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.

15.5.3 Coursework Overview

Aluminum alloys continue to be the most prevalent material in aircraft structure. Table 15.6 is a conservative presentation of typical percentages of composite use for the coursework project. The table reflects the typical current practice, although there are some newer designs that perform better than what is listed.

The introductory coursework exercise may use the following strategy. The weight equations provided in Chapter 8 are valid.

Civil Aircraft Design

For civil aircraft, the design is all-metal construction. If nonmetals (i.e., composites) are introduced, they may be limited to secondary and tertiary structures up to only approximately 25% of the OEW. Typical nonmetal structures include the floorboards, control surfaces, complex fairings, and empennage.