PART 14

STRUCTURES AND SYSTEMS

Since the P-51D and P-51K were the definitive versions of the Mustang employed in both the Second World War and the Korean War, this part will focus on them. These versions of the Mustang are identical with the exception of their propellers, slight differences in their cockpit canopy shaping and their production factories with a few other minor exceptions. The P-51D was manufactured at both the Inglewood, California, and Dallas, Texas, facilities while the P-51K was manufactured exclusively in Dallas, Texas.

The P-51 Mustang became a legitimate force to be contended with after it was wedded to the Packard-built Rolls-Royce V-1650 Merlin engine. Nevertheless, it did suffer from a few aerodynamic flaws that showed up during combat. To cure the worst of these, in mid-1944, NACA-Ames was asked by the USAAF Air Materiel Command to investigate a rather serious handling problem with the Mustang. One factory-fresh production P-51B-1-NA (43-12490) was flown to Naval Air Station Moffett Field in Sunnyvale, California, from Mines Field near the NAA production facility in Inglewood, California, and it was placed into the NACA-Ames wind tunnel for aerodynamic testing. Even though manoeuvring and handling characteristics were very good, the horizontal stabilisers of several P-51Bs and P-51Cs had failed structurally during slow aileron rolls. These failures had occurred at a time when earlier NACA-Ames flight tests revealed that the Mustang had unsatisfactory directional characteristics, including a reversal of rudder force at large angles of sideslip. It was decided that in a high-speed rolling pullout, adverse aileron yaw could generate sufficient sideslip to inadvertently cause a snap roll and thus impose large enough stresses to cause horizontal tail failure.

The USAAF AMC requested that NACA-Ames improve the directional characteristics of the P-51 to reduce sideslip excursions in rolling manoeuvres while retaining existing rudder force change with airspeed. The modification(s) were to be simple in order to facilitate alterations to aircraft on the production lines and in service. This P-51B was tested with nine modifications in thirteen flight conditions in sequence so that the relative merit of each could be evaluated. The addition of a dorsal fin, rudder trailing-edge bulges and a rudder anti-boost tab ratio of 1:2 gave it the best overall flight behaviour. The dorsal fin eliminated rudder-force reversals in sideslips and had a favourable effect on structural loads. The NACA-Ames modifications essentially eliminated horizontal tail failures in manoeuvring flight. This aerodynamic modification was implemented on the P-51D and P-51K production lines and the dorsal fin and rudder modification kits were sent to all of the P-51 fighter groups throughout all theatres of war. The following information has been gleaned from the North American P-51 Mustang Pilot Training Manual.

Air Induction System

The air induction system provides a selection of two kinds of carburettor air: cold ram air and cold-filtered unrammed air. Normal operation is with cold ram air. However, under moisture freezing conditions, operation should be with unrammed filtered air. With the cockpit control in this position, the ram air door is closed and the filter air door opened. A hairpin-type icing screen is located in the rear duct section. This screen will ice over under icing conditions and suction created in the induction system will open emergency doors, located aft of the icing screen, and allow warm air from the engine compartment to enter the carburettor. Under moisture freezing conditions, or at any time that temperatures and conditions indicate the possibility of carburettor ice, flight should be with the carburettor air control in the ‘Unrammed Filtered Air’ position.

Armament and Gunnery Equipment

The main objective of a pursuit airplane, as the designation implies, is to pursue and destroy. Although its ability to carry bombs and rockets is of great importance, a fighter is primarily a flying gun platform – a means of taking its firepower into the air to destroy enemy aircraft and targets of opportunity on the ground. A fighter pilot’s success is measured not only by how well he flies, but how he shoots.

The P-51D and P-51K were equipped with a basic armament of six wing-mounted air-cooled disintegrating link belt-fed Browning M2 .50 calibre heavy machine guns with a maximum load of 1,880 rounds of ammunition for all six guns: three fixed guns housed within each wing on either side of the fuselage. This gun features a 750 to 850 rounds per minute rate of fire, each weighing 61 lb. The maximum ammunition capacity is 400 rounds for each of the inboard guns and 270 rounds each for the centre and outboard guns. The guns were adjustable on the ground so that they could be harmonised to different firing patterns for various tactical situations. Usually they were aligned to converge at a range of from 250 to 300 yards.

