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Bristol Mercury Radial Engine

Bristol Mercury Radial Engine


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Bristol Mercury Radial Engine

A view of the Bristol Mercury Radial Engine as used in the Blenheim Mk IV


Bristol Mercury

The Bristol Mercury is an air-cooled 9-cylinder radial engine from the British manufacturer Bristol Aircraft Company . The Mercury is a further development of the Bristol Jupiter . The Bristol Mercury from 1925 should not be confused with the fourteen- cylinder Cosmos Mercury from 1919, which was also designed by Roy Fedden .


Contents

The Mercury was developed by the Bristol Aeroplane Company in 1925 as their Bristol Jupiter was reaching the end of its lifespan. Although the Mercury initially failed to attract much interest, the Air Ministry eventually funded three prototypes and it became another winner for the designer Roy Fedden.

With the widespread introduction of superchargers to the aviation industry in order to improve altitude performance, Fedden felt it was reasonable to use a small amount of boost at all times in order to improve performance of an otherwise smaller engine. Instead of designing an entirely new block, the existing Jupiter parts were re-used with the stroke reduced by one inch (25 mm). The smaller capacity engine was then boosted back to Jupiter power levels, while running at higher rpm and thus requiring a reduction gear for the propeller. The same techniques were applied to the original Jupiter-sized engine to produce the Pegasus.

The Mercury's smaller size was aimed at fighter use and it powered the Gloster Gauntlet and its successor, the Gloster Gladiator. It was intended that the larger Pegasus would be for bombers but as the power ratings of both engines rose, the Mercury found itself being used in almost all roles. Perhaps its most famous use was in a twin-engine light bomber, the Bristol Blenheim.

In 1938 Roy Fedden pressed the Air Ministry to import supplies of 100 octane aviation spirit (gasoline) from the USA. This new fuel would allow aero engines to run at higher compression ratios and supercharger boost pressure than the existing 87-octane fuel, thus increasing the power. The Mercury XV was one of the first British aero engines to be type-tested and cleared to use the 100-octane fuel in 1939. This engine was capable of running with a boost pressure of +9 lbs/sq.in and was first used in the Blenheim Mk IV. [1]

The Mercury was also the first British aero engine to be approved for use with variable-pitch propellers.

The Bristol company and its shadow factories produced 20,700 examples of the engine. [2] Outside the United Kingdom, Mercury was licence-built in Poland and used in their PZL P.11 fighters. It was also built by NOHAB in Sweden and used in the Swedish Gloster Gladiator fighters and in the Saab 17 dive-bomber. In Italy, it was built by Alfa Romeo as the Mercurius. In Czechoslovakia it was built by Walter Engines. In Finland, it was built by Tampella and mainly used on Bristol Blenheim bombers.

Mercury I (1926) 808 hp, direct drive. Schneider Trophy racing engine. Mercury II (1928) 420 hp, compression ratio 5.3:1. Mercury IIA (1928) 440 hp Mercury III (1929) 485 hp, compression ratio 4.8:1, 0.5:1 reduction gear.

Mercury XV (1938) 825 hp, developed from Mercury VIII. Converted to run on 100 Octane fuel (previously 87 Octane). Mercury XVI 830 hp. Mercury XX (1940) 810 hp Mercury 25 (1941) 825 hp. Mercury XV with crankshaft modifications. Mercury 26 825 hp. As Mercury 25 with modified carburettor. Mercury 30 (1941) 810 hp, Mercury XX with crankshaft modifications. Mercury 31 (1945) 810 hp, Mercury 30 with carburettor modifications and fixed pitch propeller for Hamilcar X.


