page_title: North American P-51 Mustang variants
section_title: P-51B and P-51C
hierarchical_section_title: North American P-51 Mustang variants / Merlin-engined Mustangs / P-51B and P-51C
caption_attribution_description: A USAAF North American P-51B-1-NA Mustang (s/n 43-12433, code AJ-M) of the 356th Fighter Squadron, 354th Fighter Group, named "Miss Pea Ridge" (pilot Mack Tyner). 42-12433 was originally destined for the Royal Air Force as Mustang Mk III, s/n FX905. It did not serve with the RAF but was returned to the USAAF on 30 December 1943.
context_page_description: Over twenty variants of the North American P-51 Mustang fighter were produced from 1940, when it first flew, to after the Second World War, some of which were employed also in the Korean War and in several other conflicts. Numerous examples of the aircraft survive to this day, often as heavily modified air racers.
context_section_description: The two XP-51Bs were a more thorough conversion than the Mustang X, with a tailor-made engine installation and a complete redesign of the radiator duct. 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. The new engine drove a four-bladed 11 ft 2 in (3.40 m)-diameter Hamilton Standard propeller that featured cuffs of hard molded rubber. To cater for the increased cooling requirements of the Merlin, a new fuselage duct was designed. This housed a larger radiator, which incorporated a section for the supercharger coolant, and, forward of this and slightly lower, an oil cooler was housed in a secondary duct which drew air through the main opening and exhausted via a separate exit flap.
  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. This was carried out by inserting the airplane, with the outer wing panels removed, into the 16-foot wind tunnel. A test engineer would sit in the cockpit with the wind tunnel running and listen for the duct rumble. 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. The production P-51B inlet was lowered even further, to give a separation of  2.63 in (67 mm) from the bottom of the wing. In addition, the shelf above the oil cooler face was removed and the inlet highlight swept back.
  It was decided new P-51Bs (NA-102s) would continue with the same armament and ammunition load of the P-51A, while the bomb rack/external drop tank installation was adapted from the A-36 Apache; the racks were rated to carry up to 500 lb (230 kg) of ordnance and were also piped for drop tanks. The machine guns were aimed using the electrically illuminated N-3B reflector sight fitted with an A-1 head assembly which allowed it to be used as a gun or bomb sight through varying the angle of the reflector glass. Pilots were also given the option of having ring and bead sights mounted on the top engine cowling formers. This option was discontinued with the P-51D.
  The first XP-51B flew on 30 November 1942. Flight tests confirmed the potential of the new fighter, with the service ceiling being raised by  10,000 ft (3,000 m), with the top speed improving by 50 mph (43 kn; 80 km/h) at 30,000 ft (9,100 m). American production was started in early 1943 with the P-51B (NA-102) being manufactured at Inglewood, California, and the P-51C (NA-103) at a new plant in Dallas, Texas, which was in operation by summer 1943. The RAF named these models Mustang Mk III. In performance tests, the P-51B reached 441 mph (383 kn; 710 km/h) at 30,000 ft (9,100 m). In addition, the extended range made possible by the use of drop tanks enabled the Merlin-powered Mustang to be introduced as a bomber escort with a combat radius of 750 mi (1,210 km) using two 75 US gal (62 imp gal; 280 l) 2-piece, sheet-metal stamped construction drop tanks.
  The range would be further increased with the introduction of an 85 US gal (71 imp gal; 320 l) self-sealing fuel tank aft of the pilot's seat, starting with P-51B-5-NA ("block 5"). When this tank was full, the center of gravity of the Mustang was moved dangerously close to the aft limit. As a result, maneuvers were restricted until the tank was down to about 25 US gal (21 imp gal; 95 l) and the external tanks had been dropped. Problems with high-speed "porpoising" of the P-51Bs and P-51Cs with the fuse
caption: An early P-51B assigned to the 356th FS, 354th FG, Eighth Air Force in England.
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