DESIGN ANALYSIS NO 7
The North American P-51 Mustang

By William R Nelson,
West Coast Editor, Aviation

Aviation's graphic and thoroughly detailed engineering dissection of NAA's great fighter — initially completed less than four months after design inception, then fourfoldly hailed for low-altitude cooperation, dive-bombing versatility, high-altitude fighting speed and long-range prowess.

Prior to World War II it was generally agreed among aeronautical engineers and military aviation authorities that it was impracticable, if not impossible, to design an airplane capable of accomplishing more than one type of military operation.

The record of North American's P-51 Mustang fighter proves, however, that it is both possible and practical to create a single basic design that can be modified, as military needs dictate, to keep abreast of requirements. Among single-seat, single engine fighters, the Mustang has been credited as the best low-altitude cooperational craft, the most versatile dive bomber, the fastest high-altitude fighter, and the plane with the greatest range.

All this has been achieved by a plane whose basic design and most of its original specifications remained unchanged. New models incorporated equipment and design refinements but retained the desirable advantages of preceding versions.

The original Mustang, designed and built for the British in less than 120 days, was intended for low and medium altitude work. It was a low-wing all-metal monoplane powered by a 12-cyl, V-type Allison engine of 1,150 hp, and it was credited with close to 400 mph speed. RAF pilots said it was highly maneuverable, had no "cranky" characteristics; and, for those early days, it was heavily armed, with .50-cal guns, one on each side of the engine, and one .50- and two .30-cal guns in each wing. A gun camera was mounted in the left wing.

In its P-51 version for the USAAF, the Mustang — retaining its high maneuverability with somewhat increased speed — quickly acquired fame as a "train buster" by virtue of its two 20-mm cannon in each wing. Stripped of guns and with a K-24 camera, it became a widely used scouting and reconnaissance plane.

The invasion of Sicily was highlighted by reports of a phenomenal new "secret" fighter-dive bomber. This craft — which combat men quickly dubbed the Invader — was the A-36 version of the Mustang. Retaining the characteristics of its fighter and level-bomber forbears, the A-36 was equipped with dive brakes, two 500-lb bombs, and six 50-cal machine guns.

Next revision retained the basic design and uses of previous models, but increased the speed with single-stage, single-speed supercharger. Auxiliary fuel tanks gave considerably increased range. Carried were wing bomb racks and four 50-cal guns, two in each wing, maintaining striking effectiveness against ground targets.

Another revision introduced the 1,500-hp Packard-built, Rolls-Royce Merlin engine with two-speed two-stage supercharger. Also added were removable bomb racks, depth charges, chemical tanks, and auxiliary fuel tanks. Armament was four 50-cal guns. Speed and ceiling both went up, whereupon the Mustang was officially credited with the highest ceiling ("over 40,000 ft") and the greatest speed ("over 425 mph") of any fighter in existence. Soon the Mustang was accompanying our heavy bombers on their longest missions, since it possessed the longest range of any single-engine fighter in the war.

These achievements are, from an engineering standpoint, remarkable — because they were accomplished by a plane that does not to any extent embody previously unknown engineering features, but rather employed refinements of know accepted practices.

Such performance is attained by close attention to aerodynamic cleanness of design, employing an efficient, low-drag, laminar-flow airfoil, a modification of an NACA design. Second-degree curves, calculated as mathematical expressions, are employed for external lines of fuselage, fillets, ducting, and air scoop.

The air scoop is located below and just aft of the center of the fuselage, where it was found by wind tunnel tests to create less drag, while operating efficiently. Both oil and coolant radiators are contained in the air scoop.

An identifying feature of the Mustang is the square wing and tail surface tips, which tended to prevent stalling and to maintain excellent aileron control.

The fuselage, of semi-monocoque construction, is divided into three main sections: Engine mount, main, and rear section, all joined with bolts. With exception of cockpit armor fore and aft, fuselage is entirely Alclad and aluminum alloy extrusions.

Engine mount is a box beam of Alclad sheet and extruded parts, designed so that the engine can be removed as a unit. Mount is attached at the firewall by four bolts.

Main fuselage section is constructed around four 24ST extruded longerons, intermediate frames, Alclad covering, and stringers. Stainless steel sheet and armor plate firewall form forward bulkhead. A turnover truss of 24ST extrusions and formed sheet protect the pilot. Upper longerons are extruded H-sections which extend aft from firewall, tapering to T-section and terminating near rear section. Lower longeron, H-beam and U-channel, extends full length of section.