Machine gun loading/bore sighting instruction placard found inside the main doors of all gun bays on P-51D/K aircraft. (USAF)

Browning M2 heavy machine gun details. (USAF)

Typical 500 lb bomb installation for all versions of the Mustang. (USAF)

An alternative installation was implemented for ground target strafing missions where duration of fire rather than air-to-air combat firepower was of paramount importance. For ground target strafing missions, the centre guns were removed. This allowed 500 rounds of ammo to be carried for each outboard gun. Firepower was reduced to four guns, but the total ammunition load remained around the same: 1,800 rounds.

Under each wing on either side of the fuselage are three- to four-mounting attachments depending on the combat mission and the ordnance to be carried. For example, for ground attack, two 500 lb high explosive bombs and six five-in.-diameter high-velocity aerial rockets (HVARs) could be carried.

For long-range bomber escort, two 75, 110, 125 or 165-US gallon auxiliary drop-type fuel tanks could be carried. Some P-51D and P-51K Mustangs were armed with two ‘Bazooka’ M10 rocket tubes – one under either wing, that housed and fired three M8 4.5-in. rockets each.

Gun Camera

A gun camera is mounted in the leading edge of the left or port wing and is accessible for loading film and making adjustments from inside the left-hand wheel well. A small door covers the camera aperture in the wing. This door remains open during flight, but is closed by a mechanical linkage when the landing gear is extended, thus protecting the lens from blown sand, pebbles or debris when the airplane is on the ground.

The six guns and camera are controlled by a three-position switch on the front switch panel. This same switch also turns on the lamp in the optical sight. When the switch is flicked up to ‘guns’, ‘camera’ and ‘sight’, the guns fire and camera operates when the pilot presses the trigger on his control stick. When the switch is down to ‘camera’ and ‘sight’, the pilot will be taking pictures by pressing the trigger, but the guns will not fire. Middle position of the switch is ‘off’; the pilot must be sure to keep it there during take-off, landing and all ground operations. The guns and camera are heated electrically so their operation is not affected by extreme cold encountered at high altitude. Gun heaters are controlled by a switch on the right switch panel. The camera heater is built into the camera and works automatically whenever the camera switch is turned on.

Gunsight

Early P-51D airplanes employed the N-9 gunsight, but later P-51D/K airplanes used the K-14 computing gunsights mounted on the instrument hood centreline.

Cockpit

The cockpits of fighter-type airplanes are generally cramped and the cockpit of the Mustang is no different. Concentration of numerous instruments and controls into a small space is unavoidable. In the case of the P-51D/K, the controls are simplified and their grouping has been planned to give a combat pilot the greatest possible efficiency. As fighter airplanes go, the Mustang cockpit is comparatively comfortable.

The cockpit can be both heated and ventilated. Cold air is fed into the cockpit through a small scoop located between the fuselage and the big air scoop. Warm air is fed into the cockpit from inside the large water cooling air scoop just behind the radiator. Warm air from this source also serves to defrost the windshield. Controls for regulating cold and warm air and the defroster are on the floor of the cockpit on either of the seat. The pilot’s seat is designed to accommodate either a seat-type or back-pack parachute. The back cushion is kapok-filled and can be used as a life preserver. The seat is adjustable vertically and the seat lock is on the right-hand side of the seat. No fore-and-aft adjustment is possible.

Pilot comfort on long flights is increased by a small, folding arm rest on the left-hand side of the seat. A standard safety belt and shoulder harness are provided. There is a lever on the left side of the seat for relaxing the tension on the shoulder harness. This permits the pilot to lean forwards whenever necessary – for example, to look out of the canopy while taxiing.

Cockpit Instruments

Most of the P-51D/K instruments are mounted on the instrument panel, flight instruments being grouped at the centre and to the left, engine instruments to the right. Exceptions are the hydraulic pressure gauge, which is below the pilot’s switch pane; the fuel gauges on the floor and aft of the cockpit; the ammeter on the electrical switch and circuit breaker panel. The instruments can be classified into four general groups as follows:

The primary parts of the K-14A computing gunsight. (USAF)

Vacuum System Instruments

These are operated by a vacuum pump powered by the engine and include: 1) the flight indicator; 2) the bank-and-turn indicator; 3) the directional gyro; and 4) the suction gauge. The suction gauge shows whether the vacuum pump is providing sufficient vacuum for the system. If the gauge reads more than 4.25 or less than 3.75, the pilot knows that the vacuum instruments are not functioning properly and are not giving reliable readings. Normal suction reading is 4.00.

