Profile – North American P-51 Mustang

The worst fears of the British were realised on September 3, 1939, when war was declared on Germany after that country’s invasion of Poland and suddenly the Royal Air Force found itself in desperate need of more combat aircraft and once again turned to the USA as a source of supply. Ostensibly continuing their policy of strict neutrality, the Americans placed an embargo on the export of military equipment in 1939, but this was easily circumvented as Congress had left a loophole for the benefit of the British (and French) as a demonstration of goodwill. The loophole provided for the export of equipment provided it left American shores in the holds of ships owned by the purchasing country.

With this in mind, the RAF and French Armee Air in late 1939 ordered a total of 1740 Curtiss P-40 Hawks to be shared between the two services. Although suffering by being powered by the supercharged Allison V-1710 in-line liquid-cooled engine, the Hawk – then Just entering service with the United States Army Air Corps – was adjudged the best aircraft available, although deliveries to Europe could not be made until late 1940 at the earliest due to Curtiss’ USAAC commitments.

The British, therefore, decided to attempt to establish a P-40 production line with another, less committed manufacturer Remembering their favourable experiences with North American Aviation and the Harvard purchase, they put the idea of P-40 licence manufacturer to NAA’s president, James H (‘Dutch’) Kindelberger, an idea which did not overly impress him or his company’s board of directors, who responded with a proposal that a new and more capable fighter utilising the P-40’s Allison powerplant be developed, a project on which some preliminary design work had already been done.

This was January 1940, and following discussions with the British Direct Purchasing Commission’s Colonel William Cave and Air Commodore G B A Baker, North American set about completing the design under the direction of Kindelberger, vice president Leland Atwood and chief engineer Larry Waite.

Although North American had not previously designed a high speed fighter, the British DPC was sufficiently impressed by the company’s work to sign a letter of intent for 320 NA-73 aircraft on April 10, following this with a formal contract on May 29 worth over $15 million.

Some of the conditions of the contract were restrictive, the prototype having to meet all specification goals and be flying within eight months, the cost per aircraft should not exceed $50,000 and North American was required to obtain all current data on P-40 development from Curtiss as a hedge against NAA’s lack of experience with high speed fighter aircraft.

If the required timescale wasn’t already tight enough, the company took the decision to tighten it even further, setting itself a limit of 120 days from contract signing to roll-out, the same time required if the P-40 had been built under licence.

By working 16 hours a day, seven days a week, North American was in fact able to roll out the prototype NA-73X airframe (the manufacturer’s designation as the US military was not yet involved in the project) just 102 days after the contract was signed, although a delay in delivery of the 1150 hp Allison V-1710 engine (due to Allison’s commitments to Lockheed P-38, Bell P-39 and Curtiss P-40 production) caused a further 20 days to pass before the complete aircraft emerged from the factory. Nevertheless, 122 days from contract signing to roll out was a remarkable performance and well within the contract’s stipulations.

Advanced Design

Underneath the shiny aluminium skin of the new fighter – which bore the civil registration NX19998 – was a detail design which featured several innovations.

The outstanding feature was the aircraft’s laminar flow wing, a concept developed by the National Advisory Committee for Aeronautics (NACA) and involving moving the thickest section of the aerofoil aft to a point near centre chord. This provided a long sloping incline on the upper wing surface which gave an increase in the area of even airflow before breaking up at the thickened point. The theory was that this made for reduced drag and resultant better overall performance and longer range.

Many of the aircraft’s other design features were also intended to reduce drag and increase performance; the tightly cowled engine, the low-set canopy, the minimal fuselage cross section, the ventral radiator air scoop and the use of secondary curves in the design of the aeroplane instead of the more usual straight angles (which were easier to manufacture) and single curvatures. ‘Conic geometry’ was the term for this kind of design, the compound and non-uniform curvatures providing smooth airflow over the NA-73’s surfaces.

Construction was all metal with a semi-monocoque fuselage and two-spar stressed skin wing. All control surfaces were metal covered. The prototype had provision for two 0. 50 calibre Browning MG53-2 machine guns in the lower engine cowling with 400 rounds per gun, a single example of this weapon in each wing close to the propeller disc and a further pair of 0. 30 Browning MG40s in each wing with 500rpg. Internal fuel capacity was 180 US gallons (150 Imperial gallons) divided between a self-sealing tank in each wing. The Allison engine drove a three-bladed Curtiss constant-speed propeller of 10 ft 6 in (3. 20 m) diameter.

