Project Number : PS 1.1b
Active Tiltrotor Aeroelastic and
Aeromechanical Stability
Augmentation
PI:
Dr. Farhan Gandhi
Phone: (814) 865-1164
E-mail: fgandhi@psu.edu
Graduate Student Researchers:
Rupinder Singh (funded by NRTC)
Eric Hathaway (Boeing Philadelphia)
2005 Penn State RCOE Program Review
May 3, 2004
Background
• Tiltrotors susceptible to whirl flutter instability at high forward speeds
• Alleviating whirl flutter allows higher cruise speeds and/or reduced structural
weight (greater payload/range)
• Proposed soft-inplane tiltrotor configurations vulnerable to aeromechanical
instabilities (ground/air resonance)
• Passive design techniques which improve soft-inplane aeromechanical stability
have been reported to reduce whirl flutter stability
Technical Barriers / Physical Mechanisms to Solve
• Ground Resonance characteristics of soft-inplane tiltrotors not been fully
explored
• Modern Adaptive Controllers may be capable of providing the required stability
augmentation, complexity of these systems not attractive for production
• Simpler controllers may have lower benefits, may not be sufficiently robust
• Which actuation mechanism to use?
Overall Objectives
• Evaluate effectiveness of active control in improving the damping of critical
modes in various flight regimes, including:




High-speed (whirl flutter)
Low- to moderate-speed (air resonance)
Ground contact and Hover (ground/air resonance)
Increasing speed, reducing weight, allowing for soft-inplane designs
Approaches
• Develop, validate simple tiltrotor stability analysis, suitable for closed-loop
control
• Extend analysis for active control via wing-mounted trailing edge flap and
swashplate
• Verify active control results with available experimental data
• Examine the effectiveness of active control for improving tiltrotor whirl flutter/
aeromechanical stability, considering both swashplate/wing-flaperon actuation
• Compare performance of simple controllers to full-state LQR control, evaluate
robustness and performance
Current Motivation
•
Recent active tiltrotor stability augmentation efforts employ simple single-state
feedback schemes or complex modern adaptive controllers
 What about LQR optimal control (what is the best you can do)?
 How much performance loss if feedback of few (easily measured) states used?
 How robust would such a controller be? or do you need adaptive control?
•
How does the flaperon compare to a swashplate-based actuation system?
•
Recent tests on active alleviation of aeromechanical instabilities of soft-inplane
tiltrotor configs., but limited analysis and understanding
Analytical Model
•
Rotor blades  rigid flap/lag dynamics represented
 Distribution of stiffness inboard/outboard of pitch bearing allows first principles derivation of
variation of frequencies with collective and aeroelastic couplings
•
Gimbal motions represented
•
FEM wing model – reduced to three fundamental wing modes (b,c,t)
•
Quasi-Steady/Unsteady Aerodynamics options (quasi-steady results compare well
with unsteady aero results, as reported in 2004)
•
Model extensively validated in previous years using XV-15 data,
M-222 data, WRATS data, as well as Johnson’s and Nixon’s elastic blade
analysis results (AHS J, July 2003). Model well-suited for control studies
•
Wing vertical bending mode: Tip disp 2.5% R
Wing chord mode: Tip disp 1% R
Modeled actuation through wing-flaperon (sized to match XV-15 flaperon)
Wing
torsion
mode:
Extends over outer half of wing
and 25%
of the
chordTip rotation 1 deg
•
Modeled actuation through the swashplate
•
Limits on swashplate motions (1 deg cyclic) and flap delections (+/-6 deg) determine
maximum controller gains (for typical disturbances levels)
Baseline / No-Control Results
Cruise (458) RPM
Critical Flutter Speed = 330 knots
At 380 knots airspeed