Eight riveted and bolted assemblies which comprise main fuselage section may be removed and replaced as units. They are: Firewall, turn-over truss, upper deck, left and right side panel subassemblies, radio shelf, web assembly, and lower section with air scoop.

Comfort and safety are given consideration in the design of the cockpit seat, which accommodates the seat-type parachute and has a kapok back-cushion life preserver, provisions for heating and cooling, and protecting armor plate at the firewall and seat.

The combination armor-plate firewall protects the pilot from line of level flight to approximately 20° below it, also against fire from the engine. Firewall is face-hardened steel armor, except a section at center of stainless steel to provide room for oil tank. Aft protection is provided by two plates of face-hardened steel behind the seat.

Protection and visibility are afforded by windshield, rear window, and cockpit enclosure. Forward flat section of windshield is bullet-proof, 5-ply laminated glass, 1½ in thick, slanted 31° from vertical, being the best compromise for visibility, protection, and aerodynamic contouring. Side and upper panels of windshield are of 3/16-in safety plate and transparent plastic. Windshield cowling extends from lower forward end of glass to firewall and down to upper longeron.

Over instrument panel a shroud, integral with windshield, extends aft with a circular rubber extrusion to protect pilot. This shroud supports windshield defroster, optical gun sight, and hand-holds, and it also eliminates instrument glare in the window glass.

Cockpit enclosure consists of upper and side plastic panels, each in two sections, forward one forming a sliding window with locking handle. Right upper panel hinges upward; left panel hinges downward against fuselage. Both have locks controlled from inside and outside. Hood is attached by four hinges on upper longerons. An emergency release permits enclosure to be removed or jettisoned in emergency.

Aft windows of molded Lucite fit fuselage contour and are removable for access to radio behind pilot. Aft of radio, plywood bulkhead prevents draft and keeps objects from rolling aft and fouling controls. Nut plates at center of bulkhead secure oxygen bottles.

The rear fuselage section consists of two 24ST longerons, a shelf and five formers of 24ST, three solid bulkheads, and Alclad skin.

Wing is cantilever stressed-skin design in two sections, bolted at center. Each consists of main panel, removable tip, aileron, and full trailing-edge flap. Wing has angle of incidence of approximately 1° at root and dihedral of 5° along 25% chord line, to give stability.

The 25% chord line is perpendicular to longitudinal axis of airplane. Wing area, including ailerons, is 233.19 sq ft, with span of 36 ft 5/16 in and a taper ratio of .499.

Main wing panel consists of a main spar, rear spar, pressed ribs, and extruded stringers covered with alloy sheet. Space is provided at inboard end for self-sealing fuel cell, part of which is located in fuselage. A gun bay is in each wing panel to accommodate guns, ammunition containers, and chutes. Main landing gear retracts into wheel bay in the inboard leading edge.

Main structural member of wing is forward or main spar, of two sections of of 24ST sheet spliced together. Inboard spar section is fabricated of .129-in thick 24ST, with angle flanges along both upper and lower edges for spar caps. A .25-in thick 24ST bar is riveted to inner side of upper cap between stations 0 and 85.5.

Rear spar is formed of two sheets of 24ST spliced at station 128.6. Upper cap is reinforced by a .091 24SO angle between stations 0 and 92.5. Ribs and formers are approximately 12.5 in apart and are of 24SO, heat-treated after forming to 24ST. At leading edge, wing has sweepback of 3° 35' 32". Aspect ratio is 5.815.

Each aileron has two spars and twelve flanged ribs covered with 24ST. The forward spar is U-shaped 24ST Alclad, and ribs are 24SO heat-treated to 24ST. Trailing edge is 24ST sheet reinforced with aluminum supports and plastic ribs. Three aileron hinge brackets bolted to the forward spar provide bearing attachment points.

Ailerons are dynamically and statically balanced. Internal aerodynamic balance is obtained by a diaphragm attached to forward edge of aileron and sealed to the rear spar by fabric strip.

Phenol-fiber trim tabs are mounted in each aileron by three hinge bearings. A metal horn tab provides attachment for actuating rod. Left tab, adjustable in flight, is operated by a knob on control pedestal. Angular travel, 10° up and 10° down, is limited by stops on cables.

Ailerons are conventionally controlled by the stick, and to meet variations in specifications they can be connected for angular travel of 10°, 12°, or 15°.

Structure of the 24ST Alclad-covered wing flaps is two Alclad spars, 13 nose ribs of 24SO heat-treated after forming to 24ST, 15 main ribs, and a series of rolled-section stringers, all of 24ST. The flap trailing edge is formed from a single 24ST sheet reinforced with 27 tapered hat-section supports. The flaps are hinged on three sealed ball bearings, and are hydraulically controlled by a lever selects and holds any corresponding position of the flaps.