Pitot Static System Instruments

These instruments are operated by pressure or static air from the pitot tube that is located under the right or starboard wing and from static plates on the fuselage skin. They include: 1) the airspeed indicator; 2) the altimeter; and 3) the rate-of-climb indicator.

Engine Instruments

The engine instruments include: 1) the tachometer; 2) the carburettor air-temperature indicator; 3) the coolant temperature gauge; and 4) the engine gauge. The engine gauge consists of three instruments in one, showing oil temperature, oil pressure and fuel pressure.

Miscellaneous Instruments

Miscellaneous instruments include: 1) the remote indicator compass; 2) the hydraulic pressure gauge; 3) the oxygen pressure gauge; 4) the fuel gauges; 5) the ammeter; and 6) clock.

Once the bubble-type canopy was adopted by the Mustang pilot, visibility was vastly improved. This illustration shows the major parts associated with the cockpit canopy assembly for the NA-106 aircraft. (NAA via USAF)

Detailed phantom view of the first full production version of the bubble-top Mustang: the NA-109 P-51D. (NAA via USAF)

Details of the NA-122 P-51D cockpit canopy. (NAA via USAF)

Cockpit Canopy

The cockpit enclosure is of the half-teardrop or bubble-shaped type. It consists of an armour-glass windshield and a sliding canopy formed from a single piece of transparent plastic. The canopy is designed to give the pilot the best possible vision in all directions since obstructions above, at the sides and to the rear have been eliminated. (Earlier models of the Mustang had framed window-type cockpits that hindered pilot visibility.)

The pilot enters the cockpit from the left or port side of the airplane. To assist the pilot up onto the wing, there is a handhold in the left or port side of the fuselage. The pilot can step onto the fairing when climbing up onto the wing, but has to be careful as to not step on the wing flap. To open the canopy from the outside, push the spring-loaded button at the right forward side of the canopy and slide the canopy aft. The pilot will control the canopy from within by means of a hand crank. Depressing the latch control on the crank handle unlocks the canopy, after which the pilot can turn the crank to slide the canopy open or closed. Releasing the latch control locks the canopy into any position.

To warn a pilot against taking off without the canopy properly secured, there are two red indicator pins, one at each side of the canopy. If these pins are visible the canopy is properly locked. The pilot should never take off if the pins are not visible otherwise the canopy will disengage in flight. The emergency release for the canopy is the long red handle on the pilot’s right, above the oxygen system controls. When a pilot pulls on this handle, the entire canopy ejects from the airplane. The handle is safe tied with light (easily breakable) wire.

Radio Equipment

The radio equipment consists of a VHF (Very High Frequency) transmitter and receiver, a Detrola receiver, an AN/APS-13 rear-warning radio set and an IFF (Identification, Friend or Foe) unit. All radio equipment is in the fuselage, aft of the cockpit. Controls are in the cockpit, and are grouped on the right-hand side of the seat. Each set has its own antenna arrangement. The VHF antenna mast extends vertically above the fuselage aft of the cockpit, the Detrola wire antenna runs from the back armour plate to the top of the vertical tail, the AN/APS-13 antenna rods extend horizontally from the sides of the vertical tail, and the IFF antennae project from the undersides of the wings.

The VHF Set

The pilot operates the VHF set by means of a control box that has five push buttons: an off switch and ABC and D switches for selection of four different frequencies. These frequencies are crystal controlled and, therefore, cannot be adjusted in flight. The pilot can transmit and receive only on one channel at a time.

Channel A is for communications with CAA (Civil Aviation Authority) radio ranges.

Channel B is the ‘American common’ frequency. The pilot uses it for connecting all towers in the continental United States equipped with VHF facilities. It is also used for emergency homings.

Channel C is for interplane communications.

