## 6 Design and Analysis of Main Landing Gear Strut
### Design and Analysis of Retractable (Curved) Main Landing Gear Strut  
After the failure of the retraction test of the straight main landing gear strut, a retractable (curved) landing gear strut model was developed in CATIA V5(r). The CAD model of the retractable main landing gear strut, along with other sub-assemblies, is shown in Figure 8. This model helped in stress analysis of the strut within the allowable limits of thickness and width under the applied loading conditions. In the first phase of this analysis, only composite landing gear struts without mounting brackets were analyzed computationally. Landing gear struts having different thickness-to-width ratios were analyzed in order to obtain the most suitable design in terms of the Tsai-Wu failure criterion, mass and deflection of the strut. Based on the results obtained from the straight landing gear strut, the same dimensions in terms of thickness-to-width ratio were used for the retractable landing gear strut design as a first option. However, using the same dimensions could not meet the Tsai-Wu failure criterion, and, hence, struts having different dimensions were analyzed. The dimensions of the strut in terms of thickness and width were varied within the allowable limits to ensure smooth retraction of the landing gear strut. The results of these analysis are tabulated in Table 8. The Tsai-Wu failure criterion expresses the effect of all the stresses in each mesh element. Figure 9 depicts the variation in the Tsai-Wu criteria factor and the mass of different models of landing gear struts. From these results, the T300 fiber/epoxy composite material strut having dimensions of 52.8 mm \(\times\) 75 mm just meeting the Tsai-Wu failure criterion with a value of 0.98 with a minimum mass of 7.56 kg was recommended for further analysis to evaluate the effects of joining mounting brackets of the landing gear assembly. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline \multirow{2}{*}{**Model No.**} & \multirow{2}{*}{**Lamina**} & \multirow{2}{*}{\begin{tabular}{c} **No. of** \\ **Piles** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **The** \\ **Thickness of** \\ **Ply (mm)** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Thickness** \\ **to Width** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Strut** \\ **Thickness of** \\ **Ratio** \\ \end{tabular} } & \multirow{2}{*}{\begin{tabular}{c} **Tsai Wu** \\ **Failure** \\ **(mm)** \\ \end{tabular} } & \multirow{2}{*}{\
\begin{tabular}{c} **Mass of** \\ **Failure** \\ **Citeria** \\ \end{tabular} } & \multirow{2}{*}{
\begin{tabular}{c} **Deflection** \\ **(cm)** \\ \end{tabular} } \\ \hline
1 & T300 Carbon Fiber/Epoxy & 210 & 0.24 & 1.5 & 50.4 & 1.09 & 2.3 & 5.8 \\
2 & T300 Carbon Fiber/Epoxy & 220 & 0.24 & 1.5 & 52.8 & 0.97 & 2.52 & 4.8 \\
3 & T300 Carbon Fiber/Epoxy & 220 & 0.24 & 1 & 52.8 & 0.58 & 3.79 & 3.2 \\
4 & S-Glass Fiber/Epoxy & 210 & 0.24 & 1.5 & 56.7 & 1.11 & 3.43 & 9.6 \\
5 & S-Glass Fiber/Epoxy & 220 & 0.24 & 1.5 & 59.4 & 1.02 & 3.6 & 8.7 \\
6 & S-Glass Fiber/Epoxy & 220 & 0.24 & 1 & 59.4 & 0.72 & 5.4 & 8 \\ \hline \hline \end{tabular}
\end{table} 
Table 7: Parametric study of different models of straight main landing gear strut composed of S-glass fiber/epoxy and T300 carbon fiber/epoxy upon impact. Figure 8: CAD Model of a retractable main landing gear assembly. In the second phase of the analysis, the stress analysis of retractable composite main landing gear strut assembled with mounting bracket was carried out. To validate the trend of this analysis, recommended dimensions of the model (52.8 mm \(\times\) 75 mm) were used. When the analysis was performed for the strut and mounting bracket joined together with the help of Qty 4 bolts, the assembly could not meet the Tsai-Wu failure criterion as its value was found to be 4.6. In order to reduce the value of the Tsai-Wu failure factor for qualification of the assembly, different mechanical options of joining composite strut and metal mount including bolts, nuts, bolt loads and washers were also used.