When it comes to sheer size, power, and engineering ambition, few piston engines can compare to the Lycoming XR-7755. Designed during the final years of World War II, this 36-cylinder beast was meant to power America’s next-generation bombers, including what would become the Convair B-36 Peacemaker. But despite its jaw-dropping specifications, the XR-7755 never entered full production—leaving behind a legacy of what could have been one of the most powerful aircraft engines ever built.
The Mind-Blowing Specifications of the XR-7755
The Lycoming XR-7755 was a true engineering colossus, dwarfing nearly every other piston aircraft engine in history. Its specifications were staggering:
- Displacement: 7,750 cubic inches (127 liters)
- Configuration: 36 cylinders arranged in a four-bank radial layout
- Power Output: 5,000 horsepower
- Length: 10 feet (3 meters)
- Diameter: 5 feet (1.5 meters)
- Weight: 6,050 pounds (2,744 kg)
To put this into perspective, Lycoming’s largest production engine today displaces just 720 cubic inches, meaning the XR-7755 was more than 10 times larger!
A Titan Built for the “European Bomber”
The XR-7755 was developed for what was then called the “European Bomber” project, an ultra-long-range aircraft that could strike deep into enemy territory from U.S. bases. That bomber eventually became the Convair B-36 Peacemaker, but by the time the aircraft was finalized, jet engines had become the future of aviation, making piston-powered giants like the XR-7755 obsolete.
However, at the time, piston engines still ruled the skies, and Lycoming’s engineers aimed to create the most powerful aircraft piston engine ever conceived. The XR-7755 wasn’t just large—it was one of the most complex aircraft engines ever built, featuring liquid cooling, a four-bank radial layout, and three-speed supercharging for maximum efficiency at high altitudes.
How the XR-7755 Compared to Other Aircraft Engines
1. Pratt & Whitney R-4360 “Wasp Major”
- Displacement: 4,360 cubic inches
- Cylinders: 28
- Power Output: 3,500-4,300 horsepower
Comparison: The XR-7755 was nearly twice as large in displacement and produced up to 5,000 horsepower—eclipsing the Wasp Major, which powered aircraft like the Boeing B-50 Superfortress and the Douglas C-124 Globemaster II.
2. Wright R-3350 “Cyclone”
- Displacement: 3,350 cubic inches
- Cylinders: 18
- Power Output: 2,200-3,700 horsepower
Comparison: The XR-7755 had more than double the displacement of the Cyclone, which powered aircraft like the B-29 Superfortress.
In every metric, the XR-7755 was the pinnacle of piston engine design—but its sheer size and complexity also proved to be its downfall.
Why the XR-7755 Never Made It Into Production
Despite its incredible potential, the XR-7755 never flew in an operational aircraft. Here’s why:
- The Jet Age Arrived – By the late 1940s, turbojet engines were rapidly proving superior in speed, power, and efficiency. The B-36, initially designed for piston engines, ended up using six turbojets in addition to its piston engines.
- Weight and Complexity – The XR-7755 was massive and mechanically complex, making it difficult to integrate into aircraft designs efficiently.
- Maintenance and Reliability Concerns – With 36 cylinders and a highly intricate cooling system, the engine was prone to maintenance challenges.
- Shifting Military Priorities – The Cold War era shifted U.S. military strategy toward faster, jet-powered bombers, leaving piston engines behind.
Because of these factors, only two XR-7755 prototypes were ever built, and neither was used in an actual flight test.
What Happened to the XR-7755?
Today, the last remaining Lycoming XR-7755 prototype is preserved at the Smithsonian National Air and Space Museum. It serves as a testament to the peak of piston aircraft engine engineering, a glimpse into an era where bigger meant better—until jet engines changed the game forever.
The Legacy of the XR-7755
Although the XR-7755 never saw active service, its development contributed to advancements in aircraft engineering and pushed the boundaries of piston engine technology. It remains a symbol of an era when engineers dared to dream big—even if those dreams were eventually overtaken by newer technology.
Today, as the aviation world moves toward electric propulsion, hybrid engines, and advanced jet technology, the XR-7755 remains a fascinating piece of history—a reminder of what could have been the most powerful piston aircraft engine ever to take flight.
