Smart Helicopter Rotor with Active Blade Tips by Andreas Paul Friedrich Bernhard Dissertation ...

638 Pages·2002·10.21 MB·English

Smart Helicopter Rotor with Active Blade Tips
by
Andreas Paul Friedrich Bernhard
Dissertation submitted to the Faculty of the Graduate School of the
University of Maryland, College Park in partial ful(cid:12)llment
of the requirements for the degree of
Doctor of Philosophy
February 2000
Advisory Committee:
Professor Chopra, Chairman/Advisor
Professor Alfred Gessow
Professor Daryll J. Pines
Professor Norman M. Wereley
Professor Amr M. Baz, Dean’s Representative
c Copyright by
(cid:13)
Andreas Paul Friedrich Bernhard
February 2000
ABSTRACT
Title of Dissertation: Smart Helicopter Rotor with Active Blade Tips
Andreas Paul Friedrich Bernhard, Doctor of Philosophy, February 2000
Dissertation directed by: Professor Chopra
Department of Aerospace Engineering
The smart active blade tip (SABT) rotor is an on-blade rotor vibration re-
duction system, incorporating active blade tips that can be independently pitched
with respect to the main blade.
The active blade tip rotor development included an experimental test program
culminating in a Mach scale hover test, and a parallel development of a coupled,
elastic actuator and rotor blade analysis for preliminary design studies and hover
performance prediction.
The experimental testing focussed on a small scale rotor on a bearingless Bell-
412 hub. The fabricated Mach-scale active-tip rotor has a diameter of 1.524 m,
a blade chord of 76.2 mm and incorporated a 10% span active tip. The nominal
operating speed is 2000 rpm, giving a tip Mach number of 0.47. The blade tips are
driven by a novel piezo-induced bending-torsion coupled actuator beam, located
spanwise in the hollow mid-cell of the main rotor blade.
In hover at 2000 rpm, at 2 deg collective, and for an actuation of 125 V ,
rms
the measured blade tip de(cid:13)ection at the (cid:12)rst four rotor harmonics is between
1.7 and 2.8 deg, increasing to 5.3 deg at 5/rev with resonant ampli(cid:12)cation.
(cid:6) (cid:6) (cid:6)
The corresponding oscillatory amplitude of the rotor thrust coe(cid:14)cient is between
0.7 10(cid:0)3 and 1.3 10(cid:0)3 at the (cid:12)rst four rotor harmonics, increasing to 2.1 10(cid:0)3 at
(cid:1) (cid:1) (cid:1)
5/rev.
In general, the experimental blade tip frequency response and corresponding
rotor thrust response are well captured by the analysis. The (cid:13)exbeam root (cid:13)ap
bending moment is predicted in trend, but is signi(cid:12)cantly over-estimated.
The blade tips did not de(cid:13)ect as expected at high collective settings, because
of the blade tip shaft locking up in the bearing. This is caused by the high (cid:13)ap
bending moment on the blade tip shaft. Redesign of the blade tip shaft assembly
and bearing support is identi(cid:12)ed as the primary design improvement for future
research.
The active blade tip rotor was also used as a testbed for the evaluation of an
adaptive neural-network based control algorithm. E(cid:11)ective background vibration
reduction of an intentional 1/rev hover imbalance was demonstrated. The control
algorithm also showed the capability to generate desired multi-frequency control
loads on the hub, based on arti(cid:12)cial signal injection into the vibration measure-
ment.
The research program demonstrates the technical feasibility of the active blade
tip concept for vibration reduction and warrants further investigation in terms of
closed loop forward (cid:13)ight tests in the windtunnel and full scale design studies.
ACKNOWLEDGEMENTS
I would like to thank Professor Chopra, academic advisor and mentor,
for his invaluable guidance in my graduate studies. I would also like
to express my gratitude to Professor Alan Nurick, Aeronautical Engi-
neeringDepartment, UniversityoftheWitwatersrand, whowasmyun-
dergraduate advisor and who originally introduced me to rotary-wing
research. I thank the Army Research O(cid:14)ce, with Drs Tom Doligalski
and Gary Anderson as Technical Monitors, for providing the (cid:12)nancial
support for this research.