Variants

Mercury I (1926) 808 hp, direct drive. Schneider Trophy racing engine. Mercury II (1928) 420 hp, compression ratio 5.3:1. Mercury IIA (1928) 440 hp Mercury III (1929) 485 hp, compression ratio 4.8:1, 0.5:1 reduction gear. Mercury license built by NOHAB Mercury IIIA Minor modification of Mercury III. Mercury IV (1929) 485 hp, 0.656:1 reduction gear. Mercury IVA (1931) 510 hp. Mercury IVS.2 (1932) 510 hp. Mercury (Short stroke) Unsuccessful experimental short stroke (5.0 in) version, 390 hp. Mercury V 546 hp (became the Pegasus IS.2) Mercury VIS (1933) 605 hp, see specifications section. Side view showing valve gear detail. Mercury VISP (1931) 605 hp, 'P' for Persia. Mercury VIS.2 (1933) 605 hp. Mercury VIA (1928) 575 hp (became the Pegasus IU.2) Mercury VIIA 560 hp (became the Pegasus IM.2) Mercury VIII (1935) 825 hp, compression ratio 6.25:1, lightened engine. Mercury VIIIA Mercury VIII fitted with gun synchronisation gear for the Gloster Gladiator Mercury VIIIA 535 hp, second use of VIIIA designation, (became the Pegasus IU.2P) Mercury IX (1935) 825 hp, lightened engine. Mercury X (1937) 820 hp. Mercury XI (1937) 820 hp. Mercury XII

Mercury XV (1938) 825 hp, developed from Mercury VIII. Converted to run on 100 Octane fuel (previously 87 Octane). Mercury XVI 830 hp. Mercury XX (1940) 810 hp Mercury 25 (1941) 825 hp. Mercury XV with crankshaft modifications. Mercury 26 825 hp. As Mercury 25 with modified carburettor. Mercury 30 (1941) 810 hp, Mercury XX with crankshaft modifications. Mercury 31 (1945) 810 hp, Mercury 30 with carburettor modifications and fixed pitch propeller for Hamilcar X.


From Graces Guide

The Bristol Engine Company manufactured aeroengines.

The company was originally a separate entity called Cosmos Engineering which had been formed from the pre-First World War automobile company, Brazil, Straker and Co.

1917 Cosmos was asked to investigate air-cooled radial engines, producing what would become the Bristol Mercury, a 14 cylinder two-row (helical) radial, which they launched in 1918. This engine saw little use, but a smaller and simpler 9 cylinder version known as the Bristol Jupiter was clearly a winning design.

With the post-war rapid contraction of military orders Cosmos Engineering went bankrupt, and the Air Ministry let it be known that it would be a good idea if the Bristol Aeroplane Co purchased them.

The Jupiter engine competed with the Armstrong Siddeley Jaguar throughout the 1920s, but Bristol put more effort into their design, and by 1929, the Jupiter was clearly superior.

1930s developed a new line of radials based on the sleeve valve principle, which would develop into some of the most powerful piston engines in the world, and continued to be sold into the 1960s.


Bristol Mercury Radial Engine - History

Almost 21,000 engines were produced, with a number also being built in Europe under licence.

Design and development
The Mercury was developed by the Bristol Aeroplane Company in 1925 as their Bristol Jupiter was reaching the end of its lifespan. Although the Mercury initially failed to attract much interest, the Air Ministry eventually funded three prototypes and it became another winner for the designer Roy Fedden.

With the widespread introduction of superchargers to the aviation industry in order to improve altitude performance, Fedden felt it was reasonable to use a small amount of boost at all times in order to improve performance of an otherwise smaller engine. Instead of designing an entirely new block, the existing Jupiter parts were re-used with the stroke reduced by one inch (25 mm). The smaller capacity engine was then boosted back to Jupiter power levels, while running at higher rpm and thus requiring a reduction gear for the propeller. The same techniques were applied to the original Jupiter-sized engine to produce the Pegasus.

The Mercury's smaller size was aimed at fighter use and it powered the Gloster Gauntlet and its successor, the Gloster Gladiator. It was intended that the larger Pegasus would be for bombers but as the power ratings of both engines rose, the Mercury found itself being used in almost all roles. Perhaps its most famous use was in a twin-engine light bomber, the Bristol Blenheim.

In 1938 Roy Fedden pressed the Air Ministry to import supplies of 100 octane aviation spirit (gasoline) from the USA. This new fuel would allow aero engines to run at higher compression ratios and supercharger boost pressure than the existing 87-octane fuel, thus increasing the power. The Mercury XV was one of the first British aero engines to be type-tested and cleared to use the 100-octane fuel in 1939. This engine was capable of running with a boost pressure of +9 lbs/sq.in and was first used in the Blenheim Mk IV.

