Project Number : PS 1.1b Active Tiltrotor Aeroelastic and Aeromechanical Stability Augmentation PI: Dr. Farhan Gandhi Phone: (814) 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 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% R Wing chord mode: Tip disp 1% R Wing 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 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 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 Damping Design Variables For Gains G2 For 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) Feedback Wing-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 RPM Full-State Feedback and Wing-State Feedback Compare Well for Wing-Flaperon Actuation

Output (Wing-State) Feedback Swashplate Actuation

Result: all modes stable from 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 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. 46 th AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics & Materials Conference, 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 31 st European Rotorcraft Forum, Dynamics Session, 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 Uncoupled Wing/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 stiffness MBDyn – 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