My thanks also to Dr V.T. Nagaraj, who always had an open door to
discuss theoretical and experimental rotorcraft topics, and also for his
assistance with the hover test stand and the Bell-412 Mach scale rotor
hub. It was a great honour and privilege to have the opportunity to
interact with Professor Alfred Gessow and Professor Marat Tishchenko
in the 1998 AHS/Boeing student design competition and in matters
related to my research.
During the course of my studies I have made many wonderful friends
at the Alfred Gessow Rotorcraft Center, and all of them, in one form
or another had an input in my research, in particular Pete Chen and
ii
TaeohLee. MyspecialthanksalsotoNikhilKoratkar,forhisassistance
during the hover tests.
In 1996 I was awarded the (cid:12)rst Gustave J. Hokenson Fellowship, and
would like to dedicate this dissertation, in part, to the memory of
Gustave \Gus" J. Hokenson, a 1971 Ph.D. graduate of the University
of Maryland Aerospace Department.
Aboveall,Idedicatethisresearchdissertationtomyparents,Andyand
Karin Bernhard, my brothers, Maxi and Nicky, and my (cid:12)ance(cid:19)e, Ruta.
You have supported me in my endeavours to make my dreams come
true and have always inspired me to achieve my greatest ambitions.
I am truly grateful for your involvement in my life and your sel(cid:13)ess
kinship.
iii
TABLE OF CONTENTS
List of Tables ix
List of Figures xi
1 INTRODUCTION 1
1.1 PROBLEM STATEMENT . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 BACKGROUND AND MOTIVATION . . . . . . . . . . . . . . . . 3
1.2.1 Main Rotor Vibration . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 Vibration Reduction Concepts . . . . . . . . . . . . . . . . . 7
1.2.3 Active Vibration Reduction . . . . . . . . . . . . . . . . . . 8
1.2.4 Higher Harmonic Control and Individual Blade Control . . . 10
1.3 SMART STRUCTURES TECHNOLOGY . . . . . . . . . . . . . . 13
1.3.1 Smart Materials . . . . . . . . . . . . . . . . . . . . . . . . . 15
1.3.2 Piezoelectric Materials . . . . . . . . . . . . . . . . . . . . . 16
1.4 SMART ROTOR SYSTEMS . . . . . . . . . . . . . . . . . . . . . . 20
1.4.1 Active Flap Rotors . . . . . . . . . . . . . . . . . . . . . . . 22
1.4.2 Active Twist Rotors . . . . . . . . . . . . . . . . . . . . . . 31
1.4.3 Actuators based on Elastic Couplings . . . . . . . . . . . . . 35
1.4.4 Other Active Rotor Concepts . . . . . . . . . . . . . . . . . 36
1.5 FREE-TIP ROTOR . . . . . . . . . . . . . . . . . . . . . . . . . . 38
1.6 SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
1.7 OBJECTIVE . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
1.8 SCOPE OF RESEARCH . . . . . . . . . . . . . . . . . . . . . . . . 45
1.9 DISSERTATION ROAD-MAP . . . . . . . . . . . . . . . . . . . . . 49
2 ACTUATOR BEAM 53
2.1 BENDING-TORSION COUPLED ACTUATOR . . . . . . . . . . . 56
2.2 ACTUATOR BEAM MECHANICS . . . . . . . . . . . . . . . . . . 58
2.3 ADVANTAGES AND DISADVANTAGES . . . . . . . . . . . . . . 61
