Bristol Pegasus

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Bristol Pegasus

The Bristol Pegasus was an aircraft engine from a series of air-cooled nine-cylinder radial engines made by the British manufacturer Bristol Airplane Co. Ltd in Filton / Bristol.


The basis for the development of the Pegasus was the Bristol Mercury . Roy Fedden , Bristol's chief engineer, increased the stroke while maintaining the same bore from 165 to 190.5 mm, increasing the total displacement to 28.7 l. Development began in 1928 and ended in 1931 when the engine, now known as the Pegasus, was ready for series production.


The engine had cylinder heads forged from light metal , each with four overhead valves, which were operated via bumpers. The outlet valves were sodium-cooled and, like the valve seats , were made of hard metal . The two-row cam ring ran in front of the cylinders with the opposite direction of rotation to the crankshaft and was geared down 1: 8. He operated the bumpers encased in oval protective tubes via roller tappets . The cylinders were machined from a steel forging and had hardened running surfaces. The cylinder heads were shrunk onto the cylinders and fastened there. The full-shaft pistons forged from light metal have been completely machined. They each had two compression and wiper rings. The piston pins were case-hardened . The I-profile connecting rods were drop-forged, as was the two-part crankshaft mounted in roller bearings. Its front part with the crank pin was completely surface hardened. The crankcase , cast from light metal, was divided vertically. The engine was supplied with lubricating oil through a dry sump circulation system. Both for the pressure and for the return, there were gear pumps that sat together in one housing. Each had its own oil filter . The propeller was driven by a planetary gear with reduced speed.

The carburetor used was a Claudel-Hobson model equipped with an acceleration pump and mixture control. A radial fan made of aluminum, which was connected to the crankshaft via a centrifugal clutch and a gearbox , took care of the charging . It was mounted behind the engine. The two-speed blower was actuated by three hydraulically operated clutches which were supplied with pressure oil from the engine lubrication circuit. The two ignition systems had an ignition timing adjustment and were completely shielded . The engine was started either electrically or manually via a starter gear.

On the engine there was a gearbox with several drives for the auxiliary equipment. A fuel pump, two air compressors, an alternator, a hydraulic pump and a vacuum pump could be mounted on it. From the engine version Mk. XVIII the propeller adjustment was operated from this gearbox.


The engine was built in a number of designs. A separate type code existed for their designation.

The first number indicated the series. A letter or number that followed in the 2nd or 3rd position meant:

F fan with motor speed
U bottom loader, gear ratio 4.5: 1, full pressure height 150 m
L floor loader, ratio 5.7: 1, full pressure height 450 m
M loader, 7: 1 ratio, full pressure altitude 1500–2100 m
P air screw
S Höhenlader, 10: 1 ratio, full pressure altitude 3050–4300 m
2 propeller reduction gears i = 0.655
2A propeller reduction gear i = 0.666
3 propeller reduction gears i = 0.5
4 propeller reduction gears i = 0.572

The engine initially developed 546 hp, but was continuously developed. The Pegasus XVII from 1937 with a 2-speed charger already had 815 hp, the XXV already had 1010 hp. The increase in performance was also made possible by the introduction of higher octane fuels. The engine design was initially based on 73 octane. From the Mk. X version, which already required 87 octane fuel, all engines could be operated with controllable pitch propellers . From 1940 fuel with 100 octane became available, which required a corresponding adaptation of the engines. Older engines were partially rebuilt. The speeds could also be increased in the course of development. If the upper limit was initially 2300 min-1, it was later increased to 3120 min-1, albeit only permissible for 20 seconds.

The first to be built in series was the Pegasus Mk. I, which, depending on the charger, delivered between 535 and 590 hp. With the S3 version of this engine, it was possible in September 1932 to set the absolute altitude record of 13,403 m with a Vickers Vespa VII . In 1933 a Westland-Houston PV3, also equipped with this engine, was able to fly over Mount Everest for the first time . The engine was widely used, for example in the Hawker Hart , the Blackburn Ripon or the Boulton & Paul Overstrand .

In 1934 the Mk. II version was added, which delivered a higher output between 595 hp and 625 hp with the same octane number. One version, the Mk. IIP, was designed for pneumatic screws and was used in the Supermarine Walrus .

