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Napier Sabre

From Academic Kids

The Sabre was a 24-cylinder sleeve valve piston aircraft engine designed by Major Frank B Halford and built by Napier & Son during WWII. It was one of the most powerful piston aircraft engines in the world, especially for inline designs, developing over 3,500 horsepower (2,200 kW) in its later versions. However, the rapid conversion to jet engines after the war led to the quick demise of the Sabre, as Napier also turned to jets.

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

They followed the Lion with two new H-block designs: an H-16 engine known as the Rapier, and a H-24 known as the Dagger. The H-block has a compact layout, as it essentially consists of two horizontally opposed inline engines lying one atop another. Since the cylinders are opposed, the motion in one is balanced by the opposite motion in the one on the opposite side, leading to smooth running. However, in these new designs, Napier oddly decided to use air cooling; in service, the rear cylinders proved to be impossible to cool properly, leading to terrible reliability problems.

During the 1930s, designers were looking to the future of engine development. Many studies showed the need for engines that could produce 1 hp per cubic inch (50 kW/L), in order to be able to provide the power needed to equip large aircraft which could carry enough fuel for long-range use. This design goal became known as the hyper engine, and 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 in³ (30 L), so an advance of some 50% would be needed. This called for radical changes, and while many companies tried to build the hyper engine, none were successful.

In 1927 Harry Ricardo, at the RAE, published a seminal study on the concept of the sleeve valve engine. In it they essentially stated that traditional poppet valve engines would likely have a hard time producing much beyond 1,500 hp (1,100 kW), a figure 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. In the mid-1930s, Napier set about developing the Dagger into the most powerful engine in the world, by redesigning it with the sleeve valve system and adding water cooling. The H-block layout's inherent balance allowed it to run at higher RPM, to deliver more power from a smaller displacement (more bangs per second means more power delivered); the sleeve valve would allow these higher RPMs to be reached.

The first Sabre engines were ready for running in January 1938, although at a severely limited 1,350 hp (1,000 kW). By March they were already passing tests at 2,050 hp (1,500 kW), and by June 1940 the first production-ready versions were delivering 2,200 hp (1,640 kW) from their 2,238 in³ (37 L), close enough to 1 hp/in³ (50 kW/L) to be the first hyper engine to actually work. 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), and the most powerful engines in the world all developed around 1,200 hp (900 kW).

Problems started to appear as soon as volume production started. Up to that point the engines had been hand-assembled by Napier craftsmen, and it proved to be rather difficult to adapt it to assembly line production techniques. In particular, the sleeves tended to fail quite often, seizing the engine in the process. At that time Bristol were developing their own sleeve valve designs, and their Taurus engine had the same bore. At first Bristol refused to work with Napier, but eventually, under intense pressure from the Air Ministry, they relented, and the problems soon disappeared with the addition of Bristol's well-machined sleeves.

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. Mechanics were constantly overworked trying to keep Sabres running, and during cold weather they had to run them every two hours during the night so that the engine oil wouldn't congeal and prevent the engine starting the next day (unlike 'multigrades' today, the oils available tended to become thick at low temperatures, creating increased viscous-drag that prevented the Sabre from 'picking-up' when started - the firing of just one or two cylinders being insufficient to overcome the drag of the thickened oil) 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 were unfamiliar with the very different nature of this engine, and tended to blame the Sabre for problems which were caused by incorrect handling. This was all exacerbated by the representatives of the competing Rolls-Royce company, who had their own agenda.

The problems were eventually addressed, however, and the engine started to reliably allow higher and higher boost settings. By 1944 the Sabre V was delivering 2,400 hp (1,800 kW) consistently, and the reputation of the engine started to improve. This was the last version to see service, however. The later Sabre VII delivered 3,500 hp (2,600 kW) with a new supercharger, and the final test articles delivered 4,000 hp (3,000 kW). By the end of the war there were several engines of the same power class; the Pratt & Whitney R-4360 Wasp Major was at that time producing about 3,055 hp (2,280 kW), but used over twice the displacement, at 4,360 in³ (71 L).

The Sabre's primary use was in the Hawker Typhoon and its derivative, the Tempest. While the former was not the fastest plane in the air, the Sabre engine drove it past anything whilst flying at lower altitudes, where it could reach about 412 mph (663 km/h). At higher altitudes, the thick wing of the Typhoon made it slower, and so it was primarily used as a strike fighter. The later Tempest added a new low-drag wing, and the otherwise similar plane became the fastest propellor-driven fighter of the war, at least for a short time.

Specifications

For Napier Sabre II, the first production version:

Bore by stroke: 5.0 by 4.75 in (127 by 121 mm)
Displacement: 2,238 in³ (36.7L)
Compression ratio: 7 to 1
Power: 2,180 hp (1,630 kW) at 3700 rpm
Weight: 2360 lb (1,070 kg)

References:

Sabre Sleeve Valve Engine (http://www.cpmac.com/~cmcgarry/napier/napierwelc.html)
LJK Setright: The power to fly: the development of the piston engine in aviation. Allen & Unwin, 1971.
Graham White: Allied Aircraft Piston Engines of World War II . SAE, 1995.


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