(An arbitrarily selected target cruise speed
up to which flutter-free operation is desired)
Hover (565) RPM
Critical Flutter Speed = 315 knots
Wing-Flaperon Actuation
Full-State Feedback Airspeed (and RPM) Scheduled LQR Optimal Control
Wing- Flaperon Actuation, At Cruise
(458) RPM
Stability Boundary = 415 knots, determined
by airspeed at which required actuation
input exceeds prescribed limits, increase of
85 knots over baseline
Wing- Flaperon Actuation, At Hover
(565) RPM
Stability Boundary = 375 knots, determined by
airspeed at which required actuation input
exceeds prescribed limits, increase of 60
knots over baseline
Full-State Feedback Constant Gain Controller (458 RPM, 380 knots LQR
Optimal Gains Used)
Wing- Flaperon Actuation, At Cruise (458)
RPM
Critical Flutter Speed = 420 knots, airspeed at which
wing chord mode unstable, increase of 90 knots
over baseline
Similar Increase at Hover RPM
Wing-Flaperon Actuation, At 380 knots
airspeed
Increase in operating range (all modes stable
from 400-575 RPM) compared to baseline
Constant Gain Controller Robust to Changes in Airspeed
and RPM
Swashplate Actuation
Full-State Feedback Airspeed (and RPM) Scheduled LQR Optimal Control
Swashplate Actuation, At Cruise (458)
RPM
Stability Boundary = 400 knots, determined by
airspeed at which required actuation input
exceeds prescribed limits, increase of 70
knots over baseline
Swashplate Actuation, At Hover
(565) RPM
Stability Boundary = 390 knots, determined
by airspeed at which required actuation
input exceeds prescribed limits, increase of
75 knots over baseline
Full-State Feedback Constant Gain Controller (458 RPM, 380 knots LQR
Optimal Gains Used)
Swashplate Actuation, At Cruise (458)
RPM
Critical Flutter Speed = 405 knots, airspeed at
which wing chord mode unstable, increase of 75
knots over baseline
Similar Increase at Hover RPM
Swashplate Actuation, At 380 knots
airspeed
Increase in operating range (all modes stable
from 400-555 RPM) compared to baseline
Constant Gain Controller not as robust to changes in
RPM, possible solution: Moving-Point Optimization
Swashplate Actuation
(with Moving-Point Optimization)
• Objective function to be minimized, F (K )  
j
|
min RPM  400 620
• Design variables K are the control gains
j
•
 min is the minimum damping of the least damped mode at any point
during the iteration process
Damping
For Gains G2
For Gains G1
Design Variables
Current value of design variables optimizer is working with
Swashplate Actuation, At 380 knots
airspeed
Increase in operating range (all modes
stable over ENTIRE operating range)
Controller very robust to
changes in RPM
Swashplate Actuation, At Cruise (458)
RPM
Stability Boundary = 395 knots, determined
by airspeed at which required actuation input
exceeds prescribed limits, increase of 65
knots over baseline
Similar Increase at Hover RPM
Possible to Design Constant Gain Controllers that are
Robust to Variations in RPM and Airspeed
Output (Wing-State) Feedback
Wing-Flaperon Actuation
Full-State
Wing-State
Wing-Flaperon Actuation, At 380 knots airspeed
(Wing-State Feedback Gains Obtained using Moving-Point Optimization)
Result: all modes stable from 400-580 RPM
Full-State Feedback and Wing-State Feedback Compare
Well for Wing-Flaperon Actuation
Output (Wing-State) Feedback
Swashplate Actuation
Swashplate Actuation, At 380 knots airspeed
(Wing-State Feedback Gains Obtained using Moving-Point Optimization)
Result: all modes stable from 400-555 RPM
Wing-State Feedback not as Robust as Full-State
Feedback for Swashplate Actuation
Suggests need for measurement/estimation of some rotor states or using higher
actuation limits of  2 deg
Summary of Active Control Results
• Constant Gain Controllers: Effective in increasing critical flutter speed.
Robust to variations in RPM, airspeed and wing frequencies.
