North American P-51D Mustang

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North American P-51D Mustang

With the threat of war in Europe growing day by day at the end of the 1930s, British manufacturers found that they could not keep up with the demands of a rapidly expanding air force so attention turned to the United States as a source of additional aircraft. After assessing the available fighters, the New York based British Purchasing Commission ordered substantial numbers of the Curtiss P-40 in 1939. The US Army Air Corps had already placed a large order for the P-40 and Curtiss was unable to produce the number of aircraft that the Royal Air Force required so the British decided to look for a manufacturer to licence build it for them. Attention turned to the North American Aviation Corporation. North American was already building NA-16 Harvard trainers for the RAF but was not keen on the idea of building P-40s. Earlier studies had indicated that the company could design and build an entirely new aircraft that would be better than the P-40 while still using the same Allison V-1710 engine, all in the same time it would have taken to set up a P-40 production line. James "Dutch" Kindelberger, the president of North American, submitted the proposal to the British in January 1940 and on April 10 of the same year, approval was granted to design and build a prototype on the condition that it had to be ready in 120 days. The design was assigned the company project name of Model NA-73X. At that time, the USAAC reserved the right to block any foreign aircraft sales that it regarded as "not in the army's interest". On May 4, 1940, they agreed not to block the British sale on the condition that two examples of the new aircraft were transferred (free of charge) to them for testing and evaluation.

The prototype contract was signed on May 23, 1940 followed six days later by a provisional RAF procurement order (dependant on satisfactory testing of the prototype) for 320 production aircraft, the Mustang I. Final assembly and engine installation began on September 9, 1940, only 117 days after the initial British order. The speed of the design and construction of the NA-73X was mainly due to the fact that, as mentioned previously, North American had already spent almost a year doing initial design work during their fighter project studies. In a contract approved on September 20, 1940, it was agreed that the fourth and tenth production Mustang Is would be the aircraft transferred to the USAAC and receive the designation XP-51. Meanwhile, the RAF increased their Mustang I order to 620 aircraft on September 24, 1940 and then in December of the same year they increased the order by a further 300. On October 26, 1940, test pilot Vance Breese flew the prototype for the first time and in spite of having the same engine, it was 25 mph faster than the P-40. Further testing, modifications and fine tuning were carried out before the first production Mustang I took to the air on May 1, 1941. This aircraft remained at North American Aviation for technical evaluation with deliveries to Britain beginning in November 1941.

Special attention was paid to features that would make the Mustang I simple and inexpensive to manufacture. It featured all-metal stressed-skin construction, with the wings having a sheet-web main spar and a rear spar that was almost as strong to carry the ailerons and the flaps. The two wing spars were far enough apart to accommodate the length of a 0.5 in machine gun, leaving only the barrel protruding ahead of the main spar. Fuel was housed in two self-sealing tanks, one in each inboard section of the wing. Total capacity was 180 US gallons, almost twice that of a Spitfire. The main gear retracted inward into wheel wells in the wing and had a track of almost 12 feet, which made landing the Mustang easier than the narrow track Spitfire and Bf 109. When retracted, the wheels were covered by doors hinged near the aircraft centreline that closed again when the landing gear was fully extended. The tail wheel was steerable (linked to the rudder) and was fully retractable into a compartment with twin doors.

One feature that made the Mustang unique among most of its contemporaries was its use of a laminar flow wing. This was an aerofoil that was made as smooth as possible and had a thickness that kept on increasing far beyond the usual location. (50 percent chord rather than the usual 20 percent). In addition, the wing had very little camber with the underside being almost a mirror image of the upper. In theory, this wing was much more "slippery" than the old profiles, providing less aerodynamic drag at high speeds. However, it provided less lift at low speeds so large and powerful slotted flaps had to be fitted to keep landing speeds from being impractically high. While the laminar flow principles may have worked well during carefully controlled tests, in the real world many of the properties were lost. Simple things like manufacturing tolerances would cause the smooth laminar layer next to the wing surface to jump over to a turbulent, high drag condition. Throw in scratches, dents and general wear and tear from everyday use and the wind tunnel theories quickly went out the window. Having said this, it was still far more efficient than conventional wings at very high speeds, especially in terms of aileron effectiveness.