The Mercury was also the first British aero engine to be approved for use with variable-pitch propellers.

The Bristol company and its shadow factories produced 20,700 examples of the engine. Outside the United Kingdom, Mercury was licence-built in Poland and used in their PZL P.11 fighters. It was also built by NOHAB in Sweden and used in the Swedish Gloster Gladiator fighters and in the Saab 17 dive-bomber. In Italy, it was built by Alfa Romeo as the Mercurius. In Czechoslovakia it was built by Walter Engines. In Finland, it was built by Tampella and mainly used on Bristol Blenheim bombers.

  • Mercury I: (1926) 808 hp, direct drive. Schneider Trophy racing engine.
  • Mercury II: (1928) 420 hp, compression ratio 5.3:1.
  • Mercury IIA: (1928) 440 hp
  • Mercury III: (1929) 485 hp, compression ratio 4.8:1, 0.5:1 reduction gear.
  • Mercury IIIA: Minor modification of Mercury III.
  • Mercury IV (1929) 485 hp, 0.656:1 reduction gear.
  • Mercury IVA: (1931) 510 hp.
  • Mercury IVS.2: (1932) 510 hp.
  • Mercury (Short stroke): Unsuccessful experimental short stroke (5.0 in) version, 390 hp.
  • Mercury V: 546 hp (became the Pegasus IS.2)
  • Mercury VIS: (1933) 605 hp, see specifications section.
  • Mercury VISP: (1931) 605 hp, 'P' for Persia.
  • Mercury VIS.2: (1933) 605 hp.
  • Mercury VIA: (1928) 575 hp (became the Pegasus IU.2)
  • Mercury VIIA: 560 hp (became the Pegasus IM.2)
  • Mercury VIII: (1935) 825 hp, compression ratio 6.25:1, lightened engine.
  • Mercury VIIIA: Mercury VIII fitted with gun synchronisation gear for the Gloster Gladiator
  • Mercury VIIIA: 535 hp, second use of VIIIA designation, (became the Pegasus IU.2P)
  • Mercury IX: (1935) 825 hp, lightened engine.
  • Mercury X: (1937) 820 hp.
  • Mercury XI: (1937) 820 hp.
  • Mercury XII: (1937) 820 hp
  • Mercury XV: (1938) 825 hp, developed from Mercury VIII. Converted to run on 100 Octane fuel (previously 87 Octane).
  • Mercury XVI: 830 hp.
  • Mercury XX: (1940) 810 hp
  • Mercury 25: (1941) 825 hp. Mercury XV with crankshaft modifications.
  • Mercury 26: 825 hp. As Mercury 25 with modified carburettor.
  • Mercury 30: (1941) 810 hp, Mercury XX with crankshaft modifications.
  • Mercury 31: (1945) 810 hp, Mercury 30 with carburettor modifications and fixed pitch propeller for Hamilcar X.

Pistons - Full-skirted type. Machined inside and outside from aluminum-alloy forgings. One sindle and one double scrapper ring and two gas rings. Robust fully-floating, case-hardened gudgeon pins.

Connecting rods - "I" section, machined from steel alloy stampings.

Crankshaft - Two-piece, machined from alloy steel stampings. Front half incorporates a large diameter crank-pin, is surface hardened all over. Seperate tail-shaft for auxiliary drives. Carried on two main roller-bearings, with a deep-groove journal-bearing at the reduction end and steadying bearing in rear.

Crankcase - Machined from aluminum-alloy forgings, split on centre-line of cylinders and held with nine through-bolts.

Valve Gear - Two inlet and two sodium-cooled stellited exhaust valves and stellited seats per cylinder. Clearances bewteen rockers and valves automatically compensated for expansion. A two-row cam runs concentric with the crankshaft in front of the crank at one eigth engine speed in an anti-crank direction. It operates the tappets through rollers on floating bronze bushes, and thence by push-rods enclosed in oval tubes.

Carburation - Claudell-Hobson carburetter, with delayed action acceleration pump, variable datum automatic boost and mixture control with slow-running runout. controlable hot and cold air-intakes. Hot oil circulated around the chokes.

