Acoustic Scaling for Small Rotors in Hover

This paper discusses an effort to determine useful scaling laws for predicting the sound pressure levels at various observer positions for small scale rotors in hover. Surveys of rotor thrust, rotor torque and near-field acoustics are acquired using a series of identically shaped two-bladed rotors ranging between 8 and 18 inches in diameter. Rotor speeds were varied between 50 and 90 revolutions per second which equate to blade tip Mach numbers as high as 0.56. Common sound metrics like overall sound pressure level, sound pressure level and blade-pass frequency harmonic noise were correlated with both dimensional variables (rotor thrust, torque, power and blade tip speed) and non-dimensional variables (thrust and power coefficients, Reynolds number, blade tip Mach number and figure of merit) pertaining to rotor performance. The findings revealed a new correlation parameter for predicting rotor noise based on the product of figure of merit with blade tip Mach number. It is postulated that an increase in rotor collective translates to an increase in loading noise. So long as the flow remains attached, the increased loading noise is expressed by an increase in figure of merit. This new correlation parameter is shown to provide a reasonable collapse of all rotor noise data. In order to account for the directivity of the sound field, a second order response surface model of this new correlation parameter was developed and validated.