# Parametric Analysis on Landing Gear Strut Friction of Light Aircraft for Touchdown Performance
## 2 Landing Gear Modeling and Validating
### Drop Test and Model Validation  
To verify the accuracy of the numerical model of the landing gear, a single landing gear drop test system is established in this subsection, as shown in Figure 6. The truss structure installed on the vertical slide rail is used to simulate the landing gear fuselage installation point and the mass of the aircraft body. The vertical landing velocity of the aircraft is  
Figure 5: Schematic of reaction forces on upper and lower surfaces of bearings. simulated through a fixed height free fall. Turn the wheel of landing gear in reverse to simulate the aircraft landing longitudinal speed. A suitable rough plate is installed on the impact plate to simulate the dry runway. The load sensors are installed at the bottom of the impact flat to measure the ground vertical load and longitudinal load. The buffer stroke is measured by a displacement sensor mounted on the landing gear. The various response data are collected in the buffering process, including the displacement of the sprung mass and the unsprung mass, sprung mass acceleration, ground vertical force and longitudinal force, and buffer stroke. We used a multi-channel signal collector to acquire real-time test data of each dynamic response. The drop test is carried out in two landing conditions with various vertical descending velocities and sprung mass, shown in Table 1. The LC-1 is the regular landing condition with lower vertical descending velocity and sprung mass. The LC-2 is the extreme worst design landing condition with the highest design vertical descending velocity and sprung mass. The calculation results of the numerical model of landing gear dynamic responses in two landing conditions were obtained using the same values of landing gear parameters as the drop test, and the parameter values used in this work are shown in Table 2. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Landing Condition**} & \multirow{2}{*}{**Sprung Mass (kg)**} & **Vortical Desending** & **Longitudinal Velocity** \\  & & **Velocity at Initial** & **at Initial Contact** \\  & & **Contact (m s\({}^{-1}\))** & **(Linear Velocity of** \\  & & & **Tire) (m s\({}^{-1}\))** \\ \hline LC-1 & 112.35 & 2.85 & 1.0 & 65.8 \\ LC-2 & 151.83 & 2.85 & 1.8 & 65.8 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Landing condition properties. Figure 6: Drop test system of the single landing gear. To verify the accuracy of the numerical model, the comparison of the landing gear landing dynamic responses obtained from the drop test with the results calculated by the numerical model is shown in Table 3 and Figure 7. Table 3 shows the landing dynamic response results of landing gear acquired from two methods and the relative errors between the results. The ground vertical force and shock absorber stroke are two crucial dynamic responses of landing gear in the landing process. Figure 7 is the corresponding curve of the two crucial responses, which can express the quantity and the energy absorption efficiency. According to the result in Table 3, it can be seen that the numerical simulated results have good agreement with the drop test results. The maximum discrepancy error is 4.58% in the sprung mass displacement of LC-1. As shown in Figure 7, the numerical simulated corresponding curve has good consistency with the drop test corresponding curve. Therefore, the numerical model can predict the dynamic response of the landing gear in the landing process accurately, which can be used to analyze the friction effect on touchdown performance in the following research.