Design Breakdown of
the P-51 Mustang

by George Gehrkens,
Project Engineer, North American Aviation
Project Engineer George Gehrkens has seen the P-51 grow from the drafting board stage to its present design. A graduate of the University of California at Los Angeles, he has been with North American Aviation for seven years.

The single-seater, all-metal P-51, one of the world's fastest combat planes, is analyzed in technical detail by one of the project engineers

Performance of the Mustang fighter squadrons in escorting heavy bombers far into Germany since the start of the 1944 Allied air blitz has demonstrated the importance of close attention to aerodynamic cleanness of design.

A considerable part of the P-51's performance as the fastest fighter in existence is due to its high-lift, low-drag airfoil which was evolved from an original NACA design. The wing section developed by North American's aerodynamicists permits a smooth flow of air and avoids shock waves of destructive intensity at high speeds.

The Mustang fighter is a low-wing, single-seat, high altitude combat fighter powered with a 1520 hp Packard-built Rolls-Royce engine with a two-stage, two-speed supercharger. In actual combat, Mustang pilots are maintaining overwhelming odds against German fighters.

The A-36 Invader has been praised by the pilots who provided exceptional support for the infantry in Sicily and Italy. Previous to the A-36, Mustangs equipped with four 20-mm cannon were used by RAF pilots to wreck German communication lines over the low countries two years ago. In all designs, the laminar flow wing has given it exceptional speed as well as range and maneuverability. Speed is in excess of 425 mph; altitude is about 40,000 ft.

The Mustang wing is a full cantilever type with a span of 37'-3/10", and a total area of 233 sq ft, including ailerons; chord at the root is 8'8", and at the tip 4'2". It is constructed in two halves and bolted together at the center line of the airplane. Skin over the wing and elsewhere on the plane is butt-fitted and flush-riveted.

The main wing spar, located at approximately 30% of the wing chord, is the main structural member of the wing section. It is constructed of two sections of .129" 24ST aluminum alloy, and reinforced with .25" 24ST bar. The aft spar, of the same material, is made up in two sections and spliced. There is an auxiliary reinforcing spar at the landing gear wheel well of each wing panel.

Between these spars are stringers of extruded aluminum alloy and pressed hat sections. Flanged ribs formed from aluminum alloy give the structure rigidity. Some of these have stiffening beads while others have lightening holes. A solid rib is placed at the inboard side of the gun compartment of each wing section and at the attaching point of each wing half.

Self-sealing fuel cells are located inboard between the main and rear spar in each wing; flush, structural doors under the wings are provided for access. The rear spar supports the hinges for the wing flaps and ailerons. Wing tips consist of a spar and pressed flanged at ribs and are attached with screws.

Angle of incidence is 1° at the root, and the dihedral is 5° with reference to the 25% chord line to give lateral stability. Each wing section has a 58' negative angle of incidence at the wing tips. A fast-drying glazing putty is applied to both the upper and lower surfaces of the wing as far back as 40% of the chord line to minimize turbulence in flight. North American engineers developed the spray-application of this glazing putty.

Ailerons

Ailerons are constructed of two spars and 12 flanged ribs, entirely covered with 24ST aluminum alloy sheet. Each aileron has an area of 6.36 sq ft, including .731 sq ft of tab area. The forward spar is a bent-up sheet channel of .032" 24ST Alclad and the rear spar channel is formed of the same material. Ribs are 24SO alloy, heat treated to 24ST. Plastic ribs are used to reinforce the trailing edge, which is formed from a single sheet of 24ST.

Each aileron is hinged at three points by brackets bolted to the forward spar, and has an angular travel of 12° up and down from neutral. In addition to static and dynamic balance, each aileron has internal aerodynamic balance.

Aileron trim tabs are of phenol fiber, the left tab being adjustable by a knob on the control pedestal on the left side of the cockpit through a series of link rods and cables. An actuating rod extends from the outboard cable drum to a bracket on each tab, which has an angular travel of 10° up and down from neutral.

Wing Flaps

Wing flaps are 24ST Alclad covered, sealed-aileron type, hinged to the wings on three sealed-type bearings between the fuselage and the ailerons. Each flap is constructed of two formed 24ST Alclad spars, 12 nose ribs, 11 main ribs and a series of rolled-section stringers, all of 24ST.

The forward spar consists of a single .040" aluminum sheet which has both edges braked at right angles to the surface of the spar. Flanged lightening holes in the spar permit access to the inner structure after the skin has been applied. The aft spar is a U-channel formed from .032" Alclad.

