ANALYSIS OF A HIGHER HARMONIC CONTROL TEST TO REDUCE BLADE-VORTEX INTERACTION NOISE

A noise study using an aeroelastically scaled BO-105 rotor was conduct ed in the German-Dutch Wind Tunnel to examine the use of higher harmon ic control (HHC) of blade pitch to reduce impulsive blade-vortex inter action (BVI) noise. The noise directivity was measured over a large pl ane underneath the rotor using a traversing inflow microphone array. N oise and vibration measurements were made for a range of matched rotor operating conditions where prescribed (or open loop) HHC pitch, at va rious amplitudes and phases, was superimposed on normal (baseline) col lective and cyclic trim pitch. Acoustic data are presented for 3, 4, a nd 5P HHC applied to a typical landing approach rotor operating condit ion where BVI noise is normally intense. Noise reductions of up to 6 d B were found for the advancing side BVI noise radiating upstream of th e rotor, and also for the retreating side BVI noise radiating below an d downstream of the rotor. The relative levels between the sides were modified by HHC control phase. To help give insight to the physics of the HHC/BVI noise problem, high-resolution loading and noise predictio n results are presented for comparison to the data. The predictions ar e based on a new high-resolution version of the CAMRAD rotor performan ce program under development at Langley, called HIRES.