EFFECTS OF UNEVEN BLADE SPACING ON DUCTED TAIL ROTOR ACOUSTICS

A reduced-noise ducted tail rotor helicopter antitorque system is described which employs uneven blade spacing and low rotor tip speeds. The blade spacing scheme, based on a sinusoidally varying pattern, redistributes the tonal content of the rotor noise across the audible spectrum, which reduces annoyance. The blade spacing design and analytical predictions used are described, along with validations from model test results. The validations indicate that the noise spectra of unevenly spaced rotor blades can be reasonably predicted. Model tests show that rotor configurations with uneven blade spacings reduce ducted tail rotor harmonics by up to 5.4 dBA. Rotor tip speed reductions and blade tip modifications further reduce noise levels another 5.9 dBA. Full-scale flight tests demonstrate total test aircraft noise reductions of up to 6 dBA, and ducted tail rotor harmonic noise reduction of 5 to 20 decibels. In addition to reduced noise levels, a significant improvement in the sound quality is observed.