One hundred and twenty days from the beginning of preliminary design to delivery of the first plane completely designed for quantity production. This was the record established by North American Aviation on its P-51 Mustang.
Although large numbers of the type have now been in action for some time, and a dive bomber version has already gone into production, design and construction details have just been released.
The race against time began in April, 1940, when the British Purchasing Commission opened negotiations with North American for production of a fighter plane incorporating all the combat knowledge gained to that time by the RAF, the AAF, and the company. The commission originally asked that North American build a fighter type already in production, but J L Atwood, NAA vice-president, sold its members on the new craft, which didn't yet exist.
To complete the design, for which more than 2,800 original drawings were made, Chief Design Engineer Edgar Schmued divided the work among specialized engineering groups, who worked with skeleton specifications made up from sketches and verbal instructions.
Despite the time limit, North American decided to use a laminar-flow wing, even though this type had never been used before. Although the laminar-flow section designed and wind tunnel tested by the NACA to which NAA engineers give full credit for research was used as a basis, the P-51 wing section, as finally perfected, differed considerably
Of full cantilever, stressed skin construction, the wing consists of two panels bolted together at the center plane of the fuselage. Both main and rear spars are flanged aluminum alloy sheet construction, with flap and aileron hinge supports mounted on the rear spar. Remainder of the wing structure consists of extruded stringers and pressed ribs, with skin covering of aluminum alloy. Fuel tanks are located between the spars on both sides of the centerline, with a structural door in the under side of each wing section to facilitate their installation and removal.
Next to the laminar-flow wing, North American engineers consider the outstanding factor in the P-51's aerodynamic efficiency to be the fuselage, which has what is believed to be the smallest cross-sectional area ever put behind an Allison engine.
In keeping with size and shape of the fuselage, a new idea was tried in the form of a semi-spherical, molded plastic windshield. Wind tunnel tests delighted the engineers, but flight test proved tough on pilots, for the curved glass distorted ground appearance and made landings extremely difficult. A conventional windshield, placed at a 40° angle to horizontal line of flight, was then installed.
The cockpit itself is under a flush type canopy with an upper and right side section hinged to open for pilot's entrance and exit. Sliding windows are built into both side sections, and the entire enclosure may be jettisoned as a unit for emergency egress of the pilot.
The fuselage is divided into three sections: engine, main, and tail, all of which are attached by bolts. At the cockpit, or main section, the fuselage construction consists of two beams. The structure comprises four longerons, two on each side of the cockpit forming the beam caps and the skin forming the webs, reinforced by vertical frames. Aft of the cockpit, the longerons extend into semi-monocoque structure reinforced by vertical frames.
The ethylene glycol engine coolant and oil radiators are set in the bottom of the fuselage, aft of the cockpit, enclosed in a duct with an adjustable air scoop. On initial flight tests the engine overheated, and wind tunnel tests showed that the disturbed boundaries of air under the wing and fuselage prevented a clean flow of air through the scoop. Lowering the entrance lip of the scoop approximately 1" from the fuselage bottom cured the trouble without affecting performance.
Empennage is a full cantilever structure with semi-monocoque fin and stabilizer. The full cantilever, metal-covered horizontal stabilizer consists of two spars, aluminum alloy ribs, and extruded stringers, and it is built as one unit with detachable tips. Elevators are of fabric covered aluminum allow construction, consisting of a front spar, short intercostal rear spar, flanged ribs, and metal leading and trailing edge sections. Both elevators which are interchangeable are statically balanced and fitted with trim tabs controllable from the cockpit.
Vertical stabilizer is a full cantilever, semi-monocoque structure comprised of forward and rear spar, flanged ribs, and extruded stringers.
Power plant it one Allison V-1710-F3R liquid-cooled engine equipped with ramming type air intake for altitude operation, swinging a three-blade Curtiss electric constant speed propeller of 10' 9" dia. The close-fitting cowling around the power plant consists of a forward ring and seven detachable panels for maximum accessibility.
To speed engine installation and removal and also to provide case of access, light weight, and simplicity of construction, and original structural aluminum mount was designed to replace the conventional welded steel.
To eliminate the chance of engine failure because of long lines from the oil tank to the engine the tank was located just above the engine ahead of the firewall. Oil pressure at any attitude of flight was assured through a swivel unit arranged to be under oil at all times, so the liquid would feed in a vertical climb even though the tank was only one-quarter full.
In the original design, the carburetor air intake was set over the engine, with the opening well back from the propeller. On flight tests, however, the engine cut out under certain high speed conditions and instruments indicated a peculiar pulsation effect in the air scoop. Further flight and wind tunnel tests indicated a repetition of the old boundary-layer air trouble and revealed that an air "beat" from the propeller was being transmitted through the intake to the carburetor. To overcome the first condition, the scoop was raised slightly. the second trouble was eliminated by lengthening the scoop to a point just behind the propeller where air was picked up before the pulsation had been set up.
All three units of the hydraulically operated landing gear rare fully retractable, with the main units being fitted with hydraulic brakes. Wheel wells are covered by hydraulically operated fairing to eliminate drag when the gear is in down position. The tail wheel, steerable within the range of rudder pedal travel can swivel 360°.
The tail wheel also employs a simple surging orifice replacing the normal metering pin to regulate the amount of oil flowing from one strut cylinder to the other under landing impact. Its development was brought about during design, when it was found the originally scheduled unit could not be delivered on time by the manufacturer. NAA engineers designed and built the type now used, then turned the drawings over to the original producer.
|Span||37 ft 5/16 in|
|Length||32 ft 2-7/8 in|
|Height||8 ft 8 in|
|Total wing area||233.19 sq ft|
|High speed (approx||400 mph|
|Engine||Allison V-1710-F3R V-type ethylene glycol cooled|
This article was originally published in the June, 1943, issue of Aviation magazine, vol 42, no 6, pp 234, 237, 361, 362, 363.
A PDF of this article is included in the P-51 PDF draft. It includes 7 photos and the data table above.
Photos are not credited, but are probably from North American Aviation.