Rotorcraft Center of Excellence
Helicopter Blade Lag Damping Using
Embedded Inertial Dampers
Dr. Edward C. Smith
Jason S. Petrie
Professor of Aerospace
Engineering
ecs5@psu.edu
MS
jpetrie@psu.edu
Dr. George A. Lesieutre
Professor of Aerospace
Engineering
g-lesieutre@psu.edu
2004 National Rotorcraft Technology Center Review
May 3, 2005
Rotorcraft Center of Excellence
Presentation Outline
 Background




Embedded Damper Concept
Objectives
Technical Approach
Accomplishments
 Embedded Fluidlastic Damper Design
 Experiment Hardware and Resuts
 Conclusions
Rotorcraft Center of Excellence
Aeromechanical Instabilities
Major design considerations in the
development of both Articulated and
Hingeless Rotor Systems are
Aeromechanical Instabilities
(Ground Resonance and Air Resonance)
An effective method to avoid these
instabilities is the addition of
Blade Lag Damping
Lag Damper
Rotorcraft Center of Excellence
State-of-the-Art Lag Dampers
Extremely High Maintenance
Many Critical Flight Conditions / Loads
Limited Life / High Cost of Replacement
Stroke Limits for Elastomeric Dampers
No Breakthrough Advances in Passive Rotor
Blade Lag Damper Technology in the Last 20 Years
Rotorcraft Center of Excellence
Embedded Inertial Dampers
Simplified Hub Design
Fewer Parts
Less Constraints
Chordwise Motion of the Mass
Out of Phase with Rotor Blade
Lag Motion
Large
Moment
Arm
Blade Cavity
Mass
Ma
Elastomeric
Spring
Embedded Damper System

