Editing Bristol Pegasus (section)

==Design and development==

The Pegasus was designed by Sir [[Roy Fedden]] as the follow-on to the [[Bristol Aeroplane Company]]'s very successful [[Bristol Jupiter]], using lessons learned in development of the [[Bristol Mercury|Mercury]]. Although having a capacity (25 L) almost 15% smaller, the Mercury produced about as much power as the Jupiter, through a combination of [[supercharger|supercharging]] to improve the "charge", and various changes to increase the operating [[revolutions per minute|RPM]]. 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 improved [[power-to-weight ratio]] due to better [[volumetric efficiency]].{{sfnp|Lumsden|2003|p=104}}

[[File:BristolPegasus.JPG|thumb|left|Bristol Pegasus fitted to a [[Fairey Swordfish]]]]
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 {{convert|580|hp|kW|abbr=on}}, to the first '''Pegasus II''' with {{convert|635|hp|kW|abbr=on}}, to {{convert|690|hp|kW|abbr=on}} in the first production model '''Pegasus III''', and eventually to the late-model '''Pegasus XXII''' with {{convert|1,010|hp|kW|abbr=on}} thanks to the two-speed [[supercharger]] (introduced on the '''Pegasus XVIII''') and [[octane rating|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 IV|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 [[Państwowe Zakłady Lotnicze|PZL]] company in [[Poland]]. It was used on the [[PZL.23 Karaś]] and [[PZL.37 Łoś]] bombers.

In Italy [[Alfa Romeo Avio]] built both the Jupiter (126-RC35) and the Pegasus under licence, with the engine based on the Pegasus designated as the [[Alfa Romeo 125|Alfa Romeo 126-RC34]] with the civil version as the 126-RC10.<ref name="aroca-qld.com">{{Cite web
  |url=http://www.arocaqld.com/library_articles/alfa_romeo_aero_engines.php
  |title=Alfa Aero Engines
  |access-date=19 December 2010
  |website=aroca-qld.com
}}</ref>  In [[Czechoslovakia]] it was built by [[Walter Engines]] and was known as the '''Pegas'''.<ref>[http://www.walterengines.com/about-us/history.htm Walter engines history - Walterengines.com] {{webarchive |url=https://web.archive.org/web/20080509084946/http://www.walterengines.com/about-us/history.htm |date=9 May 2008 }} Retrieved: 3 August 2009</ref>

Approximately 32,000 Pegasus engines were built.{{sfnp|Lumsden|2003|p=104}} The Pegasus set three height records in the [[Bristol Type 138]]: in 1932, 1936 and 1937. It was used for the first flight over [[Mount Everest]] in the [[Westland Wallace]], and in 1938 set the world's long-distance record in [[Vickers Wellesley]]s.{{sfnp|Bridgman|1998|p=270}}

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.<ref >{{cite book
  |last= Frame  |first= Alex
  |title= Flying Boats: My Father's War in the Mediterranean
  |location= London  |publisher= Victoria University Press
  |year = 2007
  | pages = 50
  |isbn= 978-0-86473-562-1
}}</ref>