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Bristol Pegasus (radial)
Walter Engines Pegas


The nine-cylinder, single-row, air-cooled radial Pegasus was designed by Sir Roy Fedden as the follow-on to the Bristol Aeroplane Company's Bristol 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 due to better volumetric efficiency.

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.

Some notable users of the Pegasus were 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 by the PZL company in Poland. It was used on the PZL.23 Karaś and PZL.37 Łoś bombers. In Italy Alfa Romeo built both the Jupiter (126-RC35) and the Pegasus under licence, with the engine based on the Pegasus designated as the Alfa Romeo 126-RC34 with the civil version as the 126-RC10. In Czechoslovakia it was built by Walter Engines and was known as the Pegas.

Approximately 32,000 Pegasus engines were built. The Pegasus set three height records: in 1932, 1936 and 1937. It was used for the first flight over Mount Everest, and in 1938 set the world's long-distance record.

The Pegasus was produced in many variants, early prototype engines were unsupercharged but the majority used a geared supercharger, either single-speed or two-speed. Variant differences included compression ratios, propeller reduction gear ratios and accessories.

Blackburn Baffin
Blackburn Ripon
Blackburn Shark
Boulton Paul Mailplane
Boulton Paul Overstrand
Boulton Paul Sidestrand
Bristol Bombay
Bristol Type 118
Bristol Type 120
Bristol Type 138
Douglas DC-2
Fairey TSR I
Fairey Seal
Fairey Swordfish
Fokker C.X
Fokker D.XXI-5
Gloster Goring
Handley Page H.P.43
Handley Page H.P.47
Handley Page H.P.51
Handley Page Hampden
Handley Page H.P.54 Harrow
Hawker Audax
Hawker Hart
Hawker Osprey
Junkers Ju 52
Junkers Ju 86K-4
Koolhoven FK.52
Letov Š-328
LWS-6 Żubr
PZL.23 Karaś
PZL.37 Łoś
Saro London
Savoia-Marchetti SM.95
Short Mayo Composite
Short Sandringham
Short Sunderland
Short Empire
Short Syrinx
Supermarine Stranraer
Supermarine Walrus
Vickers Type 253
Vickers Valentia
Vickers Vanox
Vickers Vellox
Vickers Vespa
Vickers Viastra X
Vickers Victoria
Vickers Vildebeest
Vickers Vincent
Vickers Virginia
Vickers Wellington
Vickers Wellesley
Westland Wallace
Westland Wapiti
Westland PV.7
Westland-Houston PV-3


Pegasus XVIII

Type: 9-cylinder, single-row, supercharged, air-cooled radial engine
Bore: 5.75 in (146 mm)
Stroke: 7.5 in (190 mm)
Displacement: 1,753 in³ (28.7 L)
Length: 61 in (1,549 mm)
Diameter: 55.3 in (1,405 mm)
Dry weight: 1,111 lb (504 kg)
Valvetrain: Four pushrod-actuated valves per cylinder – two intake and two sodium-filled exhaust
Supercharger: Two-speed centrifugal type supercharger
Fuel system: Claudel-Hobson carburettor
Fuel type: 100 Octane petrol
Oil system: Dry sump with one combination pressure/scavenge pump
Cooling system: Air-cooled
Reduction gear: Farman epicyclic gearing, 0.5:1
Propeller: Rotol three-blade with variable pitch
Power output:
965 hp (720 kW) at 2,475 rpm for takeoff at sea level
835 hp (623 kW) at 2,250 rpm at 8,500 ft (2,590 m), maximum continuous climb power
965 hp (720 kW) at 2,600 rpm at 13,000 ft (3,960 m), maximum power (emergency combat - 5 minutes only)
Specific power: 0.55



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