Miss Shilling's orifice
Encyclopedia
Miss Shilling's Orifice was a very simple technical device made to counter engine cut-out in early Spitfire
Supermarine Spitfire
The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and many other Allied countries throughout the Second World War. The Spitfire continued to be used as a front line fighter and in secondary roles into the 1950s...

 and Hurricane
Hawker Hurricane
The Hawker Hurricane is a British single-seat fighter aircraft that was designed and predominantly built by Hawker Aircraft Ltd for the Royal Air Force...

 fighter aeroplanes during the Battle of Britain
Battle of Britain
The Battle of Britain is the name given to the World War II air campaign waged by the German Air Force against the United Kingdom during the summer and autumn of 1940...

. While it was officially called the R.A.E restrictor, it was normally referred to under various names, such as Miss Tilly's Diaphragm or the Tilly orifice in reference to its inventor, Beatrice "Tilly" Shilling
Beatrice Shilling
Beatrice Shilling OBE PhD MSc CEng was an aeronautical engineer who was responsible for correcting a serious defect in the Rolls-Royce Merlin engine during the Second World War. She also was a well-known racing motorcyclist....

.

Engine cut out problems

The early versions of the Rolls-Royce Merlin
Rolls-Royce Merlin
The Rolls-Royce Merlin is a British liquid-cooled, V-12, piston aero engine, of 27-litre capacity. Rolls-Royce Limited designed and built the engine which was initially known as the PV-12: the PV-12 became known as the Merlin following the company convention of naming its piston aero engines after...

 engine came equipped with a SU carburettor. When these aeroplanes performed a negative G force
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...

 manoeuvre (pitching the nose hard down), fuel was forced upwards to the top of the float chamber of the carburettor rather than into the engine, leading to loss of power. If the negative G continued, the fuel would collect in the top of the float chamber, forcing the float to the floor of the chamber. This would in turn open the needle valve to maximum, flooding the carburettor with fuel and drowning the supercharger
Supercharger
A supercharger is an air compressor used for forced induction of an internal combustion engine.The greater mass flow-rate provides more oxygen to support combustion than would be available in a naturally aspirated engine, which allows more fuel to be burned and more work to be done per cycle,...

 with an over-rich mixture. This would lead to a rich mixture cut-out, which would shut down the engine completely, a serious drawback in combat.

Negative G commonly occurs when manoeuvring to fire on an enemy aircraft in a dogfight
Dogfight
A dogfight, or dog fight, is a form of aerial combat between fighter aircraft; in particular, combat of maneuver at short range, where each side is aware of the other's presence. Dogfighting first appeared during World War I, shortly after the invention of the airplane...

. Moving the stick forward would starve the engine of fuel, producing a sudden loss of power. This would let the enemy get away, and if continued the maneuver would cause the carburettor valve to open, provide far too rich a mixture and kill the engine. During the Battles of France
Battle of France
In the Second World War, the Battle of France was the German invasion of France and the Low Countries, beginning on 10 May 1940, which ended the Phoney War. The battle consisted of two main operations. In the first, Fall Gelb , German armoured units pushed through the Ardennes, to cut off and...

 and Britain
Battle of Britain
The Battle of Britain is the name given to the World War II air campaign waged by the German Air Force against the United Kingdom during the summer and autumn of 1940...

, the German fighter aeroplanes had fuel injected engines and therefore did not suffer from this problem as the fuel injection pumps kept the fuel at a constant pressure whatever manoeuvre were made. The German pilots could exploit this by pitching steeply forward while pushing the throttle wide open, the pursuing British aircraft being left flat footed since trying to emulate the maneuver would result in loss of power, or fuel flooding and engine shutdown. The British countermeasure, a half roll so the aircraft would only be subjected to positive G as they followed German aircraft into a dive, took enough time to let the enemy escape in most instances.

The Tilly orifice

Complaints from the pilots led to a search for a solution. Beatrice 'Tilly' Shilling
Beatrice Shilling
Beatrice Shilling OBE PhD MSc CEng was an aeronautical engineer who was responsible for correcting a serious defect in the Rolls-Royce Merlin engine during the Second World War. She also was a well-known racing motorcyclist....

, a young engineer working at Royal Aircraft Establishment
Royal Aircraft Establishment
The Royal Aircraft Establishment , was a British research establishment, known by several different names during its history, that eventually came under the aegis of the UK Ministry of Defence , before finally losing its identity in mergers with other institutions.The first site was at Farnborough...

 at Farnborough
Farnborough Airfield
Farnborough Airport or TAG London Farnborough Airport is an airport situated in Farnborough, Rushmoor, Hampshire, England...

, came up with a disarmingly simple solution. She introduced a simple flow restrictor: a small metal disc much like a plain metal washer. The restrictor orifice was made to accommodate just the fuel needed for maximum engine power, the power setting usually used during dogfights. It came in two versions, one for 12 psi
Pounds per square inch
The pound per square inch or, more accurately, pound-force per square inch is a unit of pressure or of stress based on avoirdupois units...

 manifold pressure and one for boosted engines with 15 psi manifold pressure.

While not solving the problem fully, the restrictor, along with modifications to the needle valve, permitted pilots to perform quick negative G manoeuvres without loss of engine power, removing the annoying drawback the British fighters had had in comparison to the German Messerschmitt Bf 109 machine, which was equipped with fuel injection
Fuel injection
Fuel injection is a system for admitting fuel into an internal combustion engine. It has become the primary fuel delivery system used in automotive petrol engines, having almost completely replaced carburetors in the late 1980s....

. Miss Shilling with a small team travelled around the countryside in early 1941 fitting the restrictors, giving priority to front-line units. By March 1941 the device had been installed throughout RAF Fighter Command
RAF Fighter Command
RAF Fighter Command was one of three functional commands of the Royal Air Force. It was formed in 1936 to allow more specialised control of fighter aircraft. It served throughout the Second World War, gaining recognition in the Battle of Britain. The Command continued until 17 November 1943, when...

. Officially named the 'R.A.E restrictor', the device was immensely popular with pilots, who affectionately named it 'Miss Shilling's orifice' or simply the 'Tilly orifice'.

This simple and elegant solution was only a stopgap: it did not allow inverted flight for any length of time. The problems were not finally overcome until the introduction of Bendix
Bendix Corporation
The Bendix Corporation was an American manufacturing and engineering company which during various times in its 60 year existence made brake systems, aeronautical hydraulics, avionics, aircraft and automobile fuel control systems, radios, televisions and computers, and which licensed its name for...

 and later Rolls Royce pressure carburettors in 1943.

Further reading

  • Negative Gravity, the Life of Beatrice Shilling, by Matthew Freudenberg (ISBN 0-9546165-0-2)
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