North American P-51 Mustang variants

Unlike later models, Allison-powered Mustangs were characterized by the carburetor air intake placed on the dorsal surface of the nose, immediately behind the propeller.[7] Fifty-five of these P-51-1s were outfitted with a pair of K.24 cameras in the rear fuselage for tactical low-level reconnaissance and re-designated F-6A (the "F" for photographic, although confusingly also still referred to as the P-51 or P-51-1[7]).After the late-June 1941 reorganization of the USAAC into the United States Army Air Forces, roughly six months before the attack on Pearl Harbor changed the outlook for the United States regarding involvement in global hostilities against the Axis overnight, priority had to be given to building as many existing fighters – P-38s, P-39s, and P-40s – as possible while simultaneously training pilots and other personnel, which meant evaluation of the XP-51 did not begin immediately.[6] On 16 April 1942, Fighter Project Officer Benjamin S. Kelsey ordered 500 A-36As, a redesign that included six .50 in (12.7 mm) M2 Browning machine guns, dive brakes, and the ability to carry two 500 lb (230 kg) bombs.In April 1942, the RAF's Air Fighting Development Unit (AFDU) tested the Mustang and found its performance inadequate at higher altitudes.The airframe itself was strengthened, with the fuselage and engine mount area receiving more formers because of the 355 lb (161 kg) greater weight of the Packard V-1650-3 compared with the V-1710.The engine cowling was completely redesigned to house the Packard Merlin, which, because of the intercooler radiator mounted on the supercharger casing, was 5 in (130 mm) taller and used an updraft carburetor, rather than the downdraft variety of the Allison.[23] A "duct rumble" heard by pilots in flight in the prototype P-51B resulted in a full-scale wind-tunnel test at NACA's Ames Aeronautical Laboratory.It was eventually found that the rumble could be eliminated by increasing the gap between the lower surface of the wing and the upper lip of the cooling system duct from 1 to 2 in (25 to 51 mm).They concluded part of the boundary layer on the lower surface of the wing was being ingested into the inlet and separating, causing the radiator to vibrate and producing the rumble.Problems with high-speed "porpoising" of the P-51Bs and P-51Cs with the fuselage tanks would lead to the replacement of the fabric-covered elevators with metal-covered surfaces and a reduction of the tailplane incidence.[29] Despite these modifications, the P-51Bs and P-51Cs, and the newer P-51Ds and P-51Ks, experienced low-speed handling problems that could result in an involuntary "snap-roll" under certain conditions of air speed, angle of attack, gross weight, and center of gravity.Also incorporated was a change to the rudder trim tabs, which would help prevent the pilot over-controlling the aircraft and creating heavy loads on the tail unit.[42] The addition of the 85 US gallon (322 l) fuselage fuel tank, coupled with the reduction in area of the new rear fuselage, exacerbated the handling problems already experienced with the B/C series when fitted with the tank, and led to the same fillet being added to -B, -C and initial -D-series versions in the field, to be quickly standardized as a normal rear-fuselage airframe component on later production blocks of the -D version.[44] The hollow-bladed Aeroproducts propeller was unreliable, due to manufacturing problems, with dangerous vibrations at full throttle and was eventually replaced by the Hamilton Standard.In November 1944 the Australian government decided to order Australian-built Mustangs, to replace its Curtiss Kittyhawks and CAC Boomerangs in the South West Pacific theatre.In October 1953, six Mustangs, including A68-1, the first Australian built CA-17 Mk 20, were allotted to the Long Range Weapons Development Establishment at Maralinga, South Australia, for use in experiments to gauge the effects of low-yield atomic bombs.Modifications included changes to the cowling, a simplified undercarriage with smaller wheels and disc brakes, a larger canopy,[clarification needed] and an armament of four .50 Brownings.This aircraft, with minor differences as the NA-129, came too late to participate in World War II, but it brought the development of the Mustang to a peak as one of the fastest production piston-engine fighters to see service.[53] The P-51H used the new V-1650-9 engine, a version of the Merlin that included Simmons automatic supercharger boost control with water injection, allowing War Emergency Power as high as 2,218 hp (1,500 kW).The canopy resembled the P-51D "bubble" style, over a raised pilot's position, and the aircraft was given a new propeller with wider, uncuffed blades and rounded tips to allow the additional power to be better used.Many of the aerodynamic advances of the P-51 (including the laminar flow wing) were carried over to North American's next generation of jet-powered fighters, the Navy FJ-1 Fury and Air Force F-86 Sabre.The wings, empennage and canopy of the first straight-winged variant of the Fury (the FJ-1) and the unbuilt preliminary prototypes of the P-86/F-86 strongly resembled those of the Mustang before the aircraft were modified with swept-wing designs.When the undercarriage was retracted, the main gear skis were housed in the space in the lower engine compartment made available by the removal of the fuselage .50 in (12.7 mm) Brownings from the P-51As.The entire installation added 390 lb (180 kg) to the aircraft weight and required that the operating pressure of the hydraulic system had to be increased from 1,000 to 1,200 psi (6,900 to 8,300 kPa)).Flight tests showed ground handling was good, and the Mustang could take off and land in a field length of 1,000 ft (300 m); the maximum speed was 18 mph (29 km/h) lower, although it was thought that fairings over the retracted skis would compensate.[55] Concern over the USAAF's inability to escort Boeing B-29 Superfortresses all the way to mainland Japan resulted in the highly classified "Seahorse" project (NAA-133), an effort to "navalize" the P-51.Landings were found to be easy, and, by allowing the tail wheel to contact the deck before the main gear, the aircraft could be stopped in a minimum distance.The power rating for the engines could vary according to the type of supercharger fitted, the size of the compressor impeller(s), and the gear speeds selected.As the critical altitude was passed a pressure-operated aneroid capsule operated the gearbox which changed up to 'high blower', driving the impellors faster, compressing a greater volume of the air-fuel mixture.