Intensity, acoustical radiation spectrum, as well as single-rotor external noise level are analyzed in the paper based on the experimental data obtained from testing actual helicopters. The helicopter acoustical far-field is shown to be a superposition of the main and tail rotor fields. The external noise spectrum measured on the ground contains discrete and broadband components. Discrete component frequencies are multiples of the main and tail rotor passage frequencies and fall within the frequency ranges 20-160 Hz and 100-500 Hz respectively. Continuous spectrum radiation can be seen within the whole sound frequency range, but the maximum intensity is registered at 500-3,000 Hz frequencies. It has been found that the intensity of the helicopter acoustical radiation in the direction corresponding to the maximum external noise level and the power required by the main and tail rotors change according to the same law. The helicopter acoustical field is asymmetric relative to the vertical plane running through the longitudinal axis of the aircraft. The helicopter external noise level measured in PNdb has been found to be defined by the tail rotor acoustical radiation with a discrete frequency spectrum within 100-500 Hz and the continuous spectrum radiation within 500-3,000 Hz. The tail rotor configuration is shown to affect the helicopter acoustic response. Experimental studies of the external noise produced by the same helicopter first equipped with a tail rotor with symmetrical layout of the blades and then with an X-shaped tail rotor have allowed to establish that its tail rotor is the dominating source of the helicopter broadband noise. The X-shaped tail rotor has allowed to reduce the helicopter noise by 3-5 PNdb in level flight.
External Noise of Single Rotor Helicopters
1995
8 pages
Report
No indication
English
Acoustics , Aircraft , Aeroacoustics , Aerodynamic noise , Continuous spectra , Far fields , Frequency ranges , Helicopters , Noise intensity , Noise measurement , Noise reduction , Noise spectra , Rotary wings , Sound pressure , Tail rotors , Aircraft noise , Blade tips , Emission spectra , Frequency distribution , Russian federation , Sound fields , Sound waves , X wing rotors , Foreign technology
External Noise of Single Rotor Helicopters
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