Sunday, October 14, 2007

Largest Lycoming

You think you have oil consumption problems? Meet the Lycoming R-7755, a 36-cylinder, 5000-hp, turbosupercharged monster displacing 7,755 cubic inches (bore/stroke 6.375 X 6.75 in.) and weighing a mere three tons, give or take a beer keg.

Two of these babies were built in 1946 (one carbureted, one fuel-injected), for the Convair B-36. Pratt & Whitney won the engine contract, ultimately, with its 28-cylinder R-4360 after the Lycoming proved too unreliable. (Think about that; the R-4360 won on reliability.) Had Lycoming gotten the contract, the B-36 would have gone into the air with 216 cylinders and 432 spark plugs. Imagine trying to keep 432 spark plugs clean, operating on postwar 115/145 avgas.

The R-7755 was innovative in a number of ways. It was liquid-cooled, which is why the cylinders line up in a perfect line (in 9 rows of 4). Each bank of cylinders had an overhead camshaft. (I don't know of another radial with an overhead cam, do you?) Each cam, in turn, had two sets of lobes: one for high power, the other for long-distance economy cruise. When the pilot chose a different setting, the entire cam would slide lengthwise a couple inches to engage the other set of lobes.

The Air Force spent 10 years battling engine problems in the B-36, many of them related to poor cylinder cooling, others involving carb ice and carburetor fires. None of which would have been a problem with the Lycoming R-7755.

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