Specific strength is not always the only consideration. Strength over weight is the factor here, if a material has a third of the weight but also a third of the strength of another material, they have the same specific strength. Aluminium, titanium and steel are the metal alloys for aircraft construction, and they have roughly the same specific strength. There is a large variety within alloys of course, 2024 aluminium alloy has a yield strength of 324 MPa and is over 30 times stronger than pure aluminium.

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A wing in flight is bent upwards by the aerodynamic lift forces, and this bending moment is resisted by the upper and lower skin in the wing. The lower skin is loaded in tension, the upper skin in compression. With a tension loaded construction, we're only concerned with the cross sectional area: how much force does it see, in order to get to the yield strength. With a compression loaded construction however, the failure mechanism is not yield strength but buckling.

In order to prevent buckling, we need a larger cross section than what would be necessary for pure yield strength, and this cross section is the same regardless of the material used. So for compression, purely the lightest material is the best, and of the three mentioned above that is aluminium.

Specific strength is not always the only consideration. Strength over weight is the factor here, if a material has a third of the weight but also a third of the strength of another material, they have the same specific strength. Aluminium, titanium and steel are the metal alloys for aircraft construction, and they have roughly the same specific strength. There is a large variety within alloys of course, 2024 aluminium alloy has a yield strength of 324 MPa and is over 30 times stronger than pure aluminium.

Image source

A wing in flight is bent upwards by the aerodynamic lift forces, and this bending moment is resisted by the upper and lower skin in the wing. The lower skin is loaded in tension, the upper skin in compression. With a tension loaded construction, we're only concerned with the cross sectional area: how much force does it see, in order to get to the yield strength. With a compression loaded construction however, the failure mechanism is not yield strength but buckling.

In order to prevent buckling, we need a larger cross section than what would be necessary for pure yield strength, and this cross section is a function of the elasticity modulus, not the yield strength. So now the ratio of weight over elasticity sets the dimension. For compression, aluminium has the best ratio of the three metals mentioned above.