15.4 Engine Exhaust Emissions

Currently, the civil aviation sector burns about 12% of the fossil fuels consumed by the worldwide transportation industry. It is responsible for an approximate 3% annual addition to greenhouse gases and pollutant oxide gases. The environmental debate has become intense on issues such as climate change and depletion of the ozone layer, leading to the debate on long-term effects of global pollution. Smog consists of nitrogen oxide, which affects the pulmonary and respiratory health of humans. The success of the automobile industry in controlling engine emissions is evident by dramatic improvements achieved in many cities.

The U.S. Environment Protection Agency (EPA) recognized these problems decades ago. In the 1980s, the need for government agencies to tackle the engine emissions issue was emphasized. The early 1990s brought a formal declaration (i.e., the Kyoto agreement) to limit pollution (specifically around airports). Currently, there are no regulations for an aircraft’s cruise segment. In the United States, FAR Part 35, and internationally, the ICAO (i.e., Annexure 16, Volume II), outline the emissions requirements. EPA has worked closely with both the FAA and the ICAO to standardize the requirements. Although military aircraft emissions standards are exempt, they are increasingly being scrutinized for MILSPECS standards. Emissions are measured by an emission index (EI).

Combustion of air (i.e., oxygen plus nitrogen) and fuel (i.e., hydrocarbon plus a small amount of sulphur) ideally produces carbon dioxide (i.e., CO2), water (i.e., H2O), residual oxygen (i.e., O2), and traces of sulphur particles. In practice, the combustion product consists of all of these plus an undesirable amount of pollutants, such as carbon monoxide (i.e., CO, which is toxic), unburned hydrocarbons (UHC), carbon soot (i.e., smoke, which affects visibility), oxide of sulphur, and various oxides of nitrogen (i.e., NOX, which affects the ozone concentration). The regulations aim to reduce the level of undesirable pollutants by improving combustion technology. The sustainability of air travel and growth of the industry depend on how technology keeps up with the demands for human-health preservation.

Lower and slower flying reduces the EI; however, this conflicts with the market demand for flying higher and faster. Designers must make compromises. Reduction of the EI is the obligation of the engine manufacturer; therefore, details of the airworthiness EI requirements are not provided herein (refer to the respective FAA and ICAO publications). Aircraft designers must depend on engine designers to supply certified engines that comply with regulatory standards.

15.5 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.

Figure 15.12. Material stress–strain relationship

characteristics of the stress–strain relationship. The nature of alloys, crystal formation, heat treatment, and cooling affects a materials characteristics.

A typical stress–strain characteristic of an aluminum alloy is shown in Figure 15.12. The figure shows that initially, the stress–strain relationship behaves linearly according to Hooke’s Law, which represents the elastic property of a material. Within the elastic limit, the material strength (i.e., how much stress it can bear) and stiffness (i.e., how much deformation occurs) are the two main properties considered by designers in choosing materials; of course, the cost, weight, and other properties are also factors to consider. The maximum point within which linearity holds is the yield point. Past the yield point, permanent deformation occurs: The material behaves like plastic and the slope is no longer linear. The highest point in the stress–strain graph is known as the ultimate strength, beyond which the component continues to deform and results in a rupture that is a catastrophic failure. The linear portion gives the following:

stress/strain = constant = Young’s Modulus (the slope of the graph)

Sometimes raw material is supplied with a small amount of prestretching (i.e., strain hardening) and a permanent deformation set in which the yield point is higher. Typically, some aluminum sheet metals are supplied with 0.2% built-in prestretched strain (see Figure 15.12a). However, with prestretching, the ultimate strength is unchanged. Figure 15.12b compares various types of typical aircraft materials. A steeper slope indicates higher stiffness, which often has a higher elastic limit. Brittle materials rupture abruptly with minimal strain buildup; ductile materials exhibit significant strain buildup before rupturing, thereby warning of an imminent failure. Rubber-like materials do not have a linear stress–strain relationship. The pertinent properties associated with materials follow (some are shown in Figure 15.12):

brittleness: when a sudden rupture occurs under stress application (e.g., glass)

ductility: the opposite of brittleness (e.g., aluminum)