1. Department of Aerospace Engineering Rotorcraft Center of Excellence TASK PS 2.3a Passive, Semi-Active, Active Reduction of Gearbox Vibration and Noise Principal Investigators: George A. Lesieutre, Professor Edward C. Smith, Professor tel: (814) 863-0103 tel: (814) 863-0966 email: g-lesieutre@psu.edu email: ecs@rcoe.psu.edu Graduate Students: François LeHen, MS Joseph T. Szefi, Research Associate tel: (814) 865-1986 email: fxl132@psu.edu email: szefi@psu.edufxl132@psu.edu PSU RCOE Program Review May 3, 2005 S PENNTATE 1 8 5 5

2. Background and Technical Barriers Frequency Range of Interest Ideally, Rigid at Low Frequencies, Soft at High Frequencies Problem: Irritating High Frequency Gearbox Noise Transmitted to Fuselage through Rigid Links Frequency Range of Interest BACKGROUND Helicopter gearboxes transmit irritating high frequency noise and vibration to the cabin (500 - 2000 Hz) Cabin suspended by struts, with quasi- static loads between -10 kN and +30 kN Many active control treatments proposed, some for retrofit –Issues: complexity, reliability, BW Layered isolators exhibit desirable low and high frequency behavior –Passive or semi-active TECHNICAL BARRIERS TO SOLVE At least minimum level of axial stiffness to ensure flight controls integrity Must have a low weight penalty Elastomers must stay in compression Must not exceed size constraints Must be nearly rigid at low frequencies, while attenuating high frequency vibrations (500 - 2000 Hz) BACKGROUND Helicopter gearboxes transmit irritating high frequency noise and vibration to the cabin (500 - 2000 Hz) Cabin suspended by struts, with quasi- static loads between -10 kN and +30 kN Many active control treatments proposed, some for retrofit –Issues: complexity, reliability, BW Layered isolators exhibit desirable low and high frequency behavior –Passive or semi-active TECHNICAL BARRIERS TO SOLVE At least minimum level of axial stiffness to ensure flight controls integrity Must have a low weight penalty Elastomers must stay in compression Must not exceed size constraints Must be nearly rigid at low frequencies, while attenuating high frequency vibrations (500 - 2000 Hz)

3. Quasi-static Stiffness Constraint From literature: –GW = 12,000 lbs, axial stiffness of 14 MN/m at 4 foot locations sufficient to minimize deflections of high- speed transmission shafts Maximum Mass Constraint Sikorsky correspondence –GW = 12,000 lbs, 50 lbs of added weight for high frequency vibration control acceptable Total Isolator weight < 0.5 % GW (soundproof 2-3%) Geometry Constraints Foot-type gearbox mounting –No suspension struts –Assumed d ~ 20 cm, h ~ 10 cm Strut-type gearbox connections –Typically strut diameter, d ~ 8 cm Fatigue Constraints Extensive fatigue tests performed to develop design guidelines for layered elastomeric bridge bearings Design guidelines expressed in terms of shear modulus and shape factor Included as fatigue constraint in layered isolator design Quasi-static Stiffness Constraint From literature: –GW = 12,000 lbs, axial stiffness of 14 MN/m at 4 foot locations sufficient to minimize deflections of high- speed transmission shafts Maximum Mass Constraint Sikorsky correspondence –GW = 12,000 lbs, 50 lbs of added weight for high frequency vibration control acceptable Total Isolator weight < 0.5 % GW (soundproof 2-3%) Geometry Constraints Foot-type gearbox mounting –No suspension struts –Assumed d ~ 20 cm, h ~ 10 cm Strut-type gearbox connections –Typically strut diameter, d ~ 8 cm Fatigue Constraints Extensive fatigue tests performed to develop design guidelines for layered elastomeric bridge bearings Design guidelines expressed in terms of shear modulus and shape factor Included as fatigue constraint in layered isolator design Typical Stiffness at Four Feet k axial = 14 MN/m S-76B Main Gearbox Supports GW = 12,000 lbs  S < 2.00 GS < 12.00 MPa  D < 1.00 GS 20-year-old Elastomer Bridge Bearing in UK  S = Static Stress  D = Dynamic Stress G = Shear Modulus S = Shape Factor Background: Helicopter Gearbox Design Constraints