Restoring Inertial
Moment about
the Lag Hinge
Hebert, Lesieutre & Zapfe (1996 – 1998)
Rotorcraft Center of Excellence
Embedded Inertial Dampers
Viscous Root End Dampers
m  c  k  M
Embedded Dampers
m  k  M  M Damper
M Damper  ma aRa  e 
Rotorcraft Center of Excellence
Embedded Dampers vs Root End Dampers
Root End Damper
Difficulties with the Geometry
Yes
(Especially with Bearingless Rotors)
of the Blade or Hub
Embedded Inertial Damper
Yes
(Small Blade Cavity)
Amount of Lag Damping
Small - Moderate
Moderate - Large
Hub Loads
Increases MLag
Possibly Reduce MLag
Rotor Weight
Moderate Increase
(Utilized Leading Edge Mass)
Complexity of Rotor Hub
Increases
Does Not Affect Hub
Rotor Hub Drag
Increases
Does Not Affect Hub
Size
Moderate to Large
Small
High Centrifugal Force
Loading
No
Yes
(Stiff In-Plane?)
Small Increase
Rotorcraft Center of Excellence
Embedded Devices
Embedded mechanical
devices have been
successfully integrated
into full scale rotor blades.
An embedded inertial
damper will be subject to
similar loads and
geometric constraints as
existing embedded
devices.
Reference: DARPA - Smart Rotor Program - 2004
Rotorcraft Center of Excellence
Objectives
Initial research shows that embedded inertial dampers
may be promising for lag damping of rotor blades. In
addition, embedded inertial dampers may utilize part of
the leading edge weight of the blade and simplify the
rotor hub considerably.
Current Research Objectives:
Theoretical and experimental investigation of the feasibility
of blade lag damping using embedded inertial dampers
Develop a physical understanding of blade lag damping
with embedded inertial dampers (modal properties, stability,
and response)
Establish design guidelines for rotor blade lag damping
with embedded inertial dampers
Rotorcraft Center of Excellence
Technical Approach
Theoretical Investigation of Blade Lag Damping Using
Embedded Inertial Dampers
Develop Aeromechanical Stability Analysis for the Rotor-FuselageDamper System
Aeroelastic and Aeromechanical Stability Analysis of Rotor System
with Embedded Damper
Parametric Study
Analysis Validation and Experimental Investigation of
Blade Lag Damping Using Embedded Inertial Dampers
Isolated Blade Lag Damping
Aeromechanical Stability of Rotor System
Embedded Inertial Damper Device Design and Test
Rotorcraft Center of Excellence
2004 RCOE Review
External Interactions
Lord Corporation
US Army
Sikorsky
Bell Helicopter
Rotorcraft Center of Excellence
2001 - 2002 Accomplishments
Isolated Blade Lag Damping Experiment
Validated the Analytical Model and Concept
Revealed the Excessive Static Displacement of the Damper Mass
Identified the Technical Barriers
Developed an Understanding of the Design Issues
Related to Embedded Chordwise Inertial Dampers
Modified Design Analysis to Capture Realistic Physics
Non-Linear Effects of the Static Lag Angle on Damper Response
Investigated Additional Conceptual Design Parameters
Angular and Radial Damper Response
Conducted an Initial Investigation of Blade Lag Damping Using
Embedded Fluid Elastic Dampers
Developed a pure lag blade-embedded damper model
Conducted a parametric study
Rotorcraft Center of Excellence
2003 Accomplishments
Conducted Initial Simulation of Rotor Blade Loads and Hub
Vibration in Forward Flight
Refined Fluid Elastic Damper Model to Include All Necessary Fluid
Motion Dynamics and Attributes
Conducted a Study of Blade Lag Damping Using Embedded Fluid
Elastic Dampers
Conducted a parametric study to determine the effects of the fluid elastic
element on rotor blade lag damping and the damper response
Compared the use of fluid elastic inertial dampers with elastomeric dampers
previously investigated
Conducted feasibility study of embedded fluid elastic inertial dampers
Completed Initial Design of Fluid Elastic Damper with the Lord
Corporation for Full Scale and Model Rotors
Rotorcraft Center of Excellence
2004-05 Accomplishments
Development of a New test facility to evaluate Lag Damper
Technologies
Completed Detailed Design of Fluid Elastic Damper with the Lord
Corporation for Full Scale and Model Rotors
Fabrication of Second Generation (Fluid Elastic) Embedded
Inertial Damper
Benchtop and initial rotor testing completed
Published AIAA and AHS Conference Papers, MS Thesis, and
AIAA Journal of Aircraft paper
Rotorcraft Center of Excellence
Presentation Outline
 Background
 Embedded Fluidlastic Damper Design
 Experiment Hardware and Resuts
 Conclusions
Rotorcraft Center of Excellence
Elastomeric Damper Design

ka*
ma
ao
y
a
CG
Damper Equation of Motion:
ma a  ma ra  e   ma a  ao  2  ma e 2  2ma a  ao   k a a  0
Damper Response:
a static 
a0 ma  2
k a
 ma 
2
adynamic
ma ra  e  2
 
ka  ma  2   2


Rotorcraft Center of Excellence
Elastomeric Damper Design Issues
1)
2)
The static displacement of the embedded inertial damper may be
excessive
A low damper tuning frequency is required to produce a suitable
damping band for aeromechanical stability of system
a static 
adynamic
a0 ma  2
k a  ma  2
ma ra  e  2
 