The Mk. III version was available from 1935 and was designed for 87 octane fuel. The engine now developed 690-700 hp and was used on many aircraft, the best known of which is probably the Fairey Swordfish . In 1935 the type code was adjusted and certain versions were given their own numbers. The Mk. IV corresponded to the Mk. III with a high-altitude loader and a propeller reduction gear of 0.5: 1, the Mk. V had a medium-pressure loader and a reduction of 0.572: 1. From this version, the Mk. VI was created in 1937 with a reduction gear of 0.666: 1. It was used with the Douglas DC-2 , the Hawker Audax destined for Iraq and the Junkers Ju 52 / 3m and Junkers Ju 86 , which South Africa had ordered. A pressure screw version Mk. VIP was again available for the Walrus.

A special version of the engine was the Mk. PE.VIS with an output of 500 hp. It was specially designed to attack the absolute world record in altitude. A separate, additional compressor was installed in addition to the existing high-altitude loader, which was driven by the motor via a shaft with a switchable clutch. The engine was installed in the Bristol 138A . On September 28, 1936, a height of 15,230 m could be reached, which was improved again to 16,439 m on June 30, 1937.

The Mk. X version was introduced in 1936 and was designed for 87 octane fuel. When the engine speed was increased to 2750 min-1, the output rose to 980 hp. This version was installed at the Bristol Bombay , the Handley Page Harrow and the Vickers Wellesley, among others . The XC version is a weight-reduced version required by Imperial Airways that developed 900 hp. This lighter engine was used in the Short Syrinx , Short Empire, and the flying boat that formed the lower component of the Short Mayo Composite .

Based on the lighter version, the two versions Mk. XVII and Mk. XVIII were presented in 1937, which delivered 815 hp, were designed for 87 octane fuel and differed from each other only by the different propeller ratios. Both had a 2-speed high-altitude loader and were widely used, for example in the Short Sunderland , the Vickers Wellesley and the Vickers Wellington .

The Mk. XV (725 hp), Mk. XIX (835 hp) and Mk. XX (835 hp) versions, also launched in 1937, had a smaller compressor that came from the Bristol Mercury.

The Mk. 22 version was based on the Mk. X, but had a propeller shaft with a diameter of 100 mm in order to be able to mount variable-pitch propellers from Rotol or de Havilland. The Mk. 22 LR version was created for a special long-range version of the Vickers Wellesley, which was prepared by the Long Range Development Unit for the attack on the world flight record. In 1938 the machine covered 13,263 km non-stop and without refueling.

The variants Mk. 25, Mk. 26 and Mk. 27 were the most powerful of the Pegasus series with 1010 HP continuous power and differed only in the gear reduction.

The Pegasus 38 corresponded to the Pegasus XVIII, but only had the slower loading gear. The engine was offered for commercial aviation and was used in the civilian Short Sunderland and Short Sandringham , as was the Pegasus 48, which, however, technically corresponded to the Pegasus XVIII, except for a few pieces of equipment that could only be used for civil purposes. These were the last executions of the Pegasus.

License production

The Bristol Pegasus was manufactured under license in Sweden by NOHAB and in Czechoslovakia by Walter Engines as .

Technical specifications

               Bohrung    Hub  Hubraum  Durchmesser  Gewicht Startleistung Drehzahl
                    mm     mm        l           mm       kg            PS    min-1
Pegasus XC         146  190,5     28,7         1405    476,7           920     2475
Pegasus 22         146  190,5     28,7         1405    501,6          1010     2600
Pegasus XVIII      146  190,5     28,7         1405    476,7           965     2475
                                                        Mit 100 Oktan 1050     2600


  • Alec SC Lumsden: British Piston Aero Engines and their Aircraft. Airlife, Shrewsbury 1994, ISBN 1-85310-294-6 .
  • Janes all the world aircraft, 1945. Collectors' edition. HarperCollins, London 1994, ISBN 0-00-470831-8 .
  • Introducing The Pegasus - A New "Bristol" Engine Series. In: Flight - The Aircraft Engineer & Airships. No. 1210 = Vol. 24, No. 10, March 4, 1932, ZDB -ID 160177-5 , pp. 187-190 , No. 1211 = Vol. 24, No. 11, March 11, 1932, p. 207 -211 .

See also

Web links

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