Research On Receiving And Enhancing Method Of Helicopter Rotor Aerodynamic Noise

As an air transport platform,helicopters are increasingly widely used in military and civil fields.They usually make full use of terrain and other conditions to fly at low or ultra-low altitude when carrying out combat and rescue tasks,and have the advantage of being difficult to detect by radar,infrared and other detection methods.Moreover,helicopters use rotors to provide lift,propulsion and control forces,and have unique vertical takeoff and landing,hovering,high maneuverability and low altitude low speed flight performance.The high-speed rotation of the rotor will inevitably lead to strong aerodynamic noise,and the noise has very obvious signal characteristics.The low-frequency components of the noise have less attenuation when propagating in the air,and the propagation distance is far.In practical applications,it is very easy to be detected by acoustic detection equipment and even the human ear.Therefore,it is imperative to study the helicopter rotor aerodynamic noise,and enhancing the rotor aerodynamic noise signal under strong background noise can be used for acoustic early warning to improve the early warning and defense capabilities of helicopters in China.The specific research work and results are as follows:(1)The components of helicopter noise are analyzed,and then the frequency spectrum characteristics of helicopter noise are analyzed through the wind tunnel test data of the rotor and the helicopter field flight test data.In view of the cyclostationary characteristics of helicopter rotor aerodynamic noise,the simulation modeling is carried out.(2)Aiming at the cyclostationarity of rotor aerodynamic noise,a signal extraction method based on cyclic Wiener filtering is proposed.The background noise is filtered adaptively by shifting a group of frequencies of the input signal,so that the rotor aerodynamic noise can be extracted from the strong background noise.The envelope spectrum of the filtered signal is analyzed with Hilbert transform,so that the characteristic frequency of helicopter rotor aerodynamic noise signal can be extracted.According to the harmonic characteristics of rotor aerodynamic noise,a detection function is constructed to realize the remote detection of helicopters.Combined with rotor aerodynamic noise simulation signal and helicopter field flight test,it is verified that the cyclic Wiener filter increases the detection range of Robinson R44 helicopter from 4.114 km to 17.75 km,and that of Robinson R22 helicopter from 3.49 km to 13.75 km.(3)In order to solve the problem that the new generation ratio of helicopter noise signal is extremely low in the far field,a differential beamforming method is proposed to realize the spatial filtering of helicopter noise signal,so as to improve the signal-to-noise ratio of helicopter noise signal.Combined with rotor aerodynamic noise simulation signal and helicopter outfield flight test,it is verified that differential beamforming can effectively attenuate noise signals in unexpected directions,thus successfully filtering out some clutter signals,but the enhancement effect on low frequency(rotor aerodynamic noise base frequency)is not obvious,and subsequent improvement on differential beamforming should further enhance helicopter noise signals.