Airframe Stress Analysis

Model Files

Before you begin, copy the file(s) used in this example to your working directory.

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.

Figure 1. Overview of the Design and Analysis of Aircrafts

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.

Based on these inputs and considering the aerodynamics and flight masses, an overall load case data is generated. Figure 1 shows some of the different aircraft loads that an aircraft is subjected during its flight, apart from these some of the other load cases that are to be considered include vibrations, acoustic noise, system pressures, different maneuvers and loads during ground handling. Not all of these load cases contribute to the design process and; therefore, it is important to determine the load cases that are critical to the design. These loads are analyzed multiple times for each time step to determine the critical loads accounting for the design changes. This process helps the load engineers in determining the detailed critical loads of specific loads.

Figure 2. Different Loads Acting on an Aircraft

Figure 3. Modeling Process

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.

Figure 4. GFEM Representation of a Fuselage Nose Section

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.

Figure 5. DFEM Representation of a Fuselage Nose Section
The results from GFEM and DFEM have been compared as:

Figure 6. Comparison of Results from GFEM and DFEM