Napier Sabre

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The Napier Sabre was a British H-24-cylinder, liquid cooled, sleeve valve, piston aero engine, designed by Major Frank Halford and built by Napier & Son during World War II. The engine evolved to become one of the most powerful inline piston aircraft engines in the world developing from 2,200 horsepower (1,640 kW) in its earlier versions to 3,500 hp (2,600 kW) in late-model prototypes.[1]

The first operational aircraft to be powered by the Sabre were the Hawker Typhoon and Hawker Tempest; however, the first aircraft powered by the Sabre was the Napier-Heston Racer, which was designed to capture the world speed record[nb 1]. Other aircraft using the Sabre were early prototype and production variants of the Blackburn Firebrand, the Martin-Baker MB 3 prototype and one of the Hawker Fury prototypes. The rapid conversion to jet engines after the war led to the quick demise of the Sabre, because Napier also turned to developing jet engines.

Design and development

Prior to the Sabre, Napier had been working on large aero engines for some time. Their most famous was the Lion, which had been a very successful engine between the World Wars and, in modified form, powered several of the Supermarine Schneider Trophy competitors in 1923 and 1927, as well as several land speed record cars. By the late 1920s the Lion was no longer competitive, and work started on replacements.

Napier followed the Lion with two new H-block designs: the H-16 (Rapier) and the H-24 (Dagger). The H-block has a compact layout, consisting of two horizontally opposed engines lying one atop or beside another. Since the cylinders are opposed, the motion in one is balanced by the motion on the opposing side, leading to no first order vibration or second order vibration whatever. In these new designs Napier chose to use air cooling. However, in service the rear cylinders proved to be impossible to cool properly, and this led to quite poor reliability.

Genesis

The first aircraft designed around the Sabre engine – the Napier-Heston Racer which crashed during early flight tests.

During the 1930s, designers were looking to future engine developments; for example, studies showed the need for engines capable of developing one horsepower per cubic inch of displacement (about 45 kW/litre). This specific power output was needed to propel aircraft large enough to carry large fuel loads for long range flights. It was clear that this sort of performance would not be easy to achieve. A typical large engine of the era, the Pratt & Whitney R-1830 Twin Wasp, developed about 1,200 hp (895 kW) from 1,820 cubic inches (30 litres), so an advance of some 50 percent would be needed. This called for radical changes, and while many companies tried to build such an engine, none of them were successful.[citation needed]

In 1927, Harry Ricardo published a study on the concept of the sleeve valve engine. In it, he stated traditional poppet valve engines would likely have a hard time producing much more than 1,500 hp (1,100 kW), a figure that many companies were eyeing for next generation engines. In order to pass this limit, the sleeve valve would have to be used in order to increase volumetric efficiency, as well as to decrease the engine's sensitivity to detonation prevalent with the poor quality fuels in use at the time.[citation needed] Halford's office was next to Ricardo's in London,[citation needed] and while Ricardo started work with Bristol Engines on a whole line of sleeve-valve designs, Halford started work with Napier, using the Dagger as the basis. The layout of the H-block, with its inherent balance, and the Sabre's relatively short stroke, allowed it to run at a higher rate of rotation in order to deliver more power from a smaller displacement, provided that good volumetric efficiency could be maintained (with better breathing), which sleeve valves could do.[citation needed] Another important effect of increasing the number of cylinders is that the piston area increases (for a given capacity and bore/stroke ratio) and this also aids higher power.[citation needed]

The Napier company decided first to develop a large 24 cylinder, liquid cooled engine, capable of producing at least 2,000 hp (1,491 kW) in late 1935. Although the company continued with the opposed H layout of the Dagger, this new design positioned the cylinder blocks horizontally, and it was to use sleeve valves.[2] All of the accessories were grouped accessibly above and below the cylinder blocks, rather than being located at the front and rear of the engine as in most contemporary designs.[2]

The first Sabre engines were ready for testing in January 1938, although they were limited to 1,350 hp (1,000 kW). By March, they were already passing tests at 2,050 hp (1,500 kW), and by June 1940, when the Sabre passed the Air Ministry's 100-hour test, the first production versions were delivering 2,200 hp (1,640 kW) from their 2,238 cubic inch (37 litre) displacements.[2] By the end of the year, they were producing 2,400 hp (1,800 kW). To put this in perspective, the contemporary 1940 Rolls-Royce Merlin II was generating just over 1,000 hp (750 kW) from a 1,647 cubic inch (27 litre) displacement.

