## 4 Discussion  
When the obtained results are evaluated, the use of the multi-layer hybrid flywheel structure increases the maximum RPM value and reduces the total system weight. It is also observed that the energy storage capacity of these systems has increased to a certain extent. The studies are first carried out for the steel rotor structure. In order to prevent the rotor from breaking apart by exceeding the elasticity limit of the steel, carbon fiber layers of different thicknesses are added to the outside of the steel rotor, and simulations are performed accordingly. These simulations are repeated for 2 mm, 5 mm, 10 mm and 20 mm carbon fiber thicknesses. As the thickness of the carbon fiber layer increases, the maximum RPM value that the system can withstand also increases. As a result, the energy storage capacity of the system also increases. This situation is also valid for the titanium alloy rotor material. When Table 11 is examined carefully, it is clearly seen that the maximum RPM that the rotor can withstand increases with the tensile strength properties of the material and the thickness of the carbon fiber layer. The highest RPM value was obtained when the rotor was made entirely of carbon fiber. However, carbon composite material has a significantly higher manufacturing cost. The findings show that it is necessary to select the rotor structure and material according to the specific application and requirements of the FESS. Another important point is that, when Equation (1) is taken into account, the decrease in rotor mass and the increase in speed without changing the geometric dimensions and the amount of stored energy result in a decrease in the moment of inertia of the rotor. Reducing the moment of inertia in mobile systems can be said to be a reduction in the counterforce applied during maneuvers (gyro effect). \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Amb. Temp.: 22 \({}^{\circ}\)C** & **Mild Steel** & **Titanium Alloy** & **Carbon Fiber-Reinforced Polymer** \\ \hline  & Yield Point & Yield Point & Rupture Point \\ Speed (RPM) (Analytical) & 53,047 & 132,220 & 1,047,211 \\ Speed (RPM) (ANSYS) & 50,700 & 125,700 & 952,520 \\ Weight (kg) & 2.9 & 1.7 & 0.67 \\ Maximum (MPa) & 250 & 901 & 23,000 \\ \hline \hline \end{tabular}
\end{table} 
Table 10: Comparison of analytical calculation results and simulation results for a 100 mm solid rotor, considering the yield stress values. Comparative Analysis of Systems  
A comparison is given in Table 11 for different rotor materials. When evaluated in terms of yield and rupture strength, the manufacturing steel has the lowest values. However, it also has the highest weight. As stated in Equation (1), the energy stored in FESS increases linearly with the rotor weight. However, it increases proportionally with the square of the rotor speed. Therefore, the rotor weight and rotor speed should be evaluated together in the calculation of the stored energy. Although the titanium alloy weighs 58.85% of the manufacturing steel, its yield and rupture strength values are significantly higher. The carbon composite material weighs 23% of the manufacturing steel and 39% of the titanium alloy. However, the carbon composite material has the highest strength among these materials due to its physical structure. Despite its low weight, its ability to reach very high speeds allows it to store the highest energy among all other materials. Despite its high cost, its high energy density makes it highly strategic in mobility applications. In hybrid structures, the total system weight varies depending on the percentage of materials used, as shown in Table 11. As the thickness of the carbon fiber increases, the total weight of the rotor decreases. However, RPM, manufacturing costs, and manufacturing difficulties also increase. The situations are similar for steel-carbon fiber and titanium alloy-carbon fiber hybrid structures.