P51 NORTH AMERICAN MUSTANG P-51

P51 NORTH AMERICAN MUSTANG P-51

P51 - Mustang

The P51 was designed as the NA-73 in 1940 at Britain's request. The design showed promise and AAF purchases of Allison-powered Mustangs began in 1941 primarily for photo recon and ground support use due to its limited high-altitude performance. But in 1942, tests of P-51s using the British Rolls-Royce "Merlin" engine revealed much improved speed and service ceiling, and in Dec. 1943, Merlin-powered P-51Bs first entered combat over Europe. Providing high-altitude escort to B-17s and B-24s, they scored heavily over German interceptors and by war's end, P51 s had destroyed 4,950 enemy aircraft in the air, more than any other fighter in Europe.

Mustangs served in nearly every combat zone, including the Pacific where they escorted B-29s to Japan from Iwo Jima. Between 1941-5, the AAF ordered 14,855 Mustangs (including A-36A dive bomber and F-6 photo recon versions), of which 7,956 were P-51Ds. During the Korean War, P-51Ds were used primarily for close support of ground forces until withdrawn from combat in 1953.

The P-51D on display was obtained from the West Virginia ANG in 1957 and was the last prop-driven USAF fighter assigned to a tactical unit. It is painted as the -D flown by Col. C.L. Sluder, CO of the 325th Fighter Group, 15th Air Force, in Italy in 1944. The name of this aircraft, Shimmy IV is derived from the names of Col. Sluder's daughter and wife; Sharon and Zimmy.

SPECIFICATIONS
Span: 37 ft. 0 in.
Length: 32 ft. 3 in.
Height: 13 ft. 8 in.
Weight: 12,100 lbs. max.
Armament: Six .50-cal. machine guns and ten 5 in. rockets or 2,000 lbs. of bombs.
Engine: Packard built Rolls-Royce "Merlin" V-1650 of 1,695 hp.
Cost: $54,000
Serial Number: 44-74936
Displayed as (S/N): 44-15174

PERFORMANCE
Maximum speed: 437 mph.
Cruising speed: 275 mph.
Range: 1,000 miles
Service Ceiling: 41,900 ft.

Mustang Mk.I/P-51/P-51A

It was quickly evident that performance, although exceptional up to 15,000 ft (4,600 m), was markedly reduced at higher altitudes. This deficiency was due largely to the single speed, single stage supercharger of the Allison V-1710 engine, where power diminished rapidly above the critical altitude rating. Prior to the Mustang project, the USAAC had Allison concentrate primarily on turbochargers in concert with General Electric; these proved to be exceptional in the P-38 Lightning and other high-altitude aircraft, in particular, the Air Corp's four-engine bombers. Most of the other uses for the Allison were for low-altitude designs, where a simpler supercharger would suffice. The turbocharger proved impractical in the Mustang, and it was forced to use the inadequate supercharger available. Still, 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 powerplant (the V-1710-39 producing 1,220 hp (910 kW) at 10,500 ft (3,200 m), driving a 10 ft 6 in (3.2 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]

The first production contract was awarded by the British for 320 NA-73 fighters, named Mustang Mk.I by the British (the name being selected by an anonymous member of the Purchasing Commission). Two aircraft of this lot delivered to the USAAC for evaluation were designated XP-51.[9] About 20 Mustang Mk.Is were delivered to the RAF, making their combat debut on 10 May 1942. With their long range and excellent low-level 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 15,000 ft (4,600 m).

A second British contract called for 300 more (NA-83) Mustang Mk.I fighters. In September 1940, 150 aircraft, designated NA-91 by North American, were ordered under the Lend/Lease program. These were designated by the USAAF as P-51 and initially named Apache, although this was soon dropped and the RAF name, Mustang, adopted instead. The British designated this model as Mustang Mk.IA. The Mustang Mk IA was identical to the Mustang Mk I except that the wing-mounted machine guns were removed and replaced with four long-barrelled 20 mm (.79 in) Hispano Mk II cannon.

