## 6 Discussion
### System Design Evaluation and Future Challenges during Development Process  
Integrated into an aero engine, acoustic liners have to satisfy different jet aircraft requirements. They must be reliable and effective over a wide range of operating conditions and frequencies. Acoustic liners have to reduce broadband aero-engine noise, obtain structural strength and must have a lightweight design. In addition, they have to be adaptable to restricted available space inside the aero engine structure (liner depth and area), feature low flammability and must be cost efficient. Moreover acoustic liner structures have to satisfy manufacturability and maintainability requirements. And at the same time they have to withstand, for decades of in-service life, the aero engine environment and conditions, e.g., rain, ice, oil leaks and walking-on by maintenance personnel. Furthermore, acoustic liners must have a robust and simple design, which can be adapted and specified after the details of the airframe and engine design have been finalised. For the implementation in the nacelle of the aero engine an assessment of the acoustic liners' system requirements and safety aspects is required. The results of the preliminary system safety assessment (PSSA), including an integrative concept study, back up the high potential of the broadband acoustic liners. Detailed information can be found in literature, e.g., [32; 33]. In the following,some of the most important points are addressed which are essential for further development and the exploitation of the full potential for a safe integration into the aircraft engines. The main aspects are:  
1. maintaining stability despite cross-section-reduced core structures,
2. preservation of foil properties over the entire life cycle of the aero engine,
3. possibility of draining fluids and particles of dirt, and
4. influencing sound reflections and vibration excitation on and by adjacent systems. By reducing the load bearing wall cross-sections, surrounding structures are subjected to enlarged stress levels. This can increase the probability of matrix or fibre breakage, as well as the risk of delamination of the top layer and general structural deformations. As a result of such fractures and deformations, the loss of the integrity of the system in flight operations or an unfavourable aerodynamic flow impairment of adjacent systems can have devastating consequences [34]. By suitable optimisation of the cut-outs in the honeycomb walls, the load-bearing capacity of the cross-section-reduced wall areas can be adapted to the respective loads [35; 36]. A stress reduction in the core can be achieved by a purposeful orientation of the fibre direction in the face sheets, e.g., parallel to the walls with cut-outs. The damping properties of the integrated film materials in the liner structure were demonstrated in this study. Nevertheless, in order to maintain noise reduction functionality and aviation safety, the behaviour of the film materials must be investigated over a longer period of time. The demands on the in-service lifetime of current acoustic liners are enormous. Since they are not actively required for flight operations, acoustic liners must be designed for the entire in-service lifetime of the engine of approximately 20 to 30 years [37]. All environmental and operating influences, as well as mechanical, chemical and aerodynamic loads must be borne by the liner structure and the integrated film materials throughout their entire in-service lifetime. No significant deterioration of the structural and noise-reducing properties shall occur within this period. Depending on the flight phase and the position in the aero engine, the occurring failure mode can have a more or less dangerous influence on flight operations. Through targeted drainage openings in the honeycomb core, current acoustic liners are able to rinse out liquids and dirt particles that penetrate through the perforated face sheet. Honeycomb chambers which are not able to drain would collect water, dirt and other chemicals inside.