One of the challenges that currently exists for the design of large aircraft structures under natural-laminar-flow (NLF) regime is the extremely high requirements that must be achieved in terms of tolerances with the aim of ensuring that the flow will be sufficiently stable. In systems composed of many parts (such as the Smart Fixed Wing Aircraft torsion box object of study), the assembly processes acquire a special relevance, taking into account that, depending on how they are executed, the tolerances of each part might accumulate and generate deviations much larger than those of the parts taken individually.
The objective of the present project was the development of a numerical methodology for the analysis of the assembly tolerances of aircraft components designed to operate under natural laminar flow (NLF). The methodology allowed determining the maximum acceptable components deviations to obtain assemblies within the specified tolerances, evaluating the influence of the joining techniques and the jigs, and determining the best assembly strategy or bolting sequence. For that, the proposed approach integrated tolerance accumulation techniques and mechanical/structural tools in order to consider in the analysis the parts compliance and other possible relevant effects associated with the joints.
The project SATCAS (Simulation of the Assembly Tolerances for Composite Aircraft Structures) has been defined to support the design of the assembly process for the wings of the Low Drag Demonstrator within the Smart Fixed Wing Aircraft ITD (SFWA - Integrated Technology Demonstrator of the Clean Sky JU) through the development of a numerical simulation technique/strategy. The wings proposed in the mentioned program are being designed for operating under natural-laminar-flow (NLF) conditions, which is expected to lead to significant improvements in terms of energetic efficiency. One of the challenges that currently exist for the design of large aircraft structures for NLF regimes is the extremely tight requirements that must be achieved in terms of tolerances with the aim of ensuring that the flow will be sufficiently stable. In systems that are composed of many parts, the assembly processes acquire a special relevance.
The main objective of the project was to develop a methodology for the analysis of the deviations/deformations that might be produced during assembly processes of aircraft structures caused by: deviations in the constituent parts, deviations in the assembly jigs and/or deformations introduced directly by the assembly operations. This will allow checking the fulfilment or not of the tolerances specified for the ensembles and determining, for example, the influence of considering different assembly sequences.
The method proposed in SATCAS is based on a finite element method (FEM) strategy for the simulation of the whole assembly process on which all the parts are considered as flexible bodies. For this, the approach considered in the project covers from the detailed/local analysis of the fastening techniques (WP1 and WP2) to the simulation of each of the main operations expected to be used within the assembly processes object of study (WP3). In addition, the methodology developed also covers a proposal for the definition of the cases for the cumulative tolerances analysis and the development of an application/code to perform the treatment of the FE results (WP3).
In order to validate the FEM strategy proposed, it has been applied to a small assembly (lab scale) representative of the real ones, analysing some of the basic operations both numerically and experimentally (WP4). Finally, the method has been applied to the analysis of two large assemblies (two wing boxes concepts, WP5).
As result, the methodology developed has demonstrated to respond satisfactorily to the objectives defined in the project. The FE numerical strategy proposed for the simulation of the assembly processes has shown its feasibility, adequacy and potential for this type of studies. Moreover, it has proven that it might be helpful to improve the processes (through recommendations for the fixation systems, the assembly/fastening sequences, etc) or even to support the design of the assembly jigs.
The project has been performed by ITAINNOVA under the supervision of Aernnova Engineering Division as Topic Manager. SATCAS is a sub-project associated to the activity of Assembly Simulation defined in the BLADE project – SFWA - Clean Sky programme, led by Airbus.