North American P-51 Mustang
Design: At the beginning of 1940, with the British aircraft industry attempting to cope with the demands of home defence, it became obvious to the British Government that aircraft would have to be purchased from abroad. The British Direct Purchase Commission was established in the United States. The best American fighter in operational service at that time was the Curtiss Tomahawk (P-40). With the Curtiss production line fully occupied in meeting the demands of the US Army Air Corps the Purchasing Commission approached North American to build a variant of the P-40 under licence from Curtiss. North American already had a broad outline for a fighter and submitted this design to the commission who had begun to have reservations about the suitability of the P-40 for the war in Europe. The North American design was destined to become the P-51 ‘Mustang’.
The Mustang was fitted with what is often referred to as a laminar flow wing, a concept originally developed by A.C. Robinson at NACA (National Advisory Committee for Aeronautics). The wing essentially has a symmetrical aerofoil with the same curvature on its upper and lower surfaces with the leading edge being kept as thin as possible and the wing sections thickest point being placed as far to the rear as possible. This layout allows the boundary layer airflow to adhere to the wing as far aft from the leading edge as possible with the result of a smoother airflow over the wing. To achieve laminar flow the wing surface had to be highly finished. Tests, towards the end of and after the war, showed that the drag on the Mustang wing was no less than that of a conventional wing of the same thickness and taper ratio.
The P-51 used a radiator layout which employed what was known as the ‘Meredith’ effect. F.W. Meredith had worked at the RAE (Royal Aircraft Establishment at Farnborough) and in 1935 had shown that the momentum loss in the cooling system could be largely restored when the excess cooling air was forced through the radiator at high speed. This involved closing the air exit enough to create a substantial back pressure behind the radiator. To use approximate figures the propeller thrust at full power was about 1000 pounds (455 kg), the radiator drag was about 400 pounds (205 kg) with the momentum recovery being about 350 pounds (159 kg).
A significant contribution to the P-51 design was the introduction of the eighty five gallon fuselage tank championed by Mark Bradley. When full it weighed some 600 pounds and moved the centre of gravity rearward a few inches making the Mustang longitudinally unstable. Colonel Bradley demonstrated that this could be managed by combat pilots.
Production: The wing was made in two sections that were bolted together on the fuselage centre-line, the upper wing surface forming the cockpit floor. The wing had two spars (main and rear) with smooth Alclad skin. The ailerons (metal covered with a single trim tab on the port wing) and flaps were hinged on the rear spar.
Large split trailing edge flaps took up the whole of the space between ailerons and wing roots. The main wheels and legs were located forward of the main spar. The cantilever oleo legs were hinged to forged fittings, the forgings being bolted to reinforced wing ribs.
The fuselage was built in three sections (engine, main and tail) and except for the cockpit armour was constructed entirely of Alclad and aluminium alloy extrusions. The fin and tail structure was also of Alclad and aluminium alloy extrusions with both the rudder and elevators being fabric covered. The radiator air intake was streamlined and located underneath the centre of the fuselage, below and behind the cockpit area. The intake was fitted with a retractable scoop that stood clear of the fuselage underside to avoid turbulence from the air flow. Air flow, as previously described, passed through the radiator, was expanded and accelerated before being ejected through a controllable rear flap. The radiator location also helped the overall aerodynamics. The tail wheel, located at the rear of the fuselage, could be steered and retracted forwards.
Development: On the 10th May (1940) North American received a Letter of Intent from the British Government for what was designated the NA-73X, a formal contract followed on the 23rd May. The NA-73X was to be powered by the Allison V-1710, a twelve cylinder liquid cooled power plant. This Allison power plant had not been developed to the same extent as the Merlin as American engine manufacturers had tended to concentrate on air cooled radial designs.
On the morning of the 26th October (1940) the NA-73X underwent its maiden flight. The RAF placed an initial order for 320 production aircraft and informed North American that the NA-73X was to be called the ‘Mustang’. The second production machine (AG346) was shipped to the UK for RAF evaluation. Test pilots and squadron pilots who flew AG346 were unanimous in their praise of the aircraft’s handling qualities but were not enamoured with the Mustang’s performance at altitude. A decision was made to use the Mustang as a high speed ground attack and tactical reconnaissance fighter with Army Co-Operation Command. In January (1942) 26 Squadron became the first RAF unit to receive the Mustang Mk 1.
On the 30th April 1942 Ron Harker, the Rolls-Royce service liaison pilot attached to the Air Fighting Development Unit based at Duxford, flew AG422. The next day he issued a report suggesting that if the Mustang was fitted with a Merlin 61 the performance of the P-51 would be ‘outstanding’.
Harker gained the support of Ray Dorey, the General Manager of Rolls-Royce’s Experimental Flight Test establishment at Hucknell in Nottinghamshire. Dorey encouraged Harker to go direct to Ernest Hives Director and General Works Manager at Rolls-Royce. Ernest Hives contacted the Air Ministry. The Ministry needed all Merlin 61s for the Mk IX Spitfire, which was about to be put into service as the RAF’s answer to the FW190, but offered the Merlin Mk XX as an option.
