The Eagle or the Merc.?

Which came first

Sparked off in the April issue, this controversy has caused an unexpectedly large response. Mr. M.H. Evans, Historical and Information Officer of the Rolls-Royce Public Relations Department, weighs in with some more evidence. An article which he wrote for Rolls-Royce News last winter admits that before Royce embarked on the design of the Eagle aero-engine he had examined the welded-steel water jackets of the Hon. C.S. Rolls’ Mors and the 1914 G.P. Mercedes racing-car engines; Mr. Evans also admits that Royce had looked at Austro-Daimler and Mercedes aero-engines.

It should be explained that the Eagle was a private-venture engine, which Royce sold to the Admiralty, because he refused to design an air-cooled engine when requested to do so by Mervyn O’Gorman, head of the Royal Aircraft Factory (now the R.A.E.) at Farnborough. The real point at issue is could Royce have cribbed the 1914 G.P. Mercedes design for his aero-engine ? We know that the car was taken to Derby on August 4th and that an Eagle engine first ran in February 1915, the first full testing being done early in March 1915. Mr. Evans admits that “work proceeded at an enormous pace.” He also confirms that the sequence of R.-R. aero-engines was Eagle, Hawk, Falcon, and that the Falcon wasn’t just a scaled-down Eagle but was designed by R.W. Harvey-Bailey, whose son is, today, Manager of the Machining, Assembly and Overhaul works at Derby.

About a year after the outbreak of war, Rolls-Royce Ltd. delivered a pair of Eagles (the name was devised by Claude Johnson, General Managing Director of R.-R.) to the Admiralty for testing in a Handley-Page. Production plans were in hand by December 3rd, 1915, and on January 3rd, 1916 the Admiralty placed an order for an initial 25 engines. Not long afterwards a pilot inadvertently landed an Eagle-engined Army FE biplane at Lille, and it was captured intact by the enemy. The American technical Press got hold of the data issued by Germany and the Eagle became known as the most widely publicised secret of the war! In all, 4,581 Eagle, Hawk and Falcon engines were made during the war, and the Eagle was made until June 1928, the last being the Mk. IX. Those mainly responsible for it were A.G. Elliot, Maurice Olley (later of General Motors), A.J. Rowledge, A. Wormald, and E.W. (later Lord) Hives, controlled by Royce from his retreat at St. Margaret’s Bay.

How much did this great aero-engine owe to German racing-car design ? Apart from insisting on water-cooling, Royce refused to fit detachable valve cages, as used for the inlet valves of the Wolseley and Austro-Daimler aero-engines, and for both inlet and exhaust valves of the Dorman V8 and Green aero-engines.

The Mercedes G.P. engine also eschewed detachable valve cages, but had the separate steel cylinders with welded-up water jackets adopted for the Eagle. However, this cylinder construction had appeared on Daimler-Benz aero-engines in 1912 and was used for one of the 6-cylinder 1913 Mercedes G.P. cars, so the aero-engine seems to have anticipated the racing engine in this respect. Royce may have gleaned some information from the 1914 G.P. Mercedes about cylinder construction but certainly not the overall idea. But the o.h. camshaft valve gear of the racing car may well have proved more valuable to him, for the Eagle was the first o.h.c. power unit built by Rolls-Royce.

The significant thing is that Royce, after looking at these German 4 and 6-cylinder engines, decided on a 60 deg. V12, as easier to gear down, to get a reasonable propeller speed. He originally visualised perhaps 2,250 r.p.m. for a 900-r.p.m. propeller. He was aiming for 200 b.h.p. and had specified detachable cylinders rather than removable valve cages after contemplating Panhard racing engines, the Mercedes aero-engine and the Hispano-Suiza engine, etc. Royce estimated five minutes for removing a cylinder, ten minutes for replacing it. He quoted the success of Benz, Mercedes, N.A.G. and Austro-Daimler engines in his battle with the Admiralty over non-detachable valve cages, so he appears to have been conversant with German practice long before Briggs made him a present of the racing Mercedes now owned by Philip Mann!

