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Bentley A.R.1  / BR1
Humber A.R.1  / BR1


The 130 horsepower (97 kW) Clerget 9B was an important engine for the British Royal Naval Air Service and Royal Flying Corps, being license-produced in Britain and powering a number of important British aircraft, including the Sopwith Camel. However, at £907 a copy it was expensive, and prone to overheating, so the Admiralty asked Lieutenant W. O. Bentley, an established pre-war engine designer, to produce a modified version to solve these problems.

Bentley came up with his idea of an engine - fitted with aluminium cylinders with cast iron liners, and aluminium pistons. Dual ignition was introduced to improve reliability, and the stroke increased to 6.7 inches (17 cm) which allowed power to be increased to 150 horsepower (110 kW). The cost of the engine was also reduced, falling to £605 per engine.

The resulting engine, initially known as the A.R.1 for "Admiralty Rotary", but later called the BR.1 ("Bentley Rotary") was manufactured in quantity, although initially against Admiralty orders. It was standardised for the Camel in RNAS squadrons, but unfortunately there were never enough to entirely replace the inferior and more expensive Clerget engine in British service, and most RFC Camel squadrons continued to use Clerget engines; in fact licenced production of the Clerget continued.

The BR.1 was developed as the BR.2, a heavier, more powerful engine, which powered, among other types or aircraft, the Camel's eventual replacement, the Sopwith Snipe.


Sopwith Camel
Avro 536
Westland N.1B
Port Victoria P.V.9

Type: 9-Cylinder air-cooled rotary engine
Bore: 4.72 in, (120 mm)
Stroke: 6.69 in (170 mm)
Displacement: 1,055.9 cu in (17.3 L)
Dry weight: 397 lb (180 kg)
Cooling system: Air-cooled
Power output: 150 bhp (110 kW) at 1,250 rpm
Fuel consumption: 11 gallon/hr (50 L/hr)
Oil consumption: 12 pints/hr (6.8 L/hr)
Power-to-weight ratio: 0.38 bhp/lb



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