The fin flash evolved from the rudder stripes painted on the rudders of early RFC and RAF aircraft during the First World War, the markings comprising blue, white and red vertical stripes doped on the rudder. However, with the performance of aircraft increasing considerably during the 1930s, the practice of applying painted markings onto the (then manually powered) control surfaces was discontinued because of the need to rebalance the controls - failure to do this could have adverse effects on the surface's aerodynamic balance, possibly leading to flutter of the control surface at high airspeeds. It was for this same reason that the positioning of the wing roundels was revised so that they no longer overlapped the ailerons.
In an attempt to conform to the appearance of French military aircraft, rudder stripes reappeared on aircraft (mainly Fairey Battles and Hawker Hurricanes) of the RAF based in France, starting in early September 1939. These stripes were painted in standard RAF colours in the order blue, white, red.
Fin flashes were officially adopted in June 1940. For the first six months there was no conformity in the width or height of the stripes and they were painted to cover as much of the fin area as possible. With one or two exceptions the order was red (leading edge), white, blue. In December 1940 type A fin flashes were standardised: height was 27 inches, width 24 inches, divided into three 8-inch-wide (200 mm) red, white and blue stripes (e.g.: photo six, the Sea Hurricanes show this standardised fin flash). On some aircraft, e.g.; photo reconnaissance Spitfires the fin flash was about half these dimensions.
In July 1942, with the adoption of the type C and C1 roundels the fin flash became 24 in square for RAF fighters, the stripe widths becoming 11 in red, 2 in white and 11 in blue. There were some exceptions; RAF North American Mustangs all used fin flashes which were 27 in high by 24 in wide. In early 1944 some aircraft types were painted in a "High-altitude" camouflage scheme and adopted type B roundels and fin flashes.
The then-current RAF fin flashes were also adopted for USAAF aircraft operating alongside British and Commonwealth forces in the Mediterranean theatre in 1942, appearing on US Curtiss P-40 Warhawk fighters and North American B-25 Mitchell bombers, as well as on USAAF Consolidated B-24 Liberators flying from North Africa on attacks such as 1943's Operation Tidal Wave.
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