Rotorcraft Aeroacoustics An Introduction Preliminary Remarks Rotorcraft Noise is becoming an area of considerable concern to the community United States and most European countries have stringent limitations of acceptable noise levels Any new design must be done with these limitations to avoid unpleasant surprises during certification time Some Definitions Sound Pressure Level is measured in Decibels p2 p 10 log10 SPL 20 log10 2 p p Re f Re f where pRe f p2 N 2 10 m2 Mean Square Pressure 5 Overall Sound Pressure Level OASPL Weighting A Weighting Emphasizes sound frequencies that people here best Perceived Noise Level PNL weighting The most annoying frequencies are weighted more than others Typical dB Levels Hearing Threshold 0 dBA Whisper 20 dBA Quite Neighborhood 40 dBA Normal Speech 60 dBA Busy Office 80 dBA Heavy Traffic 100 dBA Discotheque 120 dBA Flight Tests Why Flight Tests Why Flight Test Wind tunnel tests provide precise repeatable control of rotor operating conditions but accurate noise measurements are difficult for several reasons Wall effects prevent the rotor wake from developing exactly as it does in free flight This is crucial because an important contributor to rotor noise is the interaction between the rotor and its own wake such as blade vortex interaction In many wind tunnel tests the rotor test stand is not the same shape as the helicopter fuselage hence aerodynamic interference between the test stand and rotor is different than in flight The wind tunnel walls cause reflections that may corrupt the acoustic signals The wind tunnel has its own background noise caused by the wind tunnel drive and by the rotor test stand The YO 3A aircraft is actually quieter than many wind tunnels The wind tunnel turbulence level is rarely the same as in flight The rotor is frequently trimmed differently in a wind tunnel test than in flight Wind Tunnel Tests http halfdome arc nasa gov research IRAP intro html Flight Test vs Wind Tunnel Tests Noise Abatement Quite Approach Lighthill s Formulation Cabin Noise Reduction with Actuators Kirchoff Formulation f x y z t Rotor Surface Ffowcs Williams Hawkings Formulation FWH Formulation Continued FWH Formulation Continued Stress Tensor that includes pressure Comes from a CFD analysis Integration is over rotor surface Mr is Mach number of a source on the blade along r R distance between point on the blade and observer Ret Retarded time that is time at which noise left the rotor BVI Noise Predictions with Computed Loads Surface pressure input From RFS2BVI a code Jointly developed at Ga Tech And Boeing Mesa Coupling of Acoustics Solver to CFD Codes and Comprehensive Codes Provides trim Blade dynamics Elastic deformations Provides surface Pressures As a function of time all Over the blade surface Concluding Remarks Outputs from CFD codes or even lifting line blade element theory can be input into aeroacoustic codes that solve the wave equation in integral form Satisfactory agreement is obtained for thickness lift and shock noise sources with these approaches