Channel is the local homing channel used for practice homings.

Small coloured lights alongside the four buttons show which channel is in operation. These lights have a dimmer device that can dim them for night flying. The toggle switch on the control box has three positions: REM (remote), T (transmit) and R (receive). This switch is usually safety-wired in the REM position. When the toggle switch is in the REM position, the VHF set is controlled remotely by a push button on the throttle. When this throttle button is pushed in, the pilot is transmitting; when it is out of its normal position, the pilot is receiving. Under ordinary circumstances the pilot uses the remote control and the throttle button.

A tiny white light alongside the toggle switch shows the pilot if he is transmitting. This light stays on except while transmitting. If it does not go off when the throttle button is pushed, the pilot knows the transmitting equipment is not working. On the other hand, the transmitter light may stay off, indicating that the transmitter is working due to a stuck and shorted throttle switch or a jammed relay. In either case, the problem may be corrected by operating the throttle switch a few times or by turning the main radio switch on and off. If this does not help, break the safety wire and move the toggle switch from REM to R or T as required. The transmitter should not be left on when not in use. If so, the carrier wave will jam the channel so that no one else can use it.

The pilot controls the volume of the VHF set by means of a knob on the AN/APS-13 control panel located on the right-hand side of the seat. The main advantage of VHF equipment is that it is not affected nearly as much by atmospheric interference as low-frequency equipment. Accordingly, it provides much better reception in bad weather. The range of VHF equipment is normally about 200 miles at an altitude of 20,000 feet. Altitude and terrain determine range as VHF transmission only travels in straight lines. In general, operation improves with an increase in altitude. To contact VHF towers, the pilot must maintain an unobstructed line between the tower and the antenna. If the tower is below the horizon, if mountains, tall buildings or parts of the airplane are in the way, the transmission and reception will be blocked.

An advertisement telling of Du Pont’s ‘Lucite’ material that formed all of the bubble-type cockpit canopies. (Du Pont)

Canopy assembly for P-51B/C aircraft. (NAA via USAF)

The Detrola

The Detrola is a low-frequency receiver. It operates between 200 and 400 kilocycles (kc), which covers the transmission band for towers and range stations throughout the US. Operation of the Detrola is simple. It has two controls: a station selector knob and a combination on-off switch and volume control.

Rear Warning Radio

The AN/APS-13 unit is a lightweight radar set that warns the pilot, by means of an indicator light and a bell, of the presence or approach of aircraft from the rear. The red jewel-type indicator light is mounted on the left side of the instrument cowl; the bell is to the left of the seat. Control switches are provided on the panel located on the right-hand side of the seat. The equipment is automatic and is turned on and off by a toggle switch. There is also a checking switch to test the operation of the light and bell, and a rheostat for controlling the intensity of the indicator light.

The IFF Set

The IFF set is an identification device for use in combat zones. Its operation is simple so far as the pilot is concerned. It is automatic: all the pilot has to do is turn it on and off with a toggle switch. The IFF set has a detonator to destroy vital parts of the equipment if the pilot has to abandon the airplane over enemy territory. The detonator is activated by pressing two push button switches located in a box on the right side of the cockpit. Both buttons must be pressed simultaneously. The IFF set also has an impact switch to set off the detonator in a crash landing. Therefore, the equipment can be destroyed even if the pilot has to bailout and does not have time to press the two buttons. The detonator destroys the equipment internally and the explosion will not harm the pilot or the airplane.

Radio Navigation

To assist the pilot in navigation, the pilot has both a Detrola and a VHF set, which can be used independently or together. Using this equipment with regular radio navigation facilities, the pilot has no excuse for ever getting lost in the US where radio facilities are unlimited. The pilot should be thoroughly familiar with all the aids available in order to make full use of them. Too often, pilots have a tendency to concentrate on one facility. They use their VHF set, for example, almost exclusively, neglecting the Detrola. The danger of such faulty practice is obvious, so be as thoroughly experienced as possible with all the equipment available.

