Eric Greenwood

Separation of Main and Tail Rotor Noise from Ground-Based Acoustic Measurements

A new method of characterizing the external noise radiation of rotorcraft is presented, making use of ground-based acoustic measurements. The method employs time-domain de-Dopplerization to transform the acoustic pressure time-history data collected from a fixed array of ground-based microphones to the equivalent time-history signals observed by an array of virtual in-flight microphones traveling with the helicopter. The now-stationary signals observed by the virtual microphones are then periodically averaged with the main and tail rotor blade passages, which are inferred from the acoustic signals using wavelet analysis. The averaging process suppresses noise that is not periodic with the respective rotor, allowing for the separation of main and tail rotor pressure time histories. The averaged measurements are then interpolated across the range of directivity angles captured by the microphone array in order to generate separate acoustic hemispheres for the main and tail rotor noise sources. This method fa...

Measurement and Characterization of Helicopter Noise in Steady-State and Maneuvering Flight

A special acoustic flight test program was performed on the Bell 206B helicopter outfitted with an in-flight microphone boom/array attached to the helicopter while simultaneous acoustic measurements were made using a linear ground array of microphones arranged to be perpendicular to the flight path. Air and ground noise measurements were made in steady-state longitudinal and steady turning flight, and during selected dynamic maneuvers. Special instrumentation, including direct measurement of the helicopter s longitudinal tip-path-plane (TPP) angle, Differential Global Positioning System (DGPS) and Inertial Navigation Unit (INU) measurements, and a pursuit guidance display were used to measure important noise controlling parameters and to make the task of flying precise operating conditions and flight track easier for the pilot. Special care was also made to test only in very low winds. The resulting acoustic data is of relatively high quality and shows the value of carefully monitoring and controlling the helicopter s performance state. This paper has shown experimentally, that microphones close to the helicopter can be used to estimate the specific noise sources that radiate to the far field, if the microphones are positioned correctly relative to the noise source. Directivity patterns for steady, turning flight were also developed, for the first time, and connected to the turning performance of the helicopter. Some of the acoustic benefits of combining normally separated flight segments (i.e. an accelerated segment and a descending segment) were also demonstrated.

Effects of Ambient Conditions on Helicopter Rotor Source Noise Modeling

A phenomenological 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 ...