Trailing edge of each flap is formed from a single 24ST aluminum sheet reinforced with 24ST tapered hat sections. Three brackets bolted to the forward spar provide attachment supports for the sealed-type ball-bearing hinges. Metal fairings bridge the gap between the flap and the fuselage. A stainless steel rubbing strip curves forward and down on the nose ribs from the top of the forward spar.

Wing flap operating mechanism consists of a single hydraulic strut, connected to a torque tube and short connecting links to the inboard end of each flap. A handle on the pilot's control pedestal has six slotted positions to provide any setting from 10° to a maximum of 50°. The preselector linkage for the flaps is located just ahead of the flap torque tube.

Fuselage Structure

The small cross-sectioned Mustang fuselage is a semi-monocoque structure fabricated largely of aluminum alloy and divided into 3 main sections — engine mount, main center section and the tail section, all of which are separable by unbolting. The large air scoop is attached to the bottom of the center section aft of the cockpit, where it causes less drag than the more conventional types placed farther forward. The width of the fuselage is 2'11"; maximum height is 6'3-7/16"; and the length (with engine mount, tip of propeller spinner to tip of tail) is 30'9".

Main Section

The main fuselage section contains 8 riveted and bolted assemblies: firewall, turnover truss back of the pilot's seat, upper deck, right- and left-hand side panel subassemblies, the lower assembly or air scoop, radio shelf and the web assembly. Each may be installed or replaced as a unit.

This main section is built around 4 longerons. The upper two longerons are extruded H-sections 2½" x 2½", extending back from the firewall for approximately 10' and tapering to T-sections that extend back to the end of the section. Lower longerons are similar to the upper for 8', but taper into a U-section for 3' more, and from there back are bent-up U-sections. The shape of the fuselage cross-section is maintained by formers and frames of 24ST Alclad.

Skin along the sides of the cockpit is of .081" 24ST aluminum alloy, butt-fitted and flush riveted. Back of the cockpit, along the sides of the radio compartment, th skin is .064" 24ST, while above the radio compartment and to the end of the section it is .040" 24ST.

Pilot's vision forward in this model is through a 1½" thick flat section of bullet-proof 5-ply, laminated glass, which is slanted 40° from horizontal. The curved side panels are of 3/8" safety plate and the upper panels are of transparent plastic sheet. Vision toward the rear is afforded through windows in cutouts at both sides of the fuselage aft of the turnover truss, the cutout windows being removable for access to radio, battery and other equipment. The panel frames are constructed of tempered 53S aluminum alloy extrusions.

The pilot's enclosure is hinged to open outward and to the right, while a door on the left opens outward and to the right, while a door on the left opens outward and down. Sliding sections are provided in this door for partial opening during bad weather operations. The enclosure is attached to the fuselage by 4 hinges, each fitting into a U-shaped spring-loaded release cam which permit the enclosure to be removed as a unit. A release lever permits it to be jettisoned quickly in an emergency.

In addition to the windshield, the pilot is protected from forward fire by armor plate. Protection from the rear is also afforded by armor plate.

The pilot's seat is of magnesium, built for a seat-type parachute, with a kapok filled back cushion which also is a life preserver. Cockpit heating and ventilating controls are located on the floor. Over the instrument panel is a shroud which eliminates instrument panel glare in the windshield glass, supports the windshield defrosting unit, optical gun sight and two handholds.

Aft of the armor plate back of the pilot's seat is a wooden bulkhead, one of two installed in the main fuselage section to prevent drafts and to catch any fallen objects which might roll aft and foul the controls. The second bulkhead, in two sections, is attached aft of the radio shelf and extends upward to the top of the fuselage. At extreme aft end of the main fuselage section a 24ST aluminum alloy tube is installed in the lower side for lifting and tiedown purposes.

Rear Section

The aft section of the fuselage is constructed with two extruded 24ST aluminum alloy longerons at the bottom and a flat shelf, flanged at the top edges. Five 24ST formers provide for riveting the 24ST Alclad sheet skin. Solid bulkheads are installed in the front and rear ends of the tail section. The tail wheel well is located forward and at the bottom of the section, reinforced on either side by box structures which are joined together by a magnesium alloy casting to provide an attachment point for the tail wheel bearing.

