Motivation

Abstract

Alternatives to kerosene as an energy source are currently being discussed for minimum-emission—ideally zero-emission—aviation. However, their availability is likely to be limited, they will be expensive, and, according to current knowledge, they will not be completely emission-free. Energy-saving technologies for future commercial aircraft will therefore play a major role in achieving the goal.

Alternatives to kerosene as an energy source are currently being discussed for minimum-emission—ideally zero-emission—aviation. However, their availability is likely to be limited, they will be expensive, and, according to current knowledge, they will not be completely emission-free. Energy-saving technologies for future commercial aircraft will therefore play a major role in achieving the goal.

This is where lightweight system design comes in, as it enables further weight savings and significant contributions to the reduction of aerodynamic drag beyond conventional lightweight design through function integration.

Carbon-fibre reinforced polymers (CFRP) are particularly suitable for lightweight system design due to their high structural performance and their possibilities for function integration.

1.1 Lightweight System Design for Low-Emission Aviation

Definition

Lightweight system design is an extension of classic lightweight design. In lightweight system design, the aim is to integrate as many passive and active functional elements as possible into the load-bearing airframe.

In aircraft design, such functional elements include aerodynamic fairings, cabin fittings, electrical lines, antennas and energy storage systems.

In addition, technologies for aerodynamic flow control can be integrated into lightweight system design to help reduce aerodynamic drag.

Lightweight system design is understood as lightweight design in interaction with other systems of the aircraft.

CO₂ emissions in aviation are mainly caused by three classes of aircraft: regional aircraft 7%, short-/medium-range (SMR) aircraft 51%, and long-range (LR) aircraft 42%. [38]. Weight savings and drag reduction in SMR and LR aircraft are therefore of particular interest.

Possible aviation energy sources include green-energy-based synthetic fuel, also known as e-fuel, liquid hydrogen (LH2), methane, ammonia, liquid organic hydrogen carriers (LOHC), and batteries. An overview is given in Table 1.1.

Table 1.1 Possible energy sources for aviation

Costs of kerosene, e-fuel, LH2, methane [4], costs ammonia [5], primary energy use [1, 2], battery vol. energy density [32], battery grav. energy density [16, 64], battery costs [64].

e-fuel is attractive because it would enable even today’s aircraft to fly CO₂ -neutral. However, at significantly higher costs and with high primary energy input.

LH2 is two thirds lighter than kerosene in terms of gravimetric energy density, requires four times the storage volume of kerosene, is cheaper and requires less primary energy input than e-fuel, but is still more expensive than kerosene.

Methane, ammonia, dibenzyltoluene (LOHC), and batteries tend to be out of the question for medium- to long-range aviation because of their lower volumetric and gravimetric energy density.

A particular challenge when using LH2 is the resulting additional weight from the tank and piping system.

For example, for the Ariane 6 with a single-use tank, Air Liquide Energies has developed a metallic LH2 tank that holds 28 tons and weighs 5.5 tons [9]. The effective storage density is thus 28 kWh/kg.

In a DLR study [104], a total system weight of 2432 kg is assumed for an LH2 filling weight of 781 kg when a CFRP tank is positioned in the rear of the aircraft. The resulting effective storage density of 10.7 kWh/kg is on a par with kerosene, but the additional volume remains.

The required quantities of (green) e-fuel or hydrogen will be a particular challenge, in addition to the cost price and remaining residual emissions, as aviation competes with other consumers [23].

Whatever energy source is chosen or will be available in the future: it remains crucial to reduce energy consumption. To achieve this goal, there are two main influencing factors: weight reduction and drag reduction.

1.2 Potentials of Lightweight System Design

With regard to possible weight savings, a distinction is made between primary and secondary structures (Fig. 1.1). The former describes the load-bearing airframe structure and must therefore meet particularly high safety and certification requirements. Improved methods of classic lightweight design are predominantly used for primary structures. Weight savings in secondary structures and cabin elements as well as passive and active systems to support aerodynamic laminar flow are in the focus of lightweight system design.

Fig. 1.1

Typical weight distribution for a short- and medium-range (SMR) aircraft

The primary structure of a typical SMR aircraft (wings, fuselage and tail units) weighs about 15 tons in today’s metallic design. It can be shown that improved knowledge of modern lightweight materials, their structural properties and new design concepts enable a weight reduction of around 20%. From the secondary structures, the systems and the cabin (in total about 13 metric tons), the principle of function integration of lightweight system design can achieve further savings of about 10%. The unladen weight of an SMR aircraft of 44 metric tons (including landing gear, engines and operating equipment) can therefore be reduced by at least 4.3 metric tons with the current state of knowledge. Further weight-saving possibilities lie in active flight control systems for load reduction [97].

For a SMR aircraft, reducing the take-off weight by one ton for a cruise length of 2000 nautical miles, according to Brequet [3], means fuel savings of 171 L of kerosene or e-fuel. Aircraft in this class are expected to fly 60,000 flight cycles [11]. The reduction in take-off weight thus saves 10.3 million litres of fuel over the course of an aircraft’s lifetime. This does not take into account additional savings from the reduced fuel weight. Assuming 100% availability of e-fuel in 2050 and an optimistic price of 0.2 €/kWh [4], one ton of take-off weight will save almost €20 million during the aircraft’s lifetime. If the use of fossil fuels is assumed to continue, this results in a saving of 25,800 t CO₂ for the example considered, and since the price of kerosene will also increase by 2050 due to higher production costs and emission certificates, at least €10 million will also be saved in this case.

