# Helicopter Rotor Thickness Noise Control Using Unsteady Force Excitation  
[PERSON]  
1 National Key Laboratory of Rotocrraft Aeromechanics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; [EMAIL_ADDRESS] (T.L.); [EMAIL_ADDRESS] (X.H.); [EMAIL_ADDRESS] (L.D.); [EMAIL_ADDRESS] (G.X.)  
[PERSON]  
1 National Key Laboratory of Rotocrraft Aeromechanics, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China; 15803489387@163.  
including the effects of taper ratios, sweep back, tip chord length, protrusion shape, camber, and airfoil. [PERSON] et al. [PERSON] et al. (2018) designed a low-noise rotor for a moderate-weight helicopter by including the reduced noise features of modulated blade spacing and swept ogee tip shape. In reference (2018), [PERSON] discusses a series of rotor tips for blade-vortex noise reduction. [PERSON] et al. [PERSON] et al. (2018) used the multi-block computational fluid dynamics (CFD) to study the effect of anhedral tip on blade noise. Nowadays, some passive noise reduction techniques are being applied to the design of new generation helicopter rotors (2018).  
However, the passive design also affects the rotor performance because compromises should be made between noise reduction and performance improvement, and adaptation to different flight conditions should also be taken into consideration. The active control is a more attractive way to reduce rotor noise. Active rotor control has been studied for several years, but most studies were for blade-vortex interaction (BVI) noise. It can be achieved through higher harmonic control (1992; 1993; 1994), individual blade control (1995; 1996; 1997), active flaps (2000; 2001; 2002), and active twist (2001; 2002; 2003). The numerical and test results demonstrate that the active rotors are effective for BVI noise and vibration reduction as well as performance enhancement. However, the research on active control for thickness noise reduction is very limited. [PERSON] et al. [PERSON] et al. (2018, 2019) and [PERSON] (2019) separately proposed the novel control method for reducing the in-plane noise of the rotor disc in the principle of noise field cancelation and compared the noise reduction effect of single monopole and dipole sources as well as multiple controllers. [PERSON] et al. [PERSON] et al. (2019) studied the relation between the loading solution and resulting noise and proposed two active devices for in-plane noise reduction. Some experimental studies on thickness noise reduction have been carried out as well. [PERSON] et al. [PERSON] et al. (2019) conducted an experiment in a wind tunnel test using the Boeing-SMART rotor with active flaps and demonstrated that with the active flap, it is possible to reduce in-plane noise. They also analyzed the noise reduction mechanism that causes the active flap to alter the blade torsion and change 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. [PERSON] (2018) studied the noise generation mechanism of the tip blowing control and conducted noise tests. The results showed that the in-plane noise of target observation points in the rotor disk plane can be reduced by using on-blade, tip air blowing control.  
In this paper, a noise reduction technique for the rotor thickness noise using a force actuator on the blade tip is developed and verified. Firstly, the theoretical analysis on the formation of rotor noise and the mechanism of noise reduction through force excitation was performed. Then, a 2-m diameter model rotor with an active trailing edge winglet was designed, which was used to generate high-frequency unsteady control force. Through the results of the open-loop test (frequency, phase, and amplitude scanning) in a rotor anechoic chamber, the effectiveness of the unsteady force excitation on thickness noise reduction was analyzed, as well as the influence of control parameters of the winglet.