NA-102 P-51B-1-NA radio equipment. (NAA via USAF)

Back-to-front view of P-51B-1-NA radio equipment. (NAA via USAF)

The various antennas as employed by the NA-109 P-51D airplane. (NAA via USAF)

Homing

Homing facilities are usually available on two of the VHF channels: B and D. If you the pilot ever get into an emergency situation and require homing, remember these two things: 1) State definitely that you are requesting an emergency homing; and 2) Call a known station if you know the approximate locality, but call for any station if it is an extreme emergency. Many a pilot who could easily have been helped by use of homing facilities has been lost because those pilots failed to call any station that might have been listening and failed to make clear that an emergency homing was needed.

The Oxygen System

The oxygen system in the P-51D/K airplanes was the same as that used in all USAAF fighter planes at the time. It is a low-pressure, demand-type system – that is, the pilot does not have to control the oxygen manually during changing altitudes. Oxygen is supplied by oxygen-filled tubular canisters mounted aft of the pilot’s seat on the floor of the empennage.

Cooling System

The liquid cooling system uses a percentage of water and glycol – seventy per cent water and thirty per cent glycol above minus sixteen-degrees Celsius, seventy per cent glycol and thirty per cent water below sixteen-degrees Celsius. The capacity of the cooling system is sixteen-and-a-half-US gallons.

The maximum operating temperature at sea level is 121-degrees Celsius and the minimum for take-off is sixty-degrees Celsius. The high pressure cooling system employs a closed circuit that allows a continuous flow from a centrifugal pump. The system will withstand pressures up to fifty pounds per square inch. The coolant flows from the coolant radiator to the pump. From there it is pumped to the bottom of each engine block. From there it passes to the cylinder heads through fourteen brass transfer tubes and out of the cylinder heads to the header tank. From the header tank the coolant is returned back to the radiator.

P-51 cooling system for the Allison V-1710-39 Vee engine. (Allison)

Empennage Structure

The P-51D/K empennage (tail group) includes the far aft section of the fuselage to which the rear landing gear (tail wheel) assembly, horizontal and vertical stabiliser assemblies are attached including the dorsal fairing.

Fuel System

The P-51D and P-51K Mustangs carried four types of external drop-type fuel tanks: one under either wing to increase their fuel capacity and to extend their maximum range. These included seventy-five-gallon, 110-gallon, 125-gallon and 165-gallon jettisonable drop tanks.

Fuselage Structure

The main fuselage structure of the D/K airplanes is the structure between the engine mount (nose section) and the empennage which houses the cockpit and canopy, engine oil and water cooling systems, hydraulic and primary electrical systems and the engine oil and cooling air scoop.

Landing Gear

The P-51D/K variants of the Mustang are of the ‘tail dragger’ variety as are their predecessors and successors. The main landing gear, which retracts inwards toward centreline, are widely spaced (11 ft 10 in. centre-to-centre of either wheel) allowing for more than adequate ground handling while taxiing, taking off and landing. The steerable single tail wheel is fully retractable. Both the main landing gear and tail alighting gear are hydraulically operated.

Propeller

The P-51D propeller is a four-bladed Hamilton Standard hydraulically operated constant speed propeller with a diameter of 11 ft 2 in. and a blade angle of forty-two degrees. As is the case with all single-engine aircraft, the propeller cannot be feathered. Pilots control propeller rpm manually by a single lever on the throttle quadrant. P-51K airplanes use Aeroproducts propellers. These are also four-bladed hydraulically operated constant speed propellers, but with a 2-in.-less diameter of 11 ft and a prop pitch range to thirty-five degrees. The P-51D and P-51K series of airplanes are distinguished from each other solely by the propeller they employ. Although the Hamilton Standard and Aeroproducts propellers are very different in their construction, they are identical in operation.

This detailed illustration shows the P-51D/K, TP-51D and F-6D/K cooling system for the oil and water radiators; thus, the engine oil and water cooling apparatuses. Detail of the radiator scoop assembly is noteworthy. (NAA via USAF)

Another view detailing the cooling system for the V-1650 engine. (NAA via USAF)

Propulsive System

The P-51D and P-51K airplanes are both powered by 1,300hp Rolls-Royce Merlin engines built in the US under licence by the Packard Motor Car Company and designated V-1650-7. War emergency power (WEP) was around 1,700 hp. This particular engine was similar to the R-R Merlin 66 (RM 10SM) engine. This engine gave the D/K Mustangs a maximum speed of 437 mph at 25,000 ft in level attitude flight.

