Sustainable lightweight construction above the clouds

New concept for the production of window frames in aircraft fuselage shells.

The aviation industry has set the goal of achieving “net zero” carbon emissions by 2050 (zero-emission aircraft). However, this ambitious goal cannot yet be achieved with current aircraft technologies. In order to achieve this goal, the development and implementation of alternative propulsion systems and alternative energy sources are essential. In addition, innovative approaches are needed to significantly reduce the CO2 emissions of future aircraft. This is not limited to the emissions that are primarily caused by the construction of the aircraft. The promotion of the circular economy and the use of recycled materials also play an important role.

The HESTIA project addresses several of these challenges. Firstly, the empty mass of an aircraft is reduced through advanced optimization algorithms in order to reduce operational emissions during the use of the aircraft. Secondly, the use of recycled carbon fibers is being investigated in order to reduce emissions during the production of the aircraft fuselage. The concept will be verified using a window environment of an aircraft fuselage, see Figure 1.

Figure 2 shows a comparison between new carbon fibers (vCF, virgin carbon fiber) and recycled carbon fibers (rCF, recycled carbon fiber) after being placed on a curved web to highlight the special features of recycled carbon fibers. The conventional tape has no plasticity compared to the rCF tape. The extremely stiff carbon fibers cannot be stretched in the fiber direction, but can only be compressed by local buckling. This leads to inadequate preform quality or process limitation to straight lay-up paths.

In addition to the aspects of sustainability, new advantages are therefore exploited and developed on the material side when using rCF. This is because the disadvantages mentioned do not occur to the same extent when using rCF. Here, the carbon fibers with a length of 50-80 mm can slide against each other to a limited extent and with correct process control, thus forming wrinkle-free curved paths, taking into account the reduction in thickness. This advantage is particularly important for the oval windows of an aircraft, as the reinforcing fibers can be aligned to the load path without having to accept the disadvantages mentioned above.

In the production of rCF tapes, several hybrid yarns made of oriented carbon fiber pieces and pure polymer filaments are consolidated into an rCF tape in a forming and stretching system developed at IVW. IVW is also responsible for manufacturing the tape preform. Various heat sources, degrees of freedom and a laying area of up to 3,500 mm x 1,500 mm with a process speed of up to 4 m/s are available here.

The forming and overmolding process takes place on the in-house injection molding machine “ENGEL Victory 1560/400”. This is equipped with several pyrometer-controlled infrared ovens (vertical and horizontal opening) and an industrial robot. In addition to reinforcement and functionalization, the injection molding compound also offers the option of using shorter fiber lengths through a further recycling and granulation step.

Comprehensive material tests are carried out in parallel with the development of the process chain. This is done using smaller test specimens and the window frame demonstrator at the end of the project. At the same time, a finite element model of the tests is developed for a material model, which is compared with the results of the experimental tests. The aim is to validate the developed construction method, the material model and the manufacturing processes.

In summary, the following measures planned in the project will lead to a significant improvement in sustainability:

• The use of recycled fibers leads to the elimination of the energy requirement that would be necessary for the production of new fibers
• The use of thermoplastic FRP enables energy-efficient production and is also recyclable
• The use of isothermal processes to manufacture the components leads to an increase in energy efficiency
• Increased material and resource efficiency thanks to lightweight design and low material waste
• The use of mass production processes leads to cost reductions

The project is being carried out in cooperation with Airbus Operations GmbH, Deutsches Zentrum für Luft- und Raumfahrt e.V., Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V., Airbus Aerostructures GmbH and Albany Engineered Composites  GmbH. Funding reference: 20W2203E