From Wikipedia, the free encyclopedia

The Miss Shilling orifice was a very simple technical device made to counter engine cut-out in early Spitfire and Hurricane fighter aeroplanes during the Battle of Britain.

The Rolls-Royce Merlin engine originally came with a direct carburettor, prone to flooding in negative g.

The Merlin engine in both aircraft was equipped with an SU carburettor. When the aeroplane performed a negative g manoeuvre (pitching the nose hard down), fuel was forced upwards to the top of the float chamber, depriving the engine of fuel. This would produce a momentary loss of power. If the negative g continued, the excessive fuel would force the float to the floor of the float chamber, opening the needle valve and flooding the carburettor with fuel and drowning the supercharger with over-rich mixture. This would lead to a rich-mixture cut-out, which would shut down the engine completely, which was a serious drawback in combat.

Negative g commonly occurs when manoeuvring to fire on an enemy aircraft. Moving the stick forward would produce a sudden loss of power, letting the enemy get away, and even flooding the engine. The German fighter pilots, whose airplane engines were fuel injected and therefore did not suffer from this problem, could exploit this by pitching steeply forward, a manoeuvre the British fighters could not follow.

Complaints from the pilots led to a search for a solution. A young physicist working at Royal Aircraft Establishment at Farnborough, Beatrice “Tilly” Shilling, 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. It came in two versions, one for 12 psi 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 an annoying drawback the British fighters had had in comparison to the German Bf 109 machine, which was equipped with fuel injections. The restrictor became immensely popular with pilots who affectionately named it “Miss Shilling’s orifice” or the “Tilly orifice”.

This simple yet elegant solution was only a stopgap: it did not permit an aeroplane to fly inverted for any length of time. The problems were not finally overcome until the introduction of a Bendix and later R-R type carburettor.

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