Blade tip vortex measurements on actively twisted rotor blades

Abstract

Active rotor control conce pts, such as active twist actuation, have the pot ential to effectively reduce the noise and vibrations of helicopter roto rs. The present study focuses on the experimental investigation of active twist for the reduction of blade-vortex interaction (BVI) effects on a model rotor. Results of a large-sca le smart-twisting active rotor (STAR) test under hover conditions are described. This test investigated the effects of individual blade twist co ntrol on the blade tip vortices. The rotor blades were actuated with peak torsion amplitudes of up to 2° and harmonic frequencies of 1-5/rev with different phase angles. Time-resolved ste reoscopic pa rt icle image velocimetry (PIV) was ca rried out to study the effect s of active twist on the strength and t rajectories of the t ip vortices between 3.6° and 45.7° of vortex age. The analysis of t he vortex trajectories revea led that t he 1/rev active twist actuation mainly ca used a vertical deflection of the blade t ip and the corresponding vortex trajectories of up to 1.3% of the rotor radius R above and -1%R below the unactuated condit ion. An actuation with frequencies of 2/rev and 3/rev significantly affected the shapes of t he vortex trajectories and caused negative vertical displacements ofthe vortices relative to the unactuat ed case of up to 2%R within the first 35° of wake age. The 2/rev and 3/rev actuation also had the most significant effects on the vortex strength and altered the initial peak swirl velocity by up to -34% and +31% relative to the unactuated value. The present aerodynamic investigation reveals a high control authority of the active twist actuation on the strength and trajectories of the trailing blade tip vortices. The magnitude of the evoked changes indicates that the active twist actuation constitutes an effective measure for the mitigation of BVl-induced noise on helicopters.