## 4 Results and Discussion
### The Compound Coaxial Helicopter in Hover  
The propeller speed of compound coaxial helicopter is 0 RPM in hover. The rotor provides thrust, the wing and the flat tail work in the downwash of the rotor. The downwash flow of the rotor affects the lift of the wing and flat tail. The lift of the wing and flat tail are different in the two states.  
The rotor overlapping azimuth has little effect on the aerodynamic performance of isolated rotors in hover. However, the rotors produce different aerodynamic disturbances to the wing and flat tail at different rotor overlapping azimuth. At the same time, the wing and flat tail have a certain impact on the aerodynamic performance of the rotor. Therefore, it is required to study the aerodynamic interference between rotors, wing, and flat tail at different rotor overlapping azimuth in hover on compound coaxial helicopter.  
The lift on the wing and flat tail of compound coaxial helicopter in hover is shown in Figures 22 and 23.  
It can be seen from Figure 22 that the maximum value of the lift on the wing in state 2 is 8.1% smaller than that in state 1, while the minimum value is very close. It can be found from Figure 23 that the maximum value of the lift on the flat tail in state 2 is 11.2% smaller than that in state 1, and the minimum value is similar. After SMA processing, compared to state 1, the average lift of the wing and flat tail in state 2 is 1% and 1.7% smaller, respectively. However, compared with state 1, the fluctuation amplitudes of the wing lift and the flat tail lift in state 2 are 42.7% and 52.4% smaller, respectively. The rotor working in state 2 is conducive to reducing the lift fluctuations in the wing and flat tail and improving the stability of the compound coaxial helicopter in hover.  
Figures 24-26 show the upper rotor thrust coefficients, lower rotor thrust coefficients, and total rotor thrust coefficients of the compound coaxial helicopter in hover.  
Figure 22: The lift on the wing in hover.  
Figure 23: The lift on the flat tail in hover.  
Figure 24: The compound coaxial helicopter upper rotor \(C_{T}\) in hover.  
Figure 25: The compound coaxial helicopter lower rotor \(C_{T}\) in hover.  
It can be seen from Figure 24 that the peak values (maximum value and minimum value) and amplitudes of the upper rotor thrust coefficients of compound coaxial helicopter are close, and the phase difference is 90\({}^{\circ}\) in two states. It can be seen from Figure 25 that, compared to state 1, the maximum value of the lower rotor thrust coefficients is 1% smaller, the minimum value is 0.7% smaller, and the amplitude of fluctuations is 2.8% smaller in state 2, but the phase difference between the two states is 90\({}^{\circ}\). It can be seen from Figure 26 that the peak values (maximum value and minimum value) and amplitudes of the total thrust coefficients are close in two states. Due to the different distances between the upper and lower rotors and the fuselage, after SMA processing, the upper rotor's thrust coefficients, lower rotor's thrust coefficients, and total thrust coefficients of the compound coaxial helicopter were 1%, 5%, and 3% larger than that of isolated rotors in hover, respectively. In hover, the difference in rotor thrust coefficients between isolated rotors and compound coaxial helicopter is not obvious.