I'm going to quote Supermarine Chief Test Pilot Jeffrey Quill;
"Then at some indefinate time in 1942, there seemed to be a change of tactical philosophy on both sides. It was rather as if, by some sort of tacit mutual consent between enemies, it was realized that the band between 30,000 and 40,000 feet was silly place to have an air battle, and the fighting tended to drop down to the more practical regions, roughly between 15,000 and 25,000 feet. I remember how, at the time, this trend interested me very much indeed. It was clearly reflected in the LF Mk IX (Merlin 66) with the engine performance adjusted to the reduced height band. It was also reflected in the fact that, by removing the wing tips of the Spitfire, an improvement in lateral control could be achieved, but because it thereby increased the wing loading and the span loading of the wing, an aerodynamic penalty was incurred at high altitude. Such a proposition would have been unthinkable in 1940/41, but in 1942/43 the idea was enthusiastically adopted by some squadrons of 11 Group, and the 'clipped wing' Spitfire became a common sight in the sky."
