1.1B Principles and Values by wqv15485

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									         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, 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
                                  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

                              For Gains G2
                                                  For Gains G1
Damping




                                  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 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                Wing Vertical Bending Mode
(beam mode) Frequency                     (beam mode) Damping




                                           Beam Mode




    DYMORE, Masarati (2004)
    Experimental data (average), Nixon (2003)   NASA Langley 2002 test
    Present Analysis
Airplane (Cruise) Mode Results, 550 RPM, on-D/S, windmilling


 Wing Vertical Bending Mode               Wing Vertical Bending Mode
  (beam mode) Frequency                     (beam mode) Damping




                                                Beam Mode




   DYMORE, Masarati (2004)
   Experimental data (average), Nixon (2003)   NASA Langley 2002 test
   Present Analysis
         Hover Mode Results, Rotor and Wing Uncoupled
Rotor shaft-fixed (no wing)                         Wing/pylon only (no rotor)
                                                Present Analysis
    Lag Modes                                                            Pylon Yaw


                                                                     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                                            Wing mode
                                                                       frequencies
Present Analysis
                                                                        match with
                                                                     published data
                               Masarati (2004)                      (Nixon, Masarati,
                                                                          Shen)
Hover Mode Results, Rotor and Wing Coupled

            Wing Vertical Bending Mode
              (beam mode) Damping




      DYMORE, Shen (2005)
      Experimental data, Nixon (2003)    NASA Langley 2002 test

       Present Analysis
    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 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

								
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