The Bristol Hercules is a fourteen-cylinder, two-row radial aircraft engine using sleeve valves and air cooling. Roy Fedden and L. F. G. Butler began its design in March 1935. After a difficult development programme, it became Bristol's most extensively produced engine and served both combat aircraft and post-war transports.[1]Bristol Hercules VINational Air and Space Museum · airandspace.si.edu · Accessed 2026-09-13Open source details
Sleeve-valve design
Bristol had investigated the Burt-McCollum sleeve-valve principle from 1926. In this arrangement a moving sleeve between piston and cylinder controls the inlet and exhaust openings. Eccentric mechanisms drive the sleeves, replacing conventional poppet valves and their associated operating gear. The design offered smooth running and the prospect of fewer parts and improved fuel economy.[1]Bristol Hercules VINational Air and Space Museum · airandspace.si.edu · Accessed 2026-09-13Open source details[2]Bristol Hercules 763RAF Museum · rafmuseum.org.uk · Accessed 2026-09-13Open source details
Wartime applications
The Hercules powered aircraft including the Beaufighter, Stirling, Halifax and Wellington, as well as the Lancaster II. These installations extended from twin-engine fighters to four-engine heavy bombers. The Smithsonian's Hercules VI is a supercharged example listed at 1,585 hp at 2,900 rpm, illustrating one wartime stage in a family whose later ratings rose considerably.[1]Bristol Hercules VINational Air and Space Museum · airandspace.si.edu · Accessed 2026-09-13Open source details[2]Bristol Hercules 763RAF Museum · rafmuseum.org.uk · Accessed 2026-09-13Open source details
Civil transport development
The Bristol Freighter demonstrates the engine's post-war progression. The Mk I used two 1,675 hp Hercules 632 engines, the Mk 21 two 1,690 hp Hercules 672s, and the stretched Mk 32 two 1,980 hp Hercules 734s. Increasing output accompanied higher aircraft weights and greater carrying capacity.[3]Bristol Type 170 FreighterBAE Systems · baesystems.com · Accessed 2026-09-13Open source details
The Freighter became associated with transporting cars and passengers across the English Channel. Its later Superfreighter could carry three cars and twenty passengers, providing a demanding short-sector commercial role for the radial engine. Beyond the Freighter, Hercules engines also served the Handley Page Hermes. The RAF Museum's preserved Hercules 763 came from a Hermes and is identified with a 2,300 hp rating.[3]Bristol Type 170 FreighterBAE Systems · baesystems.com · Accessed 2026-09-13Open source details[2]Bristol Hercules 763RAF Museum · rafmuseum.org.uk · Accessed 2026-09-13Open source details