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. Also, two aircraft of this lot were fitted with Packard-built Merlin engines.[10][11] These were identified as the Model NA-101 by North American and XP-78 by the USAAF, later redesignated XP-51B.

On 23 June 1942 a contract was placed for 1,200 P-51As (NA-99s), later reduced to 310 aircraft. The P-51A was the first version to be procured as a fighter by the USAAF, and used a new Allison V-1710-81 engine, a development of the -39, driving a 10 ft 9 in (3.3 m) diameter, three bladed Curtiss-Electric propeller. The armament was changed to four wing-mounted .50 in (12.7 mm) Browning machine guns, two in each wing, with a maximum of 350 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 moveable fitting of previous Mustang models and the fitting of wing racks able to carry either 75 gal (284 l) or 150 gal (568 l) drop tanks, increasing the maximum ferry range to 2,740 mi (4,410 km) with the 150 gal (568 l) tanks. The top speed was raised to 409 mph (658 km/h) at 10,000 ft (3,000 m). 50 aircraft were shipped to England, serving as Mustang Mk.IIs in the RAF.[12]


P-51B and P-51C
The Mustang Mk.X AM203

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 Tomahawk in Army Cooperation Command squadrons but the commanding officer was so impressed with its manoeuvrability and low-altitude speeds that he invited Ronnie Harker from Rolls Royce's Flight Test establishment to fly it. Rolls-Royce engineers rapidly realized that equipping the Mustang with a Merlin 61 engine with its two speed, two stage supercharger would substantially improve performance and started converting five aircraft as the Mustang Mk.X. Apart from the engine installation, which utilised custom built engine bearers designed by Rolls-Royce and a standard 10 ft 9 in (3.3 m) diameter, four-bladed Rotol propeller from a Spitfire Mk.IX [13], the Mustang Mk.X was a straight-forward adaptation of the Mustang 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 Mk Xs be handed over to Carl Spaatz for trials and evaluation by the U.S. 8th Air Force in Britain.[14]
P-51B in flight showing wing planform.
USAAF P-51B-10-NA

The high-altitude performance improvement was astonishing: the Mustang Mk.X (AM208) reached 433 mph (697 km/h) at 22,000 ft (6,700 m) and AL975 tested at an absolute ceiling of 40,600 ft (12,400 m).[15]

The XP-51B prototypes were a more thorough adaptation of the airframe, 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, 1,690 lb (770 kg) compared with the Allison V-1710's 1,335 lb (606 kg). 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 updraught induction system rather than the downdraught carburetor of the Allison. The new engine drove a four bladed 11 ft 2 in (3.4 m) diameter Hamilton Standard propeller which featured cuffs of hard molded rubber. A new radiator, supercharger intercooler and oil radiator installation in a new fuselage duct was designed to cater for the increased cooling requirements of the Merlin. Also, because of the choice of a four-bladed propeller, the fuselage-mounted synchronized machine gun armament was permamently deleted, due to the near-impossibility of avoiding hits to the propeller blades.

It was decided that the armament of the new, P-51B (NA 102) would be the four .50 in (12.7 mm) M2/AN Browning machine guns (with 350 rpg for the inboard guns and 280 rpg for the outboard) of the P-51A and the bomb rack/external drop tank installation (adapted from the A-36) would also be used; the racks were rated to be able to carry up to 500 lb (230 kg) of ordnance and were also capable of carrying drop tanks. The weapons were aimed using an N-3B optical gunsight 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 later Ds.[16]
N3B gunsight with A-1 head assembly (in this case mounted in a PBJ-1H.)

