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Rotorcraft Center of Excellence
Analysis and Control of the Transient Aeroelastic Response of Rotors During Shipboard Engagement and
Disengagement Operations
Jonathan A. KellerRotorcraft Fellow
Ph.D. Thesis SeminarMarch 22, 2001
• Introduction
• Previous Research
• Objectives
• Approach
• Results
• Conclusions
Presentation Outline
Introduction
•Unique challenges in ship-based operation of helicopters
Small, moving deck area
-Strong & unsteady winds (often up to 50 knots)
-Unusual airflow patterns around ship decks
•Engagement (startup) of rotor system not mundane
Low RPM = Low CF
High winds = Potentially high Aerodynamic Forces
High blade flapping
(%NR)
Historical Motivation
•Past problems for Sea King (RN) and Sea Knight (USN)
-Blade-to-fuselage contact (114 for H-46!) - High blade loads
•Forces conservative limits to be placed on wind conditions
conducive to safe engagement operations
•Reduces operational flexibility of helicopter
H-3 Sea KingH-46 Sea Knight
Engage/Disengage Testing
• Safe conditions were determined in at-sea tests
Tests for every ship/helicopter/landing spot combo, but:
• Problems often occurred within “safe” envelopes
• Engage/Disengage testing cancelled in 1990
• Analytic methods needed!
Took 5 days, 15 people, $150k
No control of winds or seas
Calm weather = wasted tests
Styrofoam Pegs
GreasyBoard
Present Day Motivation
• H-46 tunnel strikes still frequently occur At least 3 last year aboard LHD type ships
• Use of Army helos on Navy ships (JSHIP Program) Army helos not designed for naval ops - no rotor brake?
Apache elastomeric damper loads during startup
Broken flap stop for Blackhawk during engagement op
Chinook is much like Sea Knight
H-47 ChinookH-46 Sea Knight
•USMC and USN (hopefully) purchasing V-22
V-22 blades much shorter than articulated blades
»Excessive rotor gimbal tilt angles may be a possibility
»Contact with between blades & wing/fuselage not a concern
»Contact between gimbal and restraint potentially high loads
Rubber Spring
Gimbal Restraint
Future Motivation
Introduction
• Previous Research
• Objectives
• Approach
• Results
• Conclusions
Presentation Outline
Early Engage/Disengage Research
•Willmer, Burton, & King at Westland (1960s)
Investigated Whirlwind, Wasp, and Sea King helicopters
•Leone at Boeing Vertol (1964)
Investigated H-46 Sea Knight tunnel strikes
Measured and predicted loads during blade-droop stop impacts
•Healey et al at Naval Postgraduate School (1985-1992)
Measured model-scale ship airwake for LHA, DD, AOR
Unsuccessfully investigated H-46 Sea Knight tunnel strikes
•Kunz at McDonnell Douglas (1997)
Investigated high loads in AH-64 Apache elastomeric dampers
Recent Engage/Disengage Research
•Newman at University of Southampton (1985-1995)
Developed elastic F-T code for single rotor blade
» Articulated or hingeless hubs
Articulated rotors more prone to blade sailing than hingeless
Correlated code w/ model-scale rigid R/C helicopter tests
• Geyer, Keller, Kang and Smith at PSU (1995 - Present)
Developed F-L-T code for multiple rotor blades
» Articulated, hingeless, teetering, or gimballed hubs
Simulated H-46 Sea Knight engagements and disengagements
• Botasso and Bauchau (2000)
Multi-body modeling of engagement and disengagement ops
Introduction
Previous Research
• Objectives
• Approach
• Results
• Conclusions
Presentation Outline
Objectives
•Develop unique “in-house” analysis code to:
- Increase physical understanding of engage/disengage behavior
- Accurately predict safe rotor engage/disengage envelopes
Safe Region
- Control rotor response to expand engage/disengage envelopes
Unsafe Region
wtip (%R)
Technical Barriers
• Limited data of engage/disengage ops or ship airwake
• Simulation of a complex transient aeroelastic event
- Rotor speed is a function of time 0 and (t)
» Flap/lag stop or gimbal restraint impacts at low
- Complicated ship airwake and aero environment high ,,