Supercharger - High-speed centrifugal type. Driven off the crankshaft through spring-drive and automatic centrifugal clutches. Aluminum-alloy supercharger casing with integral diffuser vanes. Aluminum-alloy volute casing. Whole unit mounted behind the rear wall of the crankcase on nine crankcase bolts.

Ignition - Dual ignition by two B.T.H. or Rotex magnetos transversely mounted on rear cover and driven by bevel gearing from crankshaft. Variable-timing device interconnected with carburetter to give best setting for various throttle openings. Completely screened ignition system.

Lubrication - Dry sump, with pressure feed. Duplex gear pump incorporating pressure and scavenge units in one assembly. Seperate feed and scavenge filters. Special device provides high initial oil pressure for rapid opening to full power.

Airscrew Drive - Self-centralising bevel-epicyclic gear. All bearings pressure-lubricated. Airscrew shafts suitable for either fixed or controllable-pitch hubs. Oil-transfer housing and internal oil-seal provided for Hamilton pitch-controll mechanism.

Accessory Drives - Provision for single or dual feed pump, high and low pressure air-compressors, shaft-driven electric generator, hydraulic pump, vacuum pump.

Starter System - Combined electric and hand turning gear.


12 September 1934

12 September 1934: Hawker Aircraft Company test pilot Flying Officer Phillip Edward Gerald Sayer made the first flight of the Gloster G.37, a prototype fighter for the Royal Air Force, designed to reach a speed of 250 miles per hour (402 kilometers per hour) while armed with four machine guns. The flight took place at Gloster’s private airfield at Brockworth, Gloucestershire.

The Gladiator was a single-place, single-engine, single-bay biplane, with fixed landing gear. The airplane was primarily of metal construction, though the aft fuselage, wings and control surfaces were fabric covered.

The production Gladiator Mk.I was 27 feet, 5 inches (8.357 meters) long with a wingspan of 32 feet, 3 inches (9.830 meters) and overall height of 11 feet, 9 inches (3.581 meters). It had an empty weight of 3,217 pounds (1,459 kilograms) and gross weight of 4,594 pounds (2,084 kilograms).

Gloster G.37 prototype, right profile

The G.37 was equipped with a left-hand tractor, air-cooled, supercharged, 1,519.083 cubic-inch-displacement (24.893 liters) Bristol Mercury IV-S2 nine cylinder radial engine. With a compression ratio of 5.3:1, the IV-S2 was rated at 505 horsepower at 2,250 r.p.m., and 540 h.p. at 2,600 r.p.m., both at 13,000 feet (3,962 meters). It developed a maximum 560 horsepower at 2,600 r.p.m. at 16,000 feet (4,877 meters). The engine had a take-off power rating of 530 horsepower at 2,250 r.p.m., at Sea Level (3-minute limit). The IV-S2 drove a two-bladed fixed-pitch propeller through a 0.655:1 gear reduction. This engine weighed 920 pounds (417 kilograms).

The G.37 was repowered with a Bristol Mercury VI-S engine, which had a 6:0:1 compression ratio and a 0.5:1 gear reduction ratio. This engine produced a maximum of 636 horsepower at 2,750 r.p.m. at 15,500 feet.

The prototype was armed with two synchronized, air-cooled Vickers .303-caliber machine guns, firing forward through the propeller arc, and two .303-caliber Lewis guns mounted under the bottom wing.

With the upgraded engine and armament, the G.37 reached 242 miles per hour (389 kilometers per hour).

The Gloster Gladiator Mk.I with an enclosed cockpit and a Bristol Mercury IX engine had a maximum speed of 257 miles per hour (414 kilometers) per hour) at 14,600 feet (4,450 meters).

This production Gloster Gladiator Mk.I, K6131, shows the cockpit enclosure. (This airplane, the second production Gladiator Mk.I, was damaged beyond repair when it ran out of fuel near RAF Church Fenton, 26 March 1938.) (Royal Air Force) Gloster Gladiator Mk.I L8032. (SDASM)

The Gladiator Mk.I entered service with the Royal Air Force in February 1937. It was the last biplane fighter to do so, and was the first fighter with an enclosed cockpit. Beginning with No. 72 Squadron, eight fighter squadrons were equipped with the type, though by the beginning of World War II, these were being phased out by more modern airplanes like the Hawker Hurricane and Supermarine Spitfire.

