Commercial Aircraft Composite Technology

The world population is growing and the trend to further urbanisation is undamped. In addition, the economic development proceeds especially in the emerging countries. Hence, a further growth of the world air traffic can be expected, and the total number of commercial aircraft is expected to double within the next 15 years. At the same time, a large number of old aircraft must be replaced. Governments have set ambitious targets with respect to highest safety and highest environmental standards at even lower travel cost. New technologies are required to tackle these challenges by the next generation of aircraft. Composites offer additional product value and have increasingly been used for different parts of the airframe for almost 100 years of aviation history. After a discussion of the market development and a summary of the history and milestones of composites used for airframe structures, the true value of light weight, the most important material requirements and the key contributors as well as the main lever arms for even lighter future airframe are introduced.

The airframe material selection is decisive to determine target weights as well as manufacturing and operating cost of aircraft components. It is a prerequisite for production planning and long lead time items. In addition, it is key for identifying any material-related certification risks and for establishing the certification process planning and means of compliance. It is therefore very important to freeze the material decision for components well before a new aircraft is offered to the market. This chapter starts with a discussion of the development process of new aircraft and the definition of the basic milestones. Fundamental aircraft requirements of operators, authorities and airframe manufacturers influencing the material decision are highlighted, and the requirement cascade is introduced. The functional analysis method as part of the design process is explained followed by the description of the general procedure of the structure stressing and certification process. Special emphasis is put on the description of the “no crack growth” concept of CFRP structures.

This chapter starts with a description of the most important material selection criteria for primary load carrying airframe structures. In addition, crucial material related risks are highlighted. Since it is of interest for many material trades and the down selection procedure, fundamental properties of common and new aluminium alloys, including fracture toughness and fatigue, are explained. After a short description of fibre metal laminates (FML) and titanium properties, basic characteristics of thermoset (epoxy) and thermoplastic (PEEK and PPS) resin are introduced. The manufacturing process of carbon fibres and their properties are highlighted. The description of the thermoset prepreg manufacturing technology is followed by the discussion of the most important laminate properties. Finally, different relevant aspects, the targets and the procedure of material qualification are explained.

The selection of the manufacturing technology will inevitably influence cycle times and manufacturing cost, i.e. non-recurring cost as well as recurring cost. It will also influence quality assurance and assembly effort. Due to the strong link of advanced design schemes and suitable manufacturing technology, maintenance cost (for example with respect to the accessibility of integrated CFRP-structures) are also affected. Different manufacturing technologies require different semi-finished materials with different mechanical properties. Hence, the selection of the manufacturing technology will also influence part weight and operational cost. In this chapter, all state of the art as well as newly developed manufacturing technologies and recent R&D are described; advantages and shortfalls are highlighted. Automated tape laying, fibre placement and pultrusion technology are explained, and examples are provided. The autoclave process technology is discussed, including unwanted defects in the cured material and the description of quality assurance technologies (dielectric analysis and non-destructive testing by ultrasound). Different textile infusion technologies are characterised. Thermoplastic stamp forming technology and thermoplastic tape laying technology are discussed in detail. The description of the filament winding technology is followed by the presentation of joining and bonding technologies. Riveting as well as adhesive bonding and welding processes are discussed. The chapter closes with the description of a possible method for the selection of “the right process technology for the right part”.

In order to certify the aircraft, the structural integrity and the safety of the airframe structure are usually demonstrated by analysis and tests. In addition, structural tests are also necessary to verify new design methods and manufacturing technologies (including acceptable tolerances of defects), to assess new materials and joining methods and for many other development and certification purposes described in this chapter. The building block approach of the test pyramid is discussed, providing examples for each level of the pyramid, i.e. coupons, elements, details, sub-components, components and major tests. Finally, a possible major test procedure for compliance demonstration is described.

A maintenance program is mandatory for commercial aircraft; thus its definition must be part of the aircraft development. This chapter describes the basic parts of the maintenance program and provides examples for interval and duration of scheduled maintenance (checks). Examples of structural damages and principles of the structural repair manual (SRM) are explained, including the definitions of cosmetic, structural, temporary and permanent repair. Important accidental damages are introduced, highlighting the necessity of a robust, impact-resistant yet not too heavy CFRP design. The detectability threshold of CFRP impact damages, crucial to decide on cosmetic or structural repair, is explained. The rationale of bolted repair and a possible repair procedure are described. Advantages and challenges of adhesive bonding repair solutions are discussed, highlighting the specific restrictions of airworthiness authorities.

The wing leading edge and trailing edge high lift system is essential for the low speed characteristics of the aircraft. Advanced CFRP technology can contribute to cost and weight savings on aircraft level as well as to improved low speed performance and added product value for the aircraft operator. In this case study, different CFRP flap design schemes and manufacturing technologies are discussed, pointing out the interdependence of CFRP material, design, manufacturing process and resulting part properties. Starting with the description of the reference flap design and the most relevant design targets, new solutions such as multi-spar and sandwich designs are characterised with their advantages and shortfalls. Advanced thermoset solutions (based on prepreg and resin infusion technology) and thermoplastic solutions (based on the tape laying technology) are included. Material and manufacturing as well as strength and stiffness aspects related to aerodynamic performance are analysed. In addition, wing integration challenges linked to new design schemes are discussed.

During the development process of a wing it is particularly important to analyse and understand the interaction of external load, resulting internal stresses and component deflection. The latter is strongly influenced by the design. Compared to aluminium, a CFRP wing design offers an additional degree of freedom to tailor the stiffness behaviour for relevant load cases. In this case study, after introducing the most fundamental aero-elastic interrelations and the relevant notions, a historic overview of tailored wing applications is given. The basics of bending-torsion coupling of tailored, non-orthotropic CFRP laminates are explained. Results of aileron efficiency, flutter and lift distribution analysis are discussed, underlining the potential of the application of non-conventional CFRP laminates for wing panels. Aspects of structure strength are analysed by experimental determination of relevant notched laminate properties. Finally, different wing panel design schemes and manufacturing technologies are described, taking into account their applicability for tailored CFRP laminates. This includes skin-stringer integration, tool-laminate interaction, quality aspects such as porosity and distortion, application of functional layers for corrosion and lightning strike protection and other important aspects.

CFRP was the prime choice for the airframe of the latest aircraft of Boeing and Airbus. However, modern aluminium alloys were developed with improved cost-performance relationships, and, in addition, todays CFRP manufacturing technology is not fully ready for high production rates of short- and medium-range aircraft. Furthermore, the light weight design potential of today’s CFRP airframe is heavily penalised by additional system installation effort, which is necessary due to the poor electrical conductivity of CFRP compared to aluminium or fibre metal laminate such as Glare^®. For the next generation of aircraft, it is necessary to reduce material cost and airframe manufacturing cost, and to improve the added product value by additional functions such as electrical conductivity. This chapter reveals latest R&D results to enable cost-effective thermoplastic composite airframe structures with blends, more electrically conductive and more damage tolerant composites with metal fibre reinforcements and up-stream research for different use cases of morphing structures by means of shaped memory alloy wire integration.