4. Background: Helicopter Gearbox Design Constraints Brennan, Pinnington, Elliot (UK) report that dominant strut vibration is axial, although lateral component is significant (‘94) Sikorsky developed Active Noise Control system (‘98) –Gearbox connected with ‘feet’ type mounting –Inertial force actuators at gearbox / fuselage connection –Sensors inside cabin –In-flight noise reduced 10-20 dB at primary gear-mesh tone Gembler, Schweitzer (Eurocopter BK117) developed smart strut concept (‘98) –Piezoelectric ceramics bonded to struts –Discrete frequency (1.9 kHz) reduced 11 dB Sutton, Elliot, Brennan, et al., (UK) used three axial magnetostrictive actuators on strut (‘97) –30-40 dB reduction in strut kinetic energy transfer –Practical for frequency range of 250 - 1250 Hz Baz, Pines, (Univ. MD) –Investigating “active periodic struts” –Experimental results suggest average transmitted vibration reduced by factor of 10 in high frequency range Brennan, Pinnington, Elliot (UK) report that dominant strut vibration is axial, although lateral component is significant (‘94) Sikorsky developed Active Noise Control system (‘98) –Gearbox connected with ‘feet’ type mounting –Inertial force actuators at gearbox / fuselage connection –Sensors inside cabin –In-flight noise reduced 10-20 dB at primary gear-mesh tone Gembler, Schweitzer (Eurocopter BK117) developed smart strut concept (‘98) –Piezoelectric ceramics bonded to struts –Discrete frequency (1.9 kHz) reduced 11 dB Sutton, Elliot, Brennan, et al., (UK) used three axial magnetostrictive actuators on strut (‘97) –30-40 dB reduction in strut kinetic energy transfer –Practical for frequency range of 250 - 1250 Hz Baz, Pines, (Univ. MD) –Investigating “active periodic struts” –Experimental results suggest average transmitted vibration reduced by factor of 10 in high frequency range SHEAR FORCES STRUT PZT’s ACTUATORS (three) STRUT Base Structure Piezoelectric Insert

5. Background: Stop Band Mode 1 Mode 2 Mode 3Mode 4 4 - Celled Isolator Transmissibility Experimental Transmissibilities First 4 Axisymmetric Mode Shapes of a 3-Celled Isolator Analytical Approx. Experimental Noise Floor Frequency (Hz) 1 Cell 2 Cells 3 Cells 4 Cells Noise Floor Frequency (Hz) Beginning of Stop Band Beginning of Stop Band End of Stop Band End of Stop Band Layered Isolator Behavior in Compression A cell is an elastomer layer plus metal layer Analogous to multi-stage Predicted high-frequency stop- bands validated experimentally for different numbers of cells (Szefi, Smith, Lesieutre, SDM 2001) Experiments suggest that damping not essential 3 or 4 cells needed in practice A cell is an elastomer layer plus metal layer Analogous to multi-stage Predicted high-frequency stop- bands validated experimentally for different numbers of cells (Szefi, Smith, Lesieutre, SDM 2001) Experiments suggest that damping not essential 3 or 4 cells needed in practice

6. Rigid Link Metal Layer of Layered Isolator Elastomer Layer Stiffness Tuned Mass on Lever Arm Embedded Inertial Amplifier Optimization routine was used to study performance limits of passive layered isolators Passive layered isolators cannot always adequately attenuate vibration at lower frequencies (~ 500 Hz ) given stiffness and mass constraints Embedded fluidic amplifiers effectively amplify inertia between layers Fluid-filled mounts provide an efficient means of inertial amplification Optimization routine was used to study performance limits of passive layered isolators Passive layered isolators cannot always adequately attenuate vibration at lower frequencies (~ 500 Hz ) given stiffness and mass constraints Embedded fluidic amplifiers effectively amplify inertia between layers Fluid-filled mounts provide an efficient means of inertial amplification 10 1 2 3 4 -8 10 -6 10 -4 10 -2 10 0 2 1e2 1e0 1e-8 1e-4 1e-6 1e-2 1,000 100 10,000 10 Minimum Desired Attenuation Level Achieved in Target Frequency Range ( < 0.01) 1 2 3 Tuned Absorber Frequencies Vibration Input Transmissibility Stop Band Begins Too High (~ 700 Hz) Frequency (Hz) Background: Passive Performance Limits of Layered Isolators