The NA-73X made its first flight from Inglewood on October 26, 1940 in the hands of freelance test pilot Vance Breeze. Breeze flew the aircraft three more times during the following month before handing it over to NAA’s own Paul Balfour. However, Balfour’s first flight in the aircraft, on November 20, ended in disaster. The NA-73X’s engine died while engaged in airspeed calibration tests and while attempting a dead stick landing in a ploughed field short of the runway it flipped over onto its back. Fuel starvation was the cause of the engine stopping, the result of Balfour failing to select the correct fuel tank.

The NA-73 was eventually repaired to fly again, Bob Chilton taking over the testing duties from the hapless Balfour.

Despite the accident, the NA-73’s potential had been proven. . . . it handled superbly and was 25 mph faster than the similarly-powered Curtiss P-40 it had been designed to replace. On the strength of this, a further 300 were ordered for the Royal Air Force (under the manufacturer’s designation NA-83) and the name ‘Mustang’ bestowed upon the aircraft by the British.

By now the belated interest of the USAAC had also been aroused and they requested a pair of production Mustang 1 s be allocated from the initial production batch for evaluation. Given the designation XP-51 and initially called ‘Apache’ by the American service, these aircraft were the first of many thousands of examples of North American Aviation’s latest product to reach the United States Army Air Force, as the service was renamed in June 1941. The USAAF also adopted the ‘Mustang’ appellation when the P-51 entered service in quantity.

The first production Mustang I (AG345) was flown in April 1941 and handed over to the British a few weeks later, although the first example to reach the RAF (AG346) didn’t arrive in Britain until late October. Following trials which confirmed the flying qualities and low-level performance (top speed 384mph at 13,000 feet) of the aircraft – and the fact that performance deteriorated badly above that height – number 26 Squadron RAF based at Gatwick began to receive the first service Mustangs from January 1942.

The lack of altitude performance restricted the Allison powered Mustang to low-level Army Cooperation, strafing and armed photo-reconnaissance duties in RAF service, roles at which it excelled due to its high speed at low altitudes, heavy eight gun armament and outstanding flying qualities. In air combat at low to medium altitudes, the aircraft proved to be a match for just about anything, including the Focke-Wulf Fw190. The Mustang’s first operational sortie occurred on May 10, 1942, when a camera-equipped 26 Squadron aircraft (AG418 flown by F/O G Dawson) undertook a reconnaissance mission

over the French coast, the pilot strafing some targets of opportunity while he was at it.

This was to be the Mustang’s forte in its early Allison-powered versions, and soon some 14 RAF Army Cooperation squadrons were flying ‘Rhubarb’ strafing and ‘Ranger’ reconnaissance sorties over France.

A Mustang also became the first single-engined RAF aircraft to fly over Germany when in October 1942 a reconnaissance was made over the Dortmund-Ems canal.

Mustangs Many and Varied

The first Mustangs to carry the official United States P-51 designation were the batch of 150 Mustang IAs ordered by the RAF for delivery from 1942. These differed from the initial 620 aircraft only in their armament installation, four wing mounted 20mm cannon replacing the eight wing and engine cowling machine guns which had been fitted before.

The reason for the adoption of the P-51 designation came about due to the introduction of the Lend-Lease Act of March 1941. In an effort to continue the illusion of its neutrality, the USA had ceased selling military equipment to foreign powers but instead drew up an agreement that allowed the equipment to be leased out. It was a thinly disguised way of helping Britain, in effect, but required the equipment – in this case aircraft – to be first ordered by an American Agency of Military Service and therefore given a US serial number and designation before being transferred to the customer.

At this still pre-Pearl Harbour stage the American military could find little use for the Mustang as it was already committed to the Lockheed P-38, Republic P-47, Curtiss P-40 and Bell P-39 by late 1941. But the events of December 7, 1941 saw the USA embroiled in a global conflict whether it liked it or not, and the USAAF immediately decided to take a further look at the two evaluation XP-51s they still had on strength from the first British order.

The result was the diversion of 57 of the RAF’s Mustang IA order to the USAAF, which fitted them with two F. 24 oblique cameras in the rear fuselage and assigned them. to photoreconnaissance units in the USA under the designation F-6A, although as the aircraft retained their cannon armament, they were still often officially referred to as P-51s.