Royce bad settled on a bore and stroke of 4 1/4 in. x 6 in. for a 200-h.p. geared-down engine and 4 3/4 in. x 7 in. for a direct-drive engine, to give, respectively, 200 h.p. at 2,000 r.p.m./130 h.p. at 1,250 r.p.m. and 200 h.p. at 1,300 r.p.m.

Briggs persuaded Royce on rather different dimensions, 4 1/2i in. x 6 1/2, from which Royce estimated 200-225 b.h.p. at 1,600 r.p.m., depending on b.m.e.p.s. of 80-90 lb./sq. in., and he spoke of a 6-cylinder using the identical cylinders, pistons and valve gear, to give 120 b.h.p. at 1,750 r.p.m. Trial crankshafts and cylinders were ordered in early September 1914.

Even so, further discussion on the ideal cylinder dimensions and crankshaft speeds took place and Royce, having insisted on epicyclic gearing down of the propeller, had much difficulty over its design, and the fixed ratios provided did not suit the Admiralty or War Office, the latter demanding a 2-to-1 reduction. Royce had already provided for a universal joint at each end of his engine for the War Office and a shorter end-bearing and universal joint for the R.A.F.

After some distress about finding hydraulic presses capable of forming the large, complicated cylinders in the 0.4-0.6 carbon steel Royce specified, altering the drive and location of water and oil pumps to comply with War Office installation dimensions and reducing the weight of the reduction gears to meet Farnborough’s requirements, and difficulties in keeping weight down to the 700 lb. (later 736 lb.) specified by the War Office, production plans for the Eagle went ahead from November 1914 onwards.

How far did Royce follow German precedents ? Certainly he wanted dual ignition from 12-wire 6-cyl. magnetos, suggesting B.T.H. as suppliers, literally copied Mercedes’ starting magneto connections, and examined a German hand-magneto, and he also had a German aero-engine tungsten steel valve analysed but was not impressed by its large carbon, tungten and chromium content.

But such matters as the method of securing the pin of the articulated con.-rods, camshaft bearing tubes camshaft drive coupling, enclosure of camshaft driving shaft, lubrication of camshaft and draining of oil down the driveshaft casing, and g.a. and details of magneto drives were regarded by Royce as probably patentable by R.-R. But he recognised the g.a. of his o.h.c. valve gear as that used by Hispano-Suiza before Mercedes, although the latter had patented it by describing a great many things in combination, even direction of rotation. Yet several details of the cylinder construction Royce regarded as new, and the Eagle’s plugs were better positioned than on the French and German engines. Likewise, Royce was proud of the epicyclic reduction gear, which was tested to destruction on an 80-h.p. works engine and incorporated an Oldham coupling balancing arrangement to anchor the sun wheel and obviate out-of-balance pressure on the bearings (Royce regarded epicyclic gears as the only ones possible, other systems being liable to overload the white-metal bearing material then in use).

There is reason to believe that Royce compared the .048 in.thick water jacketing of the Eagle with that of the Mercedes engine, not expecting to improve on it.

As November ran out the question of induction pipe length, position of the four carburetters and whether a Claudel carburetter would be as effective as an R.-R. single-jet instrument, were dealt with. Royce might well have given up when the War Office wanted all carburetters on one side, necessitating changing the exhaust valves to the centre of the engine, altering the camshaft, taking two inlet pipes under the crankcase, etc., and when they asked for a raised magneto location, calling for spur gears on the ends of the half-time shaft. He nearly did so after designing six carburetter layouts and not satisfying the War Office, and having to provide water heating, as well as hot air, at the behest of Claude! Hobson!

