North American P-51 Mustang variants

NA-73X The prototype of the Mustang, designated NA-73X, was rolled out by North American Aviation on 9 September 1940, albeit without an engine, and was first flown the following 26 October. The Mustang was originally designed to use a low-altitude rated Allison V-1710 engine. 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. Mustang Mk I (NA-73 and NA-83) factory in October 1942. The first production contract was awarded by the British for 320 NA-73 fighters, named Mustang Mk I by the British Purchasing Commission; a second British contract soon followed, which called for 300 more (NA-83) Mustang Mk I fighters. Contractual arrangements were also made for two aircraft from the first order to be delivered to the USAAC for evaluation; these two airframes, and respectively, were designated XP-51. The first RAF Mustang Mk Is were delivered to 26 Squadron at RAF Gatwick in February 1942 and made their combat debut on 10 May 1942. With their long range and excellent low-altitude performance, they were employed effectively for tactical reconnaissance and ground-attack duties over the English Channel, but were thought to be of limited value as fighters due to their poor performance above . RAF Mustangs (Mk Is, which were not drop tank capable) made history on October 22, 1942, when they escorted 22 Vickers Wellington medium bombers on a daylight raid to Germany, thus becoming the first RAF single-seat fighters to fly over the country during World War II. P-51/Mustang Mk IA (NA-91) The first American order for 150 P-51s, designated NA-91 by North American, was placed by the US Army on 7 July 1940. This was on behalf of the RAF in a Lend-Lease deal. All but the last 57 went to the British. After the attack on Pearl Harbor, the USAAF 'held back' these Mustang Mk IAs for their own use. 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 was still a relatively small, underfunded peacetime organization. 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. However, this did not mean it was neglected, or testing and evaluation mishandled. 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, removing the .50 cal (12.7 mm) "nostril"-mounted weapons. The British designated this model as Mustang Mk IA, and would fit a number with similar equipment. It was quickly evident the Mustang's performance, although exceptional up to (the supercharger's critical altitude rating), was markedly reduced at higher altitudes. This was because the single-speed single-stage supercharger fitted to the V-1710 had been designed to produce maximum power at low altitude; above that, 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 Lockheed P-38 Lightning and other high-altitude aircraft, in particular in the Air Corps' 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 available. In spite of this, the Mustang's advanced aerodynamics showed to advantage, as the Mk I was about faster than contemporary Curtiss P-40 Warhawks using the same V-1710-39 (producing at , driving a diameter, three-blade Curtiss-Electric propeller). The Mk I was faster than the Supermarine Spitfire Mk Vc at and faster at , despite the British aircraft's more powerful engine (the Rolls-Royce Merlin 45, producing at . Although it has often been stated that the poor performance of the Allison engine above 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. 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 (the US-built version of the Merlin) engines, with each receiving the designation XP-51B. P-51A/Mustang Mk II (NA-99) On 23 June 1942, a contract was placed for 1,200 P-51As (NA-99s). The P-51A used the new Allison V-1710-81 engine, a development of the V-1710-39, driving a -diameter three-bladed Curtiss-Electric propeller. The armament was changed to four .50 in (12.7 mm) Browning machine guns, two in each wing, with a maximum of 350 rounds per gun (rpg) for the inboard guns and 280 rpg for the outboard. Other improvements were made in parallel with the A-36, including an improved, fixed air duct inlet replacing the movable fitting of previous Mustang models and the fitting of wing racks able to carry either drop tanks, increasing the maximum ferry range to with the tanks. The top speed was raised to at . The USAAF received 310 and the RAF 50 (as the Mustang II) before production converted to producing the Merlin-powered P-51B. A-36A (NA-97) The A-36A was the first aircraft based on the 'Mustang' airframe ordered by the U.S. Government specifically for use by the USAAF. NAA found an unfilled 'Dive Bomber' USAAF contract, which they got mainly by their own initiative. In so doing, NAA was able to keep the production bays open with the hope the USAAF would place orders for it as a fighter. It was the first airframe of the Mustang "family" to be drop-tank capable. The 500 aircraft were designated A-36A (NA-97). This model became the first USAAF Mustang to see combat. One aircraft (British serial EW998) was passed to the British who gave it the name Mustang Mk I (Dive Bomber). ==Merlin-engined Mustangs==