2.4 DESIGN STUDY . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
2.4.1 Simpli(cid:12)ed Actuator Beam Analytic Model . . . . . . . . . . 63
2.4.2 Piezoceramic Material . . . . . . . . . . . . . . . . . . . . . 66
2.4.3 Composite Material Selection . . . . . . . . . . . . . . . . . 69
2.4.4 Actuator Geometry . . . . . . . . . . . . . . . . . . . . . . . 71
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2.5 PROOF-OF-CONCEPT ACTUATOR BEAM . . . . . . . . . . . . 72
2.5.1 Fabrication . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
2.5.2 Testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
2.6 TRAILING EDGE FLAP APPLICATION . . . . . . . . . . . . . . 78
2.6.1 Hover Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
3 ANALYTIC MODELING 87
3.1 NON-DIMENSIONALIZATION . . . . . . . . . . . . . . . . . . . . 88
3.2 STRUCTURAL ANALYSIS OF THE ACTUATOR BEAM . . . . 89
3.2.1 Kinematics . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3.2.2 Strain-Displacement Relationships . . . . . . . . . . . . . . . 95
3.2.3 Constitutive Equations . . . . . . . . . . . . . . . . . . . . . 96
3.2.4 Beam Forces . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
3.2.5 Structural Model Adjustments . . . . . . . . . . . . . . . . . 107
3.2.6 Summary: Potential Energy . . . . . . . . . . . . . . . . . . 115
3.3 KINETIC ENERGY OF THE ACTUATOR BEAM . . . . . . . . . 116
3.3.1 Summary: Kinetic Energy . . . . . . . . . . . . . . . . . . . 126
3.4 ACTUATOR FINITE ELEMENT MODEL . . . . . . . . . . . . . 128
3.5 CURRENT AND POWER REQUIREMENTS . . . . . . . . . . . . 131
3.5.1 Actuator Current . . . . . . . . . . . . . . . . . . . . . . . . 131
3.5.2 Actuator Power . . . . . . . . . . . . . . . . . . . . . . . . . 134
3.6 ACTUATOR BEAM FEM VALIDATION . . . . . . . . . . . . . . 136
3.6.1 Free Vibration Test Case . . . . . . . . . . . . . . . . . . . . 137
3.6.2 Induced-Strain Actuation Test Case . . . . . . . . . . . . . . 139
3.7 COUPLED ROTORBLADE AND ACTUATOR ANALYSIS . . . . 149
3.7.1 FEM Idealization . . . . . . . . . . . . . . . . . . . . . . . . 153
3.7.2 Solution Methodology . . . . . . . . . . . . . . . . . . . . . 155
3.7.3 Hub Loads . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157
4 DESIGN 159
4.1 GENERAL DESIGN APPROACH . . . . . . . . . . . . . . . . . . 160
4.2 ROTOR SCALING ISSUES . . . . . . . . . . . . . . . . . . . . . . 161
4.3 DESIGN CONSTRAINTS . . . . . . . . . . . . . . . . . . . . . . . 167
4.4 DESIGN OBJECTIVES . . . . . . . . . . . . . . . . . . . . . . . . 172
4.5 DESIGN ITERATION . . . . . . . . . . . . . . . . . . . . . . . . . 175
4.6 DESIGN LAYOUT . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
4.6.1 Blade Tip Assembly . . . . . . . . . . . . . . . . . . . . . . 181
4.6.2 Shaft Sub-Assembly . . . . . . . . . . . . . . . . . . . . . . 183
4.6.3 Main Blade and Blade Tip Cross Sections . . . . . . . . . . 188
4.6.4 Blade and Actuator Beam Root . . . . . . . . . . . . . . . . 192
4.6.5 Finite Element Model Revisited . . . . . . . . . . . . . . . . 194