ShipAirwake
H-46 Data
(%NR)
H-46 Data
•
Introduction
Previous Research
Objectives
• Approach
• Results
• Conclusions
Presentation Outline
Ship Airwake Modeling
• Specify speed (VWOD) & direction (WOD) relative to ship center
• Determines ship airwake (Vx, Vy, and Vz) in plane of rotor
»Vx, Vy in plane velocities, Vz vertical velocity
VWOD
Vz
Vy
Vx
WOD
Simple Ship Airwakes
• Simple airwake types derived from tests (Ref. Newman)
Vz = vertical velocity, = “gust” factor
LinearAirwake
VWOD
ConstantAirwake
HorizontalAirwake
VWOD
VWOD
Vz = VWOD
Vz
Vz
Vz = 0
Vz = VWOD
max
max
Vx = VWOD cos WOD
Vy = -VWOD sin WOD
CFD Generated Ship Airwakes
USN FFG
SFS
Flight Deck
Spot#1
Spot#2
Spot#3
WOD
VWOD
HangarFace
150ft
50ft
30ft
30ft30ft
AreaofInterest
SFS Ship Airwakes
• Along-wind airwake velocities
WOD = 0°Recirculation
ZoneVWOD
50 kts
70 kts
60 kts
50 kts
WOD = 270°Flow
AccelerationZone
VWOD
50 kts
40 kts
40 kts
20 kts
2-D Aerodynamic Modeling
• Aerodynamics modeled with Nonlinear quasi-steady aerodynamics (Ref. Prouty & Critzos)» Aero forces dependent upon instantaneous values of ,,
Nonlinear time-domain unsteady aerodynamics (Ref. Leishman)» Aero forces dependent upon time history of , , » Model only validated for small and (< 25°) and M > 0.3
» Must switch to quasi-steady at high and (> 25°) and M < 0.1
•
• • •
• •
-3
-2
-1
0
1
2
3
0 45 90 135 180 225 270 315 360(deg)
+V
AngleofAttackConvention
cl
cd
cmc/4
Structural Modeling - Element
Weight
Aero
> > > > > > >• ••
CF
• FEM used to accommodate different hub geometries
- Articulated, hingeless, teetering and gimballed
• 11 degrees of freedom per element
- 4 flap, 4 lag, & 3 twist
• Distributed blade loads
- Inertial, Aerodynamic, Weight and Centrifugal Force» Inertial loads include rotor acceleration
vb
v’b
wb
w’b
b
m
va
v’a
wa
w’a
a
•
Structural Modeling - Blade
RotorShaft
Finite Element
(t)
Flap Hinge
Conditional Flap stop springs
K
Control StiffnessSpring
K
PitchBearing
LagHinge
Conditional Lag stop springs
K
• Articulated blade modeling
Require mechanisms to restrain flap (hinge) & lag (hinge) motion
» Stops simulated with conditional springs K and K
Flap stops extend/retract at a specified rotor speed
Structural Modeling - Rotor
• Articulated or hingeless rotors
• Teetering or gimballed rotors
2
1
3
Blade motions are uncoupled
1, 2 and 3 independent
1
2
M1
[Mrotor] = M2
M3
0 0
00
00
[Mrotor] = M2
0
0
Blade motions are kinematically coupled
1 = -2
M1
Introduction
Previous Research
Objectives
Approach
• Results Baseline rotor
Passive control of H-46 rotor
Feedback control of gimballed rotor
• Conclusions
Presentation Outline
Baseline Rotor System
• Representative of a “medium-sized” naval helicopter
Nb = 4 Articulated Blades
R = 25 ft
0R = 750 ft/s
= 7.35
= 0.076
= 1.02/rev
= 0.30/rev
= 4.54/rev
FS = ±1º
LS = ±10º
0
20
40
60
80
100
0 5 10 15 20
(%NR)
Time(s)
FlapStopsRetract
Measured H-46
Baseline
Typical Engagement
• Linear airwake
VWOD = 60 knots
= 25%
• Largest wtip occur
< 25%NR
• Blade strikes flap stops repeatedly
• Majority of wtip is
elastic bending
rigid body wtip ±2%R
• Large in low even near blade tip -180
-90
0
90
180
0 2 4 6 8 10 12
50%R95%R
(deg)
Time(s)
-15
-10
-5
0
5
10
15
hinge
(deg) DS
FS
FlapStopsRetract
-30
-20
-10
0
10
20
30
wtip
(%R)
H-46 Tunnel
<25%NR
Typical Engagement
• Linear airwake
VWOD = 60 knots
= 25%
• Majority of vtip is
rigid body motion
• Blade strikes lag
stop repeatedly
• Largest torque due
to impacts
-20
-10
0
10
20
vtip
(%R)
-15
-10
-5
0
5
10
15
hinge
(deg)
LS
LS
-20000
-10000
0
10000
20000
0 2 4 6 8 10 12
Q(ft-lb)
Time (s)
-30
-20
-10
0
10
20
30
0 2 4 6 8 10
wtip
(%R)
H-46 Tunnel
Time (s)
Typical Wind Envelope
• Engagement wind envelope
Shows largest downward and upward wtip with VWOD and WOD
VWOD = 60 ktsWOD = 30°
Upward wtip
Downward wtip
Upwardwtip
Downward wtip
SFS Ship Airwake
• What effect does a “realistic” ship airwake have on rotor deflections?
