| Pegasus | |
|---|---|
 | |
| Preserved Bristol Pegasus on display at theBrooklands Museum. The red circles are temporary blanking plates removed when theexhaust manifold is fitted | |
| Type | Pistonaero engine |
| Manufacturer | Bristol Aeroplane Company |
| First run | c.1932 |
| Major applications | |
| Number built | c.32,000 |
| Developed from | Bristol Jupiter |
| Developed into | |
TheBristol Pegasus is a British nine-cylinder, single-row, air-cooledradialaero engine. Designed byRoy Fedden of theBristol Aeroplane Company, it was used to power both civil and military aircraft of the 1930s and 1940s. Developed from the earlierMercury andJupiter engines, later variants could produce 1,000 horsepower (750 kilowatts) from itscapacity of 1,750cubic inches (28 L) by use of a gearedsupercharger.
Further developments of the Pegasus created thefuel-injectedBristol Draco and thedieselBristol Phoenix, both types being produced in limited numbers. In contrast, by the end of production over 30,000 Pegasus engines had been built. Aircraft applications ranged from single-enginebiplanes to the four-enginedShort Sandringham andSunderlandflying boats. Severalaltitude anddistance records were set by aircraft using the Pegasus.
TheBristol Siddeley company reused the name many years later for theturbofan engine used in theHawker Siddeley Harrier and which became known as theRolls-Royce Pegasus whenRolls-Royce took over that company. Two Bristol Pegasus engines remain airworthy in 2010, poweringFairey Swordfish aircraft operated by theRoyal Navy Historic Flight (became Navy Wings in March 2019); other examples are preserved and on public display inaviation museums.
The Pegasus was designed by SirRoy Fedden as the follow-on to theBristol Aeroplane Company's very successfulBristol Jupiter, using lessons learned in development of theMercury. Although having a capacity (25 L) almost 15% smaller, the Mercury produced about as much power as the Jupiter, through a combination ofsupercharging to improve the "charge", and various changes to increase the operatingRPM. The 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 improvedpower-to-weight ratio due to bettervolumetric efficiency.[1]
The Pegasus was the same size, displacement and general steel/aluminium construction as the Jupiter, but various 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 firstPegasus II with 635 hp (474 kW), to 690 hp (510 kW) in the first production modelPegasus III, and eventually to the late-modelPegasus XXII with 1,010 hp (750 kW) thanks to the two-speedsupercharger (introduced on thePegasus XVIII) and100-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 theFairey Swordfish,Vickers Wellington, andShort Sunderland. It was also used on theAnbo 41,Bristol Bombay,Saro London,Short Empire,Vickers Wellesley and theWestland Wallace. Like the Jupiter before it, the Pegasus was also licensed by thePZL company inPoland. It was used on thePZL.23 Karaś andPZL.37 Łoś bombers.
In ItalyAlfa Romeo Avio built both the Jupiter (126-RC35) and the Pegasus under licence, with the engine based on the Pegasus designated as theAlfa Romeo 126-RC34 with the civil version as the 126-RC10.[2] InCzechoslovakia it was built byWalter Engines and was known as thePegas.[3]
Approximately 32,000 Pegasus engines were built.[1] The Pegasus set three height records in theBristol Type 138: in 1932, 1936 and 1937. It was used for the first flight overMount Everest in theWestland Wallace, and in 1938 set the world's long-distance record inVickers Wellesleys.[4]
In service the Pegasus was generally reliable with the exception that the valves were prone to failure. The valves were operated by rocker-boxes, which were lubricated by oil pads on the top of the cylinder heads. In hot climates the lubrication deteriorated which could lead to seizing of the valves. Also if a problem developed with the engine and the pilot shut it down it was not possible to "feather" the propeller and so it would continue to rotate creating drag and continue to turn the engine which had no lubrication being forced through it by the oil pump.[5]
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 includedcompression ratios, propeller reduction gear ratios and accessories.
Note:[6]
Bristol Pegasus engines can be viewed installed in aircraft at theRoyal Air Force Museum London and theImperial War Museum Duxford. An unrestored Pegasus recovered from the sea bed is in the care of the Bristol Aero Collection, which is closed while moving from Kemble Airport to Filton.
There is also an engine on display at theBrooklands Museum at Weybridge.
As of October 2010 two Bristol Pegasus engines remain airworthy in England. They power the two Fairey Swordfish aircraft operated by theRoyal Navy Historic Flight.[8]
Data fromLumsden.[9]
Related development
Comparable engines
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