# Helicopter Rotor Thickness Noise Control Using Unsteady Force Excitation
## 4 Conclusions  
A noise reduction technique for the rotor thickness noise using a force actuator on the blade tip was developed. Both numerical and experimental studies were carried out to study the noise reduction mechanism. From the results, the following conclusions can be drawn:  
1. The sound pressure of rotor thickness noise is characterized by a significant negative pulse caused by the time derivative of the blade surface velocity. The noise reduction can be achieved by using an unsteady excitation force at the rotating blade tip to generate an opposite sound waveform (anti-noise) as the thickness noise with a well-designed control law. The time derivative of the control force dominates the magnitude and the phase of anti-noise.
2. The trailing edge winglet with high-frequency motion is equipped at the rotor blade tip to generate the unsteady control force. When the windward area of the winglet changes periodically, anti-noise can be generated by the resultant in-plane force. A smart drive mechanism combining piezoelectric stack and diamond-shaped amplifiers was designed to drive the high-frequency motion of th ewinglet. Driving frequency up to 120 Hz (three harmonics of rotation frequency) and approximately 3\({}^{\circ}\) deflection angle of the winglet was achieved in testing.
3. The test results show that the noise in the front area of the rotor was effectively reduced, and the noise reduction up to 3 dB was achieved in the plane of the rotor disk under the control law of the first-order frequency and the 3\({}^{\circ}\) deflection angle of the winglet. Moreover, the noise reduction at the observation position strongly related to the azimuth angles, which is independent of pitch angle and distances. This indicates that the rotor noise in a certain area, but not all areas, can be reduced through designing a particular control law.
4. The deflection amplitude, frequency, and initial excitation phase angle are governing parameters that affect noise control. The magnitude of noise reduction is proportional to the deflection angle of the winglet, however, the effect of frequency and initial excitation phase angle is combined. For each frequency, there is always an optimal initial excitation phase angle to minimize the noise level, but it varies with control frequency. The relationship between control phase and frequency was derived in this paper.
5. The previous study in reference 25 states that the anti-noise is generated from the lift change of the blade, not the active flap. The flap alters the blade torsion and changes the local angle of attack and lift, which is responsible for the generation of in-plane force needed for the noise reduction at the target observer. In the present study, the small-scale rotor was stiff in torsion, and the blade torsion induced by the active flap was very small. The results indicate that noise reduction can also be achieved by relying solely on the high-frequency motion of the winglet. It is one finding from our test that needs to be further verified by sophisticated simulation work.  
The theoretical study indicated that the effectiveness of noise reduction increased with the increment of frequency when the deflection amplitude was constant, yet the experimental results herein give the opposite trend. The most probable reason is that deflection angle in this test was converted by voltage rather than direct measurement. In future research, a larger-scale rotor with an angular displacement feedback control system will be designed to monitor and adjust the deflection angle of the winglet in real time.  
Conceptualization and project administration, Y.S.; Writing-original draft preparation, T.L.; Writing-review & editing, G.X.; Validation, X.H. and L.D.  
This work was supported by the Fundamental Research Funds for the Central Universities (No. NS2018007).  
The authors will thank [PERSON] for his great help in experimental design.  
The authors declare no conflict of interest.