A CLOSED LOOP CONTROLLER FOR BVI IMPULSIVE NOISE REDUCTION BY HIGHER HARMONIC CONTROL

Within a joint research program of DLR, ECF and ECD a wind tunnel experiment was performed with the DLR rotor test rig in the German Dutch Wind Tunnel. The tests aimed on an automatic reduction of the Blade-Vortex Interaction (BVI) impulsive noise radiated by the main rotor and were based on former investigations which had shown, that the most annoying part of the helicopter noise can be reduced considerably by means of Higher Harmonic Control (HHC). However, one prerequisite for this reduction is a proper adjustment of the higher harmonic control parameters which have to be optimized according to the actual flight condition. Therefore a suited closed loop controller is required being able to keep the noise small within the whole flight envelope. Due to the nonlinear dependence of the noise level on the higher harmonic control parameters, the design of this controller becomes a nontrivial task. Therefore three different control approaches were investigated, both, within the scope of simulation and during wind tunnel tests. The corresponding results are presented for various flight conditions and parameter configurations and the long term as well as the transient behaviour of the different controllers are compared.