The Eagle was designed by Henry Royce in collaboration with his chief assistant, A. G. Elliott. It set the pattern for the smaller Falcon and the larger Condor, and in improved forms continued in service from 1917 until the 1930s.
Though designed for only 200 h.p., the first example was run in October 1915 (less than six months after work on the drawings had been started at Derby) at a brake horsepower of 225.
By March 1916 the new engine was delivering 266 b.h.p.; by July 284 h.p.; and by December 322 h.p. By September 1917 the figure had risen to 350 h.p.; in Feburary 1918 it had reached 360 h.p. An official Rolls-Royce instruction book dated December 1917 ascribes the following outputs (at normal r.p.m.) to the various series: Eagle 1, 225 b.h.p.; Eagle If, 266; Eagle Ill, 284; Eagle IV, 284; Eagle V, 322; Eagle VI, 322; Eagle VII, 322; and Eagle VIII, 350. These variants differed in the type and number of carburettors, in jet size, in the pattern of inlet pipes and pistons, and other details. By far the most famous among them was the Eagle VIII, which powered such fine aircraft as the Phomix Cork, Felixstowe F.3 and F.5, the D.H.4, 9A, 9B, 10B and IOC, the Fairey IIIC and IIII), the Handley Page 0/400 and V/1500, the Short Shirl, and the Vickers Vimy and Vernon. Other military installations were made in the Wight seaplane, F.E.2D, REA, R.E.7, Porte Baby, F.2, Felixstowe Fury (F.4), D.H.10, Fairey F.16 and F.17 Campania, Handley Page 0/100 and Vickers Vim. Civil applications included the D.H.16 and Handley Page W.8b.
Irrespective of series number, the Eagle was a twelve-cylinder, water-cooled, 60-deg vee of 20.32 litres capacity. Each cylinder, with its water-jacket, formed a separate unit; the barrel, machined from a steel forging, was retained by a base flange and studs. Inlet and exhaust ports were machined from solid forgings, welded-in at opposite sides of the head, and the water-jacket was fabricated from steel pressings, welded in place.
There was one inlet and one exhaust valve for each cylinder, and a single camshaft, carried in a casing along each bank of cylinders, operated both. There were two sparking plugs per cylinder, served, on the earlier engines, by four six-cylinder magnetos and on the later series by two twelve-cylinder magnetos.
A contemporary Rolls-Royce description continues: "The oil consumption should be taken as 1 gallon per hour. . . . The lubrication system is of the type in which one oil pump supplies pressure oil to the main bearings and other parts, while one scavenger pump evacuates the accumulation of oil in the crankcase to the oil tank. . . . The quantity of water carried in the cylinder water-jackets, water pipes, and pump is 3.1 gallons.... The Rolls-Royce carburettors are fitted with a control so that the delivery of petrol may be adjusted from the pilot's seat. This not only serves as a means of correcting the effects of decreasing atmospheric pressure with increasing altitude, but can also be used to obtain extremely economical running, and also to obtain a rich mixture for starting.... All controls, e.g., throttle, altitude, and magneto, are brought to one common location on the engine, to facilitate connection to the plane.... The carburettor throttles are fitted with springs, which, in the event of a breakage of the control, are intended to open the throttles to the full extent."
A special feature of the Eagle was the epicyclic reduction gear, for which the advantage was claimed that it imposed far less strain on the crankcase than would spur gearing. The gear was of fixed sunwheel type, in which an annulus, or internally toothed gearwheel, on the crankshaft meshed with three small spur gearwheels on the extremities of three arms formed integrally with the airscrew shaft. The three small gears were connected to three pinions which meshed with the fixed sunwheel, and the airscrew shaft was bolted to the three-armed spider which carried the planet pinion cage. An Oldharn coupling provided the anchorage of the sunwheel to the gear casing and allowed the gears to be perfectly aligned.
Late in 1922 Flight reported: "A recent model introduced by Rolls-Royce, Ltd., is the Eagle IX, which is generally similar to the famous Eagle VIII but differs from that type in certain details. The new model has been designed to be equally suitable for peace and war purposes, and to that end the following improvements have been effected: Two carburettors are fitted instead of four, so that tuning is considerably facilitated. The placing of the carburettors low down allows of using directgravity feed; the float feeds have been redesigned. The engine will now function satisfactorily with a 'head' of only 8in above the centre-line of the crankshaft. In view of the tendency m modern commercial aircraft of employing direct-gravity petrol feed from the main tanks in order to avoid pumps, piping, etc., this should be a great advantage, and should help to ensure for the Eagle IX as great popularity as that enjoyed by the previous model."
In the Eagle XV the epicyclic gear was further modified by the insertion of an additional friction clutch and gearwheel in the train, making the existing friction anchorage into a manually controllable clutch and converting the gear from a single-speed to a two-speed type. In the absence of a variable-pitch airscrew this arrangement gave a considerable advantage.
The following data for the Eagle VIII are taken from Rolls-Royce Aero Engine Instruction Book, Eagle Series I - VIII and Falcon Series I, II and III. Air Board Technical Department Book No. 244, December 1917: Dry weight, 847 lb; normal r.p.m., 1,800; max. permissible r.p.m. (5 min only), 2,000; gear ratio, 0.60; b.h.p. at normal r.p.m., 350; petrol consumption in gal/hr, at sea level, normal r.p.m. and normal b.h.p., 23; carburettors, four R.R. C.H. (Rolls-Royce, Claudel Hobson), 42 mm; magnetos, four Watford; pistons, high compression, four rings (one at bottom).
In 1925 a little-known Eagle development-the 16-cylinder, 500 h.p. Mk XVI-made its appearance. It had a stroke of 441n instead of 61in. The Mk XX was a projected scaled-up version, but was never built.