Lycoming R-7755 Powered Aircraft?

The Lycoming R-7755 was the largest piston aircraft engine ever designed, having 36 cylinders and displacing 7,755 cubic inches (127 liters). The versions tested had an output of 5,000 horsepower, and it was expected that the engine could eventually be developed to produce 7,000 horsepower. To illustrate how massive this engine was, the B-36 was the largest military combat aircraft ever produced, and it was powered by "only" six Pratt & Whitney R-4360 Wasp Major engines with 28 cylinders, a displacement of 4,363 cubic inches (71.5 liters), and an output of 3,500 horsepower with the R-4360-41 Wasp Major engines that powered the B-36B variant. Just about everything about the R-7755 was double that of the R-4360 Wasp Major. The R-7755 even required two propellers (contra-rotating), because a single propeller would have been unable to achieve ground clearance on the B-36!

To my knowledge, no aircraft was ever planned to receive the R-7755, although it was at one point planned for use on the European bomber project that led to the B-36. The challenge is thus to find a use for this massive amount of power.

Here is an image of the engine.

The h-4 flying boat for a time was to use 4 of them.

Are there any drawings of this beast anywhere? I can imagine a really dedicated or insane individual with a lathe and milling machine building a quarter scale version. Even that would probably put out 50 hp

Interesting engine and speculation.

The B-36 came with a hair's thickness of being cancelled by the Air Force after WWII and the first versions produced. It was just too slow and did not have the altitude capability needed. Had it not been for the addition of the jet engines the story would have been quite different and the B-36 just another "almost" that ate up a lot of funds.

In order to efficiently use all the power, the R-7755 would have needed contra-rotating props, the gear boxes for which were GFE. A number of aircraft projects failed or never got beyond design stage due to the lack of a functioning contra-rotating gear box, the XB-35 being on of the more notable examples but certainly not the only one by any means. This was one area where the AF completely dropped the ball. In the end everything was replaced by jets anyway, but it does make one wonder "what if?"

AlanG

Johnbr said:

The h-4 flying boat for a time was to use 4 of them.

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It's my view that the H-4 was grossly underpowered, even with eight of the most powerful engines the USA had at the time. Even if it had eight of these R-7755's is would still be marginal for power. This was at a time when big aircraft were beginning to move along at 300-350 mph so something like the H-4 doodling along at 250 mph just wasn't going to cut the mustard.

The H-4 was very nearly as long as a 747, and was/is as deep in the fuselage as an A380, and the early 747 were powered by four 50,000 lbt engines. In one of my Rolls Royce books on the Crecy engine, it states that for comparison a pound of thrust is 'about' 1 HP which would mean the early 747 had about 200,000 hp, for something that could carry twice the weight and go twice as fast as the H-4.

So for the H-4 to be able to do 300-350 mph it would have to have about half that for HP. My thinking is eight 10,000 hp engines would be about right for the H-4, or eight Northrop XT37's --- which Jack Northrop did offer to Hughes

kitnut617 said:

This was at a time when big aircraft were beginning to move along at 300-350 mph so something like the H-4 doodling along at 250 mph just wasn't going to cut the mustard.

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One should remember that the H-4 was supposed to be a partial alternate to the use of conventional ships since Allied shipping in the Atlantic Ocean was suffering heavy losses to German U-boats at the time. Therefore a comparison to other aircraft might be misplaced. Perhaps a comparison to something like a Liberty Ship should be used. In this case the H-4 going at 250mph seems like lightning compared to a Liberty ship at 10 - 15 mph.