7.  Mass Reduced  Frequencies of Interest in the Stop Band  More Compact Design Background: Layered Isolator with Fluidic Amplification Need to reduce isolator massNeed to reduce isolator mass Solution: Embedded Fluidic Motion AmplifiersSolution: Embedded Fluidic Motion Amplifiers Mass amplification due to the presence of inner cylindersMass amplification due to the presence of inner cylinders Lighter than mechanical vibration absorbersLighter than mechanical vibration absorbers Fluid acts as tuned mass of a Vibration AbsorberFluid acts as tuned mass of a Vibration Absorber Need to reduce isolator massNeed to reduce isolator mass Solution: Embedded Fluidic Motion AmplifiersSolution: Embedded Fluidic Motion Amplifiers Mass amplification due to the presence of inner cylindersMass amplification due to the presence of inner cylinders Lighter than mechanical vibration absorbersLighter than mechanical vibration absorbers Fluid acts as tuned mass of a Vibration AbsorberFluid acts as tuned mass of a Vibration Absorber

8. Background: Experimental Results for Layered Fluidic Isolator Tuned Absorber Frequencies Fluid Minimum Desired Attenuation Level in Target Frequency Range (~0.01 ) Config. 1, Fluid Config. 2, Fluid Larger inner diameters Smaller inner diameters Specimen 2, No-Fluid Experimental results are compared for Specimen 2 with no fluid, and Configurations 1 & 2 with fluid Experimental stop band beginning frequency 450 Hz Tuned absorber frequencies around 600, 710, and 1050 Hz As inner diameters increase, beginning frequencies decrease, absorber frequencies decrease However, higher amplification ratios result in reduced attenuation within the stop band Experimental results are compared for Specimen 2 with no fluid, and Configurations 1 & 2 with fluid Experimental stop band beginning frequency 450 Hz Tuned absorber frequencies around 600, 710, and 1050 Hz As inner diameters increase, beginning frequencies decrease, absorber frequencies decrease However, higher amplification ratios result in reduced attenuation within the stop band Experimental Comparison

9. Background: High Force Tension-Compression Testing Experimental results suggest that stop band characteristics are not significantly affected by the presence of high axial loads Installed layered isolators should be stiff enough to avoid driveshaft misalignment Quasi-static stiffness tests reveal that shape factor stiffness prediction method is accurate Experimental results suggest that stop band characteristics are not significantly affected by the presence of high axial loads Installed layered isolators should be stiff enough to avoid driveshaft misalignment Quasi-static stiffness tests reveal that shape factor stiffness prediction method is accurate In practice, layered isolators would be subject to high axial loads Compression and tension May be precompressed to accommodate tensile loading In practice, layered isolators would be subject to high axial loads Compression and tension May be precompressed to accommodate tensile loading Low Frequency Isolators Widely Used in Machinery McGuire, AHS 2003 Agusta A109E