(From obituary column in the ‘Daily telegraph’ July 1999: Ron Harker was born in Tynemouth on the 4th March 1909. He joined Rolls Royce at sixteen as an apprentice and took up club flying. He was laid off when the Depression hit but rejoined in 1934 moving to Hucknell as the company’s first test pilot. On the outbreak of war he joined 111 Squadron who operated Hurricanes out of Northolt. When the squadron was ordered to France Harker returned to Rolls Royce. During the Battle of Britain he organised a flow of spare parts to keep the Spitfires and Hurricanes flying. After the battle he test piloted the Merlin 60 series and helped to overcome the carburettor’s habit of cutting out in tight turns under negative g. After working on the Mustang/Merlin conversion he helped to transform the twin engine Avro Manchetser to the four engine Lancaster. After the war he stayed with Rolls Royce but was forced to retire in 1971 when the company went into receivership. Harker became an independent aviation consultant and moved to New Zealand. Two years ago, at the age of 88, he took the controls of a Mustang at an air display.)
On the 9th June the Rolls-Royce drawing office team took charge of AG518 which had been sent to Hucknell from Speke Liverpool, assembly plant for Lease Lend aircraft shipped from the USA. AG518 was found not to be up to the latest production standards, its successor was AM121. Two other aircraft (AL963 and AL975) were engaged on performance and handling trials prior to conversion. Two other Mustangs (AM203 and AM208) were allocated for modification. Of the five Mustangs that were to be fitted with Merlins, two were earmarked for evaluation by the United States Army Air Corps.
The Merlin fitted within the confines of the Mustang’s engine cowling but there were two difficulties to be overcome. The first problem was one of suspending the engine due to the width of the supercharging casing being greater than the distance between the original engine mounting pick up points on the firewall. This was overcome by the construction of an intermediate mounting that came away from the bulkhead in a shallow V fashion until the outermost point straddled the blower casing. From these points was hung the Rolls-Royce Consus (convergent suspension) mounting. The next problem was that the Allison had a downdraught air intake for the carburettor whereas the Merlin had an updraught layout.
By allowing the engine to sit higher on the bulkhead it was possible to provide a reasonable air intake elbow to the updraught carburettor and to keep the trunking that feed it within the confines of the bulkhead bottom line. By early October conversion to the Merlin 65 was complete and evaluations using two types of four bladed propeller took place.
The first flight, with AL975, took place on the 13th October 1942. AL963/975 and AM203/208 were designated Mark Tens. The Rolls Royce development data was passed to North American, who were working on their own conversion, and to the USAAF.
Packhard Motors had already been producing Merlins in the USA for about a year to provide stocks for Canadian built Lancasters and other types. In July 1942 the USAAF contracted for Packard Merlins to be installed in two P-51 air frames.
The first American conversion flew on the 30th November 1942. Modifications, similar to those undertaken at Hucknell, were carried out. These prototypes formed the basis for the P-51B of which some 2000 were initially ordered. To cope with the large production numbers North American set up two new plants in Dallas and Tulsa. Mustangs produced at Dallas were designated P-51C although they were virtually identical to the P-51B.
The first unit to operate the P-51B in England was the 354th FG with the first operational flight taking place from Boxted in Essex on the 1st December 1943.
The ability of the Merlin powered Mustang to escort the USAAF bombers all the way to the target and meet German fighters (the P-51’s having jettisoned their drop tanks) on equal terms, reduced the B-24 and B-17 losses to a fraction of those previously experienced. The drop tanks were a British development. They held 108 US gallons and were made of plastic and compressed paper held by glue. The fuel caused the tank to come apart within 24 hours but this was not a problem as the tanks were jettisoned four hours after take-off, the fuel having been put in just before take-off.
After a time the early Mustangs (P-51B/C) were replaced by the P-51D which was fitted with a large moulded plastic ‘bubble’ type sliding canopy that was combined with a cut-down rear fuselage. A dorsal fin was fitted to compensate for the reduction in fuselage side area caused by lowering the fuselage top line to accommodate the sliding canopy.
Whilst P-51D’s were used by the RAF in the Far East, they received a total of 281, most Mustangs that roamed the skies of Burma were American fighters. The Mustang’s exceptional range made it admirably suited to bomber escort work and light sweeps in the war against Japan. By 1945 the P-51D had an absolute range of 2080 miles.
RAF Mustangs: Mk 1: 520 machines powered by Alison V12, armament 0.5 and 0.3 guns, Mk 1A: 92 aircraft as Mk 1 but with four 20mm cannons, Mk II: 50 aeroplanes similar to P-51A, powered by Allison V-1710-81, four 0.5 guns in wing, Mk III: 946 Mustangs similar to P-51B/C, all powered by Packard Merlins and fitted with a larger fuselage fuel tank, MkIV/IVA: 874 Mustangs similar to P-51D, cut down rear fuselage with bubble canopy. Wings modified to take six 0.5 Brownings with 1880 rounds. All powered by Packard Merlins, MkX: see previous notes.
NB: Mustang’s were used in the Korean war. The last F51 (in 1948, the ‘F’ prefix for fighter had superseded the ‘P’ prefix for pursuit) was withdrawn from service with the United States in February 1953. The Mustang last saw service with the RAF in May 1947.
(Data from Duxford, Hendon and Rolls-Royce)