The British specified that the aircraft's powerplant had to be a liquid-cooled inline engine so the 1,150 hp Allison V-1710-F3R, as originally planned for the prototype, was fitted to the Mustang I. Unfortunately, due to the lack of a turbocharger or supercharger, the Allison suffered from a rapid drop off in power at higher altitudes. Radiators for the ethylene glycol engine coolant and lubricating oil were grouped together in a distinctive, streamlined duct underneath the rear fuselage. Due to it's plenum chamber design (the duct shape moved from narrow to wide before narrowing again aft of the radiators in an automatic, hinged exhaust port) the duct had the ability to add some propulsive thrust by heating the incoming air and expelling it out the back at a higher velocity. This cooling arrangement did have a few drawbacks, namely extra weight and (more importantly) the added combat vulnerability of the long pipes that led to and from the engine. Armament was heavier than American standards of the day. Two 0.5 in Browning M2 machine guns were installed in the underside of the nose beside the engine crankcase and synchronised to fire through the propeller arc. A further two 0.5 in guns were mounted in the wings outboard of the main gear and four 0.3 in Browning machine guns were mounted further outboard on the wing.

In January 1942, Mustang Is entered service with No. 26 Squadron, RAF Army Co-operation Command where they were fitted with an F24 oblique camera behind the pilot's head armour and operated in the low-level reconnaissance role. While the first British order had been a straight sale, the costs involved in fighting a war started to take their toll on the funds available to purchase more aircraft. Consequently, the US government instigated the Lend-Lease scheme and on July 7, 1941, the US Army Air Force (renamed from the USAAC in the previous month) ordered 150 Mustangs on behalf of the RAF. Armament was changed to four wing mounted 20 mm Hispano Suiza cannons and the aircraft entered service as the Mustang IA. Only 93 of the Mustang IAs ordered for the RAF were delivered with the remainder (apart from two used in the XP-78 project) staying with the USAAF where they were designated the P-51. The majority of the USAAF P-51s had cameras fitted and were used in the tactical reconnaissance role under the designation F-6A.

Following the Japanese attack on Pearl Harbour in December 1941 and America's entry into World War Two, military production in that country was stepped up phenomenally. Consequently, by the time the USAAF decided that they wanted the Mustang, there was no money left in the fighter budget. There was, however, still money available for attack aircraft so the A-36A, a dive bomber variant of the P-51 was developed. This was essentially just a way of getting the P-51 into USAAF service, as they had no real need for a dive bomber. Despite this, an order for 500 A-36As was placed on April 16, 1942. The A-36A was essentially a Mustang I with the 0.3 in guns removed, underwing racks to take a single 500 lb bomb under each wing added and hydraulic dive brakes installed above and below the wings. The V-1710-F3R engine was changed to a 1,325 hp Allison V-1710-87. Just over 2 months later, 310 P-51As were ordered. These were fitted with a 1,200 hp Allison V-1710-81 engine and had four wing mounted 0.5 in machine guns, the nose mounted 0.5 in and wing mounted 0.3 in guns having been removed. This model was also used in the photo-reconnaissance role when 35 were fitted with cameras and became the F-6B. Another 50 were supplied to the RAF as the Mustang II.

The Mustang (the name being adopted by the USAAF around mid 1942) soon proved to have one major weakness, its Allison engine. As mentioned previously, the Allison's poor high altitude performance and unsatisfactory rate of climb meant that the aircraft were relegated to the low-level ground attack and reconnaissance role. On April 30, 1942 Ronald W. Harker, a test pilot for Rolls Royce, took a brief flight in a Duxford based RAF Mustang and suggested that it would be a natural choice for the new 2-stage supercharged Merlin 60 series of engines that Rolls Royce were just beginning to produce. Intrigued by the idea, Rolls Royce asked to borrow three Mustangs from the RAF so that they could experiment with fitting and testing various Merlin engines. The conversion was authorised on August 12, 1942 with 3 Mustang Is being allocated to the program followed, later, by another two. Powered by a 1,635 hp Merlin 65, the first of these aircraft flew on October 13, 1942 as the Mustang X. The standard production model's 3-blade propeller was replaced with a 4-blade one to absorb the extra power of the Merlin and the supercharger intercooler radiator and air intake for the carburettor were mounted together under the nose (the carburettor intake on Allison powered Mustangs was mounted on top of the nose). The change in the performance of the Merlin powered aircraft was amazing; the power drop off problem of the Allison engine Mustangs was cured with a 490 hp increase in power at 25,000 feet. During testing at Boscombe Down, it reached a level speed of 433 mph and took 6.3 minutes to climb to 20,000 feet, about two thirds of the time taken by production models. Torque from the more powerful engine and new propeller did have a detrimental affect on the directional stability of the Mustang X, this was soon cured by the addition of a small dorsal fin just in front of the tail.