Coming back to German influence, Royce did suggest using a Mercedes engine and carburetter to test the proposed horrible inlet piping, not subsequently required, and did deliberately delete two bosses opposite the valves in order not to infringe a Mercedes patent specifying all bosses in one plane. And very definitely Royce was at some pains to avoid infringing Mercedes, Hispano-Suiza and even Peugeot patents over cylinder and o.h. camshaft details.

Towards Christmas 1914 Royce was enthusiastic about Zephyr pistons, even for the Silver Ghost car engine in c.i. form, if very considerably modified by R.-R.; he did not slavishly follow German concepts of clearances and hoped for a 3-lb. piston of 4 1/2 in. dia., which Mercedes achieved for a 4 1/8-in. piston.

Christmas over, Royce was still troubled about infringement of German patents (with a war going on!) and quoted the 1904 G.B. Mors’ cylinders as pre-dating Mercedes practice ; his oblique valves and opposite rockers were cleared by patents experts early in 1915. Even then, with an engine nearly ready to run, Royce was occupied with possible patents infringements and wondered if the Hon. C.S. Rolls’ old G.B. Mors could be found, to provide a case against the Mercedes cylinder construction !

Royce was not only concerned with getting his V12 engine into production—he had to modify it all the time at the behest of War Office and Admiralty requirements, festooning it with starter, tachometer, etc., and arrange for a separate little engine (he suggested 4-cylinder F.N. or Douglas flat-twin m/c. engines) to drive a dynamo for current for wireless and a searchlight!

German car and aero-engine practice was studied by Royce in deciding whether to use a Claudel or R.-R. carburetter for the Eagle. Welding the cylinders proved troublesome and an old man at the British Oxygen Co. got a fiver for advice he gave R.-R. on the subject! Incidentally, R.-R. had more than one Mercedes aero-engine at Derby in 1915 as well as the G.P. Mercedes car.

After some teething troubles, including big-end failure and pre-ignition, the first Eagle ran 20 hours at 216 h.p. by 19.3.15, using normal car oil, the only failure a broken tappet lever. Royce insisted on Castrol oil for further tests. Royce arranged for separate test buildings to be built at Derby, one for each Eagle on full song, to prevent air and heat from one influencing another. Royce thought £1,800 prohibitive for such bays and even concerned himself with how many windows the revised structure should have.

By July 1915 Royce was anxious to experiment with aluminium pistons, using D.F.P. pistons cast to fit R-R. patterns, both for Silver Ghost and Eagle engines. The aluminium Zephyr pistons were entirely successful but after Royce had had a momentary seizure in his Silver Ghost he put in hand a test in which one oversize piston was fitted to an aero-engine to simulate seizure, to see if broken con.-rods stemmed from piston seizure.

By January 1916 the Eagle was giving 275 h.p. at 1,600 r.p.m. with four carburetters, with 300 h.p. at 1,800 r.p.m. in sight, and fuel consumption guaranteed at 0.6 lb. per b.h.p.

And so this great British engine was perfected, after unbelievably painstaking war-work by Royce and his engineers, representing the thorough, logical, if empirical, development for which Rolls-Royce had been famous since 1906. It was giving 360 h.p. at 1,800 r.p.m. long before the end of the war. Obviously the Merc. came first but I do not consider that Royce depended all that much on these German engines for the Eagle’s design and development, and certainly he went to considerable lengths not to infringe their patents. Although there was a family likeness between the Mercedes engine and the Rolls-Royce Eagle, those with military minds will derive pleasure from the fact that the latter soon avenged the death in aerial combat of Georges Boillot, whom Mercedes had defeated in the 1914 French G.P., as well as the deaths of countless Englishmen and their Allies.

For this war contribution Royce merely received an O.B.E. But he was knighted for his Schneider Trophy-winning engines many years later which gave us domination of the skies in the Second World War (as my couplet “If the Battle of Waterloo was won on the playing fields of Eton, the Battle of Britain was decided over Calshot Water” emphasises). Thus Sir Henry Royce, O.B.E., materially contributed to a British victory in both great wars, with his Eagle and Merlin power units.—W.B.