## 3 Experiment Results and Discussion
### Waterfall Diagram of the Auto-Power Spectrum  
The two aluminum alloy blades selected for the test are identical in shape and the mass error is within 0.01 g. The auto-power spectrum waterfall diagram of the fuselage at rotor speed 1000 r/min is analyzed. Figure 7 shows the waterfall diagram of the auto-power spectrum. It can be seen that the amplitude of low frequency (1-4 \(\Omega\)) vibration is less than \(1.5\times 10^{-4}\) g\({}^{2}\)/Hz, which indicates that the rotor reaches the dynamic balance state.  
When the rotor is in work state, the counterweight adjustment has a greater impact on the vibration in the disc plane. If the rotor system consists of an even number of blades, the role of the blade-added counterweight is equivalent to that of the blade-reduced counterweight in relative position. Figure 8 shows the waterfall diagram of the fuselage auto-power spectrum caused by the counterweight. It can be seen that the amplitude of 1 \(\Omega\) and 2 \(\Omega\) vibration increases obviously when the counterweight is unbalanced.  
In order to improve the vertical vibration characteristics of the rotor, the rotor can be adjusted by changing the control input of the rotor aerodynamic force. One method is to adjust the pitch rod of the blade. The pitch rod is used to connect the automatic filter and blade on the helicopter, which can control the pitch input of the corresponding blade.  
Figure 8: Waterfall diagram of the auto-power spectrum for an unbalanced counterweight.  
Figure 7: Waterfall diagram of the auto-power spectrum for the dynamic balance state.  
Changing the length of the pitch rod changes the attack angle of the blade, and the attack angle directly affects the lift of blade. It can be seen that if the rotor has a positive vertical force in a certain direction, then the pitch rod of the blade at the opposite position of 180\({}^{\circ}\) azimuth angle should be adjusted to change the pitch so that it can generate aerodynamic force with the same size and direction. Since the pitch rod on the helicopter cannot be dynamically adjusted, the pitch must be changed when the helicopter is hovering. Figure 9 shows the waterfall diagram of the fuselage auto-power spectrum caused by the pitch rod of the blade. It can be seen that the amplitude of 1 \(\Omega\) and 2 \(\Omega\) vibration increases obviously when the pitch is unbalanced.  
The trailing-edge flap is another method to change the vertical vibration characteristics of the rotor system. The trailing-edge flap is similar to a swing device attached to the blade trailing edge. The adjustment method of the trailing-edge flap is the same as that for the aileron of a fixed-wing aircraft wing by changing the aerodynamic force attached to the blade. It can be seen that the camber of the airfoil segment with the trailing-edge flap reduces the lift when the flap is bent upward; on the contrary, the downward bending of the trailing-edge flap increases the lift of the blade in this section. Figure 10 shows the waterfall diagram of the fuselage auto-power spectrum caused by the trailing-edge flap. It can be seen that the amplitude of 1 \(\Omega\) and 2 \(\Omega\) vibration increases obviously when the trailing-edge flap is unbalanced.  
Figure 10: Waterfall diagram of the auto-power spectrum for an unbalanced trailing-edge flap.  
Figure 9: Waterfall diagram of the auto-power spectrum for an unbalanced blade pitch.  
It can be seen from Figures 8-10 that if the counterweight, pitch, and trailing-edge flap are unbalanced, then the amplitude of 1 \(\Omega\) and 2 \(\Omega\) low-frequency vibration increases.