http://en.wikipedia.org/wiki/North_American_P-51_variants
P-51/Mustang IA (NA-91)
The first American order for 150 P-51s, designated NA-91 by North American, were placed by the Army on 7 July 1940.[4] The two XP-51s (41-038 and 41-039) set aside for testing arrived at Wright Field on 24 August and 16 December 1941 respectively.[nb 1] The relatively small size of this first order reflected the fact that the USAAC was still a relatively small, underfunded peacetime organisation. After the attack on Pearl Harbor priority had to be given to building as many of the existing fighters - P-38s, P-39s and P-40s - as possible while simultaneously training pilots and other personnel, which meant that the evaluation of the XP-51s did not begin immediately. However, this did not mean that the XP-51s were neglected, or their testing and evaluation mishandled.[5] The 150 NA-91s were designated P-51 by the newly formed USAAF and were initially named Apache, although this was soon dropped, and the RAF name, Mustang, adopted instead. The USAAF did not like the mixed armament of the British Mustang Is and instead adopted an armament of four long-barrelled 20 mm (.79 in) Hispano Mk II cannon, and deleted the .50 cal engine cowling mounted weapons. The British designated this model as Mustang Mk IA. A number of aircraft from this lot were fitted out by the USAAF as F-6A photo-reconnaissance aircraft. The British would fit a number of Mustang Mk Is with similar equipment.[6]
It was quickly evident that the Mustang's performance, although exceptional up to 15,000 ft (4,600 m), was markedly reduced at higher altitudes. The single-speed, single-stage supercharger fitted to the Allison V-1710 engine had been designed to produce its maximum power at a low altitude. Above 15,000 feet, the supercharger's critical altitude rating, the power dropped off rapidly. Prior to the Mustang project, the USAAC had Allison concentrate primarily on turbochargers in concert with General Electric; the turbochargers proved to be reliable and capable of providing significant power increases in the P-38 Lightning and other high-altitude aircraft, in particular in the Air Corps's four-engine bombers. Most of the other uses for the Allison were for low-altitude designs, where a simpler supercharger would suffice. Fitting a turbocharger into the Mustang proved impractical, and Allison was forced to use the only supercharger that was available. In spite of this, the Mustang's advanced aerodynamics showed to advantage, as the Mustang Mk I was about 30 mph (48 km/h) faster than contemporary Curtiss P-40 fighters using the same engine (the V-1710-39 producing 1,220 hp (910 kW) at 10,500 ft (3,200 m), driving a 10 ft 6 in (3.20 m) diameter, three-blade Curtiss-Electric propeller).[7] The Mustang Mk I was 30 mph (48 km/h) faster than the Spitfire Mk VC at 5,000 ft (1,500 m) and 35 mph (56 km/h) faster at 15,000 ft (4,600 m), despite the British aircraft's more powerful engine.[8]
Although it has often been stated that the poor performance of the Allison engine above 15,000 ft (4,600 m) was a surprise and disappointment to the RAF and USAAF, this has to be regarded as a myth; aviation engineers of the time were fully capable of correctly assessing the performance of an aircraft's engine and supercharger.[9] As evidence of this, in mid-1941, the 93rd and 102nd airframes from the NA-91 order were slated to be set aside and fitted and tested with Packard Merlin engines, with each receiving the designation XP-51B.[10]
[4]^ Kinzey 1996, pp. 7, 17.
[5]^ Kinzey 1996, pp. 7, 17-18.
[6]^ Kinzey 1996, p. 18.
[7]^ Gruenhagan 1980, pp. 178, 180–181.
[8]^ Birch 1987, p. 11.
[9]^ Kinzey 1996, p. 8.
[10]^ Kinzey 1996, pp. 8, 18, 55.
