THE IMPORTANCE OF TAIL ROTOR INTERACTION AS AN ACOUSTIC SOURCE

Helicopter noise was considered in the 1970’s to be essentially only dependent on the main and the tail rotors and most prediction methods treated them as a separate entities. At about this time it became clear that the levels of tail rotor noise in cruise flight were often higher than predicted. In addition to the classic tail rotor whine, a modulated sound associated with the tail rotor, could also be detected on some helicopters. Research indicated that a major contribution to the tail rotor noise was associated with the interaction of the tail rotor with the tip vortex shed by the main rotor. It was established that not only did this source contribute to the high levels of noise detected on approach during cruise flight but it had a major impact on the maximum overhead noise generated by the helicopter. Once this source was identified, a practical quiet tail rotor (Q.T/R) was derived and subsequently incorporated on the Lynx Mk7 in 1986: this presented an opportunity to further examine this phenomenon and confirmed the conclusion of the initial investigation.