# Helicopter Rotor Thickness Noise Control Using Unsteady Force Excitation
## 3 Validation of Noise Reduction Method
### Results and Discussions  
The operation conditions and test cases are listed in Table 1. Three kinds of tests, i.e., frequency scanning, phase scanning, and amplitude scanning, were carried out with a total of 88 test cases. The harmonic frequency \(H_{f}\) was 1, 2, 3 \(f\) (\(f\) is rotor rotation frequency), respectively. Phase angle \(\psi_{f}\) varied from 0\({}^{\circ}\) to 360\({}^{\circ}\) with an interval of 30\({}^{\circ}\). The alternating voltages were 60 \(\pm\) 20/30/40 V. The winglet was in a neutral position when 60 V voltage was applied. It deflected upward with increasing voltage and deflected downward with decreasing voltage. As mentioned above, these voltages corresponded to different deflection angles \(A_{f}\). In the present complicated test system, some factors, including rotation fluctuation, the precisions of sensors and controllers, phase delay between actuator and controller, and rotor control system, may bring errors to the measurements. To reduce these uncertainties, repeated tests were conducted, and the averaged pressure signal over 20 revolutions was used in the post-processing of results. The signal sampling time in each condition was 10 s (about 180 revolutions of rotor). Figure 10(a) shows the input control signal from the host computer. First, the helicopter rotor operated at a baseline state where the winglet was in neutral position for 3 s, and then the controller sent a high level signal, and the amplifier applied an alternating voltage to the winglet for 5 s. The control signal was removed at the eighth second, and the rotor was recovered to the baseline state. Figure 10(b) is the noise signal collected by a microphone. The change of the sound pressure before and after the control can be clearly seen in the figure. Due to the influences of rotor vibration, random air turbulence, and mechanical transmission, the noise signals contained broadband background noise. It was necessary to filter the measured signals before further analysis. The thickness noise was low-frequency noise, thus the basic principle of the filtering wave was to filter out irrelevant high-frequency components as much as possible while ensuring that the amplitude of the low-frequency component was unchanged. The classic Butterworth low-pass filter was used in signal processing, and the cut-off frequencies of the passband and the stopband were 300 Hz and 1000 Hz, respectively. The comparison of the time history and spectrum of the noise signal from the P22 microphone before and after filtering can be seen in Figure 12. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Value** \\ \hline Rotation speed, \(\Omega\) & \(1080\pm 1\)\(rpm\) \\ Rotation frequency, \(f\) & 18 Hz \\ Collective pitch & 4\({}^{\circ}\) \\ Deflection rate, \(\Omega_{w}\) & \(1\,\Omega,2\,\Omega,3\,\Omega\) \\ Alternating voltage & \(60\pm 20/30/40\) V \\ Initial phase azimuth, \(\psi_{f}\) & 0\({}^{\circ}\)–360\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Rotor operation conditions in experiment. Figure 11: The input control signal and collected noise signal: (**a**) control signal, (**b**) noise signal. The filtered sound pressure signal became smooth, and the amplitude of the first five orders and the total noise sound pressure level (SPL) were almost unchanged (94.62 dB to 94.29 dB), which indicated the effectiveness of the test data processing. Firstly, we analyzed the test result in the condition of \(H_{f}=1\), \(\psi_{f}=270^{\circ},60\pm 40\) V. Figure 13a gives the contour plot of differential sound pressure levels (SPL\({}_{\text{on}}\)-SPL\({}_{\text{off}}\)) between the baseline and the control conditions on the 4 m spherical surface of the rotor center. It can be seen that the SPL in the observation area was effectively reduced when the control signal was applied, and the reduction of up to 3 dB was achieved near the rotor plane. The time histories of sound pressure at nine microphones are compared in Figure 13b.