# Model-Based Design of Aircraft Landing Gear System
## 1 Introduction  
Modern industrial products are smarter and more effective than in the past, but they sometimes exhibit a high level of complexity. Such complexity may derive from many different factors, such as from the many disciplines that are involved during its development, the constraints in terms of safety, the logistics of its supply chain, and many more. All of these are most certainly involved in the aerospace field, which is in charge of designing, manufacturing, and managing extremely complex products that must guarantee the highest possible level of safety and, at the same time, be able to carry out the given mission as efficiently and effectively as possible. For this reason, it has become imperative to be able to reliably design aircraft systems and predict their behavior largely in advance with respect to their industrialization. Moreover, the interaction that arises between the multitude of systems is especially critical, and it must be solidly taken into account. In fact, properly designed systems and carefully set up models that reproduce a system allow designers to better understand what is the phenomenon they are dealing with, thus supporting them in their decisional path through the design activity and particularly in the trade-off of proposed layouts. Those aspects are crucial to move the main cost of development to the earliest phases of the product life cycle in order to avoid late changes in process that might exceedingly increase the financial investment [1]. One of the most important systems of an aircraft, thus making its definition in early development a priority, is the landing gear. The landing gear is the system in charge ofsupporting the aircraft when in contact with the ground. Therefore, it is active when the aircraft is parked, taxiing, taking-off, and landing. Among these phases, the harshest ones are takeoff and landing: the former is due to the large mass of the aircraft, and the latter is due to the high impact force that is generated when the aircraft comes into contact with the runway [2]. As such, when designing landing gear, these two are the phases which are considered for all the sizing calculations required. In the last decades, there have been many methods, approaches and techniques that have been developed to carry out the design of such a complex system. Concerning its sizing, many works provide mathematical formulas for the estimation of the main parameters, thereby allowing the designer to gather a general picture of the system [3; 4; 5]. It is quite hard, however, to find a complete sizing workflow that makes it possible to estimate the necessary parameters to capture the behavior of the individual subsystems and their interaction. Regarding this purpose, many authors over the years have developed codes to predict the aircraft behavior during flight. For instance, [PERSON] [6] provides a program for the simulation of the aircraft dynamics during takeoff and landing. [PERSON] and [PERSON] [7] predicted the behavior of the shock absorber during landing. [PERSON] et al. [8] developed an experimental criterion for the analysis of the aircraft during ground maneuvers. [PERSON] performed an in-depth analysis of the dynamics of the pneumatic tire [9]; meanwhile, [PERSON] et al. studied the mechanical properties of the tires [10]. Finally, [PERSON] [11] validated numerical results with experimental data of the behavior of the landing gear main strut during landing. Those simulations, however, tended to work with already available, or at least sized, systems, to provide dynamic verification and/or characterization of the systems behavior or to focus on a specific subsystem of the landing gear. Other authors, meanwhile, such as [PERSON] et al. [12] and [PERSON] [13], analyzed individual subsystems, thus providing sizing formulas, FEM/CFD simulations, or design guidelines, usually without considering the interaction with other subsystems.