The Pennsylvania State University The Graduate School Department or Aerospace Engineering NON-HARMONIC ROOT-PITCH INDIVIDUAL-BLADE CONTROL FOR THE REDUCTION OF BLADE-VORTEX INTERACTION NOISE IN ROTORCRAFT A Dissertation in Aerospace Engineering by Brendon D. Malovrh 2012 Brendon D. Malovrh Submitted in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy December 2012 The dissertation of Brendon D. Malovrh was reviewed and approved* by the following: Farhan S. Gandhi Professor of Aerospace Engineering Dissertation Advisor Chair of Committee Kenneth S. Brentner Professor of Aerospace Engineering Edward C. Smith Professor of Aerospace Engineering Christopher Rahn Professor of Mechanical Engineering Kevin W. Noonan Aerospace Engineer NASA Langley Research Center George A. Lesieutre Professor of Aerospace Engineering Head of the Department of Aerospace Engineering *Signatures are on file in the Graduate School iii ABSTRACT One of the greatest obstacles to public acceptance of rotorcraft is the high levels of noise they produce, particularly in low-speed descent. In this flight condition, the trailing edge vortex of one blade often passes in close proximity to other blades resulting in impulsive changes in lift. This Blade-Vortex Interaction (BVI) creates high levels of both noise and vibration. The objective of this dissertation is to evaluate the effectiveness of using physically motivated pulse-type Individual Blade Control for reducing the noise associated with the BVI. First, the major parameters that affect the severity of the interaction, such as vortex strength and blade-vortex miss-distance, are analyzed. Second, inputs designed specifically to alter the parameters previously identified as key are explored, resulting in elimination of advancing side noise and overall peak BVI Sound Pressure Level (BVISPL) reductions of up to 4.6 dB. Lastly, different feedback mechanisms for closed-loop control of IBC are examined to allow implementation of the developed inputs. iv TABLE OF CONTENTS LIST OF FIGURES ..................................................................................................... vi ACKNOWLEDGEMENTS ......................................................................................... xvi Chapter 1 Introduction ................................................................................................ 1 1.1 BVI and Important Interaction Parameters ..................................................... 2 1.2 Operational Methods ...................................................................................... 4 1.3 Passive Methods ............................................................................................. 7 1.3.1 Advanced Tip Configurations .............................................................. 8 1.3.2 Other Passive Methods ......................................................................... 11 1.4 Active Methods ............................................................................................... 13 1.4.1 Higher Harmonic Control ..................................................................... 13 1.4.2 Individual Blade Control ...................................................................... 16 1.5 Overview ......................................................................................................... 20 References ............................................................................................................. 21 Chapter 2 Sensitivity of BVI-Induced Noise and Vibration to Variations in Individual Interaction Parameters ......................................................................... 29 2.1 Analysis Method ............................................................................................. 30 2.2 Baseline ........................................................................................................... 33 2.3 Vortex Strength ............................................................................................... 37 2.4 Core Radius .................................................................................................... 42 2.5 Miss-Distance ................................................................................................. 44 2.6 Blade-Shaft Plane Interaction Angle .............................................................. 50 2.7 Rotor Disk Plane Interaction Angle ................................................................ 55 2.8 Spanwise Location of the Interaction ............................................................. 60 2.9 Spanwise Length of the Interaction ................................................................ 64 2.10 Collective Pitch ............................................................................................. 68 2.11 Comparison of Effects of Various Parameters in Reducing BVI-induced Noise and Hub Vibratory Loading ................................................................ 74 2.11.1 Weak Interaction Parameters .............................................................. 75 2.11.2 Strong Interaction Parameters ............................................................ 76 2.12 Minimum Vibration Case in the HART Test ............................................... 80 2.13 Conclusions ................................................................................................... 82 References ............................................................................................................. 86 Chapter 3 Localized Individual Blade Root Pitch Control for BVI Noise Reduction .............................................................................................................. 88 3.1 Description of Analysis and Results for the Baseline Configuration ............. 89 3.2 Localized IBC Inputs to Reduce Interacting Vortex Strength ........................ 105 v 3.3 IBC Input Profile ............................................................................................ 108 3.4 Second Quadrant Vortex Strength Reduction ................................................. 117 3.5 Third Quadrant Vortex Strength Reduction ................................................... 128 3.6 Combination of Second and Third Quadrant Vortex Strength Reduction ...... 135 3.7 Effects of Retrimming .................................................................................... 142 3.8 Conclusions ..................................................................................................... 145 References ............................................................................................................. 149 Chapter 4 Metrics for BVI noise ................................................................................. 151 4.1 Skid Microphones as a Feedback Metric ........................................................ 152 4.1.1 Variable Advance Ratio, 3 deg Backward Shaft Tilt ........................... 154 4.1.2 Variable Advance Ratio, 4 deg Backward Shaft Tilt ........................... 163 4.1.3 Variable Shaft Tilt, 0.135 Advance Ratio ............................................ 172 4.1.4 Constant Flight Condition (4 deg backward Shaft Tilt, 0.135 Advance Ratio), Variable IBC Input ...................................................... 179 4.1.5 Constant Flight Condition (3 deg backward Shaft Tilt, 0.17 Advance Ratio), Variable IBC Input ...................................................... 183 4.1.6 Summary of Skid Microphone Feedback ............................................. 188 4.2 Blade Pressures as a Feedback Metric ............................................................ 189 4.2.1 First Quadrant RMS ............................................................................. 189 4.2.2 BPAP .................................................................................................... 191 4.2.2.1 BPAP Model Development ........................................................ 192 4.2.2.2 BPAP Results ............................................................................. 199 4.2.3 Summary Blade Pressure Feedback ..................................................... 203 References ..................................................................................................... 204 Chapter 5 Conclusions and Reccomendations for Future Work ................................. 206 5.1 Sensitivity of BVI-Induced Noise and Vibration to Varisations in Individual Interaction Parameters .................................................................. 206 5.2 Localized Individual Blade Root Pitch Control for BVI Noise Reduction .... 209 5.3 Metrics for BVI noise ..................................................................................... 212 5.4 Recommendations for Future Work ............................................................... 213 vi LIST OF FIGURES Figure 1.1: High-speed flight, no BVI. ........................................................................ 3 Figure 1.2: Low-speed descent, BVI condition. .......................................................... 3 Figure 1.3: Lift at 80% radius. ..................................................................................... 4 Figure 1.4: BVISPL as a function of shaft tilt and advance ratio [25]. ....................... 6 Figure 1.5: Advanced tip shapes a) ogee tip b) winglet c) tapered tip. ........................ 9 Figure 1.6: BERP blade tip. ......................................................................................... 9 Figure 1.7: Anhedral.