speed of 423 mph (681 km/h) at 18,500 ft (5,639 m) was predicted. However, constant problems with the development of the Griffon meant that the decision to proceed with building a Spitfire with this engine didn't come to fruition until 1942, with the successful flight trials of the Mk IV. The Griffon IIB which powered the Mk IV was a single-stage supercharged engine of 1,735 hp (1,293 kW). Stronger main longerons were needed to cope with the weight of the Griffon and it required a bigger radiator and oil cooler, although it kept the asymmetric under-wing radiator layout of the single stage Merlin marks. The new engine had a lower thrust-line than the Merlin and was set with 2 degrees of downthrust. The lower thrust line and larger capacity of the new engine meant that the contours of the engine cowling were completely changed, with more prominent blisters over the cylinder heads, plus a third tear-drop shaped blister on the upper forward cowling to clear the magneto, and a deeper curve down to the spinner, which was much longer than previous types. The lower cowling lost its "pigeon-chested" appearance, with a shallower curve up to the spinner. A four blade Rotol propeller of 10 ft 5 in (3.1 m) was used. Apart from these differences the Mk IV airframe was closely related to that of the Merlin-powered Mk III. One feature of the Griffon engine which was to catch a lot of pilots out was that the propeller rotated in the opposite direction to that of the Merlin; i.e.,: to the left, from the pilot's perspective, rather than to the right. This meant that the powerful slipstream swung the Spitfire to the right on the ground, requiring the rudder to be deflected to the left during takeoff. The Mark IV DP845 first flew on 27 November 1941. It had the full-span C wing combined with a small tail unit and retractable tailwheel, and also had external bracket hinges under the wings, denoting the installation of braking flaps. These were soon removed and a mock up of a proposed six-cannon armament was fitted, three in each wing. The aircraft was soon renamed Mk XX, to avoid confusion with a renamed PR type, then it became the Mk XII. Jeffrey Quill, Supermarine's chief test pilot, was the first to fly the Mk IV/Mk XII prototype DP845, Mk XII (type 366) The Mk