Production

The Hawker Typhoon was the first operational Sabre-powered aircraft, entering service with the RAF in mid-1941. Problems with both the Sabre engine and the airframe nearly led to the Typhoon's withdrawal from service.

Problems started to arise as soon as production started in volume. Up to then the prototype engines had been hand-assembled by Napier craftsmen, and it proved to be difficult to adapt it to assembly-line production techniques. In particular, the sleeves often failed, leading to seized cylinders.[3] It was just such a failure that caused the loss of the sole prototype Martin-Baker MB 3.[4] After testing some 18 different materials and manufacturing techniques a process of nitriding and lapping the sleeves helped resolve the problem.[3]

Quality control also proved to be a serious problem. Engines were often delivered with improperly cleaned castings, broken piston rings, and machine cuttings left inside the engine.[5] Mechanics were continually overworked trying to keep the Sabres running, and during cold weather they had to run them every two hours during the night so that the engine oil would not congeal and prevent the engine from starting the next day.[nb 2] These problems took too long to straighten out, and for many the engine started to attain a bad reputation. To make matters worse, mechanics and pilots unfamiliar with the very different nature of this engine tended to blame the Sabre for problems that were caused by incorrect handling. This was all exacerbated by the representatives of the competing Rolls-Royce company, which had its own agenda. In 1944, Rolls-Royce produced its own version called the Eagle.

Napier did not seem to be particularly interested in solving these sorts of problems, however, and instead it continued to tinker with the design for better performance. In 1942, it started a series of projects to improve its high-altitude performance with the addition of a three-speed, two-stage supercharger, but at this point the basic engine was still not running reliably. In December 1942, the company was purchased by the English Electric Company, and this ended the supercharger project immediately. Then the whole company was devoted to the production problems. The situation improved rapidly.

By 1944, the Sabre V was delivering 2,400 horsepower (1,800 kW) consistently, and the reputation of the engine started to improve. This was the last version to enter service, being used in the Hawker Typhoon and its derivative plane, the Hawker Tempest. Without the advanced supercharger, the engine's performance over 20,000 ft (6,100 m) fell off rapidly, and pilots flying Sabre-powered aircraft were generally instructed to enter dogfights only below this altitude. At low altitude, both planes were formidable, with the Typhoon readily outperforming its German counterpart, the FW 190. After the destruction of the Luftwaffe during early 1944, Typhoons were increasingly used as fighter-bombers, notably by the RAF Second Tactical Air Force. The Tempest became the principal destroyer of the V-1 flying bomb (Fieseler Fi 103), since it was the fastest of all the Allied fighters at low levels. Later on, the Tempest was responsible for the destruction of about 20 Messerschmitt Me 262 jet aircraft.

Development continued, and the later Sabre VII delivered 3,500 hp (2,600 kW) with a new supercharger. The final test engines delivered 5,500 hp [6](4.100 kW) at 45 lb/in2 boost. By the end of World War II, there were several engines in the same power class. The Pratt & Whitney R-4360 Wasp Major produced 3,000 hp (2,280 kW) at first, and later types produced 3,800 hp (2,834 kW), but these required almost twice the displacement in order to do so, 4,360 cubic inches (71 litres).

Variants

Note:'[nb 3]

Sabre I (E.107)
(1939) 2,000 horsepower (1,490 kW).
Sabre II
(1940) 2,300 horsepower (1,715 kW). Experimental 0.332:1 propeller reduction gear ratio.
Sabre II (production variant)
2,200 horsepower (1,640 kW). Reduction gear ratio 0.274:1: mainly used in early Hawker Typhoons.[7]
Sabre IIA
2,235 horsepower (1,665 kW). Revised ignition system: maximum boost +9 lbs.[8]
Sabre IIB
2,400 horsepower (1,790 kW). Four choke S.U. carburettor: Mainly used in Hawker Tempest V.[9]
Sabre IIC
2,065 horsepower (1,540 kW). Similar to Mk VII.
Napier Sabre III
Sabre III
2,250 horsepower (1,680 kW). Similar to Mk IIA, tailored for the Blackburn Firebrand: 25 manufactured and installed.[9]
Sabre IV
2,240 horsepower (1,670 kW). As Mk VA with Hobson fuel injection: preliminary flight development engine for Sabre V series.[9] Used in Hawker Tempest I.[10]
Sabre V
2,600 horsepower (1,940 kW). Developed MK II, redesigned supercharger with increased boost, redesigned induction system.
Sabre VA
2,600 horsepower (1,940 kW). Mk V with Hobson-R.A.E fuel injection, single-lever throttle and propeller control: used in Hawker Tempest VI.
Sabre VI
2,310 horsepower (1,720 kW). Mk VA with Rotol cooling fan: used in 2 Hawker Tempest Vs modified to use Napier designed annular radiators; also in experimental Vickers Warwick V.[11]
Sabre VII
3,055 horsepower (2,278 kW). Mk VA strengthened to withstand high powers produced using Water/Methanol injection. Larger supercharger impeller.[12]
Sabre VIII
3,000 horsepower (2,240 kW). Intended for Hawker Fury; tested in the Folland Fo.108.
Sabre E.118
(1941) Three-speed, two-stage supercharger, contra-rotating propeller; test flown in Fo.108.
Sabre E.122
(1946) 3,500 horsepower. Intended for Napier 500mph tailless fighter

