[Wolf_Rider]

^ beat me to it




The early versions of the Rolls-Royce Merlin engine came equipped with a SU carburettor. When these aeroplanes performed a negative G force 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 with over-rich mixture. This would lead to a rich mixture cut-out, which would shut down the engine completely, a serious drawback in combat.[1]

Negative G commonly occurs when manoeuvring to fire on an enemy aircraft in a dogfight. 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 and Britain, 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.

1 Price, Alfred. The Spitfire Story: Second edition. London: Arms and Armour Press Ltd., 1986. ISBN 0-85368-861-3 p78