The first XP-51Bs started test flying in December 1942.[17] After sustained lobbying at the highest level, American production was started in early 1943 with the B (NA-102) being manufactured at Inglewood, California, and the C (NA-103) at a new plant in Dallas, Texas, which was in operation by summer 1943.[18] The RAF named these models Mustang Mk.III. In performance tests, the P-51B reached 441 mph/709.70 km/h (exactly ? supersonic speed at altitude) at 25,000 ft (7.600 m)[19] and the subsequent extended range made possible by the use of drop tanks enabled the Merlin-powered Mustang to be introduced as a bomber escort.

The range would be further increased with the introduction of an 85 gal (322 l) self-sealing fuel tank aft of the pilot's seat, starting with the B-5-NA series. When this tank was full the center of gravity of the Mustang was moved dangerously close to the aft limit, as a result of which maneuvers were restricted until the tank was down to about 25 gal (95 l) and the external tanks had been dropped. Problems with high-speed "porpoising" of the P-51Bs and Cs 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.[20]

Despite these modifications the P-51 Bs and Cs and the newer Ds and Ks 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. Several crash reports tell of P-51Bs and Cs crashing because horizontal stabilizers were torn off during maneuvering. As a result of these problems a modification kit consisting of a dorsal fin was manufactured. One report stated:

"Unless a dorsal fin is installed on the P-51B, P-51C and P-51D airplanes, a snap roll may result when attempting a slow roll. The horizontal stabilizer will not withstand the effects of a snap roll. To prevent recurrence, the stabilizer should be reinforced in accordance with T.O. 01-60J-18 dated 8 April 1944 and a dorsal fin should be installed. Dorsal fin kits are being made available to overseas activities"

These kits became available in August 1944 and were fitted to Bs and Cs and to Ds and Ks. 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.[21]

P-51Bs and Cs started to arrive in England in August and October 1943. The P-51B/C versions were sent to 15 fighter groups that were part of the 8th and 9th Air Forces in England, and the 12th and 15th in Italy (the southern part of Italy was under Allied control by late 1943). Other deployments included the China Burma India Theater (CBI).

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.

One of the few remaining complaints with the Merlin-powered aircraft was a poor rearward view. This was a common problem in most fighter designs of the era, which had only been recognized by the British after the Battle of Britain proved the value of an all-around view. In order to improve the view from the Mustang at least partially, the British had field-modified some Mustangs with fishbowl-shaped sliding canopies called "Malcolm hoods" - much like those on Spitfires. Eventually all Mk IIIs, along with some American P-51B/Cs, were equipped with Malcolm Hoods.

A better solution to the problem was the "teardrop" or "bubble" canopy. Originally developed as part of the Miles M.20 project, these newer canopies were in the process of being adapted to most British designs, eventually appearing on Typhoons, Tempests and later-built Spitfires. North American adapted several NA-106 prototypes with a bubble canopy, cutting away the decking behind the cockpit, resulting in substantially improved vision to the rear. This led to the production P-51D (NA-109), considered the definitive Mustang.

A common misconception is that the cutting down of the rear fuselage to mount the bubble canopy reduced stability requiring the addition of a dorsal fin to the forward base of the vertical tail. In fact, as described, stability problems affected the earlier Bs and Cs, as well as the subsequent D/K models; this was partly attributable to the 85 gal (322 l) fuselage fuel tank which had been installed during production of the P-51B-5-NA and caused a too-far-aft center of gravity situation when filled. Other factors were the switch from the three blade propeller of the Allison powered series to the four blade propeller causing increased destabilization due to the four-bladed propeller's greater side area effect and, on the D and K the bubble canopy causing some turbulence ahead of the fin.[22]

Among other modifications, armament was increased with the addition of two more M2 machine guns, bringing the total to six. The inner pair of machine guns had 400 rpg, and the others had 270 rpg, for a total of 1,880. In previous P-51s, the M2s were mounted at an extreme side angle to allow access to the feed chutes from the ammunition trays. This angled mounting had caused problems of congestion and jamming of the ammunition and spent casings and links, leading to frequent complaints of jamming during combat maneuvers.[23] The new arrangement allowed the M2s to be mounted upright, remedying most of the jamming problems. The .50 in (12.7 mm) Browning machine guns, although not firing an explosive projectile, had excellent ballistics and proved adequate against the Focke-Wulf Fw 190 and Messerschmitt Bf 109 fighters that were the main USAAF opponents at the time. The wing racks fitted to the P-51D/K series were strengthened and were able to carry up to 1,000 lb (450 kg) of ordnance. Later models had under-wing rocket pylons added to carry up to ten rockets per plane.