• Output (wing-state) feedback controllers: Almost as effective (and robust)
as full-state feedback controllers for wing-flaperon actuation. Less so for
swashplate actuation
• Detailed results for stiff-inplane XV-15 model in “Active Tiltrotor WhirlFlutter Stability Augmentation using Wing-Flaperon and Swashplate
Actuation” (Proc. 46th AIAA/ASME/ASCE/AHS/ASC Structures, Structural
Dynamics & Materials Conference, 18-21 April 2005, Austin, Texas)
• Similar study performed for soft-inplane M-222 model, detailed results in
“Wing-Flaperon and Swashplate Control for Whirl-Flutter Stability
Augmentation of a Soft-Inplane Tiltrotor” (submitted to the 31st European
Rotorcraft Forum, Dynamics Session, 13-15 Sept. 2005, Florence, Italy)
Key Results – Flaperon greatly improves sub-critical damping in the wing
beam mode.
4-Bladed Semi-Articulated, SoftInplane (SASIP)Rotor
• A modern rotor (XV-15, M-222 – over 30 year old designs)
• Soft-inplane configuration (of interest for future tiltrotor designs)
• Tested at NASA Langley during Summer, 2002
• Our Interests:
– Modeling SASIP rotor using our rigid blade model and modal wing
– Correlation of analytical results with experimental data from
Langley tests
– Examine and evaluate active control schemes, as done for XV-15
and M-222
Airplane (Cruise) Mode Results, 550 RPM, off-D/S, windmilling
Wing Vertical Bending Mode
(beam mode) Frequency
Wing Vertical Bending Mode
(beam mode) Damping
Beam Mode
DYMORE, Masarati (2004)
Experimental data (average), Nixon (2003)
Present Analysis
NASA Langley 2002 test
Airplane (Cruise) Mode Results, 550 RPM, on-D/S, windmilling
Wing Vertical Bending Mode
(beam mode) Frequency
Wing Vertical Bending Mode
(beam mode) Damping
Beam Mode
DYMORE, Masarati (2004)
Experimental data (average), Nixon (2003)
Present Analysis
NASA Langley 2002 test
Hover Mode Results, Rotor and Wing Uncoupled
Rotor shaft-fixed (no wing)
Wing/pylon only (no rotor)
Present Analysis
Pylon Yaw
Lag Modes
Torsion/Chord
Flap Modes
Beam and Chord/Torsion
Experimental Data
NASA Langley 2002 test
MBDyn – tuned stiffness w/modal participation
DYMORE – crossover stiffness
MBDyn – crossover stiffness
Present Analysis
Masarati (2004)
Wing mode
frequencies
match with
published data
(Nixon, Masarati,
Shen)
Hover Mode Results, Rotor and Wing Coupled
Wing Vertical Bending Mode
(beam mode) Damping
DYMORE, Shen (2005)
Experimental data, Nixon (2003)
Present Analysis
NASA Langley 2002 test
Summary, SASIP Correlation
Airplane Mode
 Beam mode frequency vs. airspeed matches test data well
 No other modal freq data available (requested more data from Langley)
 Beam mode damping lower than test results
 Better at 550 RPM than 742 RPM
 Similarity between present analysis results and MBDyn results at 742 RPM
Hover Mode
• Rotor shaft-fixed frequencies, isolated wing frequencies match published values very
closely
•
Wing vertical bending mode damping vs RPM compares well against test and multibody analysis (DYMORE) , damping still over-predicted at high RPM
•
Issues remain with behavior of second wing mode (chord-torsion) when wing is
coupled to rotor, continuing to investigate
Forward Path
-- Clear up outstanding issues with regards to SASIP model and
validation
-- Examine effectiveness of Active Control for SASIP rotor (whirl
flutter and ground resonance)
-- Not proposing another 5-year 6.1 RCOE-type effort
-- Simplified analysis a great tool for examining active control on
new tiltrotor designs (relevant to quad-tiltrotors, NASA heavy-lift
program, etc.) under CRI funding
-- Would love to forge collaborations (LaRC, Bell?) on a test using
wing-flaperons for stability augmentation