**6.4 Composite materials to optimize fuel consumption and CO2 emissions**

The processing of composite materials is becoming increasingly important for aircraft construction. Even though planes are themselves tonnage heavy, every single kilogram counts. The manufacturing and processing costs of carbon fiberreinforced plastics (CFRP) in aircraft far exceed the costs of traditional metal construction. In the long term, however, the cost advantage outweighs due to the low weight and the resulting reduced fuel consumption. Nowadays, fuel consumption is a top priority for airlines because less fuel means less CO2 emissions and lower operating costs. Thus, something good is done for the environment while saving money, too. For many years CFRP has been installed on models such as the Airbus A380 (28%) or the Airbus A350 XWB (53%). Predecessors, such as the A330, weigh almost 10 tonnes more and consume more fuel than the Airbus A350 XWB with comparable payload capacity and range [62, 63].

CFRP consist of hair-thin layers of carbon fibers, which are embedded in a resin matrix (thermoset). The material scores with a very high specific strength and low weight. Mechanically, this composite material is extremely difficult to deal with, so millimeter-thin CFRP tapes have to be stacked on top of each other for the outer hull of the skin in a day-long process and then baked together under pressure and heat. Due to the extreme hardness of the material, particularly high-quality and expensive cutout drills and cutters (e.g., for external connections, doors, windows, and holes for rivets) must be used. Because of the high abrasion when drilling, even modern tools with diamond-like coating last on average only half as long as tools in metalworking [62].

#### **6.5 Aircraft engines: current technology and energy-efficient developments**

Aircraft engines must be reliable and efficient. The technology behind them is explained quickly and easily. Engines work in a similar way as rockets: The intake air is compressed and fuel is injected. The combustion of the fuel creates an exhaust gas jet, which emerges at the back. The exhaust jet drives the actual turbine (a wheel with blades). The turbine finally generates the drive for the compressor at the engine entrance. The compressor increases the pressure of the air and consists of several stages. Each of these stages includes a rotor and a stator wheel. The turbine part is also constructed like that. Depending on the engine, between 8 and 14 stages are used today. Particularly modern engines achieve compressions of 45 times the input pressure. The developments in aircraft engines initially focused on sending

*Transportation Systems Analysis and Assessment*

**6.2 Electric motors: environmentally friendly flying**

all-electric 180-passenger commercial aircraft is discussed in [58].

In aviation, aerodynamics focuses on two main forces: lift and drag. The power of lifting makes an airplane fly. This is caused by the uneven pressure on a wing's top and bottom. The drag represents the resistance that arises during movement through the airflow. Due to the high pressure under the wings, air flows over the wing tips upward and rolls off in the form of a vortex. This vortex is also called induced drag and can be so strong that it disturbs other planes. Wake turbulence can become a safety concern particularly for small aircraft. Induced air drag degrades performance and reduces the range and speed of the aircraft [59].

Winglets are more than just a striking and aesthetic design feature; they are among the most visible fuel-saving and performance-enhancing technologies in aviation introduced in recent years. According to Whitecomb, winglets can reduce induced drag by about 20% and improve carrying capacity by 6–9%. The design of the winglets can be very different. Aviation Partners Boeing (APB) has developed a special form: the Blended Winglet. The Blended Winglet's design fuses the wing into a smooth upward curve. Other winglets are shaped more like a fold or kink.

Through this smooth transition, optimal efficiency can be achieved [59].

Riblets are micro- and nanostructured surface structures that cause drag reduction. This technology comes from the field of bionics, which works by transferring phenomena from nature to technology. Riblets resemble the skin of a shark and are characterized by fine grooves on the surface. The so-called sharkskin effect causes a reduction of the friction resistance of up to 8% compared to

**6.3 Aerodynamics: winglets and riblets**

aircraft without this coating [60].

according to Sizmann. Second, the energy for the entire process is generated from solar energy. The process is very efficient and does not compete with food production as opposed to the production of other (mainly first-generation) biofuels [52].

The use of electric motors is already well advanced in parts of the transport sector. Soon, electric flying should become possible. In this regard, Siemens and Airbus announced a development cooperation in 2016 in which hybrid technology is used. In the presented test aircraft, the jet engine was replaced by a 2 megawatt electric motor (produced by Siemens), which drives the large air impellers. The 2 megawatt electric motor is only 30 centimeters long and weighs 175 kilos. To get power from the electric engine, several steps are necessary. With a gas turbine burning kerosene, an electric generator is powered, which feeds the power into a 2 tonne lithium-ion battery. Finally, the lithium-ion battery supplies the built-in electric motor. Since starting up an aircraft requires a great deal of energy, the lifting can be supported by generator and battery. During the descent, the engine blades, which work like small windmills, can be used to generate electricity. This principle is similar to that of electric cars or locomotives, which carry power back into the battery while braking. The representatives of this project are of the opinion that with their concept they can reduce the consumption of kerosene by double-digit percentages compared to conventional jet engines. Flying would therefore also become more environmentally friendly and more quiet [53, 54]. Electric power for a two-seat aircraft is discussed in [55]. The more electric aircraft (MEA) concept is discussed in [56]. Light pureelectric and hybrid-electric aircraft are presented in [57]. The MEA concept essentially aims at replacing conventional non-electric power (pneumatic, hydraulic, and mechanical) by electric power to drive aircraft subsystems more efficiently. An

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part of the air through the compressor and the combustion chamber, rather than around the engine. The first stage of the compressor, also known as a fan, works like a giant blower. The fan accelerates this circulating mantle air. The so-called turbofan engines are the current state of the art [64].

The shroud flow ideally requires a relatively low speed for the large fan and high speed in the high-pressure range. This created the two-shaft engines. The axles of these engines can rotate counter-wise. One of them is the slow-speed low-pressure shaft, which is driven by the rear turbine stages just before the exhaust outlet. At the same time, the first compressor stages are rotated. The other one is the very fast-rotating high-pressure shaft. The high-pressure shaft is operated by the turbine stages behind the combustion chamber and thus moves the high-pressure part of the compressor.

Optimization of the engine concept has been in progress for many years. First and foremost, the approach is followed to change the amount of air that has passed through. The difference in speed should not be too big between thrust and airspeed. Ideally, a very large amount of air is pushed back very slowly from the engine. Another approach for increasing efficiency is the turbine including the combustion chamber. The hotter the combustion is, the more efficient the process becomes. Here, the materials are pushed to their limits. The first stage of the turbine is under most stress because it gets the full heat of the combustion chamber. Other developments are heading back in the direction of the classic propeller. Ideas in this area run under the slogan "open rotor concept." However, the mounting size, which makes mounting on the wing difficult, as well as the noise, proves to be problematic. Aircraft could look completely different in the future, for example, with a huge propeller engine on the roof of the fuselage.
