Miss Shilling’s Orifice: The Device That Saved The Merlin

In 1940, the skies above England roared with the symphony of machine guns and aero engines as fighter planes scrambled to crown the winner of the Battle Of Britain. In such pivotal times, any advantages available were gladly exploited, especially when the Luftwaffe made the revelation that Britain’s own Merlin engines could be forced to cut out.

During its design, the engineers at Rolls-Royce determined that fitment of carburettors would result in a higher power output compared to a direct injection fed system. To put it simply, the carbs operated by ushering fuel into a float chamber, with the amount being regulated by a simple float valve, from the chamber, the fuel would be jetted out into the venturi.

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This is where the fatal flaw in the Merlin engine was hiding. As the fighter pitched nose down, negative G-forces were created, forcing everything upwards, including the fuel inside of the float chamber. The resulting temporary starvation of fuel was usually enough to cut the engine out completely, although a prolonged negative-g situation would create the correct circumstances to drown the supercharger in fuel, which would guarantee the fact.

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Supercharger about to be fitted to a Merlin engine, 1942.

The same issue plagued the Imperial Japanese Navy’s 27.9 litre radial engine that powered the Zero. After American testing of a Zero in 1942, new tactics were composed to benefit from their adversary’s pitfall.

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Captured Zero used by the Americans for testing.
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The solution to Rolls-Royce’s woes would come in the form of Miss Beatrice ‘Tilly’ Shilling. The 31 year old woman received her Master of Science degree in 1933 at the University of Manchester and had a profound love for motorbikes, racing several times throughout the 1930’s.

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Beatrice ‘Tilly’ Shilling.

Shilling, a scientific officer at the Royal Aircraft Establishment, devised that a flat washer with the correct dimensions could be used as a diaphragm. Fitted across the float chamber, the device would keep the fuel in place during a negative-G situation.

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Shilling on her Norton motorcycle.
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Accompanied by a team of mechanics, Shilling travelled the country fitting the diaphragm to every active Merlin engine, units considered front-line were given priority. Tilly accomplished this task by March, 1941, by which time, the diaphragms official name of ‘RAE Restrictor’ had been replaced to ‘Miss Shilling’s Orifice’ by pilots.

While not a permanent fix, Shilling’s makeshift solution kept the Merlin powered fighter planes in the sky, and bought the RAF enough time to build a new healthy carburettor.

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