Rotorcraft Center of Excellence Analysis and Control of the Transient Aeroelastic Response of Rotors During Shipboard Engagement and Disengagement Operations Jonathan A. Keller Rotorcraft Fellow Ph.D. Thesis Seminar March 22, 2001
Jan 11, 2016
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