Aircraft Materials

Aircraft that defy gravity necessarily must be weight-efficient, thereby forcing designers to choose lighter materials or – more precisely – those materials that give a better strength-to-weight ratio. Also implied are the questions of cost of raw mate­rials, cost of fabrication, and stability during use. This section helps readers under­stand that choosing the appropriate materials is an involved topic and therefore is an integral part of the study during the conceptual design phase. Aircraft weight and cost are affected by the choice of materials and, therefore, aircraft performance and economy. The success or failure of a new aircraft design depends largely on the choice of appropriate materials, especially when the number of those available is increasing.

In the early days of aviation, the only choice was to use an all-wood construc­tion or a fabric cover to wrap around a wooden airframe to serve as an aerody­namic surface. Being anisotropic and without enough resistance to impact, wood properties have limitation. At that time, the available metals were heavy and the lighter ones were soft and corrosive. Today, wood is no longer used except in the homebuilt-aircraft category, primarily because wood is the easiest material with which to work. Moreover, the ethical question of forest conservation discourages the use of wood.

In the 1920s, the combination of progress in engines and in aerodynamic tech­nologies allowed aircraft speed to exceed 200 mph, which required better mate­rials. Technology changed in the 1930s when Durener Metallwerke of Germany introduced duralumin, an alloy of aluminum, with a higher strength-to-weight ratio, improved anticorrosion properties, and isotropic properties. The company followed with a variety of alloys for specific manufacturability, damage tolerance, crack prop­agation, and anticorrosive properties in the form of clad-sheets, rolled bars, ingots, and so forth. The introduction of metal also resulted in a new dimension to manufac­turing philosophy. Progress in structures, aerodynamics, and engines paved the way for substantial gains in speed, altitude, and maneuverability performance. These improvements were seen primarily in the World War II designs, such as the Super­marine Spitfire, the North American P-51, the Focke Wolfe 190, and the Mitsubishi Jeero-Sen.

The last three decades have seen the appearance and increasing use of non­metals, such as fiberglass/epoxy, kevlar/epoxy, and graphite/epoxy, which are com­posite materials constructed in layers of fabric and resin. Composites have bet­ter strength-to-weight ratios compared to aluminum alloys, but they also have anisotropic properties. Because they are shaped in moulds during the fabrication of parts, difficult curvy 3D shapes can be produced relatively easily. The near future will see more variety of composite materials embedded with metal to obtain the best of both. The Bombardier CSeries, Airbus 380, Boeing 787, and Airbus 350 are examples of how extensively composite materials are used. The technology of com­posite materials is evolving at a fast rate and there will be more variety in composite materials with better properties and capabilities at a lower cost.

Typically, composites may be used in secondary and tertiary structures in which loads are low and any failure does not result in catastrophe. Figure 15.11 shows the composite materials in a Boeing 767 aircraft. As the technology progresses, more composites will appear in aircraft moving into primary load-bearing structures.

Table 15.3 compares the extent of increase in composites from an older B747 (1960s) to the relatively newer design of the B777 (1990s). The latest B787 and A350 have considerably higher percentages of composites.

Composite materials are incorporated increasingly in percentage by weight. A few smaller aircraft are made of all composite materials but the FAR Part 23/25 cer­tification procedure is more cumbersome than for metal construction. It was difficult to obtain airworthiness certification for early all-composite aircraft because there were insufficient data to substantiate the claims. Military certification standards for aircraft structures are different.

Table 15.3. Percentage mass of types of material used in the aircraft structure

The newer military aircraft designs use expensive, exotic materials (e. g., aluminum-lithium alloy and boron alloy) that have yet to prove their cost – effectiveness in commercial aircraft. More than half of the Eurofighter’s structural mass is constructed of various types of composite materials; a fifth is made of the aluminum-lithium alloy.