## 2 Study Object--General Dynamics F-111 \"Aardvark\"
### F-111 Aircraft Model for Hydrodynamic Tunnel Tests  
In order to obtain the measured data from the experiment in the hydrodynamic tunnel, a CAD model of the aircraft with a special strain gauge balance mount has to be made first (Figure 5). The balance reference point at the 25% part of the MAC projected at the symmetry plane should be properly placed, with the strain gauge fitting inside the model. This reference point placement should allow for an easy measurement of the model, since slight differences in model mounting will cause a visible difference in the measured moment. The 1:50 scale model was made using incremental technology. It consists of layers of heat-shrinkable material (PLA). The layers of material were joined together under the influence of temperature. Unfortunately, the effect of the layered structure is to create an uneven surface of the test model. In order to obtain data from the hydrodynamic tunnel test as accurately as possible, the surface quality of the model must be improved. Satisfactory roughness of the model was achieved by grinding and painting the surface. Creating aircraft models for testing in wind tunnels is a key element of the research process. Essentially, the model must accurately replicate the shape and size of the original aircraft and have similar material properties. Although hydrodynamic tunnels are an effective tool for testing the aerodynamic properties of an aircraft, the process of preparing aircraft models for testing is often complicated, time-consuming, and expensive. The choice of model production technology is crucial because different techniques present unique advantages and limitations. For example, 3D printing is fast and precise, allowing for \begin{table}
\begin{tabular}{c c c} \hline \hline
**Parameter** & **Value** & **Unit** \\ \hline Mean aerodynamic chord & MAC = 139.6 & mm \\
1/4 MAC center & (236.031, \(-\)37.284, 8.196) & mm \\ Wing span & b = 175.7 & mm \\ Lifting surface & S = 11,323.962 & mm\({}^{2}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Reference values for aerodynamic characteristics. Figure 4: The method of the mean aerodynamic chord (\(MAC\)) calculation where \(b\) is the real wing span and \(bb\) is the simplified wing span. complex shapes to be created from a variety of materials. However, the cost of 3D printers and printing materials can be very high, especially when using advanced technologies such as selective laser sintering (SLS) or 3D metal printing [22]. By comparison, CNC machining techniques are capable of producing very high-quality models. However, in order to achieve adequate precision, these techniques require expensive equipment and skilled labor. In addition, the CNC machining process is usually time-consuming, especially when producing complex shapes [23]. Traditional methods, such as carving, can be relatively cheap, but they are also time-consuming and require a high level of skill. Moreover, hand-made models may not be precise enough for certain applications [24]. Preparing a model for tunnel testing using 3D printing presents a design challenge. Such models can be prepared employing two different 3D printing technologies: Multi-Jet Modeling (MJM) and Fused Deposition Modeling (FDM). Both 3D printing technologies have unique advantages and disadvantages. MJM technology offers higher precision and accuracy, but is more costly and requires more time to produce. Alternatively, FDM technology is more economically efficient, enables faster production, and offers a greater durability of models, which can be crucial in the context of hydrodynamic research. The geometry used for CFD simulation has perfect surfaces, and the level of detail accuracy depends only on the designer preparing the model in SolidWorks 2022 CAD software. On the other hand, for a 3D-printed model intended for experimental research, there will always be certain imperfections associated with factors such as the roughness of the layered structure or the mechanical properties of the material.