Rumblings, February 1941

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Mine’s a “Merlin”

WAR from the air concerns almost everyone in this country, excepting only those isolated folk who have fled to hidey-holes where no bombers are heard and where no shell fragments are ever found.

So, naturally, the R.A.F. comes in for universal praise and well-earned admiration from all classes of the community. Motoring enthusiasts, in particular, appreciate the task of the fighter and bomber pilot and understand the splendid work which goes into the evolution and production of the aircraft with which they defend us. So some notes on the specification of the Rolls-Royce “Merlin” aero-engine, which is used in our fastest fighters and so many of our bombers, should be very opportune.

The “Merlin” is a liquid-cooled V12 motor of 5.4″x 6.0″, giving a swept volume of 27 litres. Running on 100 octane fuel, the “Merlin X” develops 1,300 h.p. for take-off, and has a maximum rating of 1,145 h.p. at 16,750 ft. That is a figure of over 42 b.h.p. per litre, which is a remarkable output for a large motor of this type. The two R.R. 50 aluminium alloy cylinder blocks carry six “wet” carbon steel cylinder liners apiece, and sit at 60° to each other on the heat-treated R.R. 50 alloy crankcase, held by fourteen holding-down studs. The cylinder heads are fixed and have screwed-in valve seats and cast-iron inlet valve guides and phosphor-bronze exhaust valve guides. The pistons carry three compression and two scraper rings each and are forged in R.R. 59 alloy, with hollow, circlip-retained, fully-floating hardened nickel-chrome-steel gudgeons, working in floating phosphor-bronze bushes. The one-piece crankshaft of Vickers chrome-molybdenum steel has integral balance weights and is nitrogen-hardened after machining. It runs in seven split mild steel bearings lined with special lead-bronze alloy. There are two nickel-steel, H-section connecting rods on each crankpin, the master rod running on the pin and the plain rod on an outer lining on the master-rod, the bearing material being special lead-bronze. The crankshaft pins and journals are lapped to a tolerance of — 0.0001 in.

There are four valves per cylinder, actuated by a single nickel-steel case-hardened camshaft above each cylinder block. The camshafts operate via rockers and run in seven bearings, the drive being by a bevel-driven shaft at the rear of the engine. Each valve is closed by two concentric coil springs, and the exhaust valves are sodium-cooled, Brightray treated and have nickel-steel hardened end-caps.

The two-speed supercharger is of centrifugal type, driven from the rear of the crankshaft through torsionally-flexible spring-drive shaft and a gear-train. In the “Merlin II” Rolls-Royce motor the supercharger runs up to approximately 24,500 r.p.m. at full speed. The “Merlin X” incorporates a two-speed gear, the change-over being operated by oil pressure and occupying about one second under full load. A friction drive protects the supercharger drive against impeller inertia on acceleration and deceleration, while the torsional shaft aforementioned damps out torque irregularities in the drive to the auxiliary components and timing gears. The supercharger draws from a twin-choke, updraught Rolls-Royce S. U. carburetter. Mixture control is automatic according to altitude and is also automatically enriched if the throttle is opened, while an accelerator pump additionally enriches the mixture as the throttle opens; there is also pilot-control over the rich and weak settings. Supercharger boost is automatically governed to altitude without, attention from the pilot, in relation to throttle opening, a cut-out being provided for emergency use. To obviate icing-up the carburetter throttle is heated by warm oil, the pressure of which is controlled by an automatic by-pass, and the chokes are coolant-jacketed. Fuel feed is by dual gear-type pumps, each one able to supply the needs of the engine independently and each driven by a separate quill-shaft. Excess fuel is by-passed to the suction side by a disc-type relief valve. Ignition is by two spigot-mounted twelve-cylinder magnetos feeding two plugs per cylinder via completely screened h.t. harness. The magnetos are driven by skew-gear from the timing gear shaft and have serrated couplings giving a vernier adjustment.

Lubrication is of the dry-sump type, employing one gear-type pressure pump and two gear-type scavenge pumps, the latter working in parallel. Oil is delivered through a triple relief-valve, at 150 lb. per square inch to the two-speed airscrew control and fuel pump bushes, at 70 lb. per square inch to the main bearings and big-ends, and at 4 to 8 lb. per square inch to the valve gear, supercharger bearings, generator, and reduction gear unit jet feeds, etc. Pistons and little-ends are splash fed and the camshaft drive, auxiliary drives, etc., are drain lubricated, the reduction gear unit bearings being lubricated by a combination of splash and drain feed.

Coolant is circulated by a centrifugal vane-type coolant pump running at 1½ times crankshaft speed, via the rear lower ends of the cylinder blocks, and three outlets on the inner sides of the blocks delivering to main outlet pipes. The electric starter drives through a 393 : 1 reduction gear and Bendix engaging device to one of the supercharger gears, and the hand-starter operates at a ratio of 14 to 1 through part of the same gear train, a multi-plate clutch on the starter layshaft safeguarding both systems in the event of a backfire. The generator is driven at 1.914 times crankshaft speed from a gear-train off the supercharger drive, and the tachometer drive is from the rear of the port camshaft, at a quarter engine speed. Provision is made for drives for a constant-speed airscrew control vacuum pump for the blind-flying instruments, automatic-pilot and de-icing equipment, a low-pressure air compressor, a high-pressure air compressor, the Lockheed undercarriage hydraulic pump, and a gun-turret pump.

The airscrew is driven from the nose of the crankshaft through a single-spur reduction gear unit and in the case of v.p. airscrews the actuating oil is fed via an internal rotating tube. The “Merlin II” motor gives 880 b.h.p. for take-off at 3,000 r.p.m., on a boost of 6.25 lb. per square inch for a nett dry weight of 1,335 lb. on 87 octane fuel, and its frontal area is a mere 5.85 square feet. This performance, and that of the later two-speed supercharger and 100 octane Rolls-Royce “Merlins,” has given our fighter aeroplanes the supremacy of the air and has enabled our bombers to so potently attack Germany’s military objectives with very small losses. The high quality materials and perfect assembly of these Rolls-Royce aero motors results in high praise of their absolute dependability, in spite of their great performance, wherever pilots of the Fighter Command and Bomber Command meet. So let the foregoing very abridged specification be respectfully digested by those who owe so much to the Royal Air Force. Unlike Leonard Engel, who, last November, in the American journal “Flying and Popular Aviation,” wrote shamelessly of “Tornado,” “Whirlwind,” “Manchester,” “Stirling,” “Vulture,” “Griffin,” and “Sabre,” we are giving away nothing by publishing these notes, for the “Merlin” was written up fully in the British aeronautical Press as long Ago as April, 1939.