Engine Mount

Engine mount consists of two full cantilever Y-shaped box beams constructed almost entirely of 24ST aluminum alloy sheet and extruded sections. Four bolts attach the Y-shaped supports at the firewall, to the upper and lower main fuselage longerons.

Empennage Structure

The empennage is a full cantilever structure with a semi-monocoque fin and stabilizer installed as a unit with the aft fuselage subassembly. The empennage is divided into four subassemblies — the horizontal stabilizer, elevators, vertical stabilizer and the rudder. Both elevators are equipped with controllable trim tabs. The complete tail assembly is removable in 45 min.

Horizontal Stabilizer

The horizontal stabilizer is constructed as a unit with detachable tips and consists of a forward and aft spar of 24ST Alclad, with integral capstrips and flanged lightening holes. Ribs are of 24ST and heat treated 24SO Alclad. Dural extruded stringers of 24ST Alclad, laid out spanwise, resist compression and provide a riveting foundation for the skin. Tips are of 52S½H aluminum alloy, with two 24ST supporting ribs. The horizontal stabilizer, which is bolted to the rear section of the fuselage, has a positive angle of incidence of 2° relative to the longitudinal axis of the airplane. Area of the horizontal stabilizer is about 27.6 sq ft; maximum chord is 2'6".

Elevators

Elevators are fabric-covered, with an area of approximately 13 sq ft. They are statically and dynamically balanced by a cast lead weight attached tot eh outboard end of their leading edge. Right and left units are interchangeable, and their angular movement is 30° up and 20° down.

Each elevator is constructed of 18 flanged aluminum rib and one main spar, a V-section trailing edge and a short intercostal beam installed forward of the trailing edge so that a cutout section is formed for the trim tab. The leading edge is covered with a formed aluminum sheet as far back as the main spar, with the exception of cutouts for the hinge fittings. The fabric covering is doped, Grade A mercerized cotton fastened along each rib with Parker-Kalon screws. At each of the three cutouts it is hand stitched to the metal leading edge. Each elevator is hinged to its respective half of the horizontal stabilizer by three sealed-type hinge bearings.

Fore and aft motion of the control stick actuates the elevators through a push-pull rod from the base of eh stick to the bellcrank on the flap torque tube. A duplicate set of cables extends from the bellcrank to the two elevator horn assemblies.

Elevator Trim Tabs

Phenol fiber trim tabs on the elevators are hinged to the respective movable surface by 3 hinge bearings attached at the cutouts provided in the leading edge of the tab. Each is actuated by a push rod extending forward from a metal bracket on the tab to a small screw jack assembly inside the elevator. The screw jack assembly is actuated by a small cable drum which is adjusted by a knob on the pilot's control pedestal. Angular travel of the trim tabs is 10° up and 25° down. Each has an area of approximately 1 sq ft.

Vertical Stabilizer

The vertical stabilizer is a full cantilever structure with an area of 9.98 sq ft, and set 1° to the left, rotated about the centerline of the rear beam, to counteract the torque of the engine. It is constructed around a forward and an aft spar of 24ST Alclad, flanged ribs of formed 23ST and heat treated 24SO Alclad, covered with 24ST Alclad sheet. The skin is stiffened with light rolled stringers of 24ST. The tip is of 52½H aluminum alloy formed about 2 supporting ribs and is non-detachable.

Rudder

The rudder is a fabric-covered structure 10.4 sq ft in area with an angular movement of 30° each side of neutral position. It is actuated by a rod which is an integral part of the lower hinge fitting, in turn controlled by cable connected to the rudder pedals. It is dynamically balanced by a 16.6-lb cast lead weight attached at the top. To reduce static unbalance an additional balance weight is attached at the bottom leading edge, concealed within the fuselage.

The rudder is constructed around a main spar, 20 flanged 24ST Alclad ribs, a V-section trailing edge and a short intercostal beam, both of 24ST Alclad. A formed aluminum sheet covers the leading edge back to the main spar with the exception of the cutouts for the hinge fittings. The rudder is hinged to the vertical stabilizer with 3 sealed-type bearings at the top, bottom and center.

Rudder Trim Tab

A phenol trim tab is hinged to the rudder by 3 sealed-type needle bearings. Angular travel of the tab is limited to 10° each side of neutral and is controlled by a knob on top of the pilot's control pedestal which adjusts control cables extending aft to the horizontal stabilizer and up inside the vertical stabilizer to the trim tab actuating drum assembly. The area of the tab is .82 sq ft.

