# Airframe Stress Analysis

## Modeling Concept

The accuracy of the Finite Element Model required for structural analysis changes throughout the design process. The flow chart below explains the process of structural analysis of an aircraft briefly.

In general determination of design, loads are the first step in the design and analysis of an aircraft. Initially, a structural design criterion is developed which accounts for the loads coming from the different operating conditions of the aircraft, flight parameters and any additional loads specified by the customer.

## Global Finite Element Model (GFEM)

The next step of the design process, a Global Finite Element Model (GFEM) or an External Loads model is developed, which is further used to analyze the external loads obtained previously. The GFEM model is a simplistic representation of the aircraft structure mostly made up of idealized frames, panels, and stringers that are represented by a coarse mesh with the use of shell and bar elements. Figure 4 shows a GFEM model of a fuselage nose section, in which the panels are represented as one single element with stringers and a frame being modeled using 1D elements. In most cases, each component of an aircraft is analyzed separately with loads applied at the reference stations. These reference stations have Multi-Point Constraints (MPCs) defined that link the grid points of the reference station to the frames. Generally, after all the external load cases are analyzed, the internal loads corresponding to these analyses can be requested and used for a detailed analysis.

## Detailed Finite Element Model (DFEM)

In this stage of the analysis, a more detailed model of the aircraft is developed, and the internal loads obtained from the GFEM are applied to the components to analyze the response. Mostly, at this stage, the 1D models are replaced with more precise 2D and 3D representation. For example, the flanges which were initially represented as a 1D element in GFEM would be updated with a 2D model in a DFEM process to obtain a 3D representation of the flange. In the subsequent sections, some of the tools and processes that can be used for DFEM simulation are discussed.
The results from GFEM and DFEM have been compared as:

## Model Files

Refer to Access the Model Files to download the required model file(s).

The model files used here includes:
• Fusealage_GFEM.fem
• Fusealage_DFEM.fem
• Fusealage_FBD_DMIG.fem
• Fusealage_FBD_DMIG_AX.pch
• Fusealage_FBD_Final_using_DMIG.fem
• Fusealage_section.fem