North American P-51 Mustang XP-51F

In 1943 North American proposed to the USAAF a redesign of the P-51D as model NA-105, exploiting a difference in requirements: British load factors for fighters were lower than American ones. The idea came, as Edgar Schmued recalled, from a USAAF enquiry as to why British aircraft were lighter; North American engineers examined the components and equipment fitted to Spitfires and, working to the lower British load factors, cut structural weight. The result was a Mustang some 730 kg lighter than the P-51D.

The changes touched almost the whole airframe. The wing adopted NACA 66,2-(1.8)15.5 a=.6 airfoils at the root and NACA 66,2-(1.8)12 a=.6 at the tip, of lower drag than the earlier NAA/NACA 45-100, with a simple trapezoidal planform and no leading-edge extension at the root. The cowling was reworked, the undercarriage simplified with smaller wheels and disc brakes, a larger canopy fitted and the armament reduced to four .50 in Brownings. In test flights the NA-105 reached 790 km/h at 6,400 m.

The XP-51F designation covered the aircraft powered by V-1650 engines; those with the Merlin RM 14 SM became the XP-51G, and a third prototype with an Allison V-1710 the XP-51J. Three XP-51Fs were built and a small number went to the British as the Mustang Mk V. None of the lightweights entered production, but their experience fed directly into the production P-51H. The reference material gives no first-flight dates or individual weights and dimensions for this variant, and this text does not invent them.