# Aerodynamic Simulation of Helicopter Based on Polyhedron Nested Grid Technology
## 3 Validation of CFD Simulation Method
### AH-1G Rotor in Forward Flight  
In this section, the flight data of the AH-1G rotor [24] are chosen for CFD calculation, and the relevant parameters and flight status are shown in Table 2. The test points with the tip Mach number of 0.65 and advance ratio of 0.19 are selected for simulation. The forward flight speed corresponding to this flight state is 151.864 km/h and the rotor speed is 2250 RPM. This test was selected because it has comprehensive load data, acoustic data, and blade harmonic data. In addition, the accuracy of the flight test data has been verified many times in other references [25; 26; 27; 28].  
In this flight state, the cyclic pitch motion equation of the blades is as follows:  
\[\theta(\psi)\ =\ 6.0^{\circ}+1.7^{\circ}\cos\psi-5.5^{\circ}\sin\psi \tag{8}\]  
Figure 7 shows the pressure contours of AH-1G blades at different azimuth angles obtained by CFD numerical simulation method.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Number of Blades** & **2** \\ \hline Plane shape of blade & Rectangle \\ Airfoil & OLS \\ Chord length \(c\) & 0.1039 m \\ Rotor radius R & 0.958 m \\ Undercut \(R_{cut}\) & 0.182 R \\ Torsion angle & \(-10^{\circ}\) \\ \hline \hline \end{tabular}
\end{table}
Table 2: Relevant parameters and flight status of AH-1G rotor.  
Figure 7: _Cont._  
Figure 7: Pressure contours of AH-1G blades at different azimuth angles. (**a**) Azimuths of 0 and 180 degrees. (**b**) Azimuths of 45 and 225 degrees. (**c**) Azimuths of 90 and 270 degrees. (**d**) Azimuths of 135 and 315 degrees.  
According to Figure 7, it is not difficult to see that the shock waves at the blade tip in the azimuth range of \(0^{\circ}\) to \(180^{\circ}\) are more obvious than those at the azimuth range of \(180^{\circ}\) to \(360^{\circ}\). The shock waves at the blade tip are the strongest at the azimuth angle of \(90^{\circ}\), which is consistent with the aerodynamic theory of the helicopter.  
Considering that only the overall trend of the simulation results can be seen in Figure 7, in order to more accurately judge the accuracy of the simulation, the pressure coefficient distribution of different azimuth angles and different blade profiles calculated by CFD numerical simulation method was compared with the experimental values, as shown in Figure 8.  
In Figure 9, the curves of normal force coefficient with azimuth angle at four different section positions on the blade calculated by CFD simulation are given and compared with the test data. According to the data in the figure, when the azimuth angle of the advancing blade is about 70\({}^{\circ}\) to 90\({}^{\circ}\) or the retreating blade is near an azimuth of about 270\({}^{\circ}\), there will be strong blade/vortex interference, resulting in abrupt change of aerodynamic force in the blade section, which makes it more difficult to simulate the rotor flow field in forward flight state.  
It can be seen from Figures 8 and 9 that the numerical calculation results at different azimuth angles and different blade sections are basically consistent with the test data, which proves that the CFD numerical simulation method and grid technology proposed in this paper achieve high accuracy in this type of simulation.