Mustang Mk X In April 1942, the RAF's Air Fighting Development Unit (AFDU) tested the Mustang and found its performance inadequate at higher altitudes. As such, it was to be used to replace the P-40 in Army Cooperation Command squadrons, but the commanding officer was so impressed with its maneuverability and low-altitude speeds, he invited Ronnie Harker (from Rolls-Royce's Flight Test establishment) to fly it. Rolls-Royce engineers rapidly realized equipping the Mustang with a high altitude Merlin 61 engine with its two-speed two-stage supercharger would substantially improve performance. The company started converting five aircraft as the Mustang Mk X with the Merlin 65. Apart from the engine installation, which utilized custom-built engine mounts designed by Rolls-Royce and initially the -diameter four-bladed Rotol propeller from a Spitfire Mk IX, the Mk X was a straightforward adaptation of the Mk I airframe, keeping the same radiator duct design. The Vice-Chief of the Air Staff, Air Marshal Sir Wilfrid R. Freeman, lobbied vociferously for Merlin-powered Mustangs, insisting two of the five experimental Mustang Xs be handed over to Carl Spaatz for trials and evaluation by the U.S. Eighth Air Force in Britain. The high-altitude performance improvement was remarkable: one Mk X (serial number AM208) reached at with full supercharger, and AL975 tested at an absolute ceiling of . P-51B and P-51C , 354th FG, Eighth Air Force in England. 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 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 taller and used an updraft carburetor, rather than the downdraft variety of the Allison. The new engine drove a four-bladed -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 . 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 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 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 , with the top speed improving by at . 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 at . The range would be further increased with the introduction of an 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 and the external tanks had been dropped. 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. One of the few remaining complaints with the Merlin-powered aircraft was a poor rearward view. The canopy structure, which was the same as the Allison-engined Mustangs, was made up of flat, framed panels; the pilot entered or exited the cockpit by lowering the port side panel and raising the top panel to the right. The canopy could not be opened in flight and tall pilots especially were hampered by limited headroom. In order to at least partially improve the view from the Mustang, the British had field-modified some Mustangs with clear, sliding canopies called Malcolm hoods (designed by R Malcolm & Co), and whose design had also been adopted by the U.S. Navy's own F4U-1D version of the Vought F4U Corsair in April 1944. Tadeusz Nowierski, CO of 133 (Polish) Wing, RAF Coolham, July 1944. The new structure was a frameless plexiglas moulding which ballooned outwards at the top and sides, increasing the headroom and allowing increased visibility to the sides and rear. Allied strategists quickly exploited the long-range fighter as a bomber escort. It was largely due to the P-51 that daylight bombing raids deep into German territory became possible without prohibitive bomber losses in late 1943. A number of the P-51B and P-51C aircraft were fitted for photo reconnaissance and designated F-6C. P-51D and P-51K Following combat experience the P-51D series introduced a "teardrop", or "bubble", canopy to rectify problems with poor visibility to the rear of the aircraft. In the United States, new moulding techniques had been developed to form streamlined nose transparencies for bombers. North American designed a new streamlined plexiglass canopy for the P-51B which was later developed into the teardrop shaped bubble canopy. In late 1942, the tenth production P-51B-1-NA was removed from the assembly lines. From the windshield aft the fuselage was redesigned by cutting