ka  ma  2   2


An ideal embedded chordwise inertial damper for helicopter blade lag
damping would have both a high static stiffness and a low dynamic stiffness
Rotorcraft Center of Excellence
Fluid Elastic Damper
 High Static Stiffness
 Low Dynamic Stiffness
Conceptual
Device
As a result of blade lag motion, the
damper mass oscillates in the lag
direction and the fluid in the tuning
port is pumped through the inner
chamber.
Fluid motion creates a force which
reduces the effective stiffness of the
damper. The fluid force increases
as the frequency of the system
increases.
References:
• Halwes (Bell Helicopter) 1980
• McGuire (Lord Corp.) 1994
• Kang (PSU) 2001
Fluid
Chamber
Elastomer
Inner
Cylinder
Damper
Amplitude
Outer
Cylinder
Mass
Tuning Port
Rotorcraft Center of Excellence
Fluid Elastic Damper Model
Mass-Spring Equivalent of a Fluid-Elastomer Damper
ap
at = (G-1)ap
mp
at
apo
ka*
mt
b
a
Reference: Halwes (Bell Helicopter) 1980
Parameters:
mp = Damper Primary Mass
mt = Tuning Mass = Fluid Mass = ALρ
A = Tuning Port Cross Sectional Area
L = Length of Tuning Port
ρ = Density of Fluid
G = b/a = Outer Cylinder-Tuning Port Area Ratio
ato
Rotorcraft Center of Excellence
Fluid Elastic Damper Design
Fluid Mass
Tuning
Frequency
Mass
Stiffness
Tuning Port
Area Ratio
Step 1
Establish an appropriate tuning frequency in order to maintain the
aeromechanical stability of the rotor system
Rotorcraft Center of Excellence
Fluid Elastic Damper Design
Fluid Mass
Tuning
Frequency
Mass
Stiffness
Tuning Port
Area Ratio
Step 2
Establish the amount of mass that can be used within the blade
cavity for the damper device
Embedded inertial dampers are intended to utilize part of the leading
edge mass or part the tip mass of a rotor blade
Rotorcraft Center of Excellence
Fluid Elastic Damper Design
Fluid Mass
Tuning
Frequency
Mass
Stiffness
Tuning Port
Area Ratio
Step 3
Set the stiffness of the elastomer such that the device will be able to
resist the centrifugal force at rotor speeds that correspond to the
tuning frequency of the device
Rotorcraft Center of Excellence
Fluid Elastic Damper Design
Fluid Mass
Tuning
Frequency
Mass
Stiffness
Tuning Port
Area Ratio
Step 4
The fluid mass and the tuning port area ratio are then determined
based on the equation for the elastomer stiffness
k f
2
m
 mt G  1
2
p

Rotorcraft Center of Excellence
Fluid Elastic Damper Design
Fluid Mass
Tuning
Frequency
Mass
Stiffness
Tuning Port
Area Ratio
The fluid mass and the tuning port area ratio will affect the overall
size of the embedded fluid elastic damper
The device must be able to
fit within the blade
Rotorcraft Center of Excellence
Fluid Elastic Damper Design
Conceptual Device
Practical Device
LORD CORPORATION
Rotorcraft Center of Excellence
Fluid Elastic Embedded Damper
•
Spar (10 lbs)
Hub
Damper
(1 lb)
Elastomer Element
Outer Cylinder
Helical Tuning Port
Inner Cylinder
Rotorcraft Center of Excellence
Lord Corp. Helical Tuning Port
Enables very high
Tuning port ratios
(G = 50+)
Suited for compact
embedded designs
Elastomeric Element:
The average stiffness
was 2058 lbs/in at +- .010"
and 5 Hz. Loss factor = .042
Rotorcraft Center of Excellence
Benchtop Damper Test
QuickTime™ and a
TIFF (Uncompressed) decompressor
are needed to see this picture.
- Clear tuning frequency at 7.5 Hz
- This shows fluid amplification effect
Rotorcraft Center of Excellence
Fluid Elastic Damper Experiment
Phase #1 – Spin Test
Scale Model
Embedded Fluid
Elastic Inertial
Damper for New PSU
Lag Test Stand
Measure Blade Lag Damping
and Frequency
Phase #2 – Bench Top Test
Full Scale
Embedded Fluid
Elastic Inertial
Damper for
Commercial Rotor
Blade System
Measure Static and Dynamic
Stiffness of Device
Examine the Stiffness Characteristics of the Damper
Validate Analytical Model and Damper Design
Rotorcraft Center of Excellence
Fluid Elastic Damper Experiment
Flexure
Blade
Actuator
Hub
Test Stand Property
Value
Mass Per Radius
m
0.0627 slugs/ft
Radius
R
4.00 ft
Lag Hinge Offset
e
10% R
Non-Rotating Frequency
ωo
4.00 Hz
Blade Damping Coefficient
Cξ
0
Chord (Actual)
c
0.1667 ft
Chord (Theoretical)
c
0.600 ft
Number of Blades
Nb
2
Rotor Speed

