Miss Shilling's orifice

Miss Shilling's orifice was a very simple technical device created to counter engine cut-outs experienced during negative G manoeuvres in early Spitfire and Hurricane fighter aeroplanes during the Battle of Britain. Officially called the R.A.E. restrictor, it was referred to under various names, such as Miss Tilly's diaphragm or the Tilly orifice in reference to its inventor, Beatrice "Tilly" Shilling.

Engine cut-out problems

Early versions of the Rolls-Royce Merlin engine came equipped with dual-choke updraught SU carburettor. When an aeroplane equipped with such an engine performed a negative G force manoeuvre (pitching the nose hard down), fuel was forced up to the top of the carburettor's float chamber rather than down into the engine, leading to loss of power. If the negative G continued, fuel collecting in the float chamber would force the float to the floor of the chamber. Since this float controlled the needle valve that regulated fuel intake, the carburettor would flood and drown the supercharger with an over-rich mixture. The consequent rich mixture cut-out would shut down the engine completely.^[1]

During the Battle of France and Battle of Britain, German fighters had gasoline direct injection engines and did not suffer from this problem as injection pumps kept fuel at constant pressure. German pilots could exploit this by pitching steeply forward while opening the throttle, a manoeuvre that pursuing British aircraft would be unable to emulate. The British countermeasure, a half roll so the aircraft would only be subjected to positive G as it followed German aircraft into a dive, could take enough time to let the enemy escape.

The RAE restrictor

Complaints from pilots over engine cut-out during dives and brief