A major contribution to minimising the energy consumption of future aircraft is made by lightweight system design in conjunction with aerodynamics in reducing frictional drag. By avoiding waviness and edges the surfaces, natural laminar flow (NLF) is enabled at maximum run length over the airfoil, and active laminar flow control (LFC) is supported with the aid of boundary layer vortex extraction and variable control of flow separation at the lift surfaces and fuselage.

According to Brequet, a 1% increase in glide ratio will result in fuel savings of 9 million litres of fuel over the lifetime of an SMR aircraft based on the above assumptions, and operating cost savings of €18 million for e-fuel.

If kerosene is used after all due to the lack of availability of e-fuel, improving the glide ratio by 1% would save around 22,500 t CO₂ and reduce operating costs by €9 million.

It goes without saying that the costs of manufacturing lightweight airframe components must also be kept within limits. Here, too, developments in lightweight system design in the field of manufacturing technologies and quality assurance offer significant potential for savings; often not against but with simultaneous weight savings.

1.3 The CFRP Lightweight Material

One possibility for lightweight system design is the substitution of materials towards higher structural performance. The use of CFRP in particular offers great potential in terms of

• weight, due to the good specific strength and stiffness,

• function integration, due to the inherent merging of multiple material components,

• laminar holding, due to possibilities of step- and gap-free integral design.

Fibre composites (FC) made of CFRP are characterised by the highest (weight-)specific strengths and stiffnesses, see Fig. 1.2. This fact, combined with the corrosion resistance and very high fatigue strength, has led to an increase in the share of CFRP in the primary structure of civil aircraft of the last generations to about 50%.

Fig. 1.2

Comparison of the specific strength (tensile length) and specific stiffness of various lightweight materials

If strength and stiffness in one load direction alone were the determining factors for an aircraft structure, CFRP structures would weigh only 20% of a comparable light-metal structure for the same load-bearing capacity. However, this very high lightweight potential cannot nearly be exploited for many reasons. One reason is that the fibers must be oriented in multiple load-bearing directions; this results in at least orthotropic layup of plies. Another reason is the lack of plasticity, which makes damage barely visible. When subjected to impact loads, CFRP structures may show delaminations in the resin area that cannot be visually detected, which reduce the load-bearing capacity, especially under compressive and shear loads. Therefore, structural properties are often set far below the theoretically possible material properties (Sect. 2.2). Another major difficulty lies in the non-fibre-composite suitable design of today’s aircraft structures. For example, current airworthiness requirements restrict the approval of structural bonding in the primary structure (Sect. 2.3) and structural bonding (Sect. 2.4).

Removing these and other limitations in the use of CFRP is of great importance for future lightweight structures. Results from research show that significant progress is possible here.

The use of CFRP in wing structures is now the rule. The use of CFRP in fuselage structures, on the other hand, is not yet established, as it has been shown that the weight advantage is not as high as initially predicted for the reasons mentioned above, among others, and the manufacturing costs are significantly higher than those of comparable metallic structures. Therefore, further possibilities for cost reduction in production must be developed and made available.

It is sometimes pointed out that the production of CFRP structures is significantly more energy-intensive than that of metallic lightweight materials [25]. However, life cycle analyses (LCA) show that in aircraft, production accounts for between 0.1% (LR aircraft) to 0.2% (SMR aircraft) of the total CO₂ footprint [53], [113]. This means that the use of CFRP in aircraft design is also advantageous for ecological reasons as a result of the energy savings over the entire lifetime of the aircraft.

1.4 The Process Chain of the Lightweight System

The lightest airframe structure consists of the most efficient materials, material-adapted design and maximum function integration. When all three aspects are addressed together, weight savings can be achieved far in excess of those achieved today.

Fibre composites belong to the class of generative materials, whose mechanical properties only arise in the manufacturing process of the components from different semi-finished products. A characteristic feature is the interaction between materials, simulation tools, design concepts, manufacturing methods, integration of functions and implementation on an industrial scale. New calculation methods enable new design concepts and new manufacturing technologies enable the use of new materials. Individual findings can be combined in very different ways to create lighter, lower-drag, lower-cost, and easier-to-maintain aircraft structures. In lightweight system design, even initially inconspicuous findings can have a major impact on the way to the airframe.

A look at the process chain and exemplary research results in the subareas is helpful in understanding the basis of the potential estimates for weight savings and drag reduction in lightweight system design. This results in a wide variety of options for the aviation industry along the entire value chain of aircraft design. In the process chain, Fig. 1.3, adaptronics exploits the potentials of function integration from a systemic perspective.

Fig. 1.3

Process chain of lightweight system design

Even if the explanations in the following chapters strive to do so, not all of the research results described here can be directly quantified in terms of their effect on weight, manufacturing costs or drag reduction because, for example, they have different effects in interaction with different aircraft concepts and design methods, but also semi-finished products or manufacturing technologies.

The examples are grouped into three main chapters:

• Contributions from classic lightweight design

• Lightweight system design with integration of passive functions

• Lightweight system design with integration of active functions

Most of the results reported in the next chapters point to potentials that have not yet been realised. The selected examples almost all originate from research work carried out over the last 15 years at the DLR Institute of Lightweight Systems together with partners. A large number of further research results on lightweight system design can be found, for instance, in [6].

The technological topics addressed in the examples are highlighted in underline in the following chapters to make them easier to find.