At the outbreak of World War I in August 1914, the Royal Aircraft Factory asked Rolls-Royce to develop a new 200 hp (150 kW) air-cooled engine. Despite initial reluctance, they agreed, on condition that it be cooled by water rather than by air, which was the company's area of expertise.^[1]

Development of the new 20 litre engine was led by Henry Royce from his home in Kent. Based initially on the 7.4 litre 40/50 Rolls-Royce Silver Ghost engine, and drawing also on the design of a 7.2 litre Daimler DF80 aero engine used in a 1913 Grand Prix Mercedes that had been acquired,^[2] the power was increased by doubling the number of cylinders to twelve and increasing their stroke to 6.5 inches (170 mm), although their bore remained at 4.5 inches (110 mm) of the 40/50. 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 valvetrain design was changed from sidevalves to a SOHC design,^[3] closely following the original "side-slot" rocker arm design philosophy used on the contemporary German Mercedes D.I, Mercedes D.II and Mercedes D.III straight-six aviation powerplants.

The engineering department of the Royal Naval Air Service (RNAS) played a key role in the success of the engine by directing by car designer W.O. Bentley,^{[clarification needed]} who had enlisted in the Royal Navy, to place his expertise in the design of aluminium pistons at the service of Rolls-Royce.^[4] This gave significant weight saving to the Eagle and contributed to its excellent performance.^[5]

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 (168 kW) at 1,600 rpm. This was quickly increased to 1,800, then in August 1915 to 2,000 rpm where it produced 300 hp (220 kW). After further testing, it was decided to approve the engine for production at 1,800 rpm and 225 hp (168 kW); 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 from 225 hp (168 kW) to 266 hp (198 kW), followed by 284 hp (212 kW), and then 322 hp (240 kW), and finally 360 hp (270 kW) by February 1918 by which time eight Eagle variants had been produced.^[6][7] Throughout World War I Rolls-Royce struggled to build Eagles in the quantities required by the War Office, but the company resisted pressure to license other manufacturers to produce it. The fears of Rolls-Royce that the engine's much admired quality would be compromised by other manufacturers is often given as an explanation for this resistance,^[8] but the commercial terms sought by Rolls-Royce for licence production were so restrictive that other manufacturers - apart from Brazil Straker - refused to accept them.^[9] When the Ministry of Munitions took over coordination of aircraft production in 1917, Sir William Weir declined to intervene in the company's commercial strategy, even though success of the engine owed much to the technology transfer directed by the RNAS. He preferred to support untested engines using cast aluminium components like the Siddeley Puma and the Sunbeam Arab, believing them to be better suited to mass production, in comparison to the intricate machining required to build the Eagle and its smaller cousin the Falcon.^[10]

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.^[11]

Time between overhaul (TBO) for later Eagles was around 100–180 hours.^[12]

Note:^[13]

Eagle I (Rolls-Royce 250 hp Mk I)

(1915), 225 hp, 104 engines produced in both left and right hand tractor versions.

Eagle II (Rolls-Royce 250 hp Mk II)

(1916), 250 hp, 36 built at Derby.

Eagle III (Rolls-Royce 250 hp Mk III)

(1917-1927), 250 hp, increased compression ratio (4.9:1), strengthened pistons. 110 built at Derby.

Eagle IV (Rolls-Royce 250 hp Mk IV)

(1916-17), 270/286 hp, 36 built at Derby.

Eagle V (Rolls-Royce 275 hp Mk I)

(1916-17), 275 hp, high-lift camshaft, 100 built at Derby.

Eagle VI (Rolls-Royce 275 hp Mk II)

(1917), 275 hp, first use of twin spark plugs, 300 built at Derby.

Eagle VII (Rolls-Royce 275 hp Mk III)

(1917-18), 275 hp, 200 built at Derby.

Eagle VIII

(1917-1922), 300 hp, extensive modifications, 3,302 built at Derby.

Eagle IX

(1922-1928), 360 hp, developed as a civil use engine, 373 built at Derby.

Examples of the Rolls-Royce Eagle are on display at the:

• Polish Aviation Museum, Kraków
• Science Museum, London
• Canada Aviation Museum
• South African National Museum of Military History, Johannesburg
• South African Air Force Museum, Port Elizabeth

One of the two Eagles that powered Alcock and Brown's historic transatlantic flight is on display at the Derby Industrial Museum.^[14]

Data from Lumsden^[15]

General characteristics

• Type: 12-cylinder liquid-cooled 60° Vee aircraft piston engine
• Bore: 4.5 in (115 mm)
• Stroke: 6.5 in (165 mm)
• Displacement: 1,239 in³ (20.32 L)
• Length: 72.6 in (1,844 mm)
• Width: 42.6 in (1,082 mm)
• Height: 46.4 in (1,178 mm)
• Dry weight: 900 lb (408 kg)

Components

• Valvetrain: Overhead camshafts
• Fuel system: Twin Claudel-Hobson carburettors
• Cooling system: Liquid-cooled

Performance

• Power output: 360 hp (268 kW) at 1,800 rpm
• Specific power: 0.32 hp/in³ (13.4 kW/L)
• Compression ratio: 5.22:1
• Fuel consumption: 24 gallons per hour (90 Litres per hour)
• Power-to-weight ratio: 0.40 hp/lb (0.66 kW/kg)

• Rolls-Royce aircraft piston engines

Related development

• Rolls-Royce Falcon
• Rolls-Royce Hawk
• Rolls-Royce Condor

Comparable engines

• Sunbeam Matabele

Related lists

• List of aircraft engines

Notes edit

Bibliography edit

• Images of the Rolls-Royce Eagle