was used in battle trials against an English Electric Lightning to determine how best a Lightning should engage piston-engined aircraft. This information was needed in case RAF Lightnings might have to engage P-51 Mustangs in the Indonesian conflict of the time. Mk XX (type 366) Mark XX was given to the original Mk IV Griffon engine prototype DP845 to avoid confusion with the retitled Spitfire PR Mk IVs. The second Mk XX, DP851, initially had a Griffon II engine and made its first flight in August 1942. In December, it was refitted with a Griffon 61 and re-designated as a Mk 21 initial prototype. Mk 21 (type 356) By early 1942, it was evident that Spitfires powered by the new two-stage supercharged Griffon 61 engine would need a much stronger airframe and wings. The proposed new design was called the Mk 21, which at first displayed poor flight qualities that damaged the excellent Spitfire reputation. The wings were redesigned with a new structure and thicker-gauge light alloy skinning. The new wing was torsionally 47 per cent stiffer, allowing an increased theoretical aileron reversal speed of . The ailerons were 5 per cent larger and the Frise balanced type were dispensed with, the ailerons being attached by continuous piano-hinges. They were extended by eight inches, meaning that with a straighter trailing edge, the wings were not the same elliptical shape as previous Spitfires. The Mk 21 armament was standardised as four 20mm Hispano II cannon with 150 rpg and no machine guns. The Griffon engine drove an -diameter five-bladed propeller, some larger than that fitted to the Mk XIV. To ensure sufficient ground clearance for the new propeller, the undercarriage legs were lengthened by . The undercarriage legs also had a wider track to help improve ground handling. The designers used a system of levers to shorten the undercarriage legs by about as they retracted, because the longer legs did not have enough space in which to retract; the levers extended the legs as they came down. The larger diameter four-spoke main wheels were strengthened to cope with the greater weights; post-war these were replaced by wider, reinforced three spoke wheels to allow Spitfires to operate from hard concrete or asphalt runways. When retracted the wheels were fully enclosed by triangular doors which were hinged to the outer edge of the wheel wells. The first production Mk 21s used the same