Stiff Inplane Tiltrotor Aeromechanics Investigation Using Two ...

Motivation
Analytical Model
Analytical Results
Summary
Stiﬀ-Inplane Tiltrotor Aeromechanics
Investigation Using Two Multibody Analyses
Jinwei Shen
shenjw@nianet.org
National Institute of Aerospace
Hampton, Virginia, USA
Multibody Dynamics 2007

Motivation
Analytical Model
Analytical Results
Summary
Authors
Pierangelo Masarati
Politecnico di Milano
Beatrice Roget
National Institute of Aerospace
David J. Piatak
NASA Langley Research Center
Mark W. Nixon
U.S. Army Research Laboratory
Jeﬀrey D. Singleton
U.S. Army Research Laboratory

Motivation
Analytical Model
Analytical Results
Summary
Outline
1
Motivation
Simulations Replace Experiments
Tiltrotor Aeromechanics with Multibody Codes
Previous Work
Objectives
2
Analytical Model
Multibody Dynamics Codes
Multibody Tiltrotor Models
3
Analytical Results
Wind Tunnel Test Used in Model Validation
Baseline Results
Parameter Study: Pitch-Flap Couplings
Parameter Study: Aerodynamic Compressibility

Motivation
Analytical Model
Analytical Results
Summary
Cost Saving by Replace Experiments with Simulations
Why uses multibody dynamics simulations in rotorcraft industry?
Experimental testing routinely performed to verify design
Experiment, either ﬂight test or wind tunnel test, becomes
prohibitively expensive
Multibody analyses provide high-level of details of complex
mechanics in rotor system
Simulation using multibody dynamics codes may become an
alternative to expensive experimental veriﬁcations.

Motivation
Analytical Model
Analytical Results
Summary
What is a Tiltrotor?
Tiltrotor uses tiltable proprotors
The Bell-Boeing V-22 Osprey,
for lift and propulsion; a
best known tiltrotor aircraft
combination of helicopter and
turboprop aircraft.
Helicopter mode: proprotor
pylon vertical to ground
Transition mode: proprotor
pylon tilts forward
Airplane mode: proprotor
pylon parallels to ground
Tiltrotor has high cruise speed and range, and has also vertical
takeoﬀ/landing and hover capability.

Motivation
Analytical Model
Analytical Results
Summary
Tiltrotor Whirl Flutter
Electra two fatal crashes in
Similar to turboprop aircraft
1959 and 1960 due to whirl
whirl ﬂutter
ﬂutter
Happens in airplane mode
Aerodynamic forces in rotor
plane drive wing beam mode
unstable
Tiltrotor has more degrees
of freedom and larger rotor
A ﬂapping proprotor on a
ﬂexibly supported pylon can
exhibit whirl ﬂutter
Tiltrotor whirl ﬂutter is an aeroelastic instability phenomenon
involving the proprotor, pylon, and wing.

Motivation
Analytical Model
Analytical Results
Summary
Important Structural Modes in Tiltrotor Whirl Flutter
Fixed System: Wing and Pylon
Rotating System: Rotor and Hub
(Pylon Mount: Downstop
(Stiﬀ or Soft Inplane Rotor)
Spring)
Rotor ﬂap mode
Wing/pylon beamwise mode
Rotor lag mode
Wing/pylon torsional mode
Rotor torsional mode

Motivation
Analytical Model
Analytical Results
Summary
Previous Work
Stiﬀ-inplane tiltrotor
Soft-inplane tiltrotor
Ghiringhelli, Masarati, et al.
Masarati, Piatak, et al.
Nonlinear Dynamics,
American Helicopter Society
Vol. 19, No. 4 1999
Forum, 2004, 2005
Pioneer work of modeling
Using two multibody
tiltrotor with multibody
analyses
analysis
Whirl ﬂutter study
Rotor natural frequencies
Rotor mast free-play
Conversion loads study
Parameter study
Lack of whirl ﬂutter study
Soft-inplane rotor has insuﬃcient stability boundaries
Current production tiltrotor uses stiﬀ-inplane rotor
Need multibody validation of stiﬀ-inplane tiltrotor model

Motivation
Analytical Model
Analytical Results
Summary
Objectives
Develop a sophisticated stiﬀ-inplane tiltrotor model
component by component using two multibody dynamics
codes
Carry out correlations of the analytical models with
experimental data
Conduct parametric investigations of key variables that are
crucial to the tiltrotor aeromechanical behavior

Motivation
Analytical Model
Analytical Results
Summary
DYMORE
Developed by Georgia Institute of Technology team,
lead by Prof. Olivier Bauchau
Nonlinear, FEM-based, multibody dynamics code
Nonlinear
FEM-based
Multibody
Large deformations
Beams
Rigid bodies
Free-play/contact
Shells
Joints
Geometry
Membranes
Revolute
Prismatic
Aerodynamics
Spherical
Cylindrical
Aerodynamics
State-space lifting line theory
Coupling with Computational Fluid Dynamics (CFD) models

Motivation
Analytical Model
Analytical Results
Summary
MBDyn
Developed by Politecnico di Milano team,
lead by Prof. Paolo Mantegazza
Multibody dynamics code with libraries: Mechanical,
Hydraulic, Controls, Aerodynamic.
Mechanical
Rigid bodies, Deformable: Lumped, Beams, Modal elements
Joints: Absolute/relative position, orientation, velocity, acceleration.
Controls
Aerodynamics
Electric motors
Blade element on rigid bodies,
beams
Strain, displacement,
acceleration
Coupling with CFD models
Programmable elements