SFS
Flight Deck
Spot#1
Spot#2
Spot#3
WOD
VWOD
HangarFace
150ft
50ft
30ft
30ft30ft
AreaofInterest
Spot #1 Engagement Envelope
• Bow and port winds have largest wtip
• Stern and 330° winds have small wtip Spot #1(Closest to hangar)
Recirculation and downflow behind hangar face
RecirculationZone
VWOD
Downflow
RecirculationZone
VWOD
Recirculation zone pushed away from flight deck
VWOD
Upflow & FlowAcceleration Zone
Large upflow component on windward side of flight deck
VWOD
Upflow & FlowAcceleration Zone
Large upflow component over flight deck and over hangar face
VWOD
FlowDeceleration
Little upflow over stern and flow decelerates near hangar face
Effect of Deck Position
• Spots closer to hangar have larger wtip
• Largest wtip consistently in port winds
• wtip for Spot #1 are ~2wtip for Spot #3Spot #1
Spot #2
Spot #3Spot #1 Spot #2 Spot #3
Introduction
Previous Research
Objectives
Approach
• Results Baseline rotor
Passive control of H-46 rotor Flap Damping
Spoilers
Feedback control of gimballed rotor
• Conclusions
Presentation Outline
Objectives
•Develop unique “in-house” analysis code to:
- Increase physical understanding of engage/disengage behavior
- Accurately predict safe rotor engage/disengage envelopes
Safe Region
- Control rotor response to expand engage/disengage envelopes
Unsafe Region
wtip (%R)
• Hydraulic flap dampers were used on 1950’s era HUP-2
Dampers only active at low
Above preset dampers became inactive
• Use same technique on H-46 Sea Knight
Not necessarily traditional hydraulic damper - MR or ER?
Use of mast causes drag penalty in forward flight
Flap Damping on HUP-2
Blade
Hub
Mast
Counterweight
Spring
Damper
Flap Damper Sizing for H-46
• Examine “worst-case” scenario - Spot #1 Airwake
-30
-20
-10
0
10
20
30
40
0 1 2 3 4 5
Maxw
tip
(%R)
Flap Damper Strength (xC)
Minwtip
(%R) H-46Tunnel
C
H-46 flap stops set at ±1º
Flap damper has stroke of only
2°
Majority of wtip is elastic
Flap damper has no effect with a
small stroke!
FS
Flap Damper Sizing for H-46
• Raise flap stop setting
Allows damper larger stroke
Keep droop stop setting at -1º No additional downward wtip
C
Raise flap stop setting
Flap damper has larger stroke
Flap damper has much large
effect!
FS
-30
-20
-10
0
10
20
30
40
0 2 4 6 8 10
Maxw
tip
(%R)
FS(deg)
Minwtip
(%R)
StandardConfiguration
C=4C
C=3C
C=5C
SFS Spot #1 Envelope
• Flap damper = 4C
• Flap stop = 6°
• Max wtip increased in
210°- 240° winds
+30%R to +34.8%R
• Min wtip decreased in
240°- 300° winds
-22.4%R to -14.8%R
• Min wtip not affected
in bow winds
Still -25.2%R
Maxwtip
Min wtip
Standard H-46 With Damper
Flap Damping in Bow Winds
• Blade does not lift
off DS until t = 5 sec
• Flap damper never
has a chance to
dissipate energy
• Summary:
Min wtip decreased
in most cases
FS must be raised
Max wtip increased
-30
-20
-10
0
10
20
30
wtip
(%R)
H-46 Tunnel
No Reduction
StandardConfiguration
FlapDamper
-2
-1
0
1
2
0 2 4 6 8 10
hinge
(deg)
Time(s)
StandardFS
DS
StandardConfiguration
FlapDamper
Introduction
Previous Research
Objectives
Approach
• Results Baseline rotor
Passive control of H-46 rotor Flap Damping
Spoilers
Feedback control of gimballed rotor
• Conclusions
Presentation Outline
• Examine reducing flapping by reducing excessive lift
• Leading-edge spoilers known to significantly reduce lift
Objectives
L
V
L
VWithout spoiler With spoiler
Leading-edge spoiler
(Ref. Brasseur)
Objectives
LowAF Low
CF
<25%SpoilersExtended
AppreciableCF
AppreciableAF
=25%SpoilersRetract High
AF
>25%SpoilersRetracted
HighCF
• Percentage of radius covered by spoilers?