Applications

The engine has been used in multiple aircraft, including the two widely deployed fighters.[13]

Applications - widely deployed

Applications - limited production and prototypes

Restoration project and engines on display

Napier Sabre engine under restoration:

Preserved Napier Sabre engines on public display:

Sectioned Napier Sabre engines on public display:

  • Imperial War Museum, Duxford
  • Birmingham Museum of Science and Industry
  • Cambridge University Engineering Department
  • RNZAF Museum, Wigram
  • Canadian Aviation and Space Museum (Ottawa)

Specifications (Sabre VA)

Data from Lumsden[15][16]

General characteristics

  • Type: 24-cylinder supercharged liquid-cooled H-type aircraft piston engine
  • Bore: 5.0 in (127 mm)
  • Stroke: 4.75 in (121 mm)
  • Displacement: 2,240 in³ (36.65 L)
  • Length: 82.25 in (2,089 mm)
  • Width: 40 in (1,016 mm)
  • Height: 46 in (1,168 mm)
  • Dry weight: 2,360 lb (1,070 kg)

Components

Performance

  • Power output:
  • 2,850 hp (2,065 kW) at 3,800 rpm and +13 psi (0.9 bar, 56") intake boost
  • 3,040 hp (2,200 kW) at 4,000 rpm war emergency power
  • Specific power: 1.36 hp/in³ (59.9 kW/L)
  • Compression ratio: 7:1
  • Fuel consumption: 117 gallons/hour (532 L/hr) at maximum cruise, F.S supercharger gear; 241 gallons/hour (1,096 L/hr) at maximum combat rating, F.S supercharger
  • Oil consumption: 47 pints/hour (27 L/hr) at maximum cruise 3,250 rpm and +7 psi (0.48 bar, 14"); 71 pints/hour (40 L/hr) at war emergency power
  • Power-to-weight ratio: 1.29 hp/lb (2.06 kW/kg)

See also


Comparable engines
Related lists

References

Footnotes

  1. ↑ The Napier-Heston Racer used the first production Sabre engine. The world-record contender crashed during early tests and the project was discontinued.
  2. ↑ Unlike current "multigrade" motor oils, the lubricants in use in the 1940s thickened up at low temperatures, thus preventing the Sabre from "picking-up" when started
  3. ↑ List from Lumsden unless otherwise noted.

Notes

  1. ↑ Wintjens, Piet. "Napier Sabre". The Hawker Tempest Page. Retrieved 17 June 2012. 
  2. ↑ 2.0 2.1 2.2 [F C Sheffield] 23 March 1944. "2,200 h.p. Napier Sabre " Flight, p. 309. www.flightglobal.com. Retrieved: 9 November 2009.
  3. ↑ 3.0 3.1 Flight 1945, p.550.
  4. ↑ Aeroplane 2010, pp. 65-66.
  5. ↑ Napier Sabre Retrieved on 17 July 2009.
  6. ↑ Setright p.134
  7. ↑ Sheffield March 1944, p. 310.
  8. ↑ Air Ministry 1943, pp. 24, 25.
  9. ↑ 9.0 9.1 9.2 Flight 1945, p. 551.
  10. ↑ Mason 1991, p. 331.
  11. ↑ Flight 1946, p. 91.
  12. ↑ Flight 1945, p. 552.
  13. ↑ Application lists from Lumsden
  14. ↑ CAHC "Workshop One" Retrieved: 21 November 2009.
  15. ↑ Lumsden 2003, p.176.
  16. ↑ Flight 1945, pp. 550-553.

Bibliography

Further reading

External links

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