motor to aid in forward momentum. This was also the case with the P-51, the P-40 and most aircraft using the Allison V-1710 inline engines.

To accomplish this Pratt and Whitney made several changes to their big radial. First they removed the mechanical supercharger. Next was a move from a carburetor to direct fuel injection and finally the addition of the variable discharge turbine to the rear of the engine. This final piece was to allow the exhaust gasses to be passed through a turbo with the exhaust exiting out as thrust.

This setup was expected to give an additional eight hundred horsepower. With this in mind several aircraft were designed for several bomber upgrades to utilize this power. Of these the final version of the Boeing B-29 Superfortress was to be called the B-54 Ultrafortress using these engines. As well as a very unusual variant of the Convair B-36 to be discussed later.

During testing issues were found with fuel distribution due to the removal of the mechanical supercharger. A larger issue was that due to this setup they had very limited control over the throttle. During testing they controlled the throttle by opening and closing of the turbine exhaust. A setup that would not work in combat or even in production. These issues could have been addressed but the writing was on the wall. The next generation of jet bombers were already on the drawing boards they found no need to put this into production.

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Specifications (R-4360-51VDT)

General characteristics

  • Type: 28-cylinder supercharged air-cooled four-row radial engine
  • Bore: 5.75 in. (146.05 mm)
  • Stroke: 6.00 in. (152.4 mm)
  • Displacement: 4,362.5 in³ (71.49 L)
  • Length: 96.5 in. (2 451 mm)
  • Diameter: 55 in (1397 mm)
  • Dry weight: 3,870 lb (1,755 kg)




Boeing B-54 Ultrafortress Mockup


This is but a footnote in the history of the large aircraft radial engine but it does lead to some interesting questions and discussion. The issues they had during testing that could have been ironed out but there really was no point. The early turboprops were rated at 5000hp and were far less complex then this monster.  The maintenance needed to keep these running would have been very intense. So now we just step back and do the classic what if’s.

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  • CaptianNemo2001

    Small correction: "used too much fuel"

    There was a planned 4800 hp version using the VDT in the C4 version of the Wasp Major. Once can only dream about how far the radial could have progressed with every engineering trick and innovation of the last 60 years. At the very least, reliability and fuel economy could be improved.

    • fodder650

      /facepalm. Fixed.

      That one PDF from the factory showed they ran the non-VDT at 4800hp successfully as well. Which makes me wonder what they could have gotten out of this setup. As for the fuel economy let me point out they could have geared it. Lycoming was attempting to do that to help the issue as well.

      • CaptianNemo2001

        Cummins is rumored to be messing with a diesel and a Variable Discharge Turbine. But I think variable veins on the turbo is more likely, rather then fixed veins, that is. I'll look into it more and try and track it down.

        Gearing the turbo would be something like this <a href="http://?” target=”_blank”>?

        Oh the unholy mechanical maddness if you used every power adder and fuel saver on the same engine.

        • fodder650

          This one got close but might have matched it if they had left the supercharger on it. Oddly what I have read said they went to direct fuel injection where Wiki lists the old pressure carburetor.

          • CaptianNemo2001

            There were some designs listed in the PDF that use Carburetors and some that used "Fuel Injection Carburetors" OR just "Injection Carburetors" &lt; This last word later got dropped and left at "Fuel Injection". But it is included in the Pilots Powerplant Manual so it has got me confused because I wanted to read about Fuel Injection.

            Scanning those photos now.

  • sporttruck

    All I can think about is the added drag from the cowling if it was kept in this configuration.

    • fodder650

      Well the aircraft were set up in tractor fashion so that this would fit in the wing and not add to the drag.

      • sporttruck

        True. I was thinking of large multi engine bombers and transports being retrofitted with these. They would work much better in a fresh design.

        • CaptianNemo2001

          I am sure the pipping/ducting could be tweaked and the whole thing streamlined a bit with the cowling. I think by changing how the exhaust gases come out and off the engine you could limit the drag even more… And by the time they get done moving things around it could be a lot less.

          Say move the turbos vertically more and slide then closer to the after coolers and make the after coolers longer in length from front to back to compensate for the slightly higher incoming heat. ect ect.

          In order for that game to make sense there must be a firewall or engine mounts in the way. (Gap from back of engine to front of after cooler.) Which explains why the turbos have to be dropped so low. With the correct wing shape and careful embedding of the engine you could have almost everything in the wing and still keep it all cool.

          Junkers G. 38 style? OR Boeing Clipper?

          • fodder650

            Considering this engine was designed to be used in a B-29 variant then the idea of it being something like a Stratocruiser only makes sense.

          • CaptianNemo2001

            I have some Stratospheric pics as well.

  • CaptianNemo2001

    B-36 wrecks and some you can go and see.

    • fodder650

      Its odd to say thank you for these but this is the first time I've seen these.