• Will rotor torque increase due to spoiler drag?
• Spoilers are used only along partial-span
• Gated spoilers are used on blade upper and lower surfaces
• Spoilers only extended at low < 25%NR and retracted into blade section at high > 25%NR
Spoiler Coverage
H-46 Engagement
SFS Spot #1 Airwake
VWOD = 40 kts
WOD = 240°
• H-46 engagement with varying amounts of spoiler coverage
• Spoilers on outer 15%R (~3½ ft) are enough to reduce wtip
0
10
20
30
Max
wtip
(%R)
-30
-20
-10
0
0 10 15 25 50 75
xspoiler (%R)
Min
wtip
(%R)
X X % R
xspoiler
H-46 Tunnel
Example Engagement
SFS Spot #1 Airwake
(Worst Case Scenario)
VWODSpot #1
VWOD = 40 kts
WOD = 240°
Conclusions:
Min and Max wtip reduced
Max torque not affected
-30
-20
-10
0
10
20
30
wtip
(%R)
Spoilers DeployedH-46 Tunnel
38%Reduction 21%
Reduction
Standard Configuration
Spoilers
-15
-10
-5
0
5
10
15
hinge
(deg)
LS
LS
SpoilersDeployed
StandardConfiguration
Spoilers
-20000
-10000
0
10000
20000
0 2 4 6 8 10
Q(ft-lb)
Time (s)
Spoilers Deployed
Standard ConfigurationSpoilers
SFS Spot #1 Airwake Envelopes
Maxwtip
Min wtip
Standard H-46 With Spoilers• Max wtip decreased
in 210°- 270° winds
+30%R to +23%R
• Min wtip decreased
in 240°- 300° winds
-25.2%R to -17.5%R
• Min wtip decreased
in bow winds
-23%R to -18.5%R
• Conclusion:
Both Min and Max wtip reduced
Introduction
Previous Research
Objectives
Approach
• Results Baseline rotor
Passive control of H-46 rotor Flap Damping
Spoilers
Feedback control of gimballed rotor
• Conclusions
Presentation Outline
Motivation
Rubber Spring
Gimbal Restraint
• V-22 blades much shorter & stiffer than articulated blades
Rotor motion due to rigid body motion, not elastic bending
• V-22 utilizes active “flap limiter” to reduce flapping in FF
Feedback from gimbal motion to swashplate inputs
• Could flap limiter be used in engagement ops?
•Rigid blade structural model
2 degrees of freedom - gimbal pitch (1c) and roll (1s)
•Linear quasi-steady aerodynamic model
Lift >> Drag
z
x
y1C
1SKb
Structural & Aerodynamic Modeling
( )20 T P TL U U U
2
γ≅ θ −
Control System Settings
Swashplate inputs
( ) )(kxsincos pp43
twis1ic1750 iβ−β+−θ+ψθ+ψθ+θ=θ
vBRKxu T1−−−=
( ) ( ) ( ) ( )ft T Tf f f 0
1 1J x t S t x t x Qx u Ru dt
2 2= + +∫
S(tf) = Final State Weight Q = State Weight R = Control Weight
• Use Matrix Ricatti Equations to find gain matrix K
( ) ( ) ( ){ } { }T
s1c175
T
s1c1s1c1 ux
tdutBxtAx
θθθ=ββββ=
++=&&
&
Disturbance d(t) due to:Airloads induced by ship
airwake effects
Equations are Linear Time Variant (LTV)
(t) and aerodynamic terms make pole placement ineffective
• Use LQR theory and define performance index J
Additional gain due to disturbance d(t)
Optimal Control Theory
• Cast equations of motion into state space form
vBRKxu T1demanded
−−−=
•Swashplate actuators typically have limits in magnitude and rate
oo
o
&
1010
5.7
7x
:Limits
c1
75
sin
maxac
<<−−>=Non-Rotating
Swashplate
RotatingSwashplate
Actuator#1
Actuator#2
Actuator#3
xac
Enforcecontrollimits
actualu dBuAxx ++= actual&
•Time integration with control system limits
Control System Limits
•Simulated V-22 engagement
Vwod = 30 kts in Bow winds
Uncontrolled case:
» 75 = 1c = 1s = 0
•Constant airwake distribution
= 25%
Conclusion:
max reduced by 50%
Min 75 limit reached
0
5
10
15
max
(deg)
GimbalRestraint
Uncontrolled
OptimalControl
-15
-10
-5
0
5
10
15
u(deg)
1c
75
1s
1cmax
1cmin
75min
-10
-5
0
5
10
0 2 4 6Time (s)
xac
(in/s)
x1x
2
x3
xac
max
xac
min
Vwod
Response in Constant Airwake
• Gain K and disturbance effect v are functions of the ship airwake
• Knowledge of the ship airwake is difficult to predict/measure
Ship anemometer reads relative wind speed and direction
Correlates to in-plane velocities Vx and Vy over flight deck
Anemometer
Vx and Vy may vary over the flight deck
Vz is unmeasured!
