Acoustic Characteristics of a Model Isolated Tiltrotor

An aeroacoustic wind tunnel test was conducted using a scaled isolated tiltrotor model. Acoustic data were acquired using an in-flow microphone wing traversed beneath the model to map the directivity of the near-field acoustic radiation of the rotor for a parametric variation of rotor angle-of-attack, tunnel speed, and rotor thrust. Acoustic metric data were examined to show trends of impulsive noise for the parametric variations. BVISPL maximum noise levels were found to increase with a for constant m and CT, although the maximum BVI levels were found at much higher a than for a typical helicopter. BVISPL levels were found to increase with m for constant a and CT. BVISPL was found to decrease with increasing CT for constant a and m, although BVISPL increased with thrust for a constant wake geometry. Metric data were also scaled for Mtip to evaluate how well simple power law scaling could be used to correct metric data for Mtip effects. Nomenclature c measured speed of sound, ft/sec CT thrust coefficient f frequency, Hz. fe equivalent flat plate drag area, ft 2 Mtip rotor tip Mach number, RW/c p/pnorm non-dimensional acoustic pressure (pnorm is arbitrarily selected constant.) R rotor radius, 4.75 ft. (1.45 m) SPL sound pressure level, dB V tunnel speed, ft/sec x/R non-dimensional x distance, origin at hub, positive downstrea