Miss Shilling's Orifice

Get your mind out of the gutter. It’s not what you think.

In the early days of the Supermarine Spitfire and Hawker Hurricane, the mighty Rolls-Royce Merlin engine had a nasty habit of losing power, or cutting out altogether, during high-G maneuvers or too much inverted flight. Unlike the fuel injected Daimler-Benz DB 605 engine of the Messerschmitt Bf-109, the Merlin was fitted with an SU carburetor (or carburettor, if you prefer the British spelling for a British engine), and during certain maneuvers, such as a hard pitch down of the nose, the fuel was forced upwards to the top of the float chamber of the carburetor, flooding the engine. This led to a loss of power, or a complete cutoff of the engine, which is really not something that you want to have happen in the middle of a pitched dogfight.

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A permanent solution was found by using a Bendix or Rolls-Royce pressure carburetor, but until that unit could be developed, a stopgap solution was discovered by Beatrice “Tilly” Shilling, an engineer working at the Royal Aircraft Establishment at Farnborough. She devised a flow restrictor (officially called the R.A.E. Restrictor, but nicknamed “Miss Shilling’s Orifice,” the “Tilly Orifice, and “Tilly’s Diaphragm”) which limited fuel flow to the maximum the engine could use during a dogfight and, along with a revised carburetor needle, it solved the fuel flooding problem until the permanent fix could be found. Miss Shilling toured England with a group of mechanics, retrofitting the engines of the RAF fighters, with priority given to front line units. By the end of 1941, all the Merlins had been modified and were back in the fight, much to the relief of the beleaguered pilots.

Beatrice Shilling with her Norton motorcycle

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