Landing Gear

The landing gear consists of 2 hydraulically-operated main gear assemblies with 27" wheels installed in the wings, and a fully retractable tail gear assembly with a 12½" wheel in the aft fuselage section. Air-oil combination shock struts are used. When retracted, both the main wheels and the auxiliary wheel are completely enclosed by fairing doors.

Magnesium castings bolted to the forward spar and to the upper and lower skin in each wing serve as trunnions for the main landing gear assemblies, the operating strut being mounted on the front spar in each wing. The steerable tail wheel may be unlocked for parking and taxiing by pushing the stick forward.

Operation of the landing gear is controlled by a handle at the lower left side of the cockpit. Pushing down on the control handle unlocks the gear and permits it to drop of its own weight. In an emergency, hydraulic pressure in th fairing-door retracting struts can be relieved by turning an emergency valve in the cockpit. The pilot, by yawing the plane from side to side, may cause the gear to lower the remainder of the way until the respective down-position locks engage. All landing gear locks except the hydraulic main gear down-lock pin are actuated by mechanical linkage from the control handle.

Brake System

Goodyear multiple disc brakes, operated by toe pedals on the rudder, are actuated by master cylinders. After braking pressure has been applied, the brakes may be locked by a parking brake which locks pressure in the brake lines and holds it after the pedals are released. Aluminum alloy 52SO tubing is used throughout the brake system for all rigid lines, and flexible hose is used between the top of the main struts and the rigid line in the wing, and between the shock cylinder and the strut fork. The brake system will operate if the regular hydraulic lines are shot away. It is separate from the high-pressure hydraulic system for operation of the plane's landing gear and for preselection of the wing flaps. A separate unit operates the Hydromatic propeller governor. For emergency use, and hand pump supplies pressure to the wing flaps and scoop, and may be use to ground check the entire system.

Engine and Accessories

The high-altitude, long-range Mustang is powered by a 12-cylinder, Packard-built Rolls-Royce Merlin V1650 liquid-cooled engine with a two-speed, two-stage centrifugal supercharger which has a low-gear ration of slightly better than 6:1 and a high-gear ratio of about 8:1. An aneroid switch automatically changes ratios, and the pilot may select cold rammed air, cold unrammed filtered air or unrammed hot air.

Ten Lord rubber mountings attach the engine to the mount. Ready access to the engine and accessories is provided by seven removable 24ST panels held in place against 24ST Alclad formers by Dzus fasteners.

The four-bladed Hamilton Standard Hydromatic propeller, 11'2" in diameter, is controlled by a governor that maintains constant propeller speed at the pilot's selection. The spinner is a spun shell of 24ST aluminum alloy which contributes to the plane's streamlining. Low pitch setting (hydraulic controlled) is 24°; high is 65°.

Fuel is carried in two self-sealing cells, one in each wing, and in a self-sealing fuselage tank. In addition, the Mustang can carry a droppable combat tank or a larger ferrying tank in each wing bomb rack, when no bombs are being carried. There is a submerged centrifugal booster pump in each main fuel cell to insure pressure and to prevent vapor lock at the high altitudes at which the plane now fights. If one tank or feed system is damaged, the other tank will operate independently.

Fuel gages for the two wing cells are located on the floor of the cockpit, while the fuselage tank has a direct-reading gage visible over the pilot's left shoulder. Pressure for altitude flying is supplied by connecting the combat or ferrying tanks to the exhaust side of the vacuum pump. The entire system is suitable for aromatic fuels.

For lubrication needs, a 12½-gal tank located on the forward side of the firewall feeds oil at any altitude. For cold weather operation, a warming compartment in the tank permits only a small quantity of oil to circulate and heat for starting. A non-congealing oil radiator for cooling, is mounted ahead of the coolant radiator, inside the scoop assembly, with its won outlet flap to regulate the flow of air. It is operated by a thermostatically-controlled electrical unit. A surge valve permits cold oil to bypass the radiator.

Cooling System

In addition to the oil cooling system, the Mustang's Rolls-Royce engine is cooled by two other separate cooling systems. The primary system employs a mixture of glycol and water under pressure of 30 psi. The coolant is pumped into the bottom of the coolant jacket on the lower exhaust side of each cylinder block where it passes to the cylinder heads through brass transfer tubes and out through manifolds on the intake side of each cylinder head. These manifolds discharge into the header tank just ahead of the engine, from where the liquid flows to the radiator under the fuselage to be cooled and then passes back to the engine pump.

