Effects of ambient conditions on helicopter rotor source noise modeling

Aphenomenological noise-modeling method called Fundamental Rotorcraft Acoustic Modeling from Experiments is used to demonstrate the changes in rotor harmonic noise generation of a helicopter operating at different ambient conditions. The method is based upon a nondimensional representation of the governing acoustic and performance equations of a single-rotor helicopter. Measured external noise is used together with parameter-identification techniques to develop a model of helicopter external noise that is a hybrid between theory and experiment. The method is used to evaluate the main rotor harmonic noise of a Bell 206B3 helicopter operating at different altitudes. The variation with altitude of blade-vortex interaction noise, known to be a strong function of the helicopter's advance ratio, depends upon which definition of airspeed is used. If normal flight procedures are followed and indicated airspeed is held constant, the true airspeed of the helicopter increases with altitude. This causes an increase in advance ratio and a decrease in the speed of sound, which results in large changes to blade-vortex interaction noise levels. Additionally, thickness noise on this helicopter becomes more intense with increasing altitude. The noise magnitude and directivity variations differ when flight conditions are defined by true airspeed. These results suggest that existing empirical helicopter rotor noise-source models may give incorrect noise estimates when they are used at conditions where data were not measured and may need to be corrected for mission land-use planning purposes.