Motivation
Analytical Model
Analytical Results
Summary
DYMORE & MBDyn Tiltrotor Models
Structural modeling
Aerodynamic modeling
Beams: Wing, blades
Aerodynamic forces on wing
Rigid bodies: rotor hub,
and rotor
control system, pylon
Aerodynamic interaction
Springs: control system
between wing and rotor
stiﬀness, pylon mount
neglected
(downstop springs)

Motivation
Analytical Model
Analytical Results
Summary
Beam Discretization
DYMORE
MBDyn
Beam
Number
Order
Number
Order
Wing
10
3
6
2
Flex-beam
5
3
10
1
Blade
10
3
15
2
Beam discretization?
Degrees of freedom
DYMORE: 3554
MBDyn: 1873
Real/Simulated time
DYMORE: 700:1
MBDyn: 70:1

Motivation
Analytical Model
Analytical Results
Summary
Wind Tunnel Model and Test
WRATS stiﬀ-inplane tiltrotor
Test Procedure
in NASA Langley TDT tunnel
Trim: adjust blade pitch
tested in 2000
angle to trim rotor speed in
windmilling
Excitation: excite
wing/pylon mode
Stability: process wing
transient response to obtain
damping
Sweep: increase air speed
until low damping or ﬂutter
“Virtual Experiment”
Analytical models use the same procedure to predict wing damping.

Motivation
Analytical Model
Analytical Results
Summary
Baseline Stability Boundary (1/3)
Wing Beam Mode Damping
5.0
Parameters
4.5
TEST
Pylon:
DYMORE
4.0
MBDyn
Oﬀ-Downstop
3.5
Rotor Speed:
3.0
2.5
742 RPM
Damping
2.0
(% critical)
1.5
Flutter Speed (kts)
1.0
TEST:
140
0.5
0.0
DYMORE: 130
-0.5
50
70
90
110
130
150
170
190
210
MBDyn:
150
Airspeed (knots)
Low ﬂutter speed when pylon is oﬀ downstop-lock
Good agreements among analytical predictions and test data

Motivation
Analytical Model
Analytical Results
Summary
Baseline Stability Boundary (2/3)
Wing Beam Mode Damping
5.0
Parameters
4.5
TEST
Pylon:
DYMORE
4.0
MBDyn
On-Downstop
3.5
Rotor Speed:
3.0
2.5
770 RPM
Damping
2.0
(% critical)
1.5
Flutter Speed (kts)
1.0
TEST:
190
0.5
0.0
DYMORE: 175
-0.5
50
70
90
110
130
150
170
190
210
MBDyn:
200
Airspeed (knots)
Higher ﬂutter speed when pylon locked on-downstop
Fair agreements among analytical predictions and test data

Motivation
Analytical Model
Analytical Results
Summary
Baseline Stability Boundary (3/3)
Wing Beam Mode Damping
5.0
Parameters
4.5
TEST
Pylon:
DYMORE
4.0
MBDyn
On-Downstop
3.5
Rotor Speed:
3.0
2.5
888 RPM
Damping
2.0
(% critical)
1.5
Flutter Speed (kts)
1.0
TEST:
155
0.5
0.0
DYMORE: 145
-0.5
50
70
90
110
130
150
170
190
210
MBDyn:
170
Airspeed (knots)
Flutter speed reduces when increasing rotor speed
Fair agreements among analytical predictions and test data

Motivation
Analytical Model
Analytical Results
Summary
Parameter Study: Pitch-Flap Couplings (δ3)
Wing Beam Mode Damping
Parameters
5.0
Pylon:
4.5
TEST, δ3=-15
TEST, δ3=-45
On-Downstop
4.0
DYMORE, δ3=-15
DYMORE, δ
3.5
3=-45
MBDyn, δ
Rotor Speed:
3=-15
MBDyn, δ
3.0
3=-45
888 RPM
2.5
Damping
2.0
Flutter Speed (kts)
(% critical)
1.5
1.0
With δ3 = −45◦
0.5
TEST:
80
0.0
DYMORE: 60
-0.5
30
50
70
90
110
130
150
170
190
210
MBDyn:
60
Airspeed (knots)
Flutter speed reduces when increasing pitch-ﬂap coupling
Analytical models capture the ﬂutter speed reduction

Motivation
Analytical Model
Analytical Results
Summary
Parameter Study: Aerodynamic Compressibility
Wing Beam Mode Damping
Parameters
5.0
Pylon:
4.5
TEST, air
TEST, R-134a
On-Downstop
4.0
DYMORE, air
DYMORE, R-134a
3.5
MBDyn, air
Rotor Speed:
MBDyn, R-134a
3.0
888 RPM
2.5
Damping
2.0
Flutter Speed (kts)
(% critical)
1.5
1.0
Test gas R-134a
0.5
TEST:
120
0.0
DYMORE: 120
-0.5
50
70
90
110
130
150
170
190
210
MBDyn:
135
Airspeed (knots)
Aerodynamic compressibility reduces whirl ﬂutter speed
Analytical models capture the ﬂutter speed reduction

Motivation
Analytical Model
Analytical Results
Summary
Summary
Develop analytical models of WRATS stiﬀ-inplane tiltrotor
using two multibody dynamics codes
Multibody dynamics analyses show consistent capabilities in
predicting tiltrotor whirl-ﬂutter stability
Parameter study using multibody dynamics analyses shows
same trend as experimental investigation
Outlook
Further improve analytical correlations
Detailed comparisons of the two multibody models in
sub-component level and in the computational aspects

Document Outline

Motivation
- Simulations Replace Experiments
- Tiltrotor Aeromechanics with Multibody Codes
- Previous Work
- Objectives
Analytical Model
- Multibody Dynamics Codes
- Multibody Tiltrotor Models
Analytical Results
- Wind Tunnel Test Used in Model Validation
- Baseline Results
- Parameter Study: Pitch-Flap Couplings
- Parameter Study: Aerodynamic Compressibility
Summary