0    450 RPM
Rotor
Slip Ring
Support
Structure
Hydraulic
Motor
Rotorcraft Center of Excellence
Fluid Elastic Damper Experiment
Steel Flexures
Dictates Lag Frequency
Interchangeable
Adds Strength
Rotorcraft Center of Excellence
Fluid Elastic Damper Experiment
Embedded Actuator
Excites Blade
Tunable
Adds Versatility
Rotorcraft Center of Excellence
Lag Damping Test Rig
Rotorcraft Center of Excellence
Fluid Elastic Design - Full Scale
•
•
•
•
•
Simulated Annealing Algorithm (derived from RCOE Mount Task)
“Comanche-’like” rotor properties (R = 20ft, Lag freq = 3.5 Hz)
3% critical damping
Absorber tuning Freq = 4.9 Hz (based on 220 RPM crossing)
Damper limit of 10% blade mass, 1%chord dynamic stroke
Rotorcraft Center of Excellence
Fluid Elastic Design - Full Scale
• Target Damping Level Achieved within realistic constraints
• Other variations possible based on modified objectives
Rotorcraft Center of Excellence
Fluid Elastic Damper- Model Test Predictions
_bo (Hz)
Fluid El astic Dam per
Property
4
mp (slugs)
0.0373 Mb
ra (ft)
R
f (Hz)
5.63

0.21
mt (slugs)
0.335 mp
G
68
k a (lbf/ft)
2.64 (10 4)
Parame ter
• Prototype damper fabricated at
Lord Corp
Rotorcraft Center of Excellence
Fluid Elastic Damper- Model Test Predictions
• Very low static displacement
(no instability)
• Proper tuning freq and
low dynamic stroke
Rotorcraft Center of Excellence
Presentation Outline
 Background




Rotor Loads and Vibration Simulation
Embedded Damper Design
Elastomeric Damper vs. Fluid Elastic Damper
Fluid Elastic Damper Design and Experiment
 Conclusions
Rotorcraft Center of Excellence
Conclusions
 An embedded fluid elastic inertial damper is capable of
producing rotor blade lag damping within a desirable frequency
band for aeromechanical stability of the system.
 The static stiffness of a fluid elastic inertial damper is large
enough to maintain a reasonable static amplitude.
 aStatic / ao < 5% of the Chord
 Static Instability Problem Resolved!
Rotorcraft Center of Excellence
Conclusions
 A new lag damping test rig was successfully designed
and brought online
 Detailed Design and Fabrication of a Compact Second
Generation (Fluid Elastic) Embedded Inertial Damper
was completed
 Benchtop testing of the new device confirmed the
dynamic characteristics predicted by design analysis
Rotorcraft Center of Excellence
Publications and Presentations
•
•
•
•
•
•
•
•
•
•
•
AIAA SDM Conference (April 2002)
Lord Corporporation (May 2002)
Sikorsky (June 2002)
ARO Aeroelasticity Workshop (November 2003)
Lord Corporation (February 2004)
AIAA Journal of Aircraft Paper (Accepted March 2004)
AIAA SDM Conference (April 2004)
Jason Petrie MS Thesis (August 2004)
Boeing, Mesa (January 2005)
Lord Corporation R&D Center (March 2005)
AHS Forum (June 2005)
Rotorcraft Center of Excellence
2005 Plans
Complete spin testing of embedded damper devices
Complete additional analysis of vibratory hub loads
and chordwise blade loads in forward flight
(Dr. Zhang)
Explore opportunities for industry team for further
development of full scale prototype (including
designs effective for both articulated and BMR)
Rotorcraft Center of Excellence
Schedule and Milestones
2001
Tasks
2002
2003
2004
STAGE ONE
Fundamental Study
System Modeling
Stability Analysis
Blade Lag Damping Test
STAGE TWO
Model Refined
Parametric Study
Concept Design of Absorber
Fluid Elastic Damper Test
STAGE THREE
Design of Absorber
Rotor Loads & Vibration
Report, Guideline of Design
Completed
Short Term
Long Term
2005
Rotorcraft Center of Excellence
Helicopter Blade Lag Damping Using
Embedded Fluid Elastic Inertial Dampers
Questions?
This project is co-funded by the Lord Corporation (Project Technical
Monitors: John Heilman,
Denny McGuire)
Rotorcraft Center of Excellence
Previous Accomplishments
Basic Study of Blade Lag Damping Using Embedded
Inertial Dampers (Kang, Smith & Lesieutre 1999 – 2001)
Parametric Study

