Aerodynamic, Weight, and Servicing Refinements
Featured in North American P-51

Continued advances in engineering of the Mustang reflect improvements of specific interest to the aircraft designer. Among these are betterments in such installations as landing gear fairing door, landing light, gun and ammunition chutes, and the armored firewall.

Fairing Door

This hydraulically operated inboard unit, covering the wheel well, is hinged close to the airplane centerline, and is designed to open to permit extension of the landing gear and then close after the gear and then close after the gear is extended. Eliminating a source of drag, the door has an area in excess of 4 sq ft and thus adds considerably to the lifting area of the wing — an important factor as takeoff with heavy bomb load under each wing.

Also, gravel and dirt from the propeller blast are prevented from entering the wheel well compartment and coming into contact with landing gear and door lock mechanisms, hydraulic lines, coolant pipes, and wiring circuits immediately accessible through the door opening.

Landing Light

NAA engineers found that the leading edge was not the most desirable location for the landing light, since the Mustang wing is comparatively thinner because of its laminar flow design. Curvature of the lens also caused refraction.

By relocating the light in the wheel well, lighting efficiency has been improved by about 40 percent.

In its new location, the installation is readily accessible for quick replacement, the light beam is not obstructed, and intersection with the propeller arc is below the pilot's line of vision.

Control switch for the light is mounted on the pilot's switch panel. A spring-loaded safety switch, actuated by the support arm for the light, is connected in series with the control switch, and breaks the circuit when the landing gear strut fairing pushes the light upward into the wheel well.

Armament Access

Gun and ammunition bay doors on the P-51 embody good design for quick and easy access to armament compartments.

Access to the bays in either wing is obtained by loosening two fasteners which safety two cover latch handles, swinging the handles up, and opening the forward cover. Rear cover of the gun bay may then be lifted out to fully expose the three .50-cal guns.

Access to the ammunition bay is provided by raising a handle in the gun bay to free one side of the ammunition bay cover, which then may be lifted out. This gives access to the three ammunition belts.

The two removable doors are replaced first, and the hinged gun bay door is closed last. This interlocking door arrangement greatly speeds work of armament men when servicing the guns and reloading between combat missions.

Shell-Ejection Chutes

Phenolic fiber shell-ejection chutes recently designed for the Mustang have been found superior to stainless steel for this purpose, after exhaustive firing tests.

Impact of the .50-cal brass shell cases against the ejection chute edges caused warping of the stainless steel, which would not return to its original formed shape. Also, peening action of the empty cases against rivets on the inside of the striking plate damaged them so severely that at the end of some 4,230 rounds the plate fell off.

The fiber chute acted as a cushion for the shell cases, and after firing more than 10,000 rounds, examination showed the new installation to be superior.

Also, phenolic fiber chutes are quicker to manufacture — requiring only about 25 min compared to more than 1 hr for the stainless steel chutes.

Firewall

Designed to save weight and material, the firewall on the P-51 is fabricated of armor plate and does double duty by serving as a structural member attaching to upper and lower longerons, and providing protection for the pilot from frontal enemy gunfire. The installation thus eliminates the need for the usual stainless steel firewall with additional backing of armor plate.

This article was originally published in the July, 1945, issue of Aviation magazine, vol 44, no 7, pp 197, 199, 201.
A PDF of this article is included in the P-51 PDF. It includes 5 photos and a detail drawing.
Photos are not credited, but are probably from North American Aviation.
Photos are of a P-51 with six guns in the wing, so it is probably a P-51D.

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