Miss Shilling's orifice

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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.

[edit] Engine cut out problems

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 a 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 momentary loss of power to the engine. 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 inconvenience 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, or 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.

[edit] The Tilly orifice

Complaints from the pilots lead to a search for a solution. A young physicist working at Royal Aircraft Establishment at Farnborough, Miss Beatrice “Tilly” Shilling, came up with a disarmingly simple solution. She introduced a 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 now permitted the pilots to perform quick negative g manoeuvres without loss of engine power, removing an annoying drawback the British fighters had compared to the German Bf 109 machine. 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 measure. 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.