Lancaster at Scampton, BBMF Spitfire and Hurricane, heated debate in The Telegraph about which was greatest. Yet they all relied on the Rolls-Royce Merlin. It is 70 years since the dambusters and 80 since drawings for the Merlin were completed the very day Sir Henry Royce died.

WO Bentley was instrumental in getting Rolls-Royce into aero engines. Working under Commander Wilfrid Briggs, head of the Admiralty Air Engine Section, he was sent to Derby, where Rolls-Royce made air-cooled Renault aero engines. WO recalled, “…a friend of mine tipped me off that one of the 1914 Mercédès racing cars, which had won the French Grand Prix, had got stuck in England at the beginning of the war and still rested at the Mercédès showroom in Long Acre. I told Briggs about it and together we went along, representing the British Crown so to speak, with a ‘search warrant’. The place was in a fine old mess, but in the basement lay a 4½ litre Grand Prix Mercédès. We dug it out, and soon it was being taken to pieces by Rolls-Royce at Derby.”

Ernest Hives (later Lord Hives) studied the Mercédès cylinder design and WO persuaded him that the resulting 200hp water-cooled Rolls-Royce Eagle engine should have aluminium pistons. In 1919 two Eagles with Bentley’s pistons were used in the Vickers Vimy that made the first non-stop crossing of the Atlantic.
Merlin in a Spitfire, Duxford
Henry Royce set up drawing offices with teams of technicians at St Margaret's Bay Kent, and later West Wittering, creating a dynasty of aero engines of the 1920s and 1930s that culminated in winning the Schneider Trophy races outright. In 1931 Royce accepted a baronetcy in recognition of his design but it was soon apparent that the RAF needed something that could be made in large numbers.

In 1932 the ailing mechanic (Royce preferred “mechanic” to designer or even engineer) persevered with a new V12 in the face of Air Ministry indifference and prevarication. Rolls-Royce could see the need for it, calling it PV for Private Venture because the government wouldn’t pay for it. Developed from the Kestrel, and the R-type that had been successful in the Schneider Trophy Supermarine S6 seaplanes, the Merlin was not named after King Arthur’s wizard, but was one of a series designated by birds of prey. A merlin is a small falcon but as an engine it was straightforward, upright, of a sort with which the Derby firm was already familiar.
Merlin in a Hurricane, Brooklands
Later ones were developed to produce substantial power increases at high altitude, and by the end of the war specialist versions produced 2640bhp (1969kW). Rolls-Royce did not have capacity in its factories at Derby, Crewe, and Hillington Glasgow to meet the demand. Four times as many Merlins were needed to equip bombers like the Lancaster, so Packard made them in America and Ford set up a plant at Urmston, Manchester, not far from Trafford Park.

Rowland Smith of Ford guessed it would cost £7million, telling chairman Lord Hives that Ford could not possibly build engines from the drawings Rolls-Royce supplied. The tolerances were much too wide. Ford production machinery would work to much closer limits than Rolls-Royce, whose hand-finished engines were often widely different in power and reliability.

Drawings for the Merlin were completed on 22 April 1933, as Royce breathed his last. Yet weak and frail as he had been, the engine (after teething troubles had been fixed) was a masterpiece. The first ran on 15 October 1933 and Royce’s vision resulted in one of the most significant aircraft power units of the Second World War. Besides Spitfire, Hurricane and Avro Lancaster, Lincoln, Manchester II, Tudor and York, the Merlin powered de Havilland Mosquito, Handley Page Halifax and North American Mustang X as a replacement for its Allison. The Mustang continued to use Merlins in the Korean War of the 1950s.

SPEC: 12-cylinders, 60deg V; front; 5.4in (137.16mm) x 6in (152.4mm), 1,648.8cu in (27,021cc); compr 6.0:1; 1030bhp (768kW) @ 3000rpm @ 16,250ft (4940m) Merlin I to 1480bhp (1104kW) @ 3000rpm @6000ft (91830m) to 12,250ft (3740m) from Merlin XX; weight from 1385lb (629kg) Merlin I to 1450lb (647kg) from Merlin XX; 1640lbs (744kg) for 1565bhp (1167kW) Merlin 61 on.
STRUCTURE 4 inclined 45deg KE965steel valves per cylinder (4 valves parallel from Merlin G); sodium-cooled exhaust valves; Stellited ends to inlet valves; double valve springs; Silchrome valve seats screwed into heads; one shaft and bevel gear-driven 7-bearing overhead camshaft per bank; two two-piece cylinder blocks cast in RR50 aluminium alloy; detachable cylinder heads; wet high carbon steel cylinder liners; aluminium crankcase split horizontally; twin choke updraught R-R/SU carburettor with anti-ice heating; gear-driven centrifugal supercharger, 2-speed from Mark X; liquid-cooled intercooler; two mechanical fuel pumps on quill shafts; two magnetos; one-piece six-throw chrome molybdenum steel 7-bearing crankshaft; dry sump lubrication; 70 per cent water 30 per cent ethylene glycol cooling; centrifugal pump; electric starter; air compressor take-off for aircraft services
TRANSMISSION single plain spur 0.477:1 or 0.42:1 reduction gears to propeller from front of crankshaft.
PRODUCTION over 30,000