The Battle of Britain Five Months That Changed History, May—October 1940 by James Holland (z-lib.org).epub

at Farnborough of two intact Me 109s captured in France, British fighters
had two-speed propellers. The angle of the propeller varied. Coarse pitch
meant its blades were angled so that as it rotated, it would ‘bite’ the air
more effectively. At fine pitch, the propeller was at a flatter angle, which
allowed the engine revs to be greater, but did not provide as much ‘bite’.
But getting the best from a plane meant striking a balance between
performance and fuel consumption in a range of different flying altitudes,
speeds and other scenarios such as, for example, diving or climbing. Thus
having just two pitch options was somewhat limiting, especially when
compared to a variable pitch propeller, which enabled a pilot to maintain a
far more varied and subtle use of the interchange between thrust, engine
revs and power. On the back of the Farnborough tests, however, first
Spitfires and then Hurricanes were hurriedly converted to De Havilland
variable pitch propellers. In another astonishing feat of production, over a
thousand existing, repaired and new-build Spitfires and Hurricanes had
been converted by 15 August, which did much to improve their
performance.
The Me 109’s ability to dive away from any mêlée quickly was largely
due to the DB601’s fuel injection, which Spitfires and Hurricanes did not
have. Rather, the Merlin relied on a carburettor. Any sudden dive
downwards created negative gravity, or negative-g, which not only pushed
the pilot up against his straps, but also forced all the fuel to the top of the
float chamber. If the negative-g was maintained, this would cause the
carburettor to become flooded with fuel because the float was no longer
controlling the fuel flow into the carburettor. This in turn then caused the
engine to cut out (called a rich-cut), producing a large puff of black smoke
as it did so. This only lasted as long as the duration of the negative-g, and
might only take a few seconds, but it was in that moment that an enemy
could make good his escape. This was not an issue for the Me 109 because
with its pioneering Bosch fuel injection fuel was pumped into the engine
consistently even when under negative-g. It was this feature of the Me 109
that saved Siegfried Bethke’s life on 2 September. He had been around
12,000 feet above the eastern edge of London, and escorting bombers, when
they were, as expected, attacked by British fighters. In a confused tussle,
Siegfried was performing a very steep left turn, and glancing up in his
mirror his blood suddenly chilled. Just behind him was a Hurricane with all
of its eight machine guns spitting bolts of lightning at him. He knew that the