Skin (aeronautics)

The skin of an aircraft is the outer surface which covers much of its wings and fuselage. The most commonly used materials are aluminum and alloys of aluminum with other metals, including zinc, magnesium and copper.

As the twentieth century progressed, aluminum became an essential metal in aircraft. The cylinder block of the engine that powered the Wright brothers’ plane at Kitty Hawk in 1903 was a one-piece casting in an aluminum alloy containing 8% copper; aluminum propeller blades appeared as early as 1907; and aluminum covers, seats, cowlings, cast brackets, and similar parts were common by the beginning of the First World War.

In 1916, L. Brequet designed a reconnaissance bomber that marked the initial use of aluminum in the working structure of an airplane. By war’s end, the Allies and Germany employed aluminum alloys for the structural framework of fuselage and wing assemblies.

The aircraft airframe has been the most demanding application for aluminum alloys; to chronicle the development of the high-strength alloys is also to record the development of airframes. Duralumin, the first high-strength, heat treatable aluminum alloy, was employed initially for the framework of rigid airships, by Germany and the Allies during World War I. Duralumin was an aluminum-copper-magnesium alloy; it was originated in Germany and developed in the United States as Alloy 17S-T (2017-T4). It was utilized primarily as sheet and plate. Alloy 7075-T6 (70,000-psi yield strength), an Al-Zn-Mg-Cu alloy, was introduced in 1943. Since then, most aircraft structures have been specified in alloys of this type. The first aircraft designed in 7075-T6 was the Navy’s P2V patrol bomber. A higher-strength alloy in the same series, 7178-T6 (78,000-psi yield strength), was developed in 1951; it has not generally displaced 7075-T6, which has superior fracture toughness. Alloy 7178-T6 is used primarily in structural members where performance is critical under compressive loading.

Alloy 7079-T6 was introduced in the United States in 1954. In forged sections over 3 in. thick, it provides higher strength and greater transverse ductility than 7075-T6. It now is available in sheet, plate, extrusions, and forgings.

Alloy X7080-T7, with higher resistance to stress corrosion than 7079-T6, is being developed for thick parts. Because it is relatively insensitive to quenching rate, good strengths with low quenching stresses can be produced in thick sections.

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Wikipedia