VWOD
Vz
Vy
Vx
WOD
Optimal Control Assumptions
Sub-Optimal Control
Conclusion:
Optimal gains max by 50%
Sub-optimal gains max by 35%
0
5
10
15
0 2 4 6
max
(deg)
Time(s)
GimbalRestraint
Uncontrolled
Sub-OptimalControl
OptimalControl
0
5
10
15
max
(deg)
GimbalRestraint
Uncontrolled
Sub-OptimalControl
OptimalControl
• V-22 Rotor Engagement
Vwod = 30 knots
Constant airwake
• Sub-Optimal Control
Vx and Vy known
Vz assumed = 0
• Optimal Control
(Best Case)
Vx, Vy and Vz known
•Anemometer measurement error
•Conclusion:
Moderate errors in anemometer reading change response by 10%
wodwodmeas
wodwodmeas VVV
Δ+=Δ+=
0
5
10
15
max
(deg)
ΔVwod=-10kts
ΔVwod=+10kts
Sub-OptimalControl
•Gains K and v calculated from
(incorrect) anemometer meas.
Error in Wind Velocity
0
5
10
15
0 2 4 6
max
(deg)
Time(s)
Δwod=-15deg
Δwod=+15deg
Sub-OptimalControl
Error in Wind Direction
Anemometererror
Robustness to Anemometer Error
Introduction
Previous Research
Objectives
Approach
Results
• Conclusions
Presentation Outline
Conclusions
• Developed transient elastic F-L-T analysis for E/D ops
Blade structure modeled with FEM
» Articulated, hingeless, teetering, or gimballed rotors
» Blade weight and acceleration included
Aerodynamics simulated with quasi-steady or unsteady models
Airwake modeled with simple types or from numerical predictions
Rotor motion time-integrated along specified (t) profile
• Investigated effect of “frigate-like” ship airwake
Blade wtip showed strong dependence on wind direction
» Winds off-bow had smallest wtip, winds over-port had largest wtip
Spots closer to hangar had larger deflections
Conclusions
• Investigated effect of flap damper for H-46
Raised flap stop setting to allow damper larger stroke
» Reduced downward wtip by 30%, but increased upward wtip by 20%
» Downward wtip not affected at all in some cases
• Investigated effect of leading-edge spoilers for H-46
Spoilers extend ( < 25%NR) and retract into blade ( > 25%NR)
Determined spoilers needed only on outer 15%R of blade
» Reduced upward and downward wtip by 20%
No significant increase in maximum rotor torque in any case
• Investigated control of gimballed rotors w/ LQR
Used feedback from gimbal motion to swashplate actuators
Resulting equations of motion were Linear Time Variant (LTV)
Enforced control system limits (magnitude and rate)
• LQR control method successful at reducing flapping
max 50% with full knowledge of ship airwake (Vx, Vy and Vz)
• Aero forces due to ship airwake contribute to control gains
max 35% with partial knowledge of ship airwake (Vx and Vy)
• Response insensitive to errors in anemometer reading
max changed ±10% with either ±10 knot or ±15° anemometer error
Conclusions
Acknowledgments
•Financial assistance
National Rotorcraft Technology Center
»Technical Monitor Dr. Yung Yu
•Technical Assistance
Dynamic Interface Group NAWC/AD Pax River, MD
»Mr. William Geyer, Mr. Kurt Long & Mr. Larry Trick
Boeing Philadelphia
»Mr. David G. Miller