down the rear fuselage formers to the same height as those forward of the cockpit; the new shape faired in to the vertical tail unit. A new simpler style of windscreen, with an angled bullet-resistant windscreen mounted on two flat side pieces improved the forward view while the new canopy resulted in exceptional all-round visibility. Wind tunnel tests of a wooden model confirmed that the aerodynamics were sound. with the "kinked" wing root leading edge and the added fin fillet on the tail present. This aircraft is famous in photos as one of "The Bottisham Four." Other alterations to the wings included new navigation lights, mounted on the wingtips, rather than the smaller lights above and below the wings of the earlier Mustangs, and retractable landing lights which were mounted at the back of the wheel wells; these replaced the lights which had been formerly mounted in the wing leading edges. The B/C subtypes' M2 guns were mounted with an inboard axial tilt, this angled mounting had caused problems with the ammunition feed and with spent casings and links failing to clear the gun-chutes, leading to frequent complaints that the guns jammed during combat maneuvers. The D/K's six M2s were mounted upright, remedying the jamming problems. In addition, the weapons were installed along the line of the wing's dihedral, rather than parallel to the ground line as in the earlier Mustangs. The wing racks fitted to the P-51D/P-51K series were strengthened and were able to carry up to of ordnance, although bombs were the recommended maximum load. Later models had removable under-wing 'Zero Rail' rocket pylons added to carry up to ten T64 5.0 in (127 mm) HVAR rockets per plane. The gunsight was changed from the N-3B to the N-9 before the introduction in September 1944 of the K-14 or K-14A gyro-computing sight. Apart from these changes, the P-51D and K series retained V-1650-7 engine used in the majority of the P-51B/C series. , 357th FG) shows the Aeroproducts propeller unit with the "uncuffed" blades and integral fin fillet on the tail 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. P-51Ds without fuselage fuel tanks were fitted with either the SCR-522-A or SCR-274-N Command Radio sets and SCR-695-A, or SCR-515 radio transmitters, as well as an AN/APS-13 rear-warning set; P-51Ds and Ks with fuselage tanks used the SCR-522-A and AN/APS-13 only. The P-51D became the most widely produced variant of the Mustang. A Dallas-built version of the P-51D, designated the P-51K, was equipped with an diameter Aeroproducts propeller in place of the Hamilton Standard propeller. 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. By the time of the Korean War, most F-51s were equipped with "uncuffed" Hamilton Standard propellers with wider, blunt-tipped blades. The remaining 20 were kept unassembled as spare parts. In addition, 84 P-51Ks were also shipped directly to the RAAF from the USA. In late 1946, CAC was given another contract to build 170 (reduced to 120) more P-51Ds on its own; these, designated CA-18 Mustang Mark 21, Mark 22 or Mark 23, were manufactured entirely in-house, with only a few components being sourced from overseas. After the USAF cancelled orders for P-51H variants, the RAAF followed suit, stopping 250 locally built CAC CA-21s. ==The "lightweight" Mustangs==

XP-51F, XP-51G and XP-51J The lightweight Mustangs had a new wing design. The airfoil was switched to the NACA 66,2-(1.8)15.5 a=.6 at the root and the NACA 66,2-(1.8)12 a=.6 at the tip. These airfoils were designed to give less drag than the previous NAA/NACA . In addition, the planform was a simple trapezoid, with no leading edge extension at the root. In 1943, North American submitted a proposal to redesign the P-51D as model NA-105, which was accepted by the USAAF. Modifications included changes to the cowling, a simplified undercarriage with smaller wheels and disc brakes, a larger canopy, and an armament of four .50 Brownings. In total the design was some lighter than the P-51D. Schmued described how it came from an enquiry by the USAAF as to why British aircraft were lighter than American ones. NAA engineers examined the various components and equipment fitted to Spitfires. In the course of this NAA found that British load factors were less than American ones. Working to the lower load factors helped them reduce structure weight. In test flights, the NA-105 achieved at . The designation XP-51F was assigned to prototypes powered with V-1650 engines (a small number were passed to the British as the Mustang V), and XP-51G to those with Merlin RM 14 SMs. A third lightweight prototype powered by an Allison V-1710-119 was added to the development program. This aircraft was designated XP-51J. Since the engine was insufficiently developed, the XP-51J was loaned to Allison for engine development. None of these experimental lightweights went into production. P-51H The P-51H (NA-126) was the final production Mustang, embodying the experience gained in the development of the XP-51F and XP-51G aircraft. 