Investigation of Propeller Vortex Noise Including the Effects of Boundary Layer Control

Abstract

An experimental investigation has been conducted on the vortex noise produced by a two-bladed 4-ft diam model propeller capable of boundary layer removal. The propeller had a spinner comprising 70% of the total propeller radius. A porous section on both surfaces of the symmetric section airfoil allowed removal of the boundary layer. Free-field measurements were made in an anechoic chamber at three field points for three tip speeds (209.5, 314.2, and 366.5 ft/sec) and four blade angles (0°, 2.5°, 5°, and 10°) both without and with boundary layer control. Agreement with theory was good (within 2 dB) showing a sixth power of tip velocity relationship and a classical dipole radiation pattern for the over-all SPL of the vortex noise. Boundary layer removal primarily affected sound levels above 3150 Hz, with no ordered effect on the over-all level.