Rolls Royce proposed the production of 500 Merlin 65 engines to be fitted to most of the RAF's Mustang fleet, bringing them up to Mark X standard. Unfortunately, because of wartime pressures, there was nowhere available in Britain to do the conversions so the idea was scrapped. However, North American Aviation, who had been had been fully briefed by Rolls Royce on the Mustang X project, had started planning their own Merlin powered Mustangs. On July 25, 1942, they received authorisation to install Merlin 65 engines imported from England into the two P-51s that had not gone to the RAF or USAAF. The aircraft initially received the designation XP-78 but this was later changed to the XP-51B. Unlike the situation in Britain, a ready supply of Rolls Royce engines was available from the Packard Motor Car Company of Detroit, Michigan who had signed a deal to licence build the Merlin in the United States. North American now had source of engines for the new XP-51B and based partly on this, as well as on estimated performance, an order for 400 P-51B Mustangs was placed in August 1942, before the first prototype had even flown.

The first XP-51B, powered by a 1,590 hp Packard Merlin V-1650-7, flew on November 30, 1942 and the increase in performance over earlier Mustangs was even greater than that achieved with the Mustang X. It achieved a level speed of 441 mph at 29,800 feet, over 100 mph faster than the Allison powered P-51 at the same altitude and at all heights, the rate of climb was almost doubled. Design changes included moving the carburettor air intake to below the nose to accommodate the new engine's updraft induction system and the addition of an intercooler radiator located in an enlarged under-fuselage duct that also housed improved coolant and oil radiators. The fuselage was strengthened to cope with higher aerodynamic loads and a new 4-blade Hamilton Standard hydromatic propeller was fitted to absorb the extra power at high altitude. Underwing racks were installed capable of carrying a 1000 lb bomb (or their equivalent weight in drop tanks) each and the four wing mounted 20 mm cannons were replaced with 0.5 in machine guns. The first production aircraft, the P-51B, flew on May 5, 1943 and the USAAF fell in love with it, promptly ordering a further 2,200 of them. The RAF also ordered it as the Mustang III and this sudden influx of orders resulted in mass production beginning at North American's Inglewood, California plant as well as at a new plant in Dallas, Texas. The Mustangs produced at the Dallas plant were designated P-51Cs, the only difference between them and the P-51B being the location they were manufactured. As well as still being an outstanding ground attack aircraft, the Mustang was now able to compete on equal (or better, depending on who you talk to) terms at all altitudes with the Focke-Wulf Fw-190 and later models of the Messerschmitt Bf-109.

By mid 1943, it was becoming clear that American heavy bombers of the 8th Air Force could not protect themselves adequately on daylight raids over Germany and were in desperate need of fighter escort. With its already exceptional range, the Mustang was a prime candidate for this role. The P-51B and C had an 85 US gallon self-sealing fuel tank installed behind the pilot's seat, bringing the total fuel capacity (including two drop tanks) to 419 US gallons. When full, this new fuel tank moved the aircraft's centre of gravity aft, which made the directional stability of the Mustang quite poor. As a result, it was normally the first tank to be used and the pilot would have to spend the first hour or so of flight concentrating on keeping his aircraft pointed in the right direction until it was empty. With the extra fuel, the Mustang was now able to accompany Boeing B-17 Flying Fortress and Consolidated B-24 Liberator bombers on the 1,100 mile round trip to Berlin. This came as a nasty shock to the German defenders. As reports of the first dogfight over German soil reached Hitler's deputy and head of the Luftwaffe, Reichsmarshall Hermann Goering, he refused to believe them. He was adamant that no Allied fighter had the range to fly so far into Germany. When he was finally convinced this was true, he was reported to have muttered, "We have lost the war."

One of the features of previous versions that had remained unchanged in the P-51B was the poor visibility from the 'birdcage' cockpit canopy. In an attempt to improve visibility, the British 'Malcolm' sliding hood (as used on the Spitfire) was fitted as a field modification to RAF Mustang IIIs as well as to many USAAF P-51Bs and Cs. This improved the pilot's field of view, however a more permanent solution was still sought. In early 1943, Colonel Mark Bradley of the USAAF was visiting England and saw the newly invented 'bubble' (or 'teardrop') canopy that gave Spitfire and Typhoon pilots unobstructed 360-degree vision. On his return to the United States, he convinced James Kindelberger that such a canopy would be ideal for the Mustang. Kindelberger agreed and North American reached an agreement with the USAAF to test a 'bubble' canopy on a Mustang, with a P-51B selected to be the test aircraft. To accommodate the new canopy, the rear fuselage of the Mustang, now designated the XP-51D, had to be cut down extensively. This led to a major loss of directional stability, bordering on unacceptable when the fuselage fuel tank was full. To correct the problem, a small dorsal fin was added ahead of the rudder. Armament changes were also made to the XP-51D with the addition of two more wing mounted 0.5 in machine guns and the ability to carry either bombs or rockets under the wings. A few other minor structural modifications were made, mainly to compensate for extra weight, and the production version of the XP-51D became the P-51D in USAAF service and the Mustang IV with the RAF. This was the most well known version of the Mustang and was also the most widely used model with a total of 8,102 P-51Ds being produced out of a grand total of 15,586 Mustangs of all versions built (including 200 assembled by Commonwealth Aircraft in Australia). Another variant, the P-51K, was built at North American's Dallas plant. This differed from the P-51D only by the use of an 11 ft diameter Aeroproducts propeller instead of the 11 ft, 2 in diameter Hamilton Standard propeller. This variant became the Mustang IVA in RAF service.

