North American P-51 Mustang

Virtually all the aircraft flying on either side in the current conflict had their development period in the days before the Munich appeasement. The Messerschmitt series first saw action in Spain, the Hurricanes and Spitfires underwent prototype development in the middle thirties. Experimental work on the Fortresses, Liberators, and even the Lancaster types of heavy bomber were under way before the first bomb fell on Poland. The first airplane to be built entirely from information garnered from the battlefield was an aerial dark horse — the North American Mustang.

This ship's development was a saga of engineering daring. More radical ideas had been packed into its slim frame than any single airplane in the history of the fighter class. Yet the wonder was so subtly blended that most experts passed the ship off as just another untried airplane. That was until early last summer, when the first P-51s showed up in England and went to work, assigned to low-level army cooperation.

Doing most of her work at zero level, the type hopped across the English Channel, frequently in the trough of the rolling channel swells. Flying lower than the treetops, they defied electronic airplane locators until they were in shooting range. Working in teams of two ships, they scouted out the enemy's hidden secrets, strafed troop and cargo trains, low-level bombed ammunition dumps, knocked out radio and locating stations, crippled anti-aircraft and coast defense installations and generally made life miserable for ground units whose anti-aircraft equipment was not built to fight airplanes making early 380 mph at treetop level.

HUNDRED DAY WONDER

The Mustang's surprising performance was no startling revelation to those who knew something of the history and background of the airplane. Six months after war had begun, the British Purchasing Commission called on North American Aviation for the mass production of a single-seater fighter already in existence. Its performance was indicated to be adequate to meet contemporary opposition. There was plenty of precedent for building what was on hand for the purpose of matching the quantitative advantage the Axis had developed from several years of extensive production. Nevertheless, the fact that a handful of Hurricanes and Spitfires had proven themselves to be the equal of superior numbers of inferior aircraft dictated a policy of planning for quality ships in the future.

North American's counter proposal was a totally new design, incorporating all the new mechanical and operational requirements found to be needed in an airplane that would be a year ahead of its time when it actually arrived on the battle line To achieve this meant daringly cutting across long established precedents, adopting new and radical idea and ruthlessly treading on the toe of aerodynamic conservatism.

The British commission did some thing unheard of up to that point They bought an airplane that did not even exist on paper, that was merely a set of ideas of what an airplane should be. They gave North American 120 days to build a prototype airplane from scratch. Its accomplishment was a saga in itself which merit lengthy discussion, and is a tribute to the courage and ingenuity of American engineering.

SHOCK WAVES

Like many modern airplanes, it would be difficult to say who designed the Mustang. Edgar Schmued, chief design engineer, laid out the general lines of the ship over a weekend late in April, 1940. The basic problem was to get a design which would outperform the aircraft projected by the Axis three years hence, This was a matter of jumping over 70 mph in cruising and nearly 100 mph in top speeds. This brought the engineering staff smack up against the stone wall of a newly discovered phenomenon — shock waves.

There is a speed, the proverbial "speed of sound" at which moving bodies or even, waves of motion can no longer "part" the air molecules but tend to compress them into a nearly solid mass. The highest velocity at which pressure disturbances can be transmitted through the air is estimated to be 1,120 ft/sec or about 764 mph at sea level, and 974 ft/sec or about 664 mph at 35,000 feet under standard conditions.

In the case of actual physical shapes moving through the air stream, long before the theoretical limit speed is reached, changes in pressure and density take place in the air surrounding the body. In the case of an airfoil, like a wing, the air seems to "pile up" at the highest point on the airfoil's upper surface, forming periodic wavelike jolts as the air progressively reaches compression points great enough to be felt. If the speed is continued under these conditions, these shock waves can be physically destructive to the airplane. Numerous otherwise unexplained accidents to high-speed aircraft have finally been attributed to shock waves.

The closeness to which a body can approach the limiting speed is determined by its shape. In the case of an airfoil, the closest approach known was the laminar-flow wing, a special thin airfoil developed by the National Advisory Committee for Aeronautics. This wing had a thin section and a special profile which allowed the air to flow in parallel and undisturbed layers. It was possible to design a wing with as much as 50% less drag or parasite resistance as a normal, conventional airfoil. Exact details on the airfoil which North American perfected, based on NACA research, are still on the US Army's confidential information list.