10. OBJECTIVES Investigate the use of periodically-layered mounts for high frequency gearbox isolation (500 - 2000 Hz) –Axisymmetric model to capture transmission of longitudinal waves through isolators Investigate passive, semi-active, and active enhancements to improve isolator performance –Embedded fluidic amplifiers lower stop-band range –Semi-active tunable fluid ports to track disturbance frequencies? –Active piezoelectric stack for improved tonal attenuation in stop band? Validate proposed isolator designs with experiment –Pre-compression, quasi-static axial stiffness –Provide rotorcraft industry with experimentally validated design tools ISSUES Can active enhancement to passive layered fluidic mount improve performance with minimal weight penalty? –Electroactive materials for increased stop band attenuation Implementation in realistic application –Maintain compression –Lateral shear and moments OBJECTIVES Investigate the use of periodically-layered mounts for high frequency gearbox isolation (500 - 2000 Hz) –Axisymmetric model to capture transmission of longitudinal waves through isolators Investigate passive, semi-active, and active enhancements to improve isolator performance –Embedded fluidic amplifiers lower stop-band range –Semi-active tunable fluid ports to track disturbance frequencies? –Active piezoelectric stack for improved tonal attenuation in stop band? Validate proposed isolator designs with experiment –Pre-compression, quasi-static axial stiffness –Provide rotorcraft industry with experimentally validated design tools ISSUES Can active enhancement to passive layered fluidic mount improve performance with minimal weight penalty? –Electroactive materials for increased stop band attenuation Implementation in realistic application –Maintain compression –Lateral shear and moments Objectives, Issues

11. Approach, Accomplishments YEAR 1 Develop axisymmetric model to predict layered isolator behavior Determine isolator design constraints –Quasi-static stiffness, mass, geometry, fatigue Determine performance limits of passive layered isolators –Use design optimization with approximate axisymmetric model Evaluate use of passive or semi-active enhancements to improve performance YEAR 2 Develop enhanced model of layered isolators to include additional components –Embedded fluid elements Validate model via fab & testing of preliminary fluidic layered specimen YEAR 3 Complete testing of preliminary fluidic layered specimen Refine layered isolator design, construct and test new compact device –Accommodate realistic constraints for gearbox isolation –Reduce weight and height of preliminary specimen by half Investigate issues associated with high-force environment –Effects of precompression; validate quasi-static axial stiffness Continue to investigate semi-active or active enhancements YEAR 1 Develop axisymmetric model to predict layered isolator behavior Determine isolator design constraints –Quasi-static stiffness, mass, geometry, fatigue Determine performance limits of passive layered isolators –Use design optimization with approximate axisymmetric model Evaluate use of passive or semi-active enhancements to improve performance YEAR 2 Develop enhanced model of layered isolators to include additional components –Embedded fluid elements Validate model via fab & testing of preliminary fluidic layered specimen YEAR 3 Complete testing of preliminary fluidic layered specimen Refine layered isolator design, construct and test new compact device –Accommodate realistic constraints for gearbox isolation –Reduce weight and height of preliminary specimen by half Investigate issues associated with high-force environment –Effects of precompression; validate quasi-static axial stiffness Continue to investigate semi-active or active enhancements

12. Approach, Accomplishments (continued) YEAR 4 Experimentally evaluate compact layered, fluidic passive isolator –Met performance goals for isolation in specified frequency range –Met realistic constraints of application –Half the weight and height of preliminary specimen Develop concept for active enhancement Use piezoelectric stack to focus on residual tonal disturbances EXPECTED RESULTS AND PRODUCTS Development of a passive layered, fluidic broadband isolator with active tonal enhancement to greatly reduce transmission of high frequency gearbox vibrations to helicopter cabins YEAR 4 Experimentally evaluate compact layered, fluidic passive isolator –Met performance goals for isolation in specified frequency range –Met realistic constraints of application –Half the weight and height of preliminary specimen Develop concept for active enhancement Use piezoelectric stack to focus on residual tonal disturbances EXPECTED RESULTS AND PRODUCTS Development of a passive layered, fluidic broadband isolator with active tonal enhancement to greatly reduce transmission of high frequency gearbox vibrations to helicopter cabins FEEDBACK FROM YEAR 4 “The task made good progress and the interaction with others is good. However, the payoff and real applications are somehow questionable. Work with Lord and using piezoelectric stack are commendable.” ACTION TAKEN RITA project with Bell & Lord initiated, with initial focus on passive mount Active enhancement using piezoelectric stack developed further

