A study is made of the various mechanisms which generate broadband noise on a range of rotors The sources considered are load fluctuations due to inflow turbulence, due to turbulent boundary layers passing the blades' trailing edges, and due to tip vortex formation Vortex shedding noises due to laminar boundary layers and blunt trailing edges are not considered as they can be prevented in most cases Various prediction methods have been reviewed and extended in some cases An extensive search was made of existing experiments and calculations based on the various prediction methods were made This study shows that present analyses are adequate to predict the spectra from a wide variety of experiments on fans, full scale and model-scale helicopter rotors, wind turbines, and propellers to within about 5 to 10 dB Better knowledge of the inflow turbulence improves the accuracy of the predictions The results of this study indicate that inflow turbulence noise depends strongly on ambient conditions and dominates at low frequencies Trailing edge noise and tip vortex noise are important at higher frequencies if inflow turbulence is weak Boundary layer trailing edge noise is important especially in the presence of large rotors; it increases slowly with angle of attack but not as rapidly as tip vortex noise, which can be important at high angles of attack for wide chord, square edge tips

Comparison of broadband noise mechanisms, analyses, and experiments on rotors

The various source mechanisms which generate broadband noise on a range of rotors are reviewed. Analyses of these mechanisms are presented and compared to existing experimental data. The sources considered are load fluctuations due to inflow turbulence, due to turbulent blade boundary layers passing the trailing edge, and due to tip vortex formation turbulence. Vortex shedding noise due to laminar boundary layers and blunt trailing edges is not considered in detail as it can be avoided in most cases. Present analyses are adequate to predict the spectra from a wide variety of experiments on fans, helicopter rotors, and wind turbines to within about 5 to 10 dB. Better knowledge of the inflow turbulence improves the accuracy of the predictions. Inflow turbulence noise depends strongly on ambient conditions and dominates at low frequencies. Trailing edge and tip vortex noise are important at higher frequencies if inflow turbulence is weak. Boundary layer trailing edge noise increases slowly with angle of attack but not as rapidly as tip vortex formation noise. Tip noise can be important at high angles of attack for wide chord, square edge tips.

Analyses of broadband noise mechanisms of rotors

A field noise measurement program has been conducted on a standard OH-6A helicopter and one that had been modified by reducing the rotor speed, altering rotor tip shape, and treating the engine exhaust and inlet to reduce the external noise levels. The modifications consisted of extensive aircraft design changes resulting in substantial noise reductions following state-of-art noise reduction techniques. The purpose of this study was to document the ground noise characteristics of each helicopter during flyover, hover, landing, and take-off operations. Based on an analysis of the measured results, the average of the overall on-track noise levels of the final modified helicopter was approximately 14 db lower than that for the standard helicopter. Narrow-band-spectra data of the hovering helicopter show a reduction in the overall noise due to the reductions achieved for the lifting main and antitorque tail rotor, engine exhaust, and gear box noise for the modified helicopter. The noise results of the test program are found to correlate generally with noise measurements made previously on this type of aircraft.

Results of the noise measurement program on a standard and modified OH-6A helicopter

: The report presents the results of the Phase 2 Quiet Helicopter Program. A Hughes OH-6A Light Observation Helicopter was extensively modified to obtain a maximum of quieting. The purpose was to apply the latest known sound-suppression techniques available to industry to an actual helicopter and then to measure the results. An acoustic goal was set which required a balanced treatment of each noise-producing source throughout the full frequency range. Noise reductions ranged from 14 to 20 db depending on the flight conditions. The report describes the detailed configuration changes, the test and development programs, and the final sound level measurements compared to the standard OH-6A.

http://www.dtic.mil/cgi-bin/GetTRDoc?AD=AD0753646&Location=U2&doc=GetTRDoc.pdf

William C. McCluskey

Papers

OH-6A Phase II Quiet Helicopter Program