4.7 ELECTRIC ACTUATION CONSIDERATIONS . . . . . . . . . . . 196
4.8 ACTUATOR BEAM DESIGN OPTIMIZATION . . . . . . . . . . 198
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4.8.1 Mach Scale Active Blade Tip Rotor . . . . . . . . . . . . . . 199
4.8.2 Mach Scale Active Twist Rotor . . . . . . . . . . . . . . . . 205
4.8.3 Reduced Tip Speed Active Tip Rotor . . . . . . . . . . . . . 207
4.8.4 Actuator Beam Structural Integrity . . . . . . . . . . . . . . 210
4.9 DESIGN SUMMARY . . . . . . . . . . . . . . . . . . . . . . . . . . 221
5 ACTUATOR BEAM AND ROTORBLADE FABRICATION 227
5.1 ACTUATOR BEAM FABRICATION . . . . . . . . . . . . . . . . . 227
5.1.1 Actuator Beam Layup . . . . . . . . . . . . . . . . . . . . . 228
5.1.2 Tip Bolt Pattern . . . . . . . . . . . . . . . . . . . . . . . . 231
5.1.3 Surface Bonding of Piezoceramics and Anchor Plates . . . . 232
5.1.4 Actuator Beam Wiring . . . . . . . . . . . . . . . . . . . . . 235
5.1.5 In(cid:13)uence of PZT Thickness . . . . . . . . . . . . . . . . . . 238
5.2 BLADE TIP FABRICATION . . . . . . . . . . . . . . . . . . . . . 239
5.2.1 Shaft Assembly . . . . . . . . . . . . . . . . . . . . . . . . . 239
5.2.2 Blade Tip Layup . . . . . . . . . . . . . . . . . . . . . . . . 243
5.3 MAIN BLADE FABRICATION . . . . . . . . . . . . . . . . . . . . 250
5.3.1 Main Blade Components . . . . . . . . . . . . . . . . . . . . 251
5.3.2 Layup Preparations . . . . . . . . . . . . . . . . . . . . . . . 253
5.3.3 Layup and Cure . . . . . . . . . . . . . . . . . . . . . . . . . 256
5.4 BLADE FINISHING . . . . . . . . . . . . . . . . . . . . . . . . . . 261
5.5 ASSEMBLY OF ACTIVE TIP BLADE . . . . . . . . . . . . . . . . 263
5.6 STRUCTURAL INTEGRITY . . . . . . . . . . . . . . . . . . . . . 266
5.6.1 Actuator Beam and Shaft Assembly Tensile Strength . . . . 266
5.6.2 Actuator Beam Electric Field Limits . . . . . . . . . . . . . 270
5.6.3 Rotorblade Bending Strength Tests . . . . . . . . . . . . . . 275
5.7 MASS AND BALANCE . . . . . . . . . . . . . . . . . . . . . . . . 278
6 NON-ROTATING CHARACTERIZATION 283
6.1 MAIN BLADE STIFFNESS MEASUREMENTS . . . . . . . . . . 284
6.2 ACTUATOR BEAM CHARACTERIZATION . . . . . . . . . . . . 287
6.2.1 PZT Free-Strain Curves . . . . . . . . . . . . . . . . . . . . 287
6.2.2 Force-Displacement Characteristics . . . . . . . . . . . . . . 290
6.2.3 Mechanical E(cid:14)ciency . . . . . . . . . . . . . . . . . . . . . . 294
6.2.4 Free Twist and Actuator Sti(cid:11)ness . . . . . . . . . . . . . . . 297
6.3 ACTIVE BLADE TIP CALIBRATION . . . . . . . . . . . . . . . . 306
6.3.1 Non-Rotating Natural Frequencies . . . . . . . . . . . . . . . 307
6.3.2 Dynamic Blade Tip Response . . . . . . . . . . . . . . . . . 310
6.3.3 Hall Sensor Calibration . . . . . . . . . . . . . . . . . . . . . 314
6.3.4 Actuator Beam Current and Power Requirements . . . . . . 315
6.3.5 Hysteresis Plots . . . . . . . . . . . . . . . . . . . . . . . . . 320
