Design and Development
Development of the new engine was led by Henry Royce from his home in Kent. Based initially on the Rolls-Royce Silver Ghost engine, and drawing also on the design of a Grand Prix Mercedes 35 hp, the power was increased by doubling the number of cylinders to twelve and increasing their stroke to 6.5 inches, although their bore remained at 4.5 inches. The engine was also run faster, and an epicyclic reduction gear was designed to keep the propeller speed below 1,100 rpm. To reduce inertia and improve performance the valve gear was changed from sidevalves to an overhead camshaft design.
On 3 January 1915 the Admiralty ordered twenty-five of the new engines. The Eagle first ran on a test bed at Rolls-Royce's Derby works in February 1915, producing 225 hp at 1,600 rpm. This was quickly increased to 1,800, then in August 1915 to 2,000 rpm where it produced 300 hp. After further testing, it was decided to approve the engine for production at 1,800 rpm and 255 hp; 1,900 rpm was allowed for short periods. The engine first flew on a Handley Page O/100 bomber in December 1915, the first flight of a Rolls-Royce aero engine.
The Eagle was developed further during 1916 and 1917, with power being progressively increased to 360 hp by February 1918 by which time eight versions had been produced. Throughout World War I Rolls-Royce struggled to build Eagles in the quantities required by the War Office, but the company resisted pressure to licence other manufacturers to produce it, fearing that the engine's much admired quality would risk being compromised.
After the War, a Mark IX version of the Eagle was developed for civilian use. Production continued until 1928, and in total 4,681 Eagle engines were built.
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