ao
18
0.200
16
y
0.180
0.160
14
Frequency (Hz)
a
Absorber Frequency
0.140
12
0.120
10
0.100
8
Blade Lag Frequency
6
0.080
0.060
4
Blade Lag Damping
2
0.040
0.020
0
0
250
500
750
Rotation Speed (rpm)
0.000
1000
Developed an analytical model of a rotor system with an embedded damper
Demonstrated that an elastomeric device could produce blade lag damping
Damping Ratio
Rigid Blade/Embedded
Damper Model
Rotorcraft Center of Excellence
Previous Accomplishments
Aeromechanical Stability Analysis for Rotor – Fuselage –
Embedded Inertial Damper (Kang, Smith & Lesieutre 2001 - 2002)
Damper Mass:
Location:
Tuned Frequency:
Loss Factor:
Consider a Hingeless Rotor
System with Embedded Inertial
Damper (AFDD Rotor)
30
0.1 (Ma/Mb)
1.0R
13.95 Hz (0.840)
0.5
2
  a
25
Damped Rotor
1.5
 
1
Decay Rate ( /s)
Frequency (Hz)
20
15
  a
10
 

5
200
400
600
800
0
0

200
400
600
Baseline
1000
-1
-5
Rotating Speed (RPM)
800
-0.5
0
0
0.5
Rotating Speed (RPM)
Indicated that embedded chordwise dampers had the potential
to maintain the aeromechanical stability of helicopters
1000
Rotorcraft Center of Excellence
Previous Accomplishments
Isolated Blade Lag Damping Tests
(Kang, Smith & Lesieutre 2001 – 2002)
Blade Properties
Number of Blades
2
Radius, in
19.5
Chord, in
0.5
Rotation Speed, RPM
0-300
Nonrotating Lag Freq., Hz
4, 6.5
Lag Damping, % Critical
0.3
Damper Properties
1
2
Mass (lbm) Frequency Loss Factor
0.0355
8.9 Hz
0.38
0.042
7.6 Hz
0.39
3
0.042
6.3 Hz
0.42
4
0.0485
5.5 Hz
0.41
Rotorcraft Center of Excellence
Previous Accomplishments
8
8
Absorber Frequency
Experimental Data, Case 3
Experimental Data, Case 1
Experimental Data, Case 2
7
Experimental Data, Case 4
6
Frequency (Hz)
Frequency (Hz)
6
5
4
Blade Lag Frequency
3
2
1
5
4
Blade Lag Frequency
3
Predicted Results, Case 1
2
Predicted Results, Case 2
1
0
Predicted Results, Case 3
Predicted Results, Case 4
0
0
50
100
150
200
250
300
0
50
Rotation Speed (rpm)
100
150
200
250
300
Rotation Speed (rpm)
0.008
0.025
Experimental Data, Case 3
0.007
Experimental Data, Case 1
0.020
Experimental Data, Case 2
Damping Ratio
Predicted Results, Case 2
0.015
Experimental Data, Case 4
0.006
Predicted Results, Case 1
Damping Ratio
R
E
S
U
L
T
S
Absorber Frequency
7
0.010
0.005
0.004
0.003
0.002
0.005
Predicted Results, Case 3
Predicted Results, Case 4
0.001
0.000
0.000
0
50
100
150
200
Rotation Speed (rpm)
250
300
0
50
100
150
200
Rotation Speed (rpm)
250
300