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Bristol Engine Co: Pegasus

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November 1932.
January 1934.
March 1934.
1942. Testing the crankcase alignment.

Note: This is a sub-section of Bristol Engine Co.

This entry relates to Pegasus piston aero engines. See Rolls-Royce Engines: Pegasus for information on the V/STOL engine developed by Bristol for the Hawker P.1127 (later designated the Hawker Siddeley: Harrier).

The Bristol Pegasus was a British 9-cylinder one-row air-cooled radial engine used in the 1930s and 1940s aircraft. Bristol reused the name many years later for the engine used in the Hawker Siddeley Harrier; that engine later became known as the Rolls-Royce Pegasus.

The Pegasus was designed by Roy Fedden as the follow-on to the Bristol Engine Co's very successful Jupiter, following lessons learned in the Mercury effort. The Mercury was a small engine that produced about as much power as the Jupiter, through a combination of supercharging that improved the "charge", and various changes to improve the operating RPM. Power of a piston engine can be calculated by multiplying the charge per cylinder by the number of cycles per second; the Mercury improved both and thereby produced more power for a given size. The primary advantage was a much improved power-to-weight ratio.

The Pegasus was the same size, displacement and general steel/aluminium construction as the Jupiter, but other improvements allowed the maximum engine speed to be increased from 1,950 to 2,600 rpm for take-off power. This improved performance considerably from the Jupiter's 580 hp (430 kW), to the first Pegasus II with 635 hp (474 kW), to 690 hp (515 kW) in the first production model Pegasus III, and eventually to the late-model Pegasus XXII with 1,010 hp (750 kW) thanks to the two-speed supercharger (introduced on the Pegasus XVIII) and 100-octane fuel. This gave rise to the claim "one pound per horsepower" reflecting the excellent power-to-weight ratio.

The most famous use of the Pegasus was on the Fairey Swordfish, Vickers Wellington, and Short Sunderland. It was also used on the Anbo 41, Bristol Bombay, Saro London, Short Empire, Vickers Wellesley and the Westland Wallace. Like the Jupiter before it, the Pegasus was also licensed to the PZL company in Poland. It was used it on PZL.23 Karaś and PZL.37 Łoś bombers. In Italy Alfa Romeo built Jupiter (126-RC35) and Pegasus licence versions, the engine based on Pegasus was named as Alfa Romeo 126-RC34 and civil version as RC10.[1]

Over 17,000 Pegasus engines were built.

General characteristics (Pegasus XVIII)

  • Type: 9-cylinder supercharged air-cooled radial engine
  • Bore: 5.75 in (146 mm)
  • Stroke: 7.5 in (191 mm)
  • Displacement: 1,753 in³ (28.7 L)
  • Diameter: 55.3 in (1,405 mm)
  • Dry weight: 1,180 lb (535 kg)


  • Valve-train: Four push-rod-actuated valves per cylinder, two inlet and two sodium-cooled exhaust valves
  • Supercharger: High-speed centrifugal, single-stage two-speed
  • Fuel system: Claudel-Hobson carburettor with automatic boost and mixture control
  • Oil system: Dry sump with one combination pressure/scavenge pump
  • Cooling system: Air-cooled


  • Power output:
    • 1,000 hp (745 kW) at 2,600 rpm at 3,000 ft (915 m) with 87-octane fuel
    • 885 hp (660 kW) at 2,600 rpm at 15,500 ft (4,730 m) with 87-octane fuel
    • 1,065 hp (795 kW) at 2,600 rpm at 1,250 ft (380 m) with 100-octane fuel
    • 965 hp (720 kW) at 2,600 rpm at 13,000 ft (3,965 m) with 100-octane fuel
  • Specific power: 0.60 hp/in³ (27.7 kW/L) at 1,250 ft (380 m) with 100-octane fuel
  • Compression ratio: 7:1
  • Power-to-weight ratio: 0.90 hp/lb (1.49 kW/kg)

See Also


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