Lufthansa Technik have teamed up with the Technical University of Darmstadt to develop new methods for load transmission into the carbon fibre aircraft fuselage structures of executive aircraft.
Over the last ten years carbon fibre composites have become increasingly used in the construction of passenger aircraft on account of their low weight and extreme load-carrying capacity. The fuselage structures of the new Boeing 787 Dreamliner and Airbus A350 types are composed almost entirely out of carbon fibre composites. Today even particularly heavily loaded areas such as the keel beams in the floor, the rear bulkheads in the fuselage and the centre wing boxes where the wings are attached to the fuselage are built out of composite materials.
Due to its advantages, the new generation of aircraft is also likely to be highly popular with VIP customers. If these customers are to be offered manufacturer-independent modifications and customised solutions for carbon fibre composite based aircraft, appropriate modifications will have to be made to the original aircraft structure to enable particular components to be installed at the required points in the aircraft and reinforce the primary structure in such a way that the additional loads can be safely introduced and distributed.
The “Fiber Force” research project is looking into new technologies for application to fibre composite structures. The primary goal of the project initiated jointly by Lufthansa Technik and the Technical University of Darmstadt in April 2010 is to develop methods and concepts for the introduction of forces into the CFC fuselage structure.
The first sub-project, entitled Floorpanel Hardpoint, addressed the question of accurately determining the maximum load of the ten millimetre thick floor panels, which vary in length from 0.5 to 3 metres. To this end extensive tests and simulations were carried out. To develop new force introduction solutions for CFC fuselages, it was necessary to examine the new materials, load-bearing components (frames), longitudinal stringers and their attachment points more closely. For this purpose special prototypes were fabricated and tested in appropriate test facilities. To ensure that the solutions would be suitable for long-term use, a computational and simulation environment (finite element simulation) for Carbon fibre composite structures was built and validated in parallel to the investigations. Further material tests were used to ascertain material characteristics as yet unknown.
To attach cabin fittings it is necessary to install hardpoints on the floor panels. These parts, which are comparable to inserts, are palm-sized and for this purpose they are glued onto the panels. In this way attachment points that offer the maximum flexibility as to the choice of installation location in the cabin floor for special items such as small cupboards, tables or partition panels capable of withstanding a maximum load of around 3000 N are created on the floor panels. Ultimately the points at which hardpoints have to be created on the cabin floor are determined by the individual design of the cabin and the interior. For example, in one current case no fewer than 40 floor panel hardpoints are being installed in a custom-fitted VIP 747-8 customer aircraft.
Another sub-project, Floorpanel Cutouts, addressed the issue of compensating for a sudden drop in pressure in the VIP cabin. Unlike conventional passenger aircraft, VIP aircraft have a lot of small compartments, so that a drop in pressure in the cabin can lead to a particularly low pressure stressing furniture, walls, doors and the floor. In order to be able to compensate quickly for such a low pressure, relatively large cutouts, known as “breather vents”, were introduced into the cabin floor panels with an appropriate frame structure. The sub-project goals were to explore the properties of the frame structure and consider how to compensate for the missing area. It was also necessary to perform function tests in different locations and under different scenarios. A virtual simulation of the problem indicated where the pressure equalization works and where the aircraft structure needs to be strengthened.
At the same time the results gained from sub-projects Floorpanel Hardpoint and Floorpanel Cutouts constitute the basis for two further sub-projects: Side Attachment, which is concerned with lateral attachment to bulkheads for the introduction of particular forces, and Upper Attachment, which focuses on the introduction of forces to the upper fuselage structure. Building the necessary virtual fuselage structure proved a special challenge, as no manufacturer data was available. With the aid of extensive research and practical tests, the development process for these projects will be completed and any data necessary for the evaluation and optimisation of the virtual FE simulation will be available by the end of the project in November 2013.
The first results from the project, which is funded by the Federal Ministry of Economics and Technology (BMWI), and the insights gained as a result are already flowing into the first VIP customer aircraft. At the same time it has also been possible to make substantial progress in the development of floor attachment points in other areas of application. Furthermore, the results from the various sub-projects of research project “Fiber Force” are already making possible the efficient installation of cabin elements and interiors. In this way Lufthansa Technik will be able to offer bespoke solutions and modifications on a competitive basis for the new generation of CFC aircraft types in the future.