Secondary cooling is employed in the engine's aftercooler which cools the supercharged fuel-air mixture. Coolant from the radiator passes through a jacket between the supercharger impellers, cooling the charged air as it passes into the second stage impeller chamber. The coolant then passes into the heat exchanger and cools the charged air as it passes into the intake manifolds, and returns to the expansion tank.

Electrical System

The 24 VDC, single-wire grounded electrical system is supplied by a 34 amp-hr storage battery located behind the pilot, and is charged by an engine-driven 100-amp generator on the left side of the engine, at a potential of 28 V. The greater part of the wiring is open, although the engine wiring is shielded and supported by conduit because of possible radio interference or vibration. Attachment to external power sources is provided for by a plug on the right side of the fuselage.

Ignition is provided by a dual magneto, the right-hand firing the intake spark plugs and the left-hand firing the exhaust plugs. A booster coil, to supply high-tension voltage during engine starting, is supplied by the right-hand magneto.

Interior lighting installations, consist of 2 fluorescent instrument instrument lights, 2 cockpit lights, a gun sight light and a compass light, with provisions for invisible light and controlled visible light. Exterior lighting consists of one landing light and 5 position lights, all controlled by switches on the pilot's switch panel.

Instruments

The rubber-mounted instrument panel contains all instruments with exception of fuel gages, which are on the floor, the oxygen gage, located at the lower right corner of the panel, and the hydraulic pressure gage, which is at the lower right corner of the switch panel mounted immediately below the instrument panel. Engine instruments are located to the right of the flight instruments in the instrument panel.

A remote-reading compass is located in the upper left corner of the instrument panel, while its transmitter is approximately 3' inboard from the end of the right wing tip, away from the magnetic interference of armor plate, electrical circuits and guns.

Engine controls are located on the pilot's left, with the constant speed control for the propeller immediately under the throttle and mixture control, on the same quadrant. The carburetor air control is located on the control pedestal, while the coolant radiator and oil radiator control switches are located above the pedestal.

Radio Equipment

Two command sets are installed in the plane, the antenna being of the vertical mast type. Receivers and transmitters are located on shelves back of the pilot's seat.

A Detrola radio range receiver is installed for ferrying. Provisions are made for a throat microphone, and the switch button is located in the throttle knob. The control panel for the radio equipment is located at the pilot's right, and incorporates push-button tuning.

Armament

Two free-firing .50-cal machine guns are mounted in a canted position in each wing in such a way that no protuberance of the outer wing surface is necessary. Electric heaters on each gun permit them to be fired in temperatures as low as -70°.

All four guns are fired by a switch on top of the pilot's control stick. They are sighted through an optical gun sight, and an auxiliary ring-and-bead sight also is provided. Cartridge cases are ejected through chutes in the under side of each wing.

Under each wing is a removable, streamlined bomb rack, each accommodating a bomb up to 550 lb, chemical tanks, combat fuel tanks or ferrying tanks. Bombs can be dropped in vertical dive, level flight or 30° climb.

This design analysis article was originally published in the July, 1944, issue of Industrial Aviation magazine, vol 1, no 2, pp 7-19, 101.
The original article includes a small portrait of the author and a photo of a P-51D, a blueprint-style 3-view and 10 detail drawings and diagrams, and 3 data tables. The drawings are of a P-51B-type aircraft.
Photos are not credited, but are probably from North American Aviation.

Photo and diagram captions:

Data tables:
Wings
Wing area (less ailerons)……220.55 sq ft
Chord at root……8' 8"
Chord at tip……4' 2"
Dihedral (at 25% line)……
Incidence (root)……
Sweepback……3° 35' 32"
Washout (tip)……
Areas (sq ft)
Ailerons (both)……12.7
Flaps……32.6
Stabilizer……27.6
Elevators (inc tabs)……13.05
Elevator trim tabs (both)……2.00
Vertical stabilizer……9.98
Rudder (inc tab)……10.4
Rudder trim tab…….82
Instruments
Flight
Gyro horizon
Bank and turn indicator
Directional gyro
Airspeed indicator
Altimeter
Rate of climb indicator
Engine
Manifold pressure indicator
Tachometer
Engine gage
Coolant temperature gage (incorporates
oil temperature, oil pressure and fuel pressure indicators)
Others
Hydraulic pressure gage
Landing gear position indicator
Suction gage
Fuel gages
Ammeter
Clock
Oxygen pressure indicator
Oxygen flow indicator
Oxygen signal assembly