Material Selection

Material selection for any engineering product depends on its function, shape, man­ufacturability (i. e., process), and cost. For aircraft applications, there is the addi­tional consideration of weight. Within the material classes, there are subclasses of alloys: composites that offer appropriate properties to suit a product – a large variety is available with ever-increasing newer types. In the conceptual design phase, engi­neers must screen and rank the types of materials that suit the requirements, listing the limitations and constraints involved and whether a change to another type is

pure aluminum

aluminum + copper (e. g., 2014-T6 Alclad sheet,* 2024-T4 extrusion)

aluminum + manganese

aluminum + silicon

aluminum + magnesium

aluminum + magnesium + silicon

aluminum + zinc – high strength, heat treatable, prone to fatigue (e. g., 7076-T6, 7076-T6 extrusion)

Table 15.6. Typical composite material usage in various aircraft classes

Aircraft type

Typical percentage of composite by weight

Typical components

Small aircraft*

20% to 40%

Control surfaces, floorboards, some skins (e. g., cowling, fillet)

Regional jets/turboprops

15% to 30%

As above, furnishing

Medium jets**

15% to 25%

As above

Large jets

15% to 20%

As above

Military trainers

20% to 30%

As above

Combat

30% to 50% or more

As above + some primary structures

Notes:

* Some smaller aircraft, including the Bizjet, are constructed of all-composite structures. ** B787 has over 50% composite material by weight.

Several options are available for appropriate materials to make the best compro­mise. Thus, aircraft-weight estimation is more complex, and engineers must identify and compute numerous parts to estimate component weights before an aircraft is built; CAD 3D modeling helps.

Choice of material affects aircraft weight and cost. The semi-empirical relation for weight estimation in Chapter 8 considers all-metal construction and describes how to adjust the prediction if some parts are made of a lighter material. For a rapid method, the OEW may be factored accordingly – only the structural weight is affected; the remainder is unchanged. Composites may be used in secondary and tertiary structures, where loads are low and failure does not result in catastrophe.

In general, for the same Young’s Modulus, metals have higher density. How­ever, when the strength-to-weight ratio (i. e., specific strength) is considered, then composites overtake metals; that is, engineers can obtain the same strength with lighter components even when the higher FS erodes the weight savings. Metals demonstrate a better Young’s Modulus for the same strength. Metals also show better fracture toughness for the same Young’s Modulus. Another important com­parison is the relative cost per unit volume versus the Young’s Modulus when metal alloys are less costly.

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