Bentley BR1

The Bentley BR.1, first known as the A.R.1 for Admiralty Rotary and later as the Bentley Rotary, was a nine-cylinder air-cooled rotary aircraft engine of the First World War. W. O. Bentley, then a Royal Navy officer acting as liaison between the Royal Naval Air Service and the engine builders, had been sent to improve the licence-built Clerget 9B, which tended to overheat and seize. At Humber's works he reworked the design with aluminium cylinders, shrunk-in liners that let ordinary piston rings replace the fragile obturator rings, and a longer stroke, lifting output to about 150 horsepower. The engine was standardised for the Sopwith Camel in Royal Naval Air Service squadrons and powered most of the Camels they flew, although supply never allowed it to replace the Clerget across British service. It cost the Admiralty far less than the Clerget and led directly to the larger BR.2.