Characterization of blade tip vortices on large-scale rotors

Abstract

The vortices produced by helicopter rotor blades dominate the aerodynamics of the rotor wake. Their interactions with the rotor blades cause sound and structural vibrations and have detrimental effects on the aircraft performance. Current research strives to better understand and reduce the effects of these blade-vortex interactions (BVI). Sub-scale model rotor tests – and an increasing number of full-scale investigations – are performed to develop measures against the interaction effects. Active rotor control concepts – such as active twist actuation – have the potential to effectively reduce the sound and vibrations of helicopter rotors. The present thesis focuses on the experimental investigation of active twist for the reduction of BVI effects on a model rotor. Results of a large-scale smart-twisting active rotor (STAR) test under hover conditions are described. This test investigated the effects of individual blade twist control 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 stereoscopic particle image velocimetry (PIV) and background-oriented schlieren (BOS) measurements were carried out to study the effects of active twist on the strength and trajectories of the tip vortices between 3.56° and 45.74° of vortex age. The analysis of the vortex trajectories revealed that the 1/rev active twist actuation mainly caused a vertical deflection of the blade tip and the corresponding vortex trajectories of up to 1.3% of the rotor radius R above and –1%R below the unactuated condition. An actuation with frequencies of 2/rev and 3/rev significantly affected the shapes of the vortex trajectories and caused negative vertical displacements of the vortices relative to the unactuated 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 BVI-induced sound on helicopters. The majority of available studies on BVI – including the STAR experiments – are based on sub-scale model rotor tests. It is challenging to correctly downscale the multitude of underlying effects contributing to BVI. Full-scale investigations with innovative, whole-field vortex visualization techniques are, consequently, required for the validation of the model test results. The present work thus also targets the advancement of optical measurement techniques for the investigation of vortices on full-scale helicopters. A series of flight tests were carried out to improve a reference-free variant of the BOS technique and demonstrate its vortex visualization capabilities for various test conditions. The goal of the main flight test was the quantitative measurement of the main rotor tip vortex system of a full-scale BO105 helicopter under maneuvering flight. The tip vortices of the helicopter were simultaneously visualized from different perspectives by a multi-camera BOS setup consisting of ten individual cameras. Based on this data set, a three-dimensional reconstruction of the main rotor vortex system of a maneuvering helicopter was realized for the first time. The flight test results thus demonstrate the potential of the BOS measurement technique for quantitative vortex investigations on full-scale helicopters under realistic flight conditions.