A total of 737 Gloster Gladiators, Mk.I and Mk.II, were built. In addition to the Royal Air Force, there were operated by several other countries in Europe, the Mediterranean and the Middle East.

Prototype Gloster Gladiator G.37 in flight, now marked K5200. A .303-caliber Lewis machine gun is visible under the right wing. (Royal Air Force) Phillip E.G. Sayer, O.B.E. (Flight)

Phillip Edward Gerald Sayer was born at Colchester, England, 2 February 1905. He was the second of three children of Edward James Sayer, a retired British Army officer and Ethel Jane Hellyar Sayer.

Sayer was granted a short service commission in the Royal Air Force as a Pilot Officer on probation, 30 June 1924. His rank was confirmed 23 May 1925. He was promoted to Flying Officer 30 March 1926. Flying Officer Sayer was transferred to the R.A.F. Reserve, 2 March 1929.

In 1930, Gerry Sayer joined Hawker Aircraft Company as a test pilot. When Hawker took over Gloster Aircraft Co., Ltd. in November 1934, he was appointed Chief Test Pilot of Gloster.

Flight Lieutenant Sayer completed his service and relinquished his commission, 2 March 1937. He was permitted to retain his rank.

On 15 May 1941, Sayer made the first flight of the Gloster-Whittle E.28/39, a prototype jet fighter.

Chief Test Pilot Phillip Edward Gerald Sayer, Esq., was appointed an Officer of the Most Excellent Order of the British Empire (OBE) in the New Years Honours list, 30 December 1941.

Gerry Sayer was flying a Hawker Typhoon from RAF Acklington, 22 October 1942, to the Druridge Bay gunnery range. He never returned.


Development Potential of Bristol Mercury and Pegasus engines

This is a question related to the better prepared Australia thread. At present I'm having the Australian Munitions Supply Board build the Bristol Mercury and Pegasus under licence in its ordnance factories from about 1933.

AFAIK both were developed up to 1,000hp. However, it would fit what I have in mind much better if they could have been developed to produce 1,200hp in 1940. Is that possible?

Paul_Sussex

Edgeworthy

NOMISYRRUC

An alternative is to built the Bristol Hercules under licence instead of the Wasp and Twin Wasp. IOTL the first Australian Wasp was produced in January 1939 (after ordering the Wirraway in January 1937) and the first Twin Wasp in November 1941 (after the July 1939 decision to build the Beaufort in Australia).

IIRC IOTL the Short Golden Hind airliner with four 1,400hp Hercules and the Saro Lerwick with two 1,500hp Hercules was in service in the middle of 1939-ish.

With a POD of 1936 is production of the Hercules in Australia from early 1938 feasible?

Edgeworthy

An alternative is to built the Bristol Hercules under licence instead of the Wasp and Twin Wasp. IOTL the first Australian Wasp was produced in January 1939 (after ordering the Wirraway in January 1937) and the first Twin Wasp in November 1941 (after the July 1939 decision to build the Beaufort in Australia).

IIRC IOTL the Short Golden Hind airliner with four 1,400hp Hercules and the Saro Lerwick with two 1,500hp Hercules was in service in the middle of 1939-ish.

With a POD of 1936 is production of the Hercules in Australia from early 1938 feasible?

Astrodragon

Tomo pauk

This is a question related to the better prepared Australia thread. At present I'm having the Australian Munitions Supply Board build the Bristol Mercury and Pegasus under licence in its ordnance factories from about 1933.

AFAIK both were developed up to 1,000hp. However, it would fit what I have in mind much better if they could have been developed to produce 1,200hp in 1940. Is that possible?

500 kg for much increase of boost/RPM/power.

added: one funky idea might've been that Bristol and De Havilland join the forces and make a V12 air cooled engine, using cylinders and rest of usable parts from Pegasus. 1300 HP in 1938?

NOMISYRRUC

500 kg for much increase of boost/RPM/power.

added: one funky idea might've been that Bristol and De Havilland join the forces and make a V12 air cooled engine, using cylinders and rest of usable parts from Pegasus. 1300 HP in 1938?

