Project Number : PS 1.1b

Active Tiltrotor Aeroelastic and Aeromechanical Stability

Augmentation

PI:

Dr. Farhan GandhiPhone: (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 ReviewMay 3, 2005

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

• Modeled actuation through wing-flaperon (sized to match XV-15 flaperon)Extends over outer half of wing and 25% of the chord

• 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)

Wing vertical bending mode: Tip disp 2.5% RWing chord mode: Tip disp 1% RWing torsion mode: Tip rotation 1 deg

Baseline / No-Control Results

Cruise (458) RPM

Critical Flutter Speed = 330 knots

Hover (565) RPM

Critical Flutter Speed = 315 knots

At 380 knots airspeed

(An arbitrarily selected target cruise speed up to which flutter-free operation is desired)

Wing-Flaperon Actuation

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 Airspeed (and RPM) Scheduled LQR Optimal Control

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

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 Airspeed (and RPM) Scheduled LQR Optimal Control

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

Full-State Feedback Constant Gain Controller (458 RPM, 380 knots LQR Optimal Gains Used)

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,

• Design variables are the control gains

• is the minimum damping of the least damped mode at any point during the iteration process

620400min|)(

RPMjKF

jK

min

Dam

pin

g

Design Variables

For Gains G2For Gains G1

Current value of design variables optimizer is working with

Swashplate Actuation, At 380 knots airspeed

Controller very robust to changes in RPM

Increase in operating range (all modes stable over ENTIRE operating range)

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) FeedbackWing-Flaperon Actuation

Wing-State

Full-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) FeedbackSwashplate Actuation

Result: all modes stable from 400-555 RPM

Swashplate Actuation, At 380 knots airspeed

(Wing-State Feedback Gains Obtained using Moving-Point Optimization)

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 deg2

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 Whirl-Flutter 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, Soft-Inplane (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

Experimental data (average), Nixon (2003)

Present Analysis

DYMORE, Masarati (2004)

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

Experimental data (average), Nixon (2003)

Present Analysis

DYMORE, Masarati (2004)

NASA Langley 2002 test

Hover Mode Results, Rotor and Wing UncoupledWing/pylon only (no rotor)

Pylon Yaw

Torsion/Chord

Beam and Chord/Torsion

Wing mode frequencies match with

published data (Nixon, Masarati,

Shen)

Present Analysis

Rotor shaft-fixed (no wing)

Experimental Data

MBDyn – tuned stiffness w/modal participation

DYMORE – crossover stiffnessMBDyn – crossover stiffness

Present Analysis

Masarati (2004)

Flap Modes

Lag Modes

NASA Langley 2002 test

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

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

Summary, SASIP Correlation

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 multi-body 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