A little background on the history of aluminum in aircraft construction. I wrote this for a piece about 2-2-V-1 that never got published. Now that the artifact is on the front burner I thought I'd share it.
Pure aluminum is too soft for use in load-bearing airplane parts but in 1908 an alloy of aluminum was invented by German researcher Alfred Wilm that included four percent copper, one half percent magnesium and on half percent manganese. Patented under the trade name “Duralumin”, the new metal was nearly as lightweight and significantly more robust than pure aluminum and could be heat treated for even greater strength. In the years immediately prior to and during World War One, frames and girders of Duralumin made possible the construction of airships of unprecedented size by the Zeppelin Luftschiffbau. Rendered as corrugated sheet, the metal had sufficient rigidity to be used in aircraft construction and, during the war, was featured in several all-metal German aircraft produced by the Junkers company.
With Germany’s defeat, the Aluminum Company of America obtained the rights to Wilm’s patent and began producing the alloy under the designation “17S” and in its fully heat-treated form “17 S-T”. The most notable use of corrugated ALCOA 17S-T sheet in post-war American aircraft was in the Ford Tri-motor series. By the early 1930s the technique of “stressed skin” construction had been developed and in 1933 the Boeing 247 and Douglas DC-1 entered service with uncorrugated skins, rivets and supporting structures made of heat treated 17S-T.
The same year ALCOA introduced an alloy that contained the same elements as the old Duralumin but in slightly different proportions. Designated “24S-T” the new product was significantly stronger and soon replaced 17S-T in most aircraft applications. To combat corrosion, ALCOA used a previously developed technique in which a sheet of alloy was “clad” with a thin protective layer of corrosion-resistant pure aluminum bonded to each side. The resulting sandwich was called Alclad. Beginning in 1934, Alclad 24ST became the most widely used material for skins on American aircraft. Known as “2024 Alclad” since 1954, it is still commonly used in aluminum aircraft construction. For that reason, metallurgists at ALCOA were not able to “date” samples taken from the artifact based upon its physical composition.
Lockheed’s Model 10 Electra made its maiden flight on February 23, 1934. The following is an excerpt from the company’s 1936 sales literature:
“The principal material used in the construction of the Electra is Alclad 24ST, a high strength duralumin (aluminum alloy) with a protective coating of pure aluminum on each side 5 per cent of the total thickness of the sheet. This is the most advanced type of aluminum alloy that has been developed. Although its aluminum coating renders Alclad highly resistant to corrosion, every part of the interior of the airplane is painted for further protection. All of the outside surface and most of the structural elements are of this material.”