6.4 ACTIVE TWIST ROTOR . . . . . . . . . . . . . . . . . . . . . . . 326
6.4.1 Non-Rotating Frequency Sweep . . . . . . . . . . . . . . . . 327
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6.4.2 Current Requirement . . . . . . . . . . . . . . . . . . . . . . 332
6.4.3 Comparison with other active twist rotors . . . . . . . . . . 336
6.5 REDUCED TIP SPEED ROTOR . . . . . . . . . . . . . . . . . . . 338
6.6 COUPLED ROTORBLADE & ACTUATOR DYNAMICS . . . . . 344
7 HOVER TESTING 354
7.1 HOVER TEST FACILITY . . . . . . . . . . . . . . . . . . . . . . . 357
7.2 DATA ACQUISITION . . . . . . . . . . . . . . . . . . . . . . . . . 359
7.3 TEST PROCEDURE AND TEST MATRIX . . . . . . . . . . . . . 363
7.4 ROTOR FAN DIAGRAMS . . . . . . . . . . . . . . . . . . . . . . 368
7.4.1 Mach Scale Active Blade Tip Rotor . . . . . . . . . . . . . . 369
7.4.2 Mach Scale Active Twist Rotor . . . . . . . . . . . . . . . . 371
7.4.3 Reduced Tip Speed Rotors . . . . . . . . . . . . . . . . . . . 372
7.5 ZERO ACTUATION HOVER CHARACTERISTICS . . . . . . . . 381
7.5.1 Steady Loads and De(cid:13)ections . . . . . . . . . . . . . . . . . 381
7.5.2 Background Vibration . . . . . . . . . . . . . . . . . . . . . 385
7.6 ACTIVE BLADE TIP OPEN LOOP TESTS . . . . . . . . . . . . 388
7.6.1 Baseline Open Loop Test . . . . . . . . . . . . . . . . . . . . 389
7.6.2 Actuator Beam and Main Blade Interference . . . . . . . . . 403
7.6.3 RPM Sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . 408
7.6.4 Collective Sweep . . . . . . . . . . . . . . . . . . . . . . . . 420
7.6.5 Blade Sweep . . . . . . . . . . . . . . . . . . . . . . . . . . . 425
7.6.6 Voltage Sweep . . . . . . . . . . . . . . . . . . . . . . . . . . 427
7.6.7 Actuation Current and Power . . . . . . . . . . . . . . . . . 436
7.7 ACTIVE TWIST OPEN LOOP HOVER TESTS . . . . . . . . . . 440
7.8 REDUCED TIP SPEED ROTOR HOVER TESTS . . . . . . . . . 457
7.8.1 Active Blade Tip Rotor SABT-F97 . . . . . . . . . . . . . . 458
7.8.2 Active Twist Rotor SABTatwNT-F97 . . . . . . . . . . . . . 472
7.9 COMPARISON OF ACTIVE ROTOR SYSTEMS . . . . . . . . . . 477
7.10 CLOSED LOOP TESTS . . . . . . . . . . . . . . . . . . . . . . . . 484
7.10.1 Description of Control Algorithm . . . . . . . . . . . . . . . 485
7.10.2 Background Vibration Reduction . . . . . . . . . . . . . . . 489
7.10.3 Pseudo Vibration Reduction . . . . . . . . . . . . . . . . . . 498
8 SUMMARY & CONCLUSIONS 505
8.1 OVERVIEW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 505
8.2 ACTUATOR BEAM . . . . . . . . . . . . . . . . . . . . . . . . . . 507
8.3 COUPLED ACTUATOR BEAM & ROTOR BLADE ANALYSIS . 509
8.4 ACTUATOR BEAM & ROTOR BLADE DESIGN . . . . . . . . . 514
8.5 ACTUATOR & ROTOR BLADE FABRICATION . . . . . . . . . 518
8.6 NON ROTATING CHARACTERIZATION . . . . . . . . . . . . . 521
8.7 OPEN LOOP HOVER TESTS . . . . . . . . . . . . . . . . . . . . 524
8.8 Closed Loop Hover Tests . . . . . . . . . . . . . . . . . . . . . . . . 530
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