THE FINAL PRODUCT

The ship that finally emerged was a slim-lined, low-wing monoplane with a wingspan of about 37', an overall length of 32' 2-7/8" and a height of 8' 8". It was powered by an 1150-hp Allison engine equipped with a ramming intake. The ship was equipped with a 10' 9" Curtiss electric constant-speed propeller.

The wing, sporting its NAA-NACA laminar-flow section was a cantilever skin-stressed unit, built in two panels and bolted together at the center of the fuselage. The main and rear spars were built of flanged aluminum alloy sheet. Flaps and ailerons were hinge-mounted to the rear spar. Stringers were of alloy extrusion, while ribs were made up of stamped metal. The ship was Alclad, aluminum-coated special aircraft alloy.

Self-sealing tanks were installed in the space between the front and rear spar on each side of a centerline arranged to hold the cells. A structural door was provided in each section's under-surface for easy installation and removal of the cells.

The fuselage lines were created in record time by a purely mathematical system known as development of secondary degree curves. This method, an advance over the formerly used method of designing fuselage lines "by eye", gave designers an absolute method for determining the best streamlining between two given points, and assured greater precision and accuracy than ever was possible previously.

The fuselage was built up in three sections: engine section, main, or cockpit section and tail unit. All three are separable and are bolted together for ease of repair.

The fuselage construction at the cockpit section consists of two beams. They are composed of two longerons on each side of the cockpit which form the beam caps, with the skin forming the web. This is reinforced by vertical frames.

Behind the cockpit, the longerons extend into a semi-monocoque structure reinforced by bulkheads and stringers. In the section behind the cockpit, a circular honeycomb radiator is installed low in the fuselage, along with the oil tank regulator. These are in a duct with an adjustable scoop.

The tail unit is also a full-cantilever structure, with a semi-monocoque fin and stabilizer. The horizontal stabilizer is built in one unit with detachable tips. It consists of two spars, stamped alloy ribs and extruded stringers, covered in a flush-riveted aluminum alloy skin. Elevators are made up of two interchangeable sections and, like the rudder, have a metal frame, fabric-covered. The rudder is statically and dynamically balanced, as are the elevators. They are equipped with trim tabs, controllable from the cockpit.

The ship's landing gear is unusually wide, consisting of two main leg assemblies and a steerable tail wheel. All three units are fully retractable into recesses flush when closed. Tail wheel is capable of swiveling 360° and steerable within the range of rudder pedal travel. All three units are hydraulically operated. Main wheels are fitted with hydraulic brakes. Fairing doors form part of the wing contours.

The cockpit is under a flush-type plastic canopy, with the upper and right section hinged for easy entrance. Sliding sections are incorporated on both sides. The entire cockpit unit may be quickly jettisoned for quick exit by the pilot in case of emergency.

The Allison engine is faired in a closely-fitted cowling, consisting of a forward wind and seven detachable panels for swift access to the power plant. For quick and easy engine installation, an aluminum alloy engine mount is used.

Armament on the Mustang varies with its use. Mixed .30- and .50-cal machine guns were used in the earlier models. Some ships used .50-cal guns and 20-mm cannon. The latest version of the Mustang is called the A-36 — a dive bomber, equipped with diving brakes and bomb racks, carrying six .50-cal machine guns, two firing through the propeller and four mounted outside the propeller arc. The dive bomber has an all-up weight of only 200 lb greater than the fighter version of this airplane.

Radio equipment is placed in a rather unique spot in the compact design. Sets are placed behind the pilot's seat in a series of sliding shelves which not only hold them firmly in place, but render them accessible for inspection and repair.

The Mustang's record as a fighter-scout and now bomber is phenomenal. To this ship and to an American pilot serving with the RAF, Pilot-Officer Hollis H Hills of Pasadena, CA, goes the distinction of having downed the first Focke-Wulf 190 — the current pride of the Luftwaffe. In the ship's design, half a decade of normal development had to be jumped under pressure. Its application to actual combat worked a telling change of pace on the Axis who had to re-rig large portions of their equipment in hopes of stemming the tide.

Nevertheless, the laminar-flow wing is not necessarily a native of low altitudes. By adjusting the power supply and providing the right supercharging, the Mustang may be made to kick the Axis at almost any altitude.

This article was originally published in the May, 1943, issue of Air Tech magazine, vol 2, no 5, pp 18-23, 68.
The original article includes 10 photos and a labeled cutaway drawing. [ lineart ], [ SVG ]
Photos credited to Kollsman, NACA, North American; cutaway drawing from Flight.

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