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. 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). Differences between the P-51D included lengthening the fuselage and increasing the height of the tailfin, which reduced the tendency to yaw. Service access to the guns and ammunition was also improved. 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. This propeller was similar to the one used on some later production P-51Ds and the majority of postwar F-51Ds. With a new airframe several hundred pounds lighter, extra power, and a more streamlined radiator, the P-51H was faster than the P-51D, able to reach at . The P-51H was designed to complement the Republic P-47N Thunderbolt as the primary aircraft for the planned invasion of Japan, with 2,000 ordered to be manufactured at Inglewood. Production was just gathering steam with 555 delivered when the war ended. Additional orders, already on the books, were canceled. With the cutback in production, the variants of the P-51H with different versions of the Merlin engine were produced in either limited numbers or terminated. These included the P-51L, similar to the P-51H but utilizing the V-1650-11 engine, which was never built; and its Dallas-built version, the P-51M, or NA-124, which used the V-1650-9A engine lacking water injection and therefore rated for lower maximum power, of which one was built out of the original 1629 ordered, serial number . Although some P-51Hs were issued to operational units, none saw combat in World War II and, in postwar service, most were issued to reserve units. One aircraft was provided to the RAF for testing and evaluation. Serial number '' was designated BuNo 09064'' and used by the U.S. Navy to test transonic airfoil designs and then returned to the Air National Guard in 1952. The P-51H was not used for combat in the Korean War despite its improved handling characteristics, since the P-51D was available in much larger numbers and was a proven commodity. 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. ==Experimental Mustangs==

In early 1944, the first P-51A-1-NA, ''''. was fitted and tested with a lightweight retractable ski kit replacing the wheels. This conversion was made in response to a perceived requirement for aircraft that would operate away from prepared airstrips. The main oleo leg fairings were retained, but the main wheel doors and tail wheel doors were removed for the tests. 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 to the aircraft weight and required that the operating pressure of the hydraulic system had to be increased from ). Flight tests showed ground handling was good, and the Mustang could take off and land in a field length of ; the maximum speed was lower, although it was thought that fairings over the retracted skis would compensate. 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. On 15 November 1944, naval aviator (and later test pilot) Lieutenant Bob Elder, in a P-51D-5-NA '''', started flight tests from the deck of the carrier . This Mustang had been fitted with an arrestor hook, which was attached to a reinforced bulkhead behind the tail wheel opening; the hook was housed in a streamlined position under the rudder fairing and could be released from the cockpit. The tests showed that the Mustang could be flown off the carrier deck without the aid of a catapult, using a flap setting of 20° down and 5° of up elevator. 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 project was canceled after U.S. Marines secured the Japanese island of Iwo Jima and its airfields, making it possible for standard P-51D models to accompany B-29s all the way to the Japanese home islands and back. While North American was concentrating on improving the performance of the P-51 through the development of the lightweight Mustangs, in Britain, other avenues of development were being pursued. To this end, two Mustang Mk IIIs (P-51Bs and P-51Cs), FX858 and FX901, were fitted with different Merlin engine variants. The first of these, FX858, was fitted with a Merlin 100 by Rolls-Royce at Hucknall; this engine was similar to the RM 14 SM fitted to