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Lycoming XR-7755

Lycoming XR-7755
posted by flyvertosset
Fri, Aug 12 2011


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Lycoming XR-7755 was the largest piston-driven aircraft engine ever produced; with 36 cylinders totaling about 7,750 in³ (127 L) of displacement and a power output of 5,000 horsepower (3,700 kilowatts). It was originally intended to be used in the "European bomber" that eventually emerged as the Convair B-36. Only two examples were built before the project was terminated in 1946.

The resulting design used 9 banks of 4 cylinders arranged around a central crankshaft to form a four-row radial engine. Unlike most multi-row radials, which "spiral" the cylinders to allow cooling air to reach them, the R-7755 was water-cooled and so each of the cylinder heads in a cylinder bank were in-line within a cooling jacket. Contrast this with the Junkers Jumo 222, which looked similar from the outside but ran on a V-style cycle instead of a radial. The XR-7755 was 10 ft (3 m) long, 5 ft (1.5 m) in diameter, and weighed 6,050 lb (2,740 kg). At full power it was to produce 5,000 hp (3,700 kW) at 2,600 rpm, maintaining that with a turbocharger to a critical altitude that was apparently never published.

Each cylinder bank had a single overhead cam powering the poppet valves. The camshaft included two sets of cams, one for full takeoff power, and another for economical cruise. The pilot could select between the two settings, which would shift the camshaft along its axis to bring the other set of cams over the valve stems. Interestingly, the design mounted some of the accessories on the "front side" of the camshafts, namely two magnetos and four distributors. The seventh camshaft was not used in this fashion, its location on the front of the engine was used to feed oil to the propeller reduction gearing.

The original XR-7755-1 design drove a single propeller, but even on the largest aircraft the propeller needed to absorb the power would have been ridiculously large. This led to a minor redesign that produced the XR-7755-3, using a new propeller gearing system driving a set of coaxial shafts to power a set of contra-rotating propellers. The propeller reduction gearing also had two speed settings to allow for a greater range of operating power than adjustable props alone could deliver. Another minor modification resulted in the XR-7755-5, the only change being the replacement of carburetors with a new fuel injection system.

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The only remaining Lycoming XR-7755-3, photographed at the Smithsonian