Stabilizer is full cantilever, no-adjustable, with detachable tips, and is fixed at a positive 2° angle of incidence relative to the longitudinal axis of the airplane. Forward and aft spars are of 24ST Alclad. Flanged ribs are formed of 24SO Alclad heat-treated to 23ST, as are six extruded stringers. Dual stringers are on the lower covering. Stabilizer tips are of 52S, 1/2-H stock on two ribs. Area is approximately 28 sq ft, and span is 13 ft 2-1/8 in.

Elevator incorporates 18 flanged ribs, front spar, trailing edge, and a short intercostal beam, all 24ST Alclad. Covering is fabric with Alclad leading edge extending to main spar, except for that portion cut out for elevator hinge fitting. Right and left elevators are interchangeable, fastened to stabilizer with five sealed ball bearing hinges, and are statically and dynamically balanced. Static balance is by a 13¼-lb lead weight attached to outboard end of leading edge. Total elevator area is approximately 13 sq ft, and angular movement by the control stick is 30° up and 20° down. Each elevator has an adjustable trim tab approximately 4-11/32 in by 32-1/16 in.

Fin is composed of front and rear spars of 24ST Alclad and ribs covered with 24ST Alclad sheet. Tip is on two ribs, and skin is stiffened spanwise by light rolled stringers. Area of fin is 9.61 sq ft, and it is set 1° to the left of center line of rear beam.

Rudder consists of spar, 20 flanged Alclad ribs, V trailing edge, and a short beam in front of trim tab, which is covered with fabric, and 24ST sheet covers leading edge back to main spar, except cut-out for rudder hinge fitting. The rudder is hinged to fin with three sealed ball bearings and is dynamically balanced by means of 16.6-lb lead at top. An additional balance weight at bottom of leading edge reduces static unbalance. Area is 10.4 sq ft and angular movement is 30° each side of neutral. Operation is by pedals through cables.

Phenol-fiber trim tabs on elevators and rudder are hinged by three sealed needle bearings. Rudder tab is controllable from cockpit, and angular travel is 10° up and 25° down, limited by stops on cable.

Landing gear is three-point, with two 27-in main wheels and full-swiveling, steerable 12.5-in tail wheel, hydraulically retractable. Main wheels retract into wing wells and tail wheel into fuselage, all fully enclosed.

Except for hydraulic main gear down-lock pin, landing gear locks are actuated from the control handle bellcrank. Cables from bellcrank actuate tail gear up-latch and down-lock pin. A push-pull rod from the lower end of the control handle works lock system in main wheel bay.

Main landing gear magnesium support casting is bolted to front spar at the outboard end of the wheel well. Hydraulic struts on the front spar retract the gear inboard. A spring-loaded, hydraulically controlled pin locks main gear down.

Tail gear is mounted on a magnesium casting bolted to lower longerons. Shock strut assembly includes cylinder, piston, torque tube, and post housing which supports axle. Gear is steered by cables from rudder bellcrank. Fairing doors are hinged at side, and a link pulls them up as gear is retracted. Tail wheel is unlocked with stick in the forward position during taxiing and parking.

Emergency lowering of landing gear is accomplished by pushing down control handle at left of seat, also relieving hydraulic pressure in retracting struts with emergency knob in cockpit, which causes dear to drop of its own weight. Pilot then yaws plane until gears engage downlocks.

Cylinder, connected to brake by aluminum alloy tubing, furnishes pressure for the Goodyear multiple-disk brake, via separate hydraulic system controlled by pedals. This pressure is relieved by a spring when pedal is released. Parking brake is controlled by depressing brake pedals and pulling knob below instrument panel. Pressure is retained until released by depressing both brake pedals.

Latest model of P-51 is powered by a 12-cyl Packard-built Rolls-Royce 1,500-hp V-1650 liquid-cooled engine having an after-cooler to reduce charge temperature.

Induction system employs a two-speed, two-stage supercharger with low-gear ratio of 6.391:1 and high-gear ratio of 8.095:1. Pilot may select cold rammed air; cold, unrammed filtered air; or unrammed hot air, as necessary. The Bendix-Stromberg double-throated, injection type, updraft carburetor is fitted with a double-diaphragm acceleration pump, automatic mixture control, fuel pressure regulator, fuel control unit, and throttle.

Automatic manifold pressure regulator limits maximum boost when below full throttle and maintains predetermined pressure for any given position of throttle lever.