the XP-51G and was capable of generating 2,080 hp (1,550 kW) at 22,800 ft (7,000 m) using a boost pressure of +25 lbf/in2 (170 kPa; 80 inHg) in war emergency setting. With this engine, FX858 reached a maximum speed of 455 mph (732 km/h) at 17,800 ft (5,425 m), and 451 mph could be maintained to 25,000 ft (7,600 m). The climb rate was 4,500 ft/min (22.9 m/s) at 1,600 ft (486 m) and 4,000 ft/min (20.3 m/s) at 13,000 ft (3,962 m). FX901 was fitted with a Merlin 113 (also used in the de Havilland Mosquito B.35). This engine was similar to the Merlin 100, fitted with a supercharger rated for higher altitudes. FX901 was capable of 454 mph (730 km/h) at 30,000 ft (9,100 m) and at 40,000 ft (12,200 m). ==Summary of P-51 variants==

;NA-73X : The initial prototype ;Mustang Mk I (NA-73/83) : The first production contract was awarded by the British for 320 NA-73 fighters. A second British contract for 300 more Mustang Mk Is was assigned a model number of NA-83 by North American. The RAF mostly used its Allison-engined Mustangs as tactical-photo reconnaissance fighters, fitting many of its Mustang Is, IAs, and IIs with camera equipment. ;XP-51 : Two aircraft of the first production batch, delivered to the USAAF. ;P-51 (NA-91) : In September 1940, 150 aircraft were ordered by the USAAF. These were designated by the USAAF as P-51 and initially named the Apache, although this name was dropped early-on for Mustang. The British designated this model as Mustang Mk IA. They were equipped with four long-barrelled 20 mm (.79 in) Hispano Mk II cannon instead of machine guns. Following British practice, a number of aircraft from this lot were fitted out by the USAAF as tactical photo reconnaissance aircraft, designated F-6A. ;A-36A (NA-97) :In early 1942, the USAAF ordered a lot of 500 aircraft modified as dive bombers and designated A-36A. This model became the first USAAF Mustang to see combat. One aircraft was passed to the British, who gave it the name Mustang Mk I (Dive Bomber). ;P-51A (NA-99) :Following the A-36A order, the USAAF ordered 310 P-51As, fifty of which were delivered to the RAF as Mustang IIs. :* P-51A-5-NA, 55 built. :* XP-51B (NA-101), two prototypes converted from P-51s. Originally designated XP-78. :* P-51B-5-NA (NA-104), with alternate air source grill on each side of the nose. 800 built. :* P-51C-5-NT (NA-103), same as B-15-NA. ;P-51D (NA-109/110/111/122/124) :As well as the modified fuselage and new canopy the production P-51Ds had modified wings compared with the P-51B/C series and became the most widely produced variant of the Mustang, with 6,502 being built at Inglewood and 1,600 at Dallas – a combined total of 8,102. Originally designated XP-51D, the designation was later changed to P-51D-NA. :* P-51D-1-NA (NA-110), initial production block of 100 unassembled aircraft sent to Australia. 80 were assembled under the designation CA-17 Mustang Mk 20, while the rest were used for parts. The first four aircraft reportedly had the razorback canopy of the P-51B/C. :* CA-18 Mustang Mk 21, Australian-built variant with V-1650-7. 26 built. :* CA-18 Mustang Mk 22, Australian-built photo reconnaissance variant. 14 converted from Mk 21s and an additional 14 new aircraft built. :* CA-18 Mustang Mk 23, Australian-built variant with Merlin 66 or 70. 66 built. :*CA-21 Mustang Mk 24, planned two-seat trainer variant. Orders canceled. ;P-51K (NA-111) :A Dallas-built variation of the P-51D equipped with an Aeroproducts propeller in place of the Hamilton Standard propeller was designated P-51K; 1,500 of these were built. :*P-51H-1-NA (NA-126), initial production block with short tail of the P-51D. 20 built. :* P-51H-5-NA (NA-126), with tall tail (some H-1s were retrofitted with this tail). 280 built. :* P-51H-10-NA (NA-126), 255 built. ;P-51L (NA-129) :The Dallas-built version of the P-51H, the P-51L, was to utilize the V-1650-11 engine. Orders for 1,445 aircraft were canceled before any were built. which has a heavily modified airframe and cockpit, Learjet wings, and an F-86 tail. ;Trans-Florida (Cavalier) Executive Mustang :Conversions of P-51Ds into two-seat personal transports. :* Cavalier 750, with approx. range of 750 miles. :* Cavalier 1200, with approx. range of 1,200 miles. :* Cavalier 1500, with approx. range of 1,500 miles. :* Cavalier 2000, with approx. range of 2,000 miles. :* Cavalier 2500, with approx. range of 2,500 miles. ;Cavalier F-51D :Re-manufactured P-51Ds for export. ;Cavalier Mustang II :Version of the Cavalier F-51D for close air support and counter-insurgency operations. : ;Piper PA-48 Enforcer :Losing contender in the Light Armed Reconnaissance Aircraft (LARA) tri-service competition, won by the North American OV-10 Bronco. A highly modified Cavalier modification powered by a Rolls-Royce Dart turboprop engine, it was originally designated Cavalier Turbo Mustang III before the project was taken over by Piper Aircraft. == Dimensions and performance ==