The gunsight was changed from the N-3B to the N-9[24] before the introduction in September 1944 of the K-14B gyro-computing sight.[25]

Alterations to the undercarriage up-locks and inner-door retracting mechanisms meant that there was a change to the shape of the inner wing leading edge, which was raked forward slightly, increasing the wing area and creating a distinctive "kink" in the leading edges of the wings.

The P51-D 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 Aeroproducts propeller in place of the Hamilton Standard propeller, as well as a larger, differently configured canopy and other minor alterations (the vent panel was different). The hollow-bladed Aeroproducts propeller was unreliable with dangerous vibrations at full throttle due to manufacturing problems 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 photo reconnaissance versions of the P-51D and P-51K were designated F-6D and F-6K respectively. The RAF assigned the name Mustang Mk.IV to the D model and Mustang Mk.IVA to K models.

The P-51D/K started arriving in Europe in mid-1944 and quickly became the primary USAAF fighter in the theater. It was produced in larger numbers than any other Mustang variant. Nevertheless, by the end of the war, roughly half of all operational Mustangs were still B or C models.

Concern over the USAAF's inability to escort B-29s all the way to mainland Japan resulted in the highly classified "Seahorse" project, an effort to "navalize" the aircraft. In late 1944, naval aviator (and later test pilot) Lieutenant Bob Elder flew carrier suitability trials with a modified P-51D. 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.

During 1945–48, P-51Ds were also built under licence in Australia by the Commonwealth Aircraft Corporation.


X-P5 1F, X-P51 G and X-P51-J

The USAAF required airframes built to their acceleration standard of 8.33 g (82 m/s?), a higher load factor than that used by the British standard of 5.33 g (52 m/s?) for their fighters. Reducing the load factor to 5.33 would allow weight to be removed, and both the USAAF and the RAF were interested in the potential performance boost.

A subtle change made in the lightweight Mustangs was the use of an improved NACA 66 series airfoil and a slightly thinner wing than that used by earlier Mustangs.[29]

In 1943, North American submitted a proposal to re-design 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, and a larger canopy. The designation XP-51F was assigned to prototypes powered with V-1650 engines (a small number of XP-51Fs were passed to the British as the Mustang V) and XP-51G to those with reverse lend/lease Merlin RM 14 SM engines.
A third lightweight prototype powered by an Allison V-1710-119 engine 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.

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 greatly reduced the tendency to yaw. The canopy resembled the P-51D style, over a somewhat raised pilot's position. Service access to the guns and ammunition was also improved. With the new airframe several hundred pounds lighter, the extra power and a more streamlined radiator, the P-51H was among the fastest propeller fighters ever, able to reach 487 mph (784 km/h or Mach 0.74) at 25,000 ft (7,600 m).

The P-51H was designed to complement the P-47N as the primary aircraft for the invasion of Japan with 2,000 ordered to be manufactured at Inglewood. Production was just ramping up with 555 delivered when the war ended. Production serial numbers:

* P-51H-1-NA 44-64160 – 44-64179
* P-51H-5-NA 44-64180 – 44-64459
* P-51H-10-NA 44-64460 – 44-64714

Additional orders, already on the books, were cancelled. 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 2,270 hp (1,690 kW) V-1650-11 engine, which was never built; and its Dallas-built version, the P-51M or NA-124 which utilized 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 45-11743.

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 44-64192 was designated BuNo 09064 and used by the U.S. Navy to test transonic airfoil designs, 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 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.

(credits: US Air Force History Support Office)