Modern Glider Development with Abaqus Finite Element Method

Please welcome our guest blogger, Jette Lorsbach from the University of Stuttgart’s Akaflieg Stuttgart, a student organization with a goal of developing, building, and flying unique and interesting aircraft prototypes. Below, they describe their current project, a workflow using Abaqus to develop a fly-by-wire glider.

The Akaflieg Stuttgart is a student organization at the University of Stuttgart. The goal of this organization is to develop, build and fly unique aircraft (glider) prototypes. Our current objective is to build a glider that experiments with an electrical control system, the fs36 FlyByWire.

The motivation for realizing this project is the idea of more safety. The development of a fly-by-wire glider can help to prevent critical situations via the installation of a stall protection or even a complete envelope protection system. Furthermore, a fly-by-wire system enables an improvement in flight performance. The replacement of a mechanical control system with electric actuators and cables provides the opportunity for new paths in terms of flexibility for the glider. For example, it is possible to control a larger amount of aileron surfaces with variable deflections without the need of designing a complicated mechanical mixer. Furthermore, by limiting the flight envelope or by adding an active flutter prevention system, it is possible to lighten the structure. The fs36 will be made of fiber composites. Fiber composite is mostly used for modern gliders thanks to its lightweight and anisotropic material behavior. This often results in complex shapes, where traditional calculation methods by hand are limited and only viable for rough estimations. Therefore, simulations are one of the key technologies in modern aircraft design. As Dassault Systèmes generously supports us with their finite element simulation software Abaqus, we have the opportunity to optimize our newly designed aircraft parts. These optimizations include the occurring loads by respecting the part wight, as the final goal is to design the structural parts as light as possible. The workflow for the load-carrying structure of the aircraft wings is described below.
To define the initial structure, traditional hand calculation methods are used to get an idea of the structure’s dimensions. This first CAD model is simplified to be suitable for finite element analysis. Therefore, small radii and surfaces are ignored, and the bonding geometries are simplified as much as possible. Subsequently, the model is imported into Abaqus, and material and element types are assigned to individual parts. The Hashin failure criterion is used for the fiber composite structures and the ply table is used to define the number of fabric layers and fabric layer orientation. The bonding between different components is defined as “solid homogeneous” due to isotropic material behavior. For the initial calculation mechanical conditions of contact are used for the entire model and the bonding between components is not considered in detail to keep the initial calculation as simple as possible.
The discretization process is of special interest due to the quality of the simulation being strongly dependent on the mesh quality. Therefore, the mesh is refined in areas where high values of tension are expected. In these areas, the goal is to achieve a fine and preferably undeformed mesh. To achieve these requirements, it is necessary to split the components into smaller partitions improving the ability to affect these special areas. The wall thickness of the fiber composite components is about 2mm in the thinnest areas. In these areas, it is important to achieve a fine enough mesh to avoid high aspect ratios which could lead to wrong simulation results. Finally, the mesh quality is checked with the Abaqus mesh quality checks and improvements were necessary.
The goal of the simulation is to reinforce the structure in the right areas and to save material in areas where the occurring loads are lower than expected. Therefore, a lightweight structure is achieved that can fulfill the requirements. To achieve this goal, it is important to know, where peaks of loads are located in the structure and what the load path looks like. This information will be gained using a finite element simulation.
However, structural tests need to be performed to calibrate the simulation. With the help of these test results, material parameters and conditions of the contract can be adapted to improve the simulation even further.
Another goal is to use the finite element method for aeroelastic tailoring. Thereby the deformations under a given load must be known exactly. If the model is very reliable, one can optimize the deformation of the wing of an aircraft under a given load to optimize the lift distribution of the wing achieving a better aerodynamic performance.
A destruction test for the aircraft wings needs to be performed, as this is a requirement for the approval of the maiden flight by the airworthiness authorities. Therefore, the final goal is the simulation of this structural test to avoid an early failure of the structure in this test saving money and time.
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