# Signal Enhancement of Helicopter Rotor Aerodynamic Noise Based on Cyclic Wiener Filtering
## 1 Introduction
BVI noise is generated due to the mutual interference between the aerodynamic characteristics of the blades and the vortex shedding from the tip of the propeller. HSI noise usually refers to a kind of impulse noise caused by the surge wave when the helicopter flies forward at a high speed [12]. Turbulence near the blade surface creates broadband noise [13]. A cyclostationary signal is a kind of special non-stationary signal with hidden periodicity. The cyclostationarity signal is a powerful tool for processing random signals with hidden periodic phenomena. Cyclostationary signal processing methods are used in various fields such as bearing fault diagnosis and mechanical source separation [14; 15; 16; 17; 18; 19; 20; 21]. However, there is little research on its application to the aerodynamic noise of helicopter rotors. Few people might have linked rotor aerodynamic noise to cyclostationarity before. While the helicopter is flying and hovering, the rotor aerodynamic noise has the characteristics of cyclostationarity ascribed to the periodic movement of the rotor [22; 23]. Therefore, the application of cyclostationary analysis methods in aeroucoustics has gradually received extensive attention [19; 20; 21; 22; 23; 24; 25]. [PERSON], as a top scholar in the field of cyclostationary signals, proposed a variety of methods to extract cyclostationary signals [24], and gradually applied them to the field of aeroucoustics [25]. However, compared with reference [25], the cyclic Wiener filter only needs one microphone to extract the acoustic signal, and the implementation of this method is simpler. Reference [26] gives a method for detecting helicopters by radar, but helicopters are not easy to detect by radar when helicopters fly at an ultra-low altitude. However, we use acoustic signals to detect the helicopter in this paper, because there must be a noise signal when the helicopter is flying. [PERSON] et al. proved the cyclostationarity of rotor aerodynamic noise [22] and proposed a cyclostationary modeling method of helicopter rotor aerodynamic noise [23]. The cyclic Wiener filter can effectively extract the signal with cyclostationarity from the noise signal without cyclostationarity [27; 28; 29]. Therefore, this method has a significant enhancement effect on the rotor aerodynamic noise signal with cyclostationary characteristics. Envelope spectrum analysis based on the Hilbert transform is effective in extracting feature frequency, and it is frequently applied in bearing fault diagnosis [30]. However, the characteristic frequencies of bearing faults are generally cyclostationary [31; 32]. Therefore, it can be adopted for rotor aerodynamic noise spectrum analysis. The innovations and main contributions of this paper are described as follows:1. Aiming at the cyclostationary characteristics of helicopter rotor aerodynamic noise, a cyclic Wiener filter is proposed to be used in the field of helicopter rotor aerodynamic noise enhancement. 2. The cyclic frequency of helicopter rotor aerodynamic noise at different distances is obtained by the cyclostationary analysis method, and it is used for frequency shifting in cyclic Wiener filtering. 3. It is proposed to use the envelope spectrum based on the Hilbert transform to analyze the effect before and after filtering, because the envelope spectrum can eliminate some unnecessary frequency interference and highlight the rotor aerodynamic noise signal with cyclostationarity. 4. According to the harmonic characteristics of helicopter rotor aerodynamic noise, a simple and practical detection method is constructed and combined with cyclic Wiener filtering, which can greatly improve the detection distance of helicopters. In this paper, a cyclic Wiener filter was constructed and used for the enhancement of helicopter rotor aerodynamic noise, and the filtering effect was analyzed through the envelope spectrum. Furthermore, a detection function was constructed under the consideration of the characteristics of rotor aerodynamic noise, so as to realize the long-distance detection of helicopters.