The North American P-51 Mustang was designed, developed and manufactured to be an out-and-out pursuit airplane. As a dedicated pursuit airplane, it was expected to have more than adequate firepower, the best combat range possible, outstanding agility and manoeuvrability as well as full energy (high speed) up to its ultimate ceiling of 40,500 ft where its rate of climb would begin to fall off – to less than 500 feet per minute. There is an old adage that states: ‘An airframe is only as good as its powerplant.’ Never a truer word spoken as has been proved by the Mustang success story. The early versions of the Mustang used a powerplant that was considered to be the best of its type being manufactured in the US. But it soon became apparent that that particular engine – the Allison V-1710 – was a disadvantage to its performance potential, especially when flying in combat above 15,000 ft which was wholly unacceptable.

Diagram of the P-51D/K fuel system. (NAA via USAF)

Exploded view of the A-36A detailing its major assembly parts. (NAA via USAF)

Turbosupercharger components for V-1710-39 engine. (Allison)

The early production Mustangs – the Mustang Mark I, P-51/Mustang Mark IA, A-36A, and P-51A/Mustang Mark II – were powered by the aforementioned Allison V-1710. And their lack of high-altitude performance was its downfall and later versions of the Mustang – the P-51B and P-51C/Mustang Mark III, the F-6C, F-6D and F-6K, and P-51D and P-51K/Mustang Mark IV, and the P-51H – featured the use of a foreign engine that allowed high-altitude operations. Thus, the Mustang series of aircraft featured the use of two different propulsive systems. This foreign engine, designed, developed and produced by Rolls-Royce in Great Britain, built under licence in America by the Packard Motor Company as the V-1650 is the famed Merlin engine. When it was eventually married to the P-51, the Mustang truly became a ‘horse of another colour’.

The Allison V-1710 Vee

The slim inline water-cooled Allison V-1710 Vee 12-cylinder engine was the right fit for the NA-73X when it was conceived. It was therefore employed for the propulsive system of this prototype that required a small frontal area for one of its most important aerodynamic features. Its biggest drawback, however, was its lack of adequate supercharging that would not allow the early Mustang to have good high-altitude performance. The Mustangs that employed the Allison V-1710 Vee for their respective propulsive systems are as follows: NA-73X (1), NA-73 Mustang Mark I (320), NA-73 XP-51 (2), NA-83 Mustang Mark I (300), NA-91 P-51/F-6A/Mustang Mark IA (148), NA-97 A-36A (500), NA-99 P-51A/F-6B/Mustang Mark II (310), and NA-105B XP-51J (2) for a grand total of 1,583.

Fuel and air-flow through the turbosupercharger. (Allison)

General V-1710 Vee Specifications

Type: liquid-cooled V-12 piston engine (F series) 

Bore: 5.5 in. 

Stroke: 6.0 in. 

Displacement: 1,710 cu in. 

Length: 85.81 in. 

Width: 29.28 in. 

Height: 37.65 in. 

Dry weight: 1,395 lb

Mustang-employed Allison V-1710 Vee Engines

V-1710-37 (F3R), NA-73X 

V-1710-39 (F3R), NA-73/NA-83 Mustang Mark I, XP-51, P-51/F-6A/Mustang Mark IA 

V-1710-81 (F20R), P-51A/F-6B/Mustang Mark II 

V-1710-87 (F21R), A-36A 

V-1710-111 (F30R), XP-51J 

V-1710-119 (F32R and F32L), XP-82A

Allison V-1710-39 (F3R) engine installation in the P-51 airplane was identical to the Mustang Mk.I and Mk.IA airplanes. (Allison)

The Rolls-Royce/Packard V-1650 Merlin

The Rolls-Royce Merlin V-12 series of piston engines was powering numerous RAF and Royal Navy combat aircraft by the time Mustang fighters first arrived in Great Britain. It was a proven and reliable propulsive system for the airframes it had already been married to such as the Spitfire and Mustang. However, it was a British-made engine and American airframe manufacturers tended to propel their designs with American-made engines. Moreover, with similar horsepower ratings, most propulsive system and aeronautical engineers at NAA thought it best to propel its Mustang fighter with the Allison V-1710 V-12 series of piston engines. This proved to be a mistake even though an honest one. Unbeknownst, as far as Mustang project personnel were concerned, a fateful change of events were afoot in Great Britain.

