Royal Air Force centenary celebrations must include Bentley. Better remembered now for cars than aero engines, at its creation on 1 April 1918 Lieutenant Walter Owen Bentley Royal Naval Volunteer Reserve (RNVR) became Captain WO Bentley RAF among all 50,000 Royal Naval Air Service (RNAS) personnel transferred into the new service. A lot of its 2,500 aircraft were equipped with engines for which in 1919 the Royal Commission on Awards to Inventors granted WO £8,000 [£156,000].
Authority on aero engines Bill Gunston described Bentley’s BRs as a pinnacle of the rotary, which spun its cylinders round like a flywheel behind the propeller. Sopwith Snipes and Camels with Bentley rotaries were among the best Allied aircraft. A total of 1,567 were delivered to the Royal Flying Corps as well as a ground attack version, the Sopwith Salamander. The engine remained in service until 1926. The Admiralty Rotary-1-150 on show (below) at Bentley Motors Crewe was found preserved and boxed, having never flown.
The Admiralty appointed Commander Wilfred Briggs RN in 1915 to look into aircraft engine manufacture and the Royal Navy. WO Bentley had experience of motor racing engines, in particular with aluminium pistons, joining Briggs’ Technical Board looking into rotary engine troubles. Rotaries had been developed by the French Société des Moteurs Gnome in 1908. Made in nickel steel they were expensive, but they were lightweight and ran smoothly. Redesigned by Pierre Clerget in 1911, his 9-cylinder 16.2 litre 9B was being made in Chiswick by Gwynnes Pumps with a better cylinder than the Gnome, deep cooling fins, and took mixture to the inlet valves through pipes instead of through the crankcase. It had dual ignition and was successful notwithstanding its gyroscopic effects.
Drawbacks included overhauls every 20 hours, heavy oil consumption and an unacceptable rate of failures. WO investigated. The front edge of the cylinder barrels were better cooled than the rear with the result that bores went oval, piston rings broke and the engines suffered seizures. Worried about making changes in case they compromised its licensing agreement with the French, Gwynne turned down WO’s suggestions.
Instead he went to work with Humber and the engine was completely redesigned to become the Bentley Rotary. Known first as the Admiralty Rotary (AR1), then BR for Bentley Rotary it used similar valvegear to the Clerget but little else, and WO was upset by allegations that his engine was no more than an imitation of the French one. “These claims originated from people who glanced only at the cam mechanism, which was the first thing they saw and, for ease of production, was the only similar feature. The crankcase, crankshaft, method of securing the cylinders as well as their heads were all fundamentally different.”
Bentley specified aluminium air-cooled cylinders with shrunk-in iron liners, redesigned the steel cylinder heads, secured them by four long bolts and to the Admirals’ delight made the engine for £605 instead of the previous £907 10/- (£907.50) The stroke was increased from the Clerget’s 6.7in (170.2mm), bringing the capacity to 1,055cu in (17,288cc). Some 150bhp (112kW) @ 1250rpm could be sustained for 100 hours between overhauls. Failures of the obturator ring, an artillery term for the sealing ring of a gun breech were fixed with a flexible piston ring of thin bronze or light alloy, providing a seal when the 0.06in (1.5mm) thin cylinder wall distorted. The flimsy cylinders were stressed to the utmost as the tips whirled at 150mph (241kph). Bentley made further improvements, then in April 1917 three prototypes of a fresh design were ordered, weighing only 93lb (42kg) more, yet with bigger cylinders and giving 200bhp (149kW) @ 1300rpm. Some 1,500 a month were planned, at £880 still well below the price of the troublesome Clerget.
Bentley’s contribution to the war effort was belatedly recognised although he was disappointed over His Majesty’s Treasury demand for £2,400 [£45,800] tax on the inventors’ award, insisting that as a serving officer he had merely been doing his job. WO contested the bill with counsel Douglas Hogg KC (the first Lord Hailsham), and the action was eventually withdrawn. “I was so disgusted that I don’t think I even celebrated.” In the New Year’s honours list of 1919 WO was awarded an MBE for his contribution to what was by now the Royal Air Force. The Complete Bentley, Eric Dymock.