Rolls-Royce Merlin

The engine for which the Packard automobile company has accepted a contract and is tooling up for production in Detroit

by Paul H Wilkinson
Consultant, Diesel Aviation

The proposal that one of our leading automobile manufacturers should tool up for the production of thousands of British Rolls-Royce Merlin aircraft engines has recently received considerable prominence in the press. Apparently the designs and patents of the Merlin are to be made available to us provided we undertake to build 6,000 of these engines for Great Britain and as an afterthought, 3,000 of them for ourselves. So far, the Ford Motor Co has considered the proposition and has declined to build the Merlin for export in this manner. Now the order has been taken by the Packard Motor Car Co which appears anxious to tackle it but has not released any definite plans on exactly what they are going to do.

From the aeronautical point of view the Merlin is beyond reproach. It has established an excellent reputation for itself in the high-performance interceptors of the Royal Air Force such as the Hawker Hurricane and the Supermarine Spitfire and two-place fighters such as the Boulton Paul Defiant. It has also been used with considerable success in light bombers such as the Fairey Battle and heavy bombers such as the Armstrong Whitworth Whitley. For this reason, it is claimed by our government experts to be a highly desirable engine in the rearmament of the air forces of the United States.

Several models of the Merlin have been produced since it was placed in service in England four years ago. For instance, the Merlin II with its one-speed supercharger has a rated output of 1,030 hp at 3,000 rpm at 16,250 ft and a take-off rating of 880 hp on 87-octane gasoline. When using 100-octane gasoline, its take-off rating is increased to 1,000 hp. Another model, the Merlin X, is equipped with a two-speed supercharger and develops 1,010 hp at 3,000 rpm at 17,750 ft and 1,075 hp for take-off when using 87-octane gasoline. A third model, the Merlin RM2.SM, is designed tor operation on 100-octane gasoline and is rated at 1,145 hp at 3,000 rpm at 16,750 ft with 1,300 h. available for take-off. All of these engines have a displacement of 1,648 cu in and their specific weight ranges from 1.3 to 1.2 per hp, depending upon their power output and equipment.

With regard to the construction of the engine, the Merlin follows conventional vee-type engine practice with a two-piece aluminum alloy crankcase and two monobloc cylinder castings with integral heads set at an angle of 60°. The crankshaft is machined from a VCM steel forging and is supported in seven lead-bronze bearings in the upper halt of the crankcase. The connecting rods are of the marine type with a plain rod and a forked rod on each throw of the crankshaft. The rods are machined from H-section nickel steel forgings. The aluminum alloy pistons are fitted with three compression rings and two oil scraper rings and have piston pins of the floating type. The four valves in each cylinder head are actuated by an overhead camshaft along each cylinder block. A spur type reduction gear contained in a housing attached to the front of the crankcase gives the propeller shaft a ratio of 0.477 to l.

The supercharger is gear-driven by means of a torsionally flexible shaft provided with a damper clutch. The impeller is machined from an aluminum alloy forging and in the case of the Merlin II, its gear ratio is 8.6 to 1. In the two-speed supercharger used on the Merlin X, the change speed mechanism is operated by means of oil pressure from the engine. The carburetor is of Rolls-Royce SU design and is of the twin-choke updraft type provided with a single air intake. Both automatic and two-position mixture controls, one incorporated in the carburetor and the throttle barrels and throttle valves are heated by warm oil circulated from the crankcase. Two gear type fuel transfer pumps are provided to supply the carburetor. The automatic boost regulator operates on the aneroid principle and each movement of the pilot's throttle produces a corresponding change in boost pressure. The pressure difference across the supercharger provides the relay force required to perform the actual throttle movements. Ignition is provided by means of two twelve-cylinder magnetos with shielded plugs and wiring.

Lubrication is on the dry sump principle with one pressure-feed and two scavenger pumps of the rotary gear type. A triple relief valve is used to regulate the oil pressure for the variable-pitch propeller (150 lb per sq in), the main bearings (70 lb per sq in) and the lower pressures required for the auxiliaries (4 to 8 lb per sq in). Ethylene glycol is used for cooling with a header tank around the upper part of the reduction gear housing. Accessory drives are provided for an electric generator, a tachometer, a low-pressure air compressor, a high-pressure air compressor, an hydraulic pump for the landing gear, an hydraulic pump for the power-operated gun-turret (if any), and a vacuum pump for the automatic pilot and the de-icers. Hand and electric starting equipment of the reduction gear type is specified. The propeller shaft is splined to take Rotol, Hamilton Standard or DeHavilland variable-pitch 3-bladed propellers.

Three factories in England are building Merlins at the present time. These are the original factory at Derby and another at Crewe, and a new $20,000,000 one near Glasgow which by now should be in full operation. All of them are laid out along modern lines and it is probable that from 25,000 to 30,000 men are employed. Inasmuch as production from these factories is insufficient to meet the demand for this type of warplane engine in Great Britain, it appears that the Merlin is somewhat difficult to produce. It might therefore be advisable for us to check up on this point before committing ourselves to a large outlay of funds for production of the Merlin in the United States.

Examination of a British-made Merlin reveals excellent workmanship but it also discloses that considerable modifications to the engine are essential if it is to be adapted to American mass production methods. The rear section and the mounting of the accessories and controls appear to be unduly complicated and the number of hand operations required during assembly appears to be excessive. Standardization of threaded parts and drives and mounts for the accessories will cause considerable delay if some of the engines have to be built to American standards and others to British standards. According to the latest reports, the Packard Motor Car Co would require a $30,000,000 expansion program and would have to employ 14,000 additional skilled craftsmen to build 9,000 Merlins by mass production methods. As it is improbable that these engines would begin to come off the assembly lines in any quantities much before 1942 and we would then have to take the last 3,000 of the 9,000 engines, the question arises: "Will present-day Rolls-Royce Merlins be sufficiently up-to-date when we get them for the high-performance warplanes required for our National Defense during the years 1942 to 1945?"

This article was originally printed in the August, 1940, issue of Aviation magazine, vol 39, no 8, pp 63-64, 116.
The original article includes a photo of the engine, a photo of the author in front of a Ju-86, and one data table.