Ignition is provided by two magnetos of the rotating magnet type, the right hand one being connected to booster coil which supplies high tension current when starting.

Engine mount consists of two Y-shaped 24ST box beams stiffened by built-up cross members of 24ST Alclad. Forward frame incorporates leading edge, and front duct section of carburetor air scoop and aft frame attaches to the two beams near the center. Engine is supported on rubber mountings between side beams. Mount is attached to firewall by four bolts.

Cowling, providing maximum accessibility to both engine and accessories, consists of 24ST Alclad formers and seven removable panels.

With adoption of Rolls-Royce engine, the Mustang was equipped with 11 ft 2 in four-blade Hamilton Standard Hydromatic propeller, controlled by a governor which maintains selected propeller speed. Spinner is a streamlined spun-shell of aluminum alloy.

Fuel is supplied from self-sealing cells, one in each wing, and form auxiliary tanks when fitted. Fuel flows from tanks through a submerged booster pump to dual check valve, then through selector valve and strainer to engine-driven fuel pump and carburetor. When auxiliary tanks are used, fuel passes through the selector valve to main fuel line. The booster pumps which boost fuel to engine pump at high altitudes operate as emergency pumps in event of engine-pump failure. Only one booster pump is operated at a time.

Fuel strainer and hand-operated engine priming pump are provided, and entire system is suitable for aromatic fuels. Droppable ferrying or combat may be installed on bomb racks when bombs are not being carried, fuel being withdrawn by engine-driven pump. Pressure is supplied to ferrying tanks by connecting them to vacuum pump exhaust.

Oil flows from bottom of 12.3-gal oil tank (forward of firewall) to oil pump which delivers through Cuno filter to moving parts of engine. Scavenger pump in sump delivers it to oil tank, either directly through a thermostatic control valve, having a by-pass, or to radiator and then back to tank, depending upon oil temperature.

Self-thawing oil radiator is forward of coolant radiator inside scoop. Airflow through scoop is regulated by outlet flap, thermostatically controlled. Flap can be controlled from the cockpit for emergency operation, and a surge valve, integral with thermostatic valve, permits cold oil at excessive pressure to by-pass radiator completely and return to tank.

Three systems — engine oil, engine coolant, and aftercooler — cool Mustang's Rolls-Royce engine.

Engine cooling system utilizes a centrifugal pump which delivers coolant into jacket on lower exhaust side of each cylinder block, whence it passes to cylinder head through transfer tubes and out through manifolds on intake side of head, discharging into header on front of engine, from which it flows through radiator.

Secondary after-cooling system, which reduces temperature of supercharged fuel-air mixture, consists of expansion tank, heat exchanger, and coolant pump. Coolant flows from expansion tank to pump, through radiator and supercharger case, and through a jacket between supercharger impellers, cooling air before it passes into second stage impeller chamber. Coolant then passes into heat exchanger and cools air from the second stage before returning to expansion tank. Scoop with thermostatic exit flaps provides air for oil, engine, and after-coolant radiators and is located beneath fuselage aft of cockpit, where it causes less drag than if farther forward. At high speed, jet action from the heated air compensates to a large degree for internal air drag. Flame-dampening exhaust stacks on either side contribute to speed by exerting a jet-propulsion effect of approximately 200 hp.

The Mustang's electrical system is 24 VDC, single wire, grounded type, most of the wiring open, supported by clips. Engine wiring, because of possible radio interference and vibrational stress, is shielded and supported by conduit.

Current is supplied by a 34 amp-hr. battery, aft of seat, charged by engine-driven generator. Connection with electrical system is through solenoid switch. A 100-amp high speed generator supplies current through a relay, which serves as generator cutout. A voltage regulator maintains potential at 28 V.

Radio equipment consists of sets for communication with other aircraft or ground. Antenna is a fore-and-aft, vertical-mast type. Receivers and transmitters are aft of seat.

Gunnery equipment consists of four fixed .50-cal guns in pairs in wings, in canted position, rotated 60° to prevent protuberances. They are adjusted to converge fire with center line of airplane at 300 yd. Lateral adjustment of 1/2° is provided on either side, and guns are quickly removable through doors in upper wing surface. Ammunition is fed to top sides of guns through stainless steel chutes. Cases and links are ejected through metal chutes in lower wing skin. All four guns fire simultaneously, control being by switch on control stick. Sighting is through optical gun sight or auxiliary ring-and-bead sight. Electric heaters are attached to each gun, permitting them to function at temperatures as low as -70°.