Beginning in spring and through autumn 1942, the engine division of Rolls-Royce Limited converted and flew several RAF Mustang Mark I airplanes powered by Rolls-Royce Merlin 61 engines instead of the Allison V-1710-39. These Rolls-Royce Mustang or Royal Air Force Mustang X aircraft as they became known tremendously outperformed their counterparts and Rolls-Royce had successfully proved that Merlin-powered Mustangs would be the final answer in the creation of a winning fighter: a 400-plus mph fighter that could soar to heights twice that of Allison-powered P-51 with improved fuel economy and nearly twice the range. After the Mustang had been crossbred with the Merlin engine, it became the thoroughbred warhorse sought by fighter pilots. While the Allison V-1710 had made the Mustang a viable low-altitude fighter, the Roll-Royce Merlin made it the high-altitude, long-ranging steed it became.

Allison V-1710-39 engine accessories. The V-1710-39 engine used a downdraft carburettor whereas the V-1650 Merlin engines used updraft carburettors. (Allison)

General V-1650 Merlin Engine Specifications

Note: The Packard-built V-1650 engines were listed as Merlin 266 or RM 10SM engines at Rolls-Royce. The number ‘2’ ahead of ‘66’ means that these engines were built by Packard, but at Rolls-Royce were known as Merlin 66 engines. At Packard, however, these were known as Merlin 68 engines.

Type: 12-cylinder, supercharged, 60-degree Vee-type water-cooled piston engine 

Bore: 5.4 in. 

Stroke: 6.0 in. 

Displacement: 1,647 cu in. 

Length: 88.7 in. 

Height: 40 in. 

Width: 30.8 in. 

Dry weight: 1,640 lb 

Supercharger: Gear-driven, two-speed, two-stage 

Maximum power: 1,700 hp at 3,000 rpm

Merlin-powered USAAF Mustangs

XP-51B – Packard-built V-1650-3 (Merlin 68) 

P-51B – Packard-built V-1650-3 (Merlin 68) 

P-51C – Packard-built V-1650-3 (Merlin 68) 

F-6C – Packard-built V-1650-3 (Merlin 68) 

XP-51D – Packard-built V-1650-3 (Merlin 68) 

P-51D – Packard-built V-1650-7 (Merlin 69) 

TP-51D – Packard-built V-1650-7 (Merlin 69) 

F-6D – Packard-built V-1650-7 (Merlin 69) 

XP-51F – Packard-built V-1650-3 (Merlin 68) 

XP-51G – Rolls-Royce RM 14SM (Merlin 100) 

P-51H – Packard-built V-1650-9 (Merlin 300) 

P-51K – Packard-built V-1650-7 (Merlin 69) 

F-6K – Packard-built V-1650-7 (Merlin 69) 

P-51M – Packard-built V-1650-9A (Merlin 300) 

XP-82 – Packard-built V-1650-23 (left-hand rotation) and V-1650-25 (right-hand rotation)

Left- and right-hand views of the Packard-built V-1650-7 Merlin engine employed by late model P-51Ds, TP-51Ds, F-6Ds and all P-51Ks and F-6Ks. (USAF)

Skeletal view of a V-1650-3 Merlin engine installation in the NA-106 P-51D-1-NA aircraft. (USAF)

Close look at an installed V-1650-7 Merlin engine in a P-51D-25-NA (44-74202) with its cowling removed. (USAF)

A total of 55,873 Packard-built Rolls-Royce Merlin engines were built at average prices of $13,286US to $17,185US for single-stage engines and $12,548US to $15,867US for two-stage engines. Continental added 897 V-1650 engines built for a grand total of 56,770 engines built.

Wing Structure

The wing structure of both the P-51D and P-51K is identical. It is attached to the bottom of the main fuselage structure and houses the main fuel tanks, main landing gear and doors, gun bays and guns, gun camera, hydraulic lines, electrical wiring, identification lights, flight control cables and actuators, flaps and ailerons.

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