Issue 43

P. Zampieri et alii, Frattura ed Integrità Strutturale, 43 (2018) 191-204; DOI: 10.3221/IGF-ESIS.43.15 192 I NTRODUCTION he design of the landing gear, according to the Airworthiness Regulations [1, 2], must take into account several requirements in terms of safety, strength, stability, etc. under all possible in-service loading conditions (weather conditions included). The landing gear is one of the main structural components characterizing an aircraft. It is aimed to support the aircraft during the landing, the tacking off and ground operations. Among such loading conditions, the landing phase defines the design specifications, since it is the most burdensome. As a result, such loading condition determines its structural size. As matter of the fact, a landing gear can be considered as an aircraft subsystem characterized by such subcomponents as brake, wheels, tyres, shock absorber and some structural kinematic components aimed to the extraction and retraction of the landing gear. The last two sub-components play an important role during the flight. In fact, the landing gear dimensions are such to get it so bulky to increase significantly the drag coefficient. Hence, it is important to retract it during the flight. An important aspect to take into account during the design phase is the landing gear weight which may reach up to the 3% of the maximum aircraft weight during the taking off. Hence the design phase of the landing gear has a heavy impact on the whole structure and on the airplane aerodynamic. For these reasons, it is developed contextually to the aircraft design phase. The landing gear development involves the use of standardized and ad-hoc components which must guarantee its kinematic, rather than the resistance to the static and dynamic loading conditions, such as the ground operations (taxing, towing, steering, etc.…) and the taking off and the landing phase (vertical load, spin up, spring back, etc.…). In literature, several works have been addressed to the investigation of landing gear performances as well as other aircraft structural components under the dynamic loads produced by a landing operation [3-10]; the State of the Art sees also the use of numerical models for landing gear design purposes. For example, Infante et al. [11] presented a detailed analysis of a nose landing gear failure, supported by FE analyses. The investigation focused on an accident in which the nose of the landing gear fork of a light aircraft failed during landing. Mohammed et al. [12], in their work, focused on the structural components, made of composite materials, of a landing gear. Structural safety for static and spectrum loads is analysed using ANSYS. Numerical methods are not only used for structural purposes. Redonnet et al. [13] proposed a numerical characterization of the aeroacoustics by a simplified nose landing gear, through the use of advanced simulation and signal processing techniques. To this end, the NLG noise physics is first simulated through an advanced hybrid approach, which relies on Computational Fluid Dynamics (CFD) and Computational AeroAcoustics (CAA) calculations. Viùdez-Moreiras et al. [14] investigated on the dynamic loads affecting main landing gear doors of an Airbus passenger aircraft. Currently, significant budget is invested by manufacturers in order to test the aerodynamic performance and the high costs associated to wind tunnel and flight testing restrict the number of test cases that can be performed. So, the authors proposed a numerical model for the unsteady aerodynamics characterized by wind tunnel testing, in order to predict the aerodynamic effect in previously untested conditions, and in this way, to allow a first stage exploration of new areas in the design space, without the need of expensive wind tunnel or flight testing. Concerning the landing gear structural investigation, generally, in the aircraft field, a preliminary step is performed with simplified FE model. Such numerical model, usually one-dimensional, is adopted to achieve the reaction forces involving each component during all aforementioned aircraft operations. After that, the design of each sub-component is carried out through detailed structural FE analyses where, once at time, each component, modelled with three-dimensional finite elements, is assembled into a one-dimensional FE model (stick model) representing the whole landing gear under the investigated operation. The reaction forces achieved by means of the multibody analysis will be applied statically to the stick FE model and, then, to each sub-component. In this paper, a new methodology is proposed. Such method allows achieving, by means of multibody simulations, rather than the reaction forces involving each sub-component, the kinematic response of whole landing gear, the coherence of the spatial dimensions of each sub-component, which should not impede the motion of another one, and the dynamic behaviour such as the in-play mass values, the equivalent stiffness and the damping coefficients of the landing gear components. Even though this approach gives a valid support to the designer during a preliminary design phase, the stick model technique is characterized by several problems, such as the approximations in the geometry modelling. Moreover, according to such approach, it is important, by a numerical point of view, to develop an isostatic FE model equivalent to the real one. In fact, if the landing gear is modelled as hyperstatic, the static equilibrium equations are insufficient for T