Just Leo

An alternative is to built the Bristol Hercules under licence instead of the Wasp and Twin Wasp. IOTL the first Australian Wasp was produced in January 1939 (after ordering the Wirraway in January 1937) and the first Twin Wasp in November 1941 (after the July 1939 decision to build the Beaufort in Australia).

IIRC IOTL the Short Golden Hind airliner with four 1,400hp Hercules and the Saro Lerwick with two 1,500hp Hercules was in service in the middle of 1939-ish.

With a POD of 1936 is production of the Hercules in Australia from early 1938 feasible?

As to the early manufacture of Hercules, they didn't look very reliable nor did they create gobs of power early on. It was discovered that the Lerwick couldn't fly on one engine, and the Short couldn't fly on two, or three when one is ablaze. The Hercules required some manufacturing expertise to be developed, using specialized machine tools, with specialized modifications, as well as newly developed alloys from your local alloy steel shops in Sheffield and Birmingham, England. The Herc wasn't something the Aussies would lust after before the Hercules VI.

I find it difficult to embrace the concept of building an original engine to fill a gap before another engine takes its place, since building engines is very hard, and takes time and gobs of money. Severe misgivings aside, I do suggest the name MSB Bungeroo for this engine. Have you given any thought to what kind of engine? I would have thought the R-1830 fills enough of the gap to preclude other options.

Tomo pauk

Just Leo

Tomo pauk

The guys that built it were confident that two 1375 HP engines can reliably power a monoplane flying boat of 845 sq ft wing, loaded weight of 28400 lbs, and max t.o. weight of 33000 lbs. The 2-engined A/C should be able to at least glide well with one engine out, and people at Saro either did not take the single engine capability in the account, or their calculations were wrong. Perhaps they lied to the costumer? Either way, their blame can't be denied.
People that bough it either believed Saro's calculations, or (and?) failed to properly test the prototype and cancel it once it was discovered that 1-engine operation is a major safety hazard. So the people that bought it were guilty, too. There is no 'manufacturer or costumer', it is 'manufacturer and costumer' in this case.
As it was the case with Blackburn Botha, or Ba.88 Lince.

Saying that 'engines were weak' in case of Lerwick is a majorization - 1375 HP in 1939-40 was no small feat. I'd also love to see a real comparison in reliablility of the engines in that time frame so we could deduce that early Hercules was really a dog.

Just Leo

NOMISYRRUC

ITTL the MSB had been developing its own engines since 1924, first via the Engine Section of the RAAF Experimental Station at Randwick and then directly after the Engine Section becomes the design department of its aero engine factory. None of the engines designed between 1924 and 1934 went into production because they were worse than the British engines that they could build under licence. The Bungeroo would have been begun in 1934. I hoped that would have been enough time to put a 1,000 hp Bungeroo into production in 1939, followed by a 1,200hp version in 1940 and the final version producing 1,500hp in 1942.

As to what kind of engine I wanted it to be a single-row, air-cooled radial using poppet valves. I.e. a development of the Pegasus with more cylinders or larger ones. I used poppet valves rather than sleeve valves because the MSB engineers were being more conservative. By basing the Bungeroo on the Pegasus the MSB could use the same production tooling to make it.

I did think of Wackett or the MSB getting a licence on the Twin Wasp earlier, but ITTL I want the Australians to develop their own engines or built more British engines under licence.

NOMISYRRUC

  1. In the 1920s the RAAF still buys De Havilland Hounds, Supermarine Seagulls and Supermarine Southamptons fitted with Napier Lion engines built by the MSB
  2. In the early 1930s the RAAF buys the De Havilland Hound Mk II, Supermarine Walrus and Supermarine Stranraer, but instead of these types being fitted with Bristol Pegasus engines built by the MSB, they have Rolls Royce Buzzard engines built by the MSB
  3. The advanced trainer that Wackett designs instead of the taking a licence out on the Harvard is powered by a Buzzard engine. The alternative to that is that the Miles Master with Buzzard or Kestrel engines was built in Australia instead of the Harvard
  4. In 1936 when the RAAF orders 300 Hampdens to be built by the MSB instead of 50 Bolingbrokes to be built by Bristol it specifies that they have Rolls Royce Merlin engines built by the MSB instead of Pegasus engines. When it orders the Short Sunderland from the MSB to replace the Stranraer the RAAF specifies that these aircraft have Merlin engines instead of the Pegasus.

