Steel is a wonderful metal that has become ubiquitous in the modern world. Ships, trains and cars are made from steel, yet the vast majority of aircraft are built from aluminium (although carbon fibre is becoming more popular and wood was used extensively through to the end of the Second World War).

 

Is there Anything Wrong with Aluminium?

Although not inherently unsuitable for building aircraft, aluminium does have a long list of disadvantages as a building material compared to steel. It is:

  • More expensive
  • More susceptible to corrosion
  • Difficult to weld
  • More susceptible to fatigue
  • Quicker to melt
  • Not as strong
  • Not as stiff

Furthermore steel aircraft are rare but indeed possible. For example the cold war era Soviet MiG-25 "Foxbat" and US XB-70 Valkyrie chose to use steel extensively to cope with the high temperatures of Mach 3 flight.

 

MiG-25 in flight

The Mikoyan-Gurevich MiG-25 could reach over Mach 3 and was made primarily from steel.

 

But Aluminium is Lighter!

So why is Aluminium so popular? The answer might at first seem obvious, aluminium is "lighter"- it has a lower density than steel and aircraft need to be as light as possible in order to fly. However a quick look at the material properties of the two metals shows a more complicated picture; steel is indeed about three times denser than aluminium, but for the critical property of stiffness, steel has a value approximately three times higher. So in other words an aircraft made from aluminium should need three times as much structure to compensate for the weaker material, leading to an increase in weight of around 3%.

  Density Stiffness
Steel

7800 kg/m3

(487 lb/ft3)

200 MPa

(29 000 psi)

Aluminium

2800 kg/m3

(175 lb/ft3)

70 MPa

(10200 psi)

Aluminium as % of Steel 35,9% 35%

Comparison of Steel and Aluminium

 

So What's so Great about Aluminium?

To understand what makes aluminium so attractive, we need to look past the simple material properties. Stiffness is the critical structural property for aircraft design, and the stiffness of a part is determined not only by the the material stiffness (e.g. steel or aluminium) but also its shape.

Total Stiffness = Material Stiffness × Shape Stiffness

This in itself is not surprising, after all it is common sense that a thicker cross section is stronger than a thinner section, even when the material used is unchanged. So changing the shape does affect the total stiffness. However the relationship is not a straightforward "more is better" rule; the distribution of the shape's area has a huge effect on the shape stiffness. For example, doubling the height of a rectangular section part makes it eight times stiffer vertically.

This is where the lower density of aluminium can finally be used to an advantage. The lower density of aluminium means that for similar parts, there is much more area (and volume) to play with for the same weight; and by skilfully distributing this extra area the designer can maximise the shape stiffness compared to a steel part. So aluminium does not provide an automatic advantage in itself, but allows designers to produce lighter aircraft by creating a structure that derives more of its stiffness from its shape rather than its material.