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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. Keller Rotorcraft Fellow Ph.D. Thesis Seminar March 22, 2001. Presentation Outline. Introduction Previous Research

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Rotorcraft center of excellence l.jpg
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


Presentation outline l.jpg
Presentation Outline

  • Introduction

  • Previous Research

  • Objectives

  • Approach

  • Results

  • Conclusions


Introduction l.jpg
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

W

(%NR)

High blade flapping


Historical motivation l.jpg
Historical Motivation

  • Past problems for Sea King (RN) and Sea Knight (USN)

    • Blade-to-fuselage contact (114 for H-46!) - High blade loads

  • Forcesconservative limits to be placed on wind conditions conducive to safe engagement operations

  • Reduces operational flexibility of helicopter

H-46 Sea Knight

H-3 Sea King


Engage disengage testing l.jpg

Styrofoam Pegs

Greasy

Board

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


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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 Chinook

H-46 Sea Knight


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Future Motivation

  • 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


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Presentation Outline

  • Introduction

  • Previous Research

  • Objectives

  • Approach

  • Results

  • Conclusions


Early engage disengage research l.jpg
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


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Recent Engage/Disengage Research

  • Newmanat 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


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Presentation Outline

  • Introduction

  • Previous Research

  • Objectives

  • Approach

  • Results

  • Conclusions


Objectives l.jpg

Objectives

  • Develop unique “in-house” analysis code to:

    • Increase physical understanding of engage/disengage behavior

    • Accurately predict safe rotor engage/disengage envelopes

wtip (%R)

Safe Region

Unsafe Region


Technical barriers l.jpg
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 W 0 andW(t)

      • Flap/lag stop or gimbal restraint impacts at low W

    • Complicated ship airwake and aero environment  high a, L, m

Ship

Airwake

H-46 Data

H-46 Data

W

(%NR)


Presentation outline14 l.jpg
Presentation Outline

  • Introduction

  • Previous Research

  • Objectives

  • Approach

  • Results

  • Conclusions


Ship airwake modeling l.jpg

Vz

Vx

Vy

VWOD

yWOD

Ship Airwake Modeling

  • Specify speed (VWOD) & direction (yWOD) 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 l.jpg
Simple Ship Airwakes

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

  • Vz = vertical velocity, k = “gust” factor

Vx = VWOD cos yWOD

Vy = -VWOD sin yWOD

Horizontal

Airwake

Vz = 0

VWOD

Vz

Constant

Airwake

Vz = kVWOD

VWOD

Vz

max

Linear

Airwake

Vz = kVWOD

VWOD

max


Cfd generated ship airwakes l.jpg
CFD Generated Ship Airwakes

USN FFG

SFS

Flight Deck


Sfs ship airwakes l.jpg

yWOD = 270°

yWOD = 0°

Flow

Acceleration

Zone

VWOD

VWOD

Recirculation

Zone

70 kts

50 kts

60 kts

50 kts

SFS Ship Airwakes

  • Along-wind airwake velocities

20 kts

50 kts

40 kts

40 kts


2 d aerodynamic modeling l.jpg
2-D Aerodynamic Modeling

  • Aerodynamics modeled with

    • Nonlinear quasi-steady aerodynamics (Ref. Prouty & Critzos)

      • Aero forces dependent upon instantaneous values of a, a, a

    • Nonlinear time-domain unsteady aerodynamics (Ref. Leishman)

      • Aero forces dependent upon time history of a, a, a

      • Model only validated for small a and L (< 25°) and M > 0.3

      • Must switch to quasi-steady at high a and L (> 25°) and M < 0.1


Structural modeling element l.jpg

va

v’a

wa

w’a

a

vb

v’b

wb

w’b

b

m

Structural Modeling - Element

  • 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 W

CF

Aero

> > > > > > >

Weight


Structural modeling blade l.jpg
Structural Modeling - Blade

  • Articulated blade modeling

    • Require mechanisms to restrain flap (bhinge) & lag (zhinge) motion

      • Stops simulated with conditional springs Kb and Kz

    • Flap stops extend/retract at a specified rotor speed

(t)