Following the perceived success of the Junkers Ju87 ‘Stuka’ in Europe and noting the stability and performance of the P-51 in the dive, NAA proposed a dive bomber variant of the Mustang to the USAAF early in 1942 and received an order for 500 such aircraft under the designation A-36A and unofficially named ‘Invader’.

Utilising the basic P-51 airframe and the 1325hp Allison V1710-87 engine, the A-36A featured a structure strengthened to cope with the stresses of dive bombing, racks to carry a 500Ib bomb under each wing, hydraulically operated dive brakes in each wing and a change of gun armament to what would later be the definitive six wing-mounted Browning MG53 0. 50 calibre machine guns.

It’s probably ironic to see a fighter aircraft being ‘downgraded’ to a dive bomber (the days of which were numbered anyway), but it does indicate the USAAF’s continued lack of appreciation of the Mustang’s true potential.

Nevertheless, the 500 A-36As were delivered between September 1942 and March 1943 and served with some distinction in North Africa and the Mediterranean, despite its troublesome air brakes being locked shut most of the time and the operational penalties which resulted. Its main claim to fame was that it was the first version of the Mustang to see combat service with the USAAF.

The final Allison-engined version of the Mustang to appear was the P-51A, 31O of which were ordered by the USAAF under the Lend-Lease scheme for the RAF as the Mustang II. Only 50 were eventually delivered to Britain from late 1942, the remainder serving with the USAAF mainly in the China-Burma-India theatre on a mixed bag of ground attack, reconnaissance, patrol and interception roles.

The P-51A was powered by the improved 1200hp V1710-81 engine which gave the aircraft a top speed of 390mph at 20,000 feet. Fixed armament was reduced to four. 0. 50 inch machine guns (with a total of 1260 rounds of ammunition) although provision for a single 500Ib bomb under each wing was retained. Of importance was the incorporation of the plumbing necessary to support 75 or 150 (for ferry flights) US gallon (62. 5/125 Imperial gallon) underwing drop tanks, the first Mustang variant to have this feature, one which was later to play such a vital role in the war over Germany.

With standard internal tankage, the P-51A had a range of 750 miles (1207km); two 75 gallon external tanks increased this to 1250 miles (2011km) and on a ferry mission with two 150 gallon tanks the aircraft’s range exceeded 2000 miles (3218km). Field modifications also allowed the fitting of three rocket launch tubes under each wing – the Mustang’s versatility was finally being appreciated. •

Included in the P-51A’s production total of 31O were 35 aircraft fitted with two F. 24 cameras. These were designated F-6B.

Merlin Magic

The Allison-engined Mustang’s inadequacies at anything higher than medium altitudes might have relegated it to tactical reconnaissance or other low level roles for the remainder its career if people in both the USA and Britain had not considered the ‘what if’ idea of installing the Rolls-Royce Merlin engine (particularly in its two-stage, two-speed supercharged ’60 series’ form) in the Mustang’s undoubtedly fine and efficient airframe.

The Merlin was already being built under licence in the USA by the Packard company as the V-1650 for installation in some versions of the Lancaster and Mosquito; so why not the Mustang? It was obvious that no matter how wonderful the Mustang’s airframe was, to exploit its potential properly required much greater performance at altitude.

A proposal for re-engining the Mustang was put to the British Air Ministry in mid-1942, the idea was approved and four Mustang Is were delivered to Rolls-Royce for conversion. The first 1565hp Merlin 61-engined Mustang (designated Mustang X) made its first flight on October J 3, 1942, and the contrast was immediately apparent, the Merlin and its 11ft 4in (3. 175m) fourbladed Rotol propeller increasing the aircraft’s top speed to a startling 432 mph at 25,500 feet.

The conversion of two Mustangs to Packard-Merlin power in the United States occurred simultaneously, the first aircraft (with 1520hp V-1650-3 engine and 11ft 2in (3. 124m) diameter Hamilton°Standard four-bladed propeller) taking to the air seven weeks after the British prototype had flown. Initially designated XP-78, these aircraft were soon officially known as the XP-518. The American conversions differed from the British ones in several ways, including the deletion of the Mustang X’s rather bulbous nose intercooler intake and moving that into an enlarged ventral scoop which incorporated both it and the main radiator. The XP-518 recorded a top speed of 453mph at 29,000 feet.

