Rotorcraft Center of Excellence Analysis and Control of the Transient Aeroelastic Response of Rotors...

transcript

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

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

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

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

Introduction

Previous Research

Objectives

• Approach

• Results

• Conclusions

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

Simple Ship Airwakes

• Simple airwake types derived from tests (Ref. Newman)

Vz = vertical velocity, = “gust” factor

LinearAirwake

ConstantAirwake

HorizontalAirwake

Vz = VWOD

Vz = 0

Vz = VWOD

Vx = VWOD cos WOD

Vy = -VWOD sin WOD

CFD Generated Ship Airwakes

USN FFG

Flight Deck

Spot#1

Spot#2

Spot#3

HangarFace

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

50 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

• • •

• •

0 45 90 135 180 225 270 315 360(deg)

AngleofAttackConvention

Structural Modeling - Element

Weight

> > > > > > >• ••

• 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

Structural Modeling - Blade

RotorShaft

Finite Element

Flap Hinge

Conditional Flap stop springs

Control StiffnessSpring

PitchBearing

LagHinge

Conditional Lag stop springs

• 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

Blade motions are uncoupled

1, 2 and 3 independent

[Mrotor] = M2

Blade motions are kinematically coupled

1 = -2

Introduction

Previous Research

Objectives

Approach

• Results Baseline rotor

Passive control of H-46 rotor

Feedback control of gimballed rotor

• Conclusions

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 5 10 15 20

Time(s)

FlapStopsRetract

Measured H-46

Baseline

Typical Engagement

• Linear airwake

VWOD = 60 knots

• 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

0 2 4 6 8 10 12

50%R95%R

Time(s)

(deg) DS

FlapStopsRetract

H-46 Tunnel

<25%NR

Typical Engagement

• Linear airwake

VWOD = 60 knots

• Majority of vtip is

rigid body motion

• Blade strikes lag

stop repeatedly

• Largest torque due

to impacts

-20000

-10000

0 2 4 6 8 10 12

Q(ft-lb)

Time (s)

0 2 4 6 8 10

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?

Flight Deck

Spot#1

Spot#2

Spot#3

HangarFace

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

Downflow

RecirculationZone

Recirculation zone pushed away from flight deck

Upflow & FlowAcceleration Zone

Large upflow component on windward side of flight deck

Upflow & FlowAcceleration Zone

Large upflow component over flight deck and over hangar face

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

Passive control of H-46 rotor Flap Damping

Spoilers

• Conclusions

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

Counterweight

Spring

Damper

Flap Damper Sizing for H-46

• Examine “worst-case” scenario - Spot #1 Airwake

0 1 2 3 4 5

Flap Damper Strength (xC)

Minwtip

(%R) H-46Tunnel

H-46 flap stops set at ±1º

Flap damper has stroke of only

Majority of wtip is elastic

Flap damper has no effect with a

small stroke!

Flap Damper Sizing for H-46

• Raise flap stop setting

Allows damper larger stroke

Keep droop stop setting at -1º No additional downward wtip

Raise flap stop setting

Flap damper has larger stroke

Flap damper has much large

effect!

0 2 4 6 8 10

FS(deg)

Minwtip

StandardConfiguration

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

H-46 Tunnel

No Reduction

FlapDamper

0 2 4 6 8 10

Time(s)

StandardFS

FlapDamper

Introduction

Previous Research

Objectives

Approach

Spoilers

• Conclusions

• Examine reducing flapping by reducing excessive lift

• Leading-edge spoilers known to significantly reduce lift

Objectives

VWithout spoiler With spoiler

Leading-edge spoiler

(Ref. Brasseur)

Objectives

LowAF Low

<25%SpoilersExtended

AppreciableCF

AppreciableAF

=25%SpoilersRetract High

>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 15 25 50 75

xspoiler (%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

Spoilers DeployedH-46 Tunnel

38%Reduction 21%

Reduction

Standard Configuration

Spoilers

SpoilersDeployed

Spoilers

-20000

-10000

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

Spoilers

• Conclusions

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

Structural & Aerodynamic Modeling

( )20 T P TL U U U

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

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

:Limits

<<−−>=Non-Rotating

Swashplate

RotatingSwashplate

Actuator#1

Actuator#2

Actuator#3

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

Conclusion:

max reduced by 50%

Min 75 limit reached

GimbalRestraint

Uncontrolled

OptimalControl

u(deg)

0 2 4 6Time (s)

(in/s)

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!

Optimal Control Assumptions

Sub-Optimal Control

Conclusion:

Optimal gains max by 50%

Sub-optimal gains max by 35%

0 2 4 6

Time(s)

GimbalRestraint

Uncontrolled

Sub-OptimalControl

OptimalControl

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+=

ΔVwod=-10kts

ΔVwod=+10kts

Sub-OptimalControl

•Gains K and v calculated from

(incorrect) anemometer meas.

Error in Wind Velocity

0 2 4 6

Time(s)

Δwod=-15deg

Δwod=+15deg

Sub-OptimalControl

Error in Wind Direction

Anemometererror

Robustness to Anemometer Error

Introduction

Previous Research

Objectives

Approach

Results

• Conclusions

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

Rotorcraft Center of Excellence Analysis and Control of the Transient Aeroelastic Response of Rotors...

Documents