Pitch

Bearing

Finite Element

Conditional Lag stop springs

Kz

Flap Hinge

Conditional Flap stop springs

K

Lag

Hinge

Control Stiffness

Spring

K

Rotor

Shaft


Structural modeling rotor l.jpg
Structural Modeling - Rotor

  • Articulated or hingeless rotors

  • Teetering or gimballed rotors

Blade motions are uncoupled

b1, b2 and b3 independent

b2

b3

M1

0

0

b1

[Mrotor] =

0

M2

0

0

0

M3

Blade motions are kinematically coupled

b1 = -b2

b1

b2

M1

0

[Mrotor] =

M2

0


Presentation outline23 l.jpg
Presentation Outline

  • Introduction

  • Previous Research

  • Objectives

  • Approach

  • Results

    • Baseline rotor

    • Passive control of H-46 rotor

    • Feedback control of gimballed rotor

  • Conclusions


Baseline rotor system l.jpg

Measured H-46

Baseline

Baseline Rotor System

  • Representative of a “medium-sized” naval helicopter

    • Nb = 4 Articulated Blades

    • R = 25 ft

    • W0R = 750 ft/s

    • g = 7.35

    • s = 0.076

    • nb = 1.02/rev

    • nz = 0.30/rev

    • nq = 4.54/rev

    • bFS = ±1º

    • zLS = ±10º


Typical engagement l.jpg
Typical Engagement

  • Linear airwake

    • VWOD = 60 knots

    • k = 25%

  • Largest wtip occur W < 25%NR

  • Blade strikes flap stops repeatedly

  • Majority of wtip is elastic bending

    • rigid body wtip ±2%R

  • Large a in low W even near blade tip


Typical engagement26 l.jpg
Typical Engagement

  • Linear airwake

    • VWOD = 60 knots

    • k = 25%

  • Majority of vtip is rigid body motion

  • Blade strikes lag stop repeatedly

  • Largest torque due to impacts


Typical wind envelope l.jpg

Upward wtip

Upward

wtip

Downward wtip

Downward wtip

Typical Wind Envelope

  • Engagement wind envelope

    • Shows largest downward and upward wtip with VWOD and yWOD

VWOD = 60 kts

yWOD = 30°


Sfs ship airwake l.jpg
SFS Ship Airwake

  • What effect does a “realistic” ship airwake have on rotor deflections?

SFS

Flight Deck


Spot 1 engagement envelope l.jpg

Large upflow component over flight deck and over hangar face

Little upflow over stern and flow decelerates near hangar face

Large upflow component on windward side of flight deck

Recirculation zone pushed away from flight deck

Recirculation and downflow behind hangar face

Spot #1 Engagement Envelope

  • Bow and port winds have largest wtip

  • Stern and 330° winds have small wtip

Spot #1

(Closest to hangar)


Effect of deck position l.jpg
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 #3

Spot #1

Spot #2

Spot #3

Spot #1

Spot #2

Spot #3


Presentation outline31 l.jpg
Presentation Outline

  • Introduction

  • Previous Research

  • Objectives

  • Approach

  • Results

    • Baseline rotor

    • Passive control of H-46 rotor

      • Flap Damping

      • Spoilers

    • Feedback control of gimballed rotor

  • Conclusions


Objectives32 l.jpg

Objectives

  • Develop unique “in-house” analysis code to:

    • Increase physical understanding of engage/disengage behavior

    • Accurately predict safe rotor engage/disengage envelopes

wtip (%R)

Safe Region

Unsafe Region


Flap damping on hup 2 l.jpg
Flap Damping on HUP-2

  • Hydraulic flap dampers were used on 1950’s era HUP-2

    • Dampers only active at low W

    • Above preset W 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

Spring

Mast

Counterweight

Damper

Hub

Blade


Flap damper sizing for h 46 l.jpg

H-46 flap stops set at ±1º

Flap damper has stroke of only 2°

Majority of wtip is elastic

bFS

Cb

Flap damper has no effect with a small stroke!

Flap Damper Sizing for H-46

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


Flap damper sizing for h 4635 l.jpg
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

Cb

bFS

Flap damper has much large effect!


Sfs spot 1 envelope l.jpg
SFS Spot #1 Envelope

  • Flap damper = 4Cz

  • 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

Standard H-46

With Damper

Max

wtip

Min wtip


Flap damping in bow winds l.jpg
Flap Damping in Bow Winds

  • Blade does not lift off bDS until t = 5 sec

  • Flap damper never has a chance to dissipate energy

  • Summary:

    • Min wtip decreased in most cases

    • bFS must be raised

    • Max wtip increased


Presentation outline38 l.jpg
Presentation Outline

  • Introduction

  • Previous Research

  • Objectives

  • Approach

  • Results

    • Baseline rotor

    • Passive control of H-46 rotor

      • Flap Damping

      • Spoilers

    • Feedback control of gimballed rotor

  • Conclusions


Objectives39 l.jpg
Objectives

  • Examine reducing flapping by reducing excessive lift

  • Leading-edge spoilers known to significantly reduce lift

L

Leading-edge spoiler

L

V

V

Without spoiler

With spoiler

(Ref. Brasseur)