After testing of the prototypes over the northern hemisphere winter of 1942/43 in both countries, the first production P-518 Mustangs were delivered in June 1943. These aircraft were very close to the XP-51B’s specification and included either four or six 0. 50 calibre machine gun$ with 1260 rounds in total, the addition of an 85 US gallon (71 Imperial gallon) fuselage tank behind the cockpit and upgraded radio equipment.

The first USAAF unit to receive the P-51B was the 354th Fighter Group at Greenham Common, England, in November. A part of the 9th Air Force, this unit immediately began escorting American bombers on their daylight raids against Germany, the role for which the Mustang is probably best remembered.

P-51B production totalled 1988 aircraft between June 1943 and February 1944 (of which some 274 went to the. RAF as the Mustang Ill), the first 1598 aircraft being powered by the V-1650-3 engine and the remainder by the slightly more powerful V-1650-7. The P-51C was identical to the ‘B’ except it was built at the North American plant at Dallas, Texas. Of the 1750 C-models built, 636 went to the RAF (also as Mustang Ills), many of them featuring the so-called ‘Malcolm Hood’, a sliding and bulged cockpit canopy that increased visibility by a substantial margin. Many USAAF Mustangs were also thus fitted. A photoreconnaissance conversion of the P-518/C was also produced, the F-6C, with the usual two F. 24 cameras. Ninety-one conversions were built.

The P-51D Mustang came about as a result of trying to permanently solve the aircraft’s visibility problem. The solution was found in a sliding ‘teardrop’ canopy, which, in conjunction with cut down fuselage decking behind the pilot, gave superb all-around visibility.

The P-51D also featured the 1695hp (at War Emergency Rating) V-1650-7 Packard Merlin as standard equipment along with the increased internal fuel of the B and C models, standardisation on six 0. 50 calibre Browning MG53-2 machine guns (with ammunition increased to 1880 rounds), a strengthened main wing spar to cater for still more underwing ordnance, extended chord wing roots and on all but the very earliest examples, a fin leading edge fillet which compensated for the reduced fuselage side area caused by the cut-down decking.

P-51D production amounted to a massive 7956 machines between February 1944 and the end of the war. Of these 6502 were built at Inglewood and 1454 at Dallas. Included in the production total were 271 for the RAF (as the Mustang IV), ten aircraft converted to two-seat tandem TP-51D combat trainer configuration and 136 photo-recce F-6Ds.

Almost identical to late model P-51Ds was the P-51K. This version featured an Aeroproducts propeller of 11ft (3. 073m) diameter and came about due to a shortage of the usual HamiltonStandard unit. All 1500 P-51Ks were built at Dallas of which 594 went the RAF and 163 were completed as F-6Ks.

As always happens with aircraft, the Mustang’s continued development had caused it to put on weight. To counter this, several lightweight experimental models were flown in 1944 and 1945. The first was the P-51F, which compared to the P-51D featured a lightened structure, thinner wings, reduced size wheel struts and brakes, the removal of two machine guns and the fuselage fuel tank and the incorporation of a lightweight three-bladed Aeroproducts propeller. All this reduced the tare weight by some 1500Ib and increased the maximum speed by 25mph, but flight stability problems stopped the programme after only three prototypes had been built.

The P-51G and P-51J (two of each built) were similar but powered by a Merlin 14SM with five-bladed Rotol propeller and 1700hp Allison V-1710-119, respectively. The J also had some streamlining of the cowling incorporated and was nine inches longer than a standard Mustang.

The final Mustang production variant was the P-51H, which incorporated many of the weight-saving innovations of the experimental models along with the reversion to six guns, an extended dorsal fin, a 50 US gallon fuselage fuel tank and a 2250hp (emergency rating) V-1650-9 engine driving a four-bladed Aeroproducts propeller. Top speed of this variant was a remarkable 487mph at 25,000 feet, 50mph faster than the P-51D.

Deliveries began in February 1945, and 555 had been built by the time hostilities ended, at which time orders for a further 1845 were cancelled. Although entering service too late to see action in Europe, the P-51H did serve in the Pacific in the final three months of the war.

The P-51L was to be a Dallas-built version of the H with a direct fuel-injected V-1650-11 but none of the 1700 ordered had been built by the Armistice; the P-51M was identical to the H and only one out of 1628 on order had rolled out of the Dallas factory when peace came.

Next issue: the Mustang in Australian service and production and a look at the many post-war variants and activities of the aircraft.