Experimental and numerical study of an aircraft seat

The aircraft certification process is a complex and costly undertaking for a company, involving a meticulous material selection process and the execution of experimental tests to prove compliance with existing regulations. This dissertation is focused on the development of a complex numericalexperimental methodology aimed at evaluating the crashworthiness of an aeronautical seat. This methodology takes into account not only the materials involved and their thickness, but also critical impact conditions described in CS 23.562. What sets this research apart is the seat configuration, which is fully integrated into the aircraft's fuselage structure. This type of seating refers to a specialized aircraft seating arrangement where passenger seats are an integral part of the aircraft's fuselage, as opposed to being standalone fixtures. This design concept is commonly found in modern commercial airliners and offers several advantages, including increased aircraft design flexibility, weight reduction, space efficiency, decreased maintenance requirements, and enhanced passenger comfort. From a safety perspective, integrated seats have the potential to provide robust safety features as they distribute loads across a larger surface area. However, due to the seat's inherent rigidity as a structural component, it can transmit high loads to the pilot. Additionally, in the specific case under consideration, the only element designed to mitigate lumbar loads is the seat cushion. Therefore, the selection of material for the seat cushion becomes a critical factor. The physical properties of these cushions, if chosen incorrectly, can amplify the pelvic load on the seated occupant during a vertical impact event. This study is structured into six chapters, each focusing on specific lines of inquiry. The first chapter is an introduction and provides a general overview of the topics covered in the thesis. The second chapter provides an overview of the regulatory framework and the regulatory authority relevant in the aviation sector. Follows an excursus of aviation safety and the update of safety standards as a direct consequence of technological progress. In fact, as will be seen, there is currently a shift towards performance standards, as this approach allows for the surpassing of outdated solutions. The focus then turns to the type certificate and light aircraft in the prospective of the discussion about the increased weight of the aircraft under analysis, Bk 160. Chapter 3 is an introduction to the seat as a safety device. It describes the impact problem and the crashworthiness based on the crash physics, involving the work-energy relationships, and analyzing the energy content of an aircraft during an impact. The core of the chapter is, therefore, the seat, seen as the primary safety device protecting the pilot and the occupant in case of emergency landing conditions. Different technical solutions for energy absorption and seat validation for the aircraft class under study are discussed considering the current regulations. Chapter 4 is entirely dedicated to the mechanical characterization of the foams used for the seat cushion. Crucial to detect the deformation of the foams during the tests is the measurement technique, namely digital image correlation (DIC), that will be presented. Polyurethane foams and their properties are introduced, and experimental and numerical results for tensile, shear, and cyclic compression tests are provided. The crashworthiness analysis is presented in chapter 5, which is dedicated to multibody modelling by Madymo. Due to the significant time and cost associated with seat certification, aviation authorities recommend the use of numerical simulations, particularly dynamic analysis, as a complementary approach to support the seat approval process. This also necessitates the use of validated virtual dummies, in this case, the Hybrid III 50th Percentile Dummy provided by Siemens. While experimental dynamic testing cannot be entirely replaced, numerical simulations offer valuable capabilities for assessing critical loads and estimating injury criteria. In this specific case, these functionalities are particularly advantageous. Since the seat is integrated into the fuselage, conducting an impact test using the entire aircraft fuselage would be required. This would entail the need for a dedicated setup and the construction of aircraft solely for this purpose. It is evident that the costs associated with such approach would be significantly higher compared to conducting the test using the seat mounted on the conventional deceleration sled. The dissertation will conclude in Chapter 6, where a summary of the results will be presented.