I've never heard of a Spitfire of any kind being able to dive 12K without ripping the wings off, or being torn into tiny pieces.
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Beginning in late 1943, high-speed diving trials were undertaken at Farnborough to investigate the handling characteristics of aircraft travelling at speeds near the sound barrier (i.e. the onset of compressibility effects). Because it had the highest limiting Mach number of any aircraft at that time, a Spitfire XI was chosen to take part in these trials. Due to the high altitudes necessary for these dives, a fully feathering Rotol propeller was fitted to prevent overspeeding. It was during these trials that EN409, flown by Squadron Leader J. R. Tobin, reached 606 mph (975 km/h, Mach 0.891) in a 45 degree dive. In April 1944 the same aircraft suffered engine failure in another dive while being flown by Squadron Leader A. F. Martindale, when the propeller and reduction gear broke off. Martindale successfully glided the Spitfire 20 miles (32 km) back to the airfield and landed safely.
That any operational aircraft off the production line, cannons sprouting from its wings and warts and all, could readily be controlled at this speed when the early jet aircraft such as Meteors, Vampires, P-80s, etc could not, was certainly extraordinary.― Jeffrey QuillOn 5 February 1952, a Spitfire 19 of No. 81 Squadron RAF based in Hong Kong reached probably the highest altitude ever achieved by a Spitfire. The pilot, Flight Lieutenant Ted Powles, was on a routine flight to survey outside air temperature and report on other meteorological conditions at various altitudes in preparation for a proposed new air service through the area. He climbed to 50,000 feet (15,240 m) indicated altitude, with a true altitude of 51,550 feet (15,712 m). The cabin pressure fell below a safe level, and in trying to reduce altitude, he entered an uncontrollable dive which shook the aircraft violently. He eventually regained control somewhere below 3,000 feet (900 m) and landed safely with no discernible damage to his aircraft. Evaluation of the recorded flight data suggested that, in the dive, he achieved a speed of 690 mph (1,110 km/h, Mach 0.94), which would have been the highest speed ever reached by a propeller-driven aircraft.
The critical Mach number of the Spitfire's original elliptical wing was higher than the subsequently-used laminar-flow-section, straight-tapering planform wing of the follow-on Supermarine Spiteful, Seafang and Attacker, illustrating that Reginald Mitchell's thoughtful and practical engineering approach to the problems of high speed flight had paid off handsomely. <<<<
