The Lycoming XR-7755 was the largest piston-driven aircraft engine ever produced, with 36 cylinders totaling about 7,750 cubic inches (a truly massive 127 Liters) of displacement. It was originally intended to be used in the US "European bomber" that eventually resulted in the Convair B-36. The engine first started testing at 5,000 hp (3,700 kW) in 1944 with the XR-7755-3 and a second example was provided in 1946. However, by this time the US Air Force had lost interest in new piston designs due to the increasing interest in jet engines.
The design, which was 10 ft (3 m) long, 5 ft (1.5 m) in diameter, and weighed 6,050 lb (2,740 kg), had 9 banks of 4 cylinders each resulting in a four-row radial engine with a single central crankshaft. As the XR-7755 was liquid cooled, the banks were in line with each other instead of being spiraled as they would be on an air-cooled engine. The original XR-7755-1 design drove a single propeller but any propeller that could absorb this much power would have been ridiculously large. A redesign resulted in the XR-7755-3 (the engine modeled here) which used a new gearing system to drive a set of contra-rotating propellers. Later, another modification resulted in the XR-7755-5 which replaced the carburetors with a new fuel injection system.
I modeled this because it represents not only an amazing engineering achievement but is also one example of the culmination of decades of piston-driven aircraft engine evolution. Unfortunately, it emerged at the end of an era as the Jet Age was beginning and therefore never flew in an airframe. The sole remaining example has been restored and is on display at the Smithsonian National Air and Space Museum in Washington, DC, USA.
#aircraft #engine #lycoming #radial #xr7755
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