By Bjorn Fehrm
May 29, 2026, ©. Leeham News: We do a series on aircraft structures and how these have shaped how our airliners can transport us around the world today.
We started the history of aircraft structures last week by observing that the development of structures is very much tied to the development of materials, with the crossover from wood to metal enabled by the discovery of copper-alloyed aluminum, which was originally patented as Duralumin.
We will now look at the stressed skin construction this enabled and the development of a second class of alloyed aluminum, the Zinc alloyed class, today classified as part of the 7000 series.
Figure 1. DC-3, the most influential stressed skin aircraft between the wars. Source: Silodrome.com
The Stressed Skin Aircraft Structure
The stressed skin aircraft structure method meant that there were new design rules and allowables to understand and design to. A thin fuselage or wing skin of Duralumin (or its cousins) was strong in tension and shear and not strong in compression and torsion, Figure 2.
Figure 2. The principal forces on a structural part. Source: FAA Structure Handbook.
The problem with compression is that the slightest non-straightness of the alloy part results in bending and, when fixed at the ends, in buckling. It meant that, in addition to frames and longerons to guide the form of a stressed-skin fuselage, the thin aluminum skins needed reinforcement with stringers to prevent buckling. Stringers are riveted at distances that, together with the frame spacing, form structural panels sized to avoid buckling under projected loads.
The Douglas DC-3 is a Good Stressed Skin Aircraft Example
The DC-3, developed from the DC-2 at the request of American Airlines for an enlarged sleeper version, was built using Alclad 24-ST. It’s now standardized as Alclad 2024-T3. It’s a 2024 alloy core metallurgically bonded with a thin layer of pure aluminum. This cladding provides exceptional corrosion resistance for Duralumin variants that otherwise have some corrosion problems.
The DC-3 fuselage structure formed a nearly cylindrical tube that was resistant to both tension and torsion (through skin stresses mainly) and compression, as the stringers and frames stabilized the skin to increase buckling load. Observe how the stringers are aligned with the principal compression load to stabilize the thin skin in the direction of the load ( Figure 1, marked part).
For a wing or a tail, the spars and ribs guide the form and, together with skin attached stringers, create the anti-buckling panels. The DC-3, known for its very strong structure, has a 3-spar tip-to-tip, one-piece wing, Figure 1. The DC-3’s wing spars, ribs, and skins create a box, called a wingbox, which is very strong in torsion and allows the wing to absorb gusts by flexing the wing up and down.
The torsional stiffness is very important to avoid wing flutter, which is when the aerodynamic center of a wing section is ahead of the torsion center. It creates a dangerous oscillation that has broken many wings since the Otto Lilienthal days.
The flexing of the wings from, e.g., a gust is not dangerous as the alloy extends in tension and contracts in compression; it, thereby, absorbs the gust energy without plastically deforming the metal. The change in a material’s dimensions per unit load is called the Young modulus, or simply the Modulus.
The Loads Decide the Alloy
A wing is loaded in tension for the lower wing skin and compression for the top skin during steady flight. In a gust, these changes, but this is from a preload of tension and compression. It means the requirements for the wing skins on the top and bottom differ. The lower shall have high tensile strength and be insensitive to fatigue, whereas the top can trade fatigue resistance for even higher tensile strength, which improves buckling resistance.
The fuselages before 1950 were unpressurized, except for high-altitude bombers like the Boeing B-29. The load for a fuselage is a mix of landing and gust load, which puts the top in tension and the bottom in compression, and other loads from tail surfaces, engines, landing gear, etc. In essence, before cabin pressurization, which came in the 1950s, the loads on fuselage skins were more of a mixed bag than on the wings and tails.
After the success of Duralumin, each aeronautical nation researched its own variants of the Cu-alloyed aluminum types. Other principal alloying elements were also tried. During WW II, the Germans were short on copper and developed zinc-alloyed aluminum types with even higher tensile strengths than Cu alloyed ones. These came to form the second major type of aeronautically used aluminum alloys, today called the 7000 series.
The zinc-alloyed aluminum was strong but had problems with stress corrosion and notch sensitivity that were not fully understood until the mid 1950s. It caused structural failures due to corrosion and fatigue in critical parts, such as spars, in the late 1940s and early 1950s.
Fatigue was known as a problem with steel, where train axles failed from fatigue. It was also known that it could appear in aluminum structures, but the exact conditions under which cracks could form and how they propagated in aluminum alloys were not fully understood.
This was not critical until the 1950s, when gas turbine engines (turboprops and then jets) enabled civil airliners to fly above 10,000ft, where a pressurized cabin was needed. There were pressure cabins on bombers and the odd high-altitude fighter/reconnaissance aircraft before, but these did not reach the cycles that a civil airliner does over decades of operational service.
The problem of fatigue in metals, especially aluminum alloys, arose when the Bristsh DeHavilland Comet was the first jet airliner to cruise at up to 35,000ft with a cabin pressure differential of over 8 PSI.
We dig into the Comet crashes in the next Corner, as they were instrumental in shaping how airliners’ structural design was developed post these crashes.
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