A removable, streamlined bomb rack is provided on each outer wing panel for bombs up to 500 lb, depth charges, chemical, or auxiliary fuel tanks. Fusing of bombs is electrically controlled from cockpit, and bombs can be dropped in a dive, level flight, or 30° climb. Sway braces are integral with racks.

This design analysis article was originally published in the July, 1944, issue of Aviation magazine, vol 43, no 7, pp 127-147.
A PDF of this article, recovered from microfilm, includes 4 detail photos, a phantom rendering and a 3-view and 27 detail drawings and diagrams, and 10 data tables. The detail drawings include 3 renderings of the panel and both cockpit side layouts.
The PDF also includes the articles "North American P-51 Has Novel Design Features" and "Aerodynamic, Weight, and Servicing Refinements Featured in North American P-51".
Photos are not credited.
The plane represented appears to be a P-51B.
Note:The A-36 was most commonly known as the Apache. It was also frequently referred to as a Mustang.

Photos and diagrams:

Data tables:

Dimensions and Leading Particulars
General
Span37.03'
Length (overall)32' 2-3/8"
Length (tail wheel on ground)30' 8"
Height (tail wheel on ground, propeller blade vertical at top)12' 6"
Wings
Airfoil sectionNAA-NACA low drag
Chord at root8' 8"
Chord near tip (215" from fuselage centerline)4' 2"
Incidence (variable)Approc 1°
Sweepback3° 35' 32"
Dihedral (at 25% line)
Stabilizer
Span13' 2-1/8"
Maximum chord2' 6"
Incidence
Dihedralnone
Fuselage
Width (max)2' 11"
Height (max)6' 3-7/16"
Length (without engine mount, front of heat exchanger to tip of tail)24' 2½"
Length (with engine mount, tip of propeller shaft to tip of tail30' 9"
Areas
Wings (less ailerons)220.55 sq ft
Ailerons (total)12.64 sq ft
Flaps (total)32.6 sq ft
Stabilizer (including elevators)27.85 sq ft
Elevators (including tabs)13.05 sq ft
Elevator trim tabs (total)2.00 sq ft
Vertical stabilizer8.83 sq ft
Rudder (including tabs)10.25 sq ft
Rudder trim tabs (total)0.82 sq ft
Settings & Range of Movement of Control Surfaces
Stabilizer, fixed
Vertical stabilizer, fixed offset (from fuselage centerline)
DegreesIn at max chord
Ailerons, wing, up travel (from neutral)10°
12°
15°
2.2
2.7
3.3
Ailerons, wing, down travel (from neutral)10°
12°
15°
2.2
2.7
3.3
Elevators, Up travel30°8.63
Elevators, Down travel20°5.9
Rudder, Right30°13.5
Rudder, Left30°13.5
Flaps50°20.6
Trim tabs
Elevator, Up10°
Elevator, Down25°
Rudder, Right10°
Rudder,Left10°
Aileron, Up10°
Aileron, Down10°
Tolerance on control surfaces movementsApprox ±¼°
Landing Gear
TypeHydraulic, retractable, conventional 3-wheel
Tread11' 10"
Shock strutsAir-oil combination
Wheels (Magnesium alloy construction) diameter27"
Tires (all-weather tread)27"
BrakesDisk, hydraulic
Oleo travel8"
Tail Gear
TypeHydraulic, retractable, steerable
Shock strutAir-oil combination
Wheel diameter12.5" × 4.5"
Tire (channel tread)12.5" × 4.5"
Oleo travel7.50"
Engine
TypePackard-built Rolls-Royce
DesignationV-1650-3
Number of cylinders12
Gear ratio21 : 44
Coolant (70% water and 30% ethylene glycol by volume)Type D — Spec AN-E-2
Propeller
TypeHamilton Standard Hydromatic
Diameter11' 2"
Blades4, paddle-type
Pitch setting (hydraulic controlled)
Low24°
High65°
Tabulated weights of principal assemblies
A (propeller)450 lb
B (landing gear)200 lb each
C (forward fuselage, engine)2,250 lb dry
D (wings)850 lb each, including fuel cells
E (wing tips)20 lb each
F (ailerons)25 lb each
G (flaps)35 lb each
H (scoop, coolers)450 lb dry
I (main fuselage)1,100 lb less radio equipment (150 lb)
J (rear fuselage)150 lb
K (horizontal stabilizer)70 lb
L (elevators)25 lb each
M (fin)20 lb
N (rudder)35 lb
Weights listed less than 100 lb given to nearest 5 lb.
Weights listed more than 100 lb given to nearest 50 lb.