Archibald

NOMISYRRUC

The guys that built it were confident that two 1375 HP engines can reliably power a monoplane flying boat of 845 sq ft wing, loaded weight of 28400 lbs, and max t.o. weight of 33000 lbs. The 2-engined A/C should be able to at least glide well with one engine out, and people at Saro either did not take the single engine capability in the account, or their calculations were wrong. Perhaps they lied to the costumer? Either way, their blame can't be denied.
People that bough it either believed Saro's calculations, or (and?) failed to properly test the prototype and cancel it once it was discovered that 1-engine operation is a major safety hazard. So the people that bought it were guilty, too. There is no 'manufacturer or costumer', it is 'manufacturer and costumer' in this case.
As it was the case with Blackburn Botha, or Ba.88 Lince.

Saying that 'engines were weak' in case of Lerwick is a majorization - 1375 HP in 1939-40 was no small feat. I'd also love to see a real comparison in reliablility of the engines in that time frame so we could deduce that early Hercules was really a dog.

I think part of the problem with the Botha and Lerwick was that they were among the types that the Air Ministry ordered into production "off the drawing board" from 1936 instead of the previous policy which was to order several competitive prototypes, test them at the A&AEE or MAEE and then put the best one or two designs into production. It did this to save time and AFAIK in the knowledge that some of them would fail or at least be severely delayed.

In the case of Specification R.1/36 the best design was tendered by Supermarine. However, the firm was too busy with the Spitfire and the heavy bomber designed to meet Spec. B.12/36 so it went to Saunders Roe by default.


THE BRISTOL family of SLEEVE-VALVE ENGINES

As far back as 1926, the Bristol Aeroplane Co. foresaw the speed and load limitations which would eventually be met in high-performance engines which have push-rod operated over-head valves. The increasing seriousness of maintenance problems with this mechanism was also foreseen. With the encouragement and support of the British Air Ministry, the Company therefore decided to develop the single sleeve-valve.

The first complete Bristol sleeve-valve engine, a nine-cylinder air-cooled radial of 24.9 liters capacity, was designed and built in 1932. It completed its official trials with great success soon afterwards. This was the Perseus. With further development it was the first sleeve-valve aero-engine in the World to be put into large quantity manufacture.

The potential advantages of the sleeve-valve for high-output two-row radial engine design were also apparent. In 1936, the Bristol Hercules fourteen-cylinder radial sleeve-valve engine of 38.7 liters capacity appeared and this was followed by the Taurus, a similar but much smaller engine of 25.4 liters. The latest type is the Centaurus, an eighteen-cylinder development of the Hercules.

After the most thorough endurance and overload testing, which make up many thousands of hours on the dynamometer and in flight, and nearly six years of operational service in the Royal Air Force, Bristol sleeve-valve aero-engines have now definitely achieved a leading position.

All Bristol sleeve-valve engines have high-speed, centrifugal, gear-driven superchargers, either single or two-speed. The supercharger is associated with a carburetor of the latest fully automatic type, incorporating variable-datum servo devices for the control of both boost-pressure and mixture strength.

Later production types employ pressure injection carburetors enabling a closer control of mixture strength under varying conditions and greater freedom from ice formation.

(Photo right: Courtesy of AirArchive.com)

An installation feature of great importance is the arrangement of engine-driven accessories. The crankcase rear cover carries only those accessories which serve the engine unit itself, namely the engine oil pump, the dual fuel pump, the magnetos and the constant-speed airscrew governor unit. All other accessories are carried by a separate accessory gear-box mounted on the bulkhead and driven by the engine through an enclosed flexibly-jointed shaft.

Several alternative arrangements of the gear-box drives are available to provide for the full range of accessories involved in modern aircraft equipment. This arrangement considerably simplifies installation work, and also lends itself to the adoption of standardized, interchangeable power units - a policy long recommended by the Bristol Company.


Bristol Mercury Radial Engine - History

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