Objectives40 l.jpg

Objectives

  • Spoilers are used only along partial-span

  • Gated spoilers are used on blade upper and lower surfaces

  • Spoilers only extended at low W < 25%NR and retracted into blade section at high W > 25%NR


Spoiler coverage l.jpg
Spoiler Coverage

  • H-46 engagement with varying amounts of spoiler coverage

  • Spoilers on outer 15%R (~3½ ft) are enough to reduce wtip

H-46 Engagement

SFS Spot #1 Airwake

VWOD = 40 kts

yWOD = 240°


Example engagement l.jpg
Example Engagement

SFS Spot #1 Airwake

(Worst Case Scenario)

VWOD = 40 kts

yWOD = 240°

Conclusions:

Min and Max wtip reduced

Max torque not affected


Sfs spot 1 airwake envelopes l.jpg
SFS Spot #1 Airwake Envelopes

  • 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

Standard H-46

With Spoilers

Max

wtip

Min wtip


Presentation outline44 l.jpg
Presentation Outline

  • Introduction

  • Previous Research

  • Objectives

  • Approach

  • Results

    • Baseline rotor

    • Passive control of H-46 rotor

      • Flap Damping

      • Spoilers

    • Feedback control of gimballed rotor

  • Conclusions


Motivation l.jpg
Motivation

  • 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?

Rubber Spring

Gimbal Restraint


Structural aerodynamic modeling l.jpg
Structural & Aerodynamic Modeling

  • Rigid blade structural model

    • 2 degrees of freedom - gimbal pitch (b1c) and roll (b1s)

  • Linear quasi-steady aerodynamic model

    • Lift >> Drag

b1C

z

y

W

x

b1S

Kb

  • Control System Settings

Swashplate inputs


Optimal control theory l.jpg
Optimal Control Theory

  • Cast equations of motion into state space form

Disturbance d(t) due to:

Airloads induced by ship airwake effects

  • Equations are Linear Time Variant (LTV)

  • W(t) and aerodynamic terms make pole placement ineffective

  • Use LQR theory and define performance index J

    • S(tf) = Final State Weight Q = State Weight R = Control Weight

  • Use Matrix Ricatti Equations to find gain matrix K

  • Additional gain due to disturbance d(t)


    Control system limits l.jpg
    Control System Limits

    • Swashplate actuators typically have limits in magnitude and rate

    • Time integration with control system limits

    Enforce

    control

    limits


    Response in constant airwake l.jpg
    Response in Constant Airwake

    • Simulated V-22 engagement

      • Vwod = 30 kts in Bow winds

      • Uncontrolled case:

        • q75 = q1c = q1s = 0

    • Constant airwake distribution

      • k = 25%

    Vwod

    Conclusion:

    bmax reduced by 50%

    Min q75 limit reached


    Optimal control assumptions l.jpg
    Optimal Control Assumptions

    • 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!

    Vz

    Vx

    Vy

    VWOD

    yWOD


    Sub optimal control l.jpg
    Sub-Optimal Control

    • 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

    Conclusion:

    Optimal gains bmax by 50%

    Sub-optimal gains bmax by 35%


    Robustness to anemometer error l.jpg
    Robustness to Anemometer Error

    • Anemometer measurement error

    Error in Wind Velocity

    Anemometer

    error

    • Gains K and v calculated from (incorrect) anemometer meas.

    Error in Wind Direction

    • Conclusion:

      • Moderate errors in anemometer reading change response by 10%


    Presentation outline53 l.jpg
    Presentation Outline

    • Introduction

    • Previous Research

    • Objectives

    • Approach

    • Results

    • Conclusions


    Conclusions l.jpg
    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 W(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


    Conclusions55 l.jpg
    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 (W < 25%NR) and retract into blade (W > 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


    Conclusions56 l.jpg
    Conclusions

    • 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

      • bmax 50% with full knowledge of ship airwake (Vx, VyandVz)

    • Aero forces due to ship airwake contribute to control gains

      • bmax 35% with partial knowledge of ship airwake (Vx and Vy)

    • Response insensitive to errors in anemometer reading

      • bmaxchanged ±10% with either ±10 knot or ±15° anemometer error


    Acknowledgments l.jpg
    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


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