Simanaitis Says

On cars, old, new and future; science & technology; vintage airplanes, computer flight simulation of them; Sherlockiana; our English language; travel; and other stuff


ENGINEERING IS a process, not a result. An excellent example of this is development of the Rolls-Royce Merlin aircraft engine, renowned for powering the Avro Lancaster, de Havilland Mosquito, Hawker Hurricane, Supermarine Spitfire, and 39 others of the world’s aircraft.

The PV-12 stood for “Private Venture, 12-cylinder,” its PV indicating that Rolls-Royce received no government funding for development. Nor was it a straightforward success: As one example, the engine’s innovative evaporative cooling proved to be unreliable. Once U.S. ethylene glycol became available, the PV-12 was converted to conventional liquid cooling because of its efficiency.

The first Hawker Hurricanes and Supermarine Spitfires, entering production in 1936, were PV-12-powered. Government funding followed.

The Merlin I and its successors are given a detailed description in Classic World War II Aircraft Cutaways, by Bill Gunston, in association with The Aeroplane and Flight magazines, Osprey Aerospace, 1995. This book contains many period photographs as well as stunning cutaway drawings by J.H. Clark of The Aeroplane and Max Millar of Flight.

“A 1936 Rolls-Royce factory photo of a pristine Merlin I unsoiled by oil leaks and glycol smears. In the frontline, powerplants were rarely cleaned to this finish once in service.” This and following images from Classic World War II Aircraft Cutaways.

Gunston notes, “Many on the Allied side would say the Rolls-Royce Merlin was the greatest aero engine of World War 2. It was certainly the most famous. Incidentally, though in wartime RAF slang anything really good was called ‘wizard,’ and Merlin was a famous wizard, the engine was named after a bird of prey, like its predecessors.” The merlin is a small Northern Hemisphere falcon (Falco columbarius).

Rolls-Royce Merlin. Image by JAW at English Wikipedia.

The Merlin was a V-12 with its vee aligned upward, unlike, for example, the Benz DB 601’s inverted-vee as installed in the Messerschmitt Bf 109. Gunston notes, “In the earliest drawings and mock-up, the engine was inverted, which has much to commend it (for example, in a single-engine aircraft it gives the pilot a better view when taxiing), but this arrangement was eventually considered un-British.”

The Merlin I. Illustration by James H. Clark.

This upright vee design had a set of reduction gears to translate the crankshaft’s rotation up to propeller height. Originally a double-helical gear, it was soon replaced with plain spur gears.

Engineering Refinements Continue. The Merlin had four valves per cylinder, two intake and two exhaust, actuated by a single overhead camshaft per bank. Though the engine featured one of the best superchargers in the world, its initial engineering displayed oddities of compressed air flow and other less- than-optimal ducting. An aerodynamicist, “Doc” Stanley Hooker, was charged with bringing things right with the supercharger and its intake ducting.

Also, notes Guston, “The carburettor float chambers incorporated the simple pierced diaphragms invented by Miss Tilly Shilling at Farnborough so that the engine kept running smoothly under negative-G (so that a [Messerschmitt] Bf 109 pilot couldn’t get away so easily by simply pushing the nose of his fighter over and diving for the deck—the favourite evasive maneuvre since the invasion of Poland.”

Messerschmitt Bf 109 with a Hawker Hurricane in the background, painting by Lou Drendel.

Just-in-Time Engineering. The result of these and other engineering refinements was the Merlin XX, which entered production in July 1940. The Battle of Britain commenced on July 10 of that year.

Guston notes, “Just in time for the Battle of Britain, 100-octane fuel became available, enabling boost pressure in combat power to be doubled, from 6 lb/sq in to 12 lb/sq in, raising power to over 1300 hp. This made a vital difference to the results of one-v-one combats against the Bf 109E.”

The Merlin XX. Illustration by James H. Clark.

Hats off to “Doc” Stanley Hooker and Miss Tilly Shilling. And also to cutaway artist extraordinaire James H. Clark. ds

© Dennis Simanaitis,, 2018


  1. Bill E.
    October 30, 2018

    Always happy to be reminded of the vital contribution that “Miss Shilling’s orifice” made to the war effort!

    • simanaitissays
      October 30, 2018

      Agreed about Beatrice Shilling. Wait until tomorrow and the next day, then….

  2. Christian
    November 1, 2018

    I still have a nice mess of Lou Drendel books. Brought them with me when my wife & I retired to Ecuador.

    I suppose the magician was named after the bird.

  3. Skip
    November 3, 2018

    Great to see Stanley Hooker recognized. He essentially doubled the output from the Merlin over its life through supercharging. And it almost didn’t happen. Hooker was sitting around with not much to do when he overlooked some basic supercharger calculations on a colleague’s desk. He sat down to improve them. Then he got a visit from the section head informing him he was now the lead man in supercharging. Very fortunate turn of events for the Brits.

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