The basic dimensions of the P-51/A-36 variants remained consistent from the XP-51 through to the experimental "lightweight" P-51s, although there were relatively minor variations in height and wing area. The P-51H was the first production version of the Mustang to feature a lengthened fuselage and taller tailfin. Power ratings 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. In the case of the V-1650, used from the P-51B on, two sets of power ratings can be quoted because these engines were fitted with two-stage, two-speed superchargers; the maximum power generated by the V-1650-3 was at the critical altitude of in low blower using of mercury (61" Hg) "boost". The "boost" is the pressure to which the air–fuel mixture is compressed before being fed through to the engine's cylinders (manifold pressure). Because air pressure and air density decrease with altitude, the efficiency of a piston engine drops because of the reduction in the amount of air that can be drawn into the engine; for example, the air density at is ⅓ of that at sea level, thus only ⅓ of the amount of air can be drawn into the cylinder and only ⅓ of the fuel can be burnt. In the case of the Mustang the air being forced through the supercharger air intake was first compressed by the supercharger's first stage, or low blower. The intercooler stopped the compressed mixture from becoming too hot and either igniting before reaching the cylinders or creating a condition known as knocking. Two-stage refers to the use of two impellors on a common driveshaft in effect constituting two superchargers in series; a Bendix-Stromberg pressure carburetor injected fuel at through a nozzle direct into the supercharger where the first-stage impellor compressed the air/fuel mixture. This was then fed to the smaller second-stage impellor which further compressed the mixture. The impellors were driven by a hydraulically operated two-speed gearbox. At low to medium altitudes, the supercharger was in 'low blower' (this referred to the speed at which the impellors were operating). Once the aircraft reached and climbed through the set critical altitude ( for the -3) the power would start to drop as the atmospheric pressure, hence the weight of air dropped. 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. This second stage blower required approximately to drive it. As a result, the maximum power generated by the V-1650-3 in 'high blower' was at , using 61" Hg. The V-1650-7 used in later P-51B/Cs and in the P-51D/K produced slightly more power but, because of the use of slightly different gear ratios for the impellers, the critical altitude ratings of the supercharger stages were lower, and , respectively, since experience showed most air-to-air combat over Europe was taking place between these altitudes. The power of the engine could also change according to the octane rating of the aviation fuel being used. Higher octane fuels allowed boost pressures to be increased without the risks of pre-ignition or knocking. Rather than use inches of mercury, the British measured boost pressure in psi. A reading of +6 meant the air/fuel mix was being compressed to 20.7 psi (6 psi more than one atmosphere) before entering the engine; +25 meant the air/fuel mix was being compressed to 39.7 psi. Reliability of performance figures When reading performance figures it should always be borne in mind that weight, the aerodynamic drag generated by different external fittings, the condition of the airframe and/or engine, and all sorts of other factors could influence how an aircraft performed. For example, the P-51's laminar flow wings needed to be kept as clean and smooth as possible; even relatively minor damage on the wing leading edges could drastically reduce top speed. The most accurate performance figures for the P-51 came from tests carried out at facilities such as the USAAF's Flight Test Engineering Branch, based at Wright Field near Dayton, Ohio and, for the RAF, the Aeroplane & Armament Experimental Establishment (A&AEE), based at Boscombe Down. North American Aviation carried out their own performance tests, as did the only other manufacturer of the P-51, the Commonwealth Aircraft Corporation (CAC) of Australia. ==See also==