# A Rational Numerical Method for Simulation of Drop-Impact Dynamics of Oleo-Pneumatic Landing Gear
###### Abstract
Finite element models represent effective tools to support the design of landing gear, enabling accurate investigation of the mechanical behavior of each subcomponent under the action of static and dynamic loads. The development of new software for parametric FEA and the introduction of elastic models in simplified multi-body dynamics packages have recently led to the possibility of accounting for some enhanced levels of detail in simulations of drop dynamics even in the preliminary design stage of the landing gear, while also allowing for trade-off studies on the effects induced by configuration changes on the overall landing performance. On the other hand, FE-based approaches become somewhat impractical at the beginning of the preliminary design phase of an aircraft, when there are too many undefined design variables to consider the generation of the landing gear's 3D CAD or structural mesh convenient for estimating its influence on ground loads. Referring to modern aircraft, characterized by ever more light and flexible structures, the possibility of predicting ground loads and vertical acceleration levels at landing is crucial from the early stages of the project, as it enables proper design choices to be made. According to airworthiness regulations, determining ground loads and acceleration follows a general process that is independent of the aircraft certification category and can be summarized in four consecutive steps:  
1. Estimate the design load factor at landing by means of conservative assumptions or rational analysis. (The design load factor at landing, or _ground load factor_, is defined as the ratio between the maximum vertical acceleration arising at the aircraft's center of gravity during landing and the acceleration of gravity.) 2. Apply standard criteria to evaluate landing loads and vertical acceleration referring to the assumed load factor and design-relevant landing conditions defined in terms of the aircraft's inertial configurations and attitudes. 3. Determine the most critical load/acceleration and landing conditions among those of the previous step. 4. After landing system design and manufacture, conduct drop tests on landing gear and experimentally evaluate the maximum ground load factor. The reference aircraft certification category has an influence only on the specific approaches and formulations that must be followed to accomplish the tasks at each step, while the general logic of the process is essentially common to all certification standards. If the experimental load factor obtained by drop tests is lower than that assumed at the beginning of the process, then the landing gear design is approved, provided that it is also shown (by test) that the landing gear is also able to withstand the most critical static/dynamic load conditions among those evaluated. Landing gear design approval is-therefore-based on demonstrating compliance with two concomitant criteria:  
1. The structure must be able to withstand the most severe loads expected in service without damage or failure (as for all components of the aircraft primary structure). 2. The maximum ground load factor developed during prescribed drop tests must be lower than that assumed for the evaluation of ground loads in the first place. If the landing gear fails to meet the first criterion, then recovery actions can be taken to strengthen the structural layout of the item and new tests can be performed to prove the adequacy of the implemented corrections. If it fails to meet the second criterion, then the entire set of ground loads is compromised and needs to be evaluated again, in correspondence of a new assumption on the load factor value; this severely impacts the design process of the landing gear and the structural components whose layout is influenced by ground loads magnitude.