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Beatrice Shilling Miss Shilling's orifice was a very simple technical device created to counter engine cut-outs experienced during negative G manoeuvres in early
Spitfire The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and other Allied countries before, during, and after World War II. It was the only British fighter produced continuously throughout the ...
and
Hurricane A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its ...
fighter aeroplanes during the
Battle of Britain The Battle of Britain () was a military campaign of the Second World War, in which the Royal Air Force (RAF) and the Fleet Air Arm (FAA) of the Royal Navy defended the United Kingdom (UK) against large-scale attacks by Nazi Germany's air force ...
. 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 The Rolls-Royce Merlin is a British Coolant#Liquids, liquid-cooled V12 engine, V-12 Reciprocating engine, piston aero engine of 27-litre (1,650 cu in) Engine displacement, capacity. Rolls-Royce Limited, Rolls-Royce designed the engine an ...
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 In an internal combustion engine, a supercharger compresses the intake gas, forcing more air into the engine in order to produce more power for a given displacement (engine), displacement. It is a form of forced induction that is mechanically ...
with an over-rich mixture. The consequent rich mixture cut-out would shut down the engine completely. During the
Battle of France The Battle of France (; 10 May – 25 June 1940), also known as the Western Campaign (), the French Campaign (, ) and the Fall of France, during the Second World War was the Nazi Germany, German invasion of the Low Countries (Belgium, Luxembour ...
and
Battle of Britain The Battle of Britain () was a military campaign of the Second World War, in which the Royal Air Force (RAF) and the Fleet Air Arm (FAA) of the Royal Navy defended the United Kingdom (UK) against large-scale attacks by Nazi Germany's air force ...
, German fighters had
gasoline direct injection Gasoline direct injection (GDI), also known as petrol direct injection (PDI), is a fuel injection system for internal combustion engines that run on gasoline (petrol) which injects fuel directly into the combustion chamber. This is distinct f ...
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
inverted flight Aerobatics is the practice of flight, flying maneuvers involving aircraft attitudes that are not used in conventional passenger-carrying flights. The term is a portmanteau of "aeroplane" and "acrobatics". Aerobatics are performed in aeroplanes ...
led to concentrated search for a solution. Engine manufacturer
Rolls-Royce Rolls-Royce (always hyphenated) may refer to: * Rolls-Royce Limited, a British manufacturer of cars and later aero engines, founded in 1906, now defunct Automobiles * Rolls-Royce Motor Cars, the current car manufacturing company incorporated in ...
produced an improved carburettor, but this failed in testing. Beatrice Shilling, an engineer working at the
Royal Aircraft Establishment The Royal Aircraft Establishment (RAE) was a British research establishment, known by several different names during its history, that eventually came under the aegis of the Ministry of Defence (United Kingdom), UK Ministry of Defence (MoD), bef ...
at
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, invented a simple device which could be fitted without taking the aircraft out of service. She designed a thimble-shaped brass flow restrictor with precisely calculated dimensions to allow just enough fuel flow for maximum engine power. It came in two versions, one for 12
psi Psi, PSI or Ψ may refer to: Alphabetic letters * Psi (Greek) (Ψ or ψ), the twenty-third letter of the Greek alphabet * Psi (Cyrillic), letter of the early Cyrillic alphabet, adopted from Greek Arts and entertainment * "Psi" as an abbreviat ...
manifold pressure and another for the 15 psi achieved by supercharged units. The design of the thimble-shaped flow restrictor was later refined to a simplified flat brass washer. While not completely solving the problem, the restrictor, along with modifications to the needle valve, permitted pilots to perform quick negative G manoeuvres without loss of engine power. This improvement solved the drawback of the RAF's
Rolls-Royce Merlin The Rolls-Royce Merlin is a British Coolant#Liquids, liquid-cooled V12 engine, V-12 Reciprocating engine, piston aero engine of 27-litre (1,650 cu in) Engine displacement, capacity. Rolls-Royce Limited, Rolls-Royce designed the engine an ...
-powered fighters versus the German
Messerschmitt Bf 109 The Messerschmitt Bf 109 is a monoplane fighter aircraft that was designed and initially produced by the Nazi Germany, German aircraft manufacturer Messerschmitt#History, Bayerische Flugzeugwerke (BFW). Together with the Focke-Wulf Fw 190, the ...
E machine. The Daimler-Benz DB 601 inverted V12 powerplant had utilised fuel injection since 1937. During early 1941, Shilling travelled with a small team to fit the restrictors in one RAF base after another, giving priority to front-line units. By March 1941 the device had been installed throughout
RAF Fighter Command RAF Fighter Command was one of the commands of the Royal Air Force. It was formed in 1936 to allow more specialised control of fighter aircraft. It operated throughout the Second World War, winning fame during the Battle of Britain in 1940. The ...
. Officially named the 'R.A.E. restrictor', the device was immensely popular with pilots, adopting the affectionate nickname 'Miss Shilling's orifice' (or simply the 'Tilly orifice'), given to the restrictor by Sir Stanley Hooker, the engineer who led supercharger development at Rolls-Royce. This simple measure was only a stopgap: it did not allow inverted flight for any length of time. The problems were not finally overcome until introduction of the Bendix pressure carburettor in 1943.


References


Sources

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Further reading

* **Originally published as * * {{refend


External links


Video
explaining the Merlins carburettor and the Tilly orifice Rolls-Royce aircraft piston engines