Large numbers of P-51Ds began arriving in Europe from March 1944 and it was one of the few allied fighters to shoot down the Messerschmitt Me-262 jet fighter/bombers. With a speed difference of almost 100 mph, the Mustang was unable to compete on equal terms in the air. It easily out turned the jet but usually only got the chance for one quick burst of fire before the jet made its getaway. The usual tactic was for the Mustangs to circle high above known Me-262 bases, pouncing on the jets as they took off or landed, or to strafe the base and try to destroy them on the ground. P-51Ds were also deployed to the Pacific theatre, but this did not start until late in 1944 as the situation in Europe "took priority". By this time, Japanese air opposition was very scarce so the Mustangs spent most of their time performing close support ground attack work, including the first land-based fighter strikes against Tokyo. By the time the war ended, Mustangs had claimed 4,950 enemy aircraft destroyed in combat and 4,131 on the ground in the course of 213,873 missions in Europe alone. One reason given for the high number of victories in the air was the P-51D and Ks use of the K-14 computing gyro gunsight. Based on a British design (Ferranti), all the pilot had to do was select the wingspan of the aircraft he was attacking, turn a handgrip on the throttle to enter the range then line up the wingtips of his target with a ring projected on the gunsight and open fire.

Following the end of the Second World War, large numbers of Mustangs remained in service with the USAAF. The re-formation of the Air National Guard (ANG) in May 1946 saw many Mustangs previously withdrawn from regular service sent to equip ANG fighter units. In 1948 the US Army Air Force became the US Air Force and replaced the P (pursuit) designation with F (fighter) on all of its fighters, consequently the P-51D became the F-51D. With the start of the Korean War on June 25, 1950, the Mustang was once again rushed into active service in the ground attack role. The Israeli Air Force also used the F-51D during the 1956 Arab-Israeli conflict and it was used by Indonesia in an unsuccessful attempt to invade Dutch New Guinea in 1962. The last American active duty Mustang (F-51D-30-NA Ser No 44-74936) was retired from service in 1957 although several other countries continued to use them as first line equipment well into the 1970s. Even though production of the Mustang ended in 1946 with the P-51H, a lightweight, more powerful and faster variant that arrived too late to see service in World War Two, the design was revived and put back into production in the late 1960s and 1970s as the turboprop powered Piper PA-48 Enforcer. 

New Zealand's involvement with the Mustang began in August - September 1945 when the RNZAF received the first 30 P-51Ds of what was to have been a total order of 370 intended to replace the F-4U Corsair. However, by the time they had finally arrived, the war was over and all 30 aircraft were promptly mothballed with the remainder of the order being cancelled. They were reactivated in 1951 and served with four Territorial Air Force squadrons until an 'alleged' undercarriage fault caused them to be removed from service in 1955 (although four remained at Ohakea with No. 42 Squadron as drogue tugs until early 1957). Finally, all surviving RNZAF Mustangs (with the exception of 3) were sold as scrap in 1958. 

The aircraft shown here ("Rudolph the Red Nose Reindeer") is based at Ardmore and is painted in the colour scheme of NZ2415 from No. 3 Squadron (TAF), flown in the early 1950's by Squadron Leader Ray Archibald, the squadron's CO. It was originally built in 1944 at Inglewood (P-51D-30-NA, Ser no  44-74829, c/n 122-41369) and served with the Royal Canadian Airforce. It was imported into New Zealand in 1984 by Sir Tim Wallis and first flew here in January, 1985. Tim Wallis subsequently sold the aircraft and it was operated by a trust set up to keep it in the country, before passing to an Ardmore based syndicate.

This syndicate disbanded and NZ2415 has been owned by Christchurch based Graham Bethell. Apart from occasional sorties south NZ2415 operates from Ardmore and provides many ‘thrill –seeker’   rides .

Text © 2002 Stuart Russell. 2014 NZ Warbirds

 

Specs. 

Power plant: One 1,133kW (1,520hp) Packard V-1650-7 

Wingspan: 11.29m (37ft 1/2in) 

Length: 9.81m (32ft 2 1/2in) 

Max T-O weight: 5,206kg (11,600lb) 

Max speed: 703km/h (437mph) 

Range: 1,529 - 3,347km (950 - 2,080mile

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