13. High-frequency noise with multiple tones (500-2000 Hz) Background: Gearbox Vibration Disturbance is Highly Tonal

14.  Solution: Active Enhancement - Replace the last cylinder link with a piezo stack actuator - No static load - Vibrations already attenuated - Focus on the main tonal disturbance frequencies - Develop a suitable control scheme - Validate with experiments - Consider power and mass penalties Piezoelectric actuator 150 V max, 55 x 20 mm Passive LimitationsPassive Limitations Sometimes impossible to satisfy:- Mass & Size Constraints - Frequency Range of Interest - Attenuation Level Active Isolator Control: Embedded Piezoelectric Stack

15. Features of this problem High frequency range (500-2000 Hz) High amplitude, periodic components Tone frequencies may vary with RPM Feedforward control Measurable disturbance reference input with sufficient propagation delay to output Models of input-output and control-output Slowly-changing Feedback control Non-measurable excitation or rapid propagation Model of control-output sensor Computational delay & model accuracy limit bandwidth Adaptive feedforward control of high-amplitude periodic components Active Control Options

16. Store measured model of the isolator (2 FRFs) FRF1 = Syst_output / Syst_input FRF2 = Syst_output / Piezo_input During each control cycle (100 ms) FFT of Syst_Input to select the main disturbance freqs For each disturbance (frequency) Measure amplitude & phase Determine ideal Piezo_Input control Determining Piezo Control Signal

17. u FFT of Syst_Output at each control cycle u For each disturbance (frequency) –Amplitude & Phase –Modify model (stored FRFs) to improve control F Adapt FRF2 (Syst_output / Piezo_input)  adapted is found via successive increments / decrements that minimize output Amplitude and Phase Adaptation

18. Features – Fast (one cycle every 100ms) – Simultaneous control of multiple disturbance tones – Adaptive Based on experimental models, measured performance Tracks changing freqs, models Active Control Scheme

19. Controller provides: –Continuous multi-channel data acquisition at 12500 Hz –Continuous data output at 12500 Hz –One control cycle every 100 ms F Data acquisition during 80 ms F Processing during 20 ms Experimental Layout

20. –2 layers for simplicity –1 fluidic layer and actuator –13 cm overall length (but design not optimized) –2.5 kg (including actuator & fluid) Fluidic Layered Isolator

21. Experimental Data vs. Theoretical 3-D Model Experimental Data vs. Theoretical 3-D Model Passive Validation

22. Very good coherence Optimum stored models For each tone (600 – 2000 Hz) 40 dB additional reduction Fast convergence to steady state (a few seconds) Low voltage needed for realistic input levels About 2.5 V for 10 g acceleration Fluid amplifies by a factor of 35 Active Validation – With Fluid

23. 3 Tonal disturbances (10 g each) + white noise input Control of 3 disturbances Average 36 dB additional active reduction Active Validation – Effective Transmissibility

24. Active Layered Isolator Stop Band over 600-2000 Hz Passive Reduction of 40 dB Active + Passive Reduction up to 80 dB Active Periodic Strut (Baz, Asiri, Pines) Stop Band over 600-2000 Hz Passive Reduction of 28 dB Active + Passive Reduction of 40 dB - Passive Layered Isolator performs as well as Active Periodic Strut - Active Layered Isolator achieves further reductions at main disturbance tones Note: This isolator has only 2 layers; 3 layers would provide even better results. Comparison to Other Solutions

25. Conclusions Periodically-layered isolators offer a unique solution to the problem of high- frequency gearbox vibration Passively provide attenuation (300-1000) over broad frequency range Add less than 0.5 % GW in mass Accommodates axial precompression Fluidic amplification lowers the stop-band start frequency and improves control Provide broadband passive attenuation over frequency range (500–2000 Hz) Does not compromise axial stiffness Active enhancement improves performance at main disturbance tones Piezo-stack cancels passively-attenuated residual vibration Adaptive feed-forward control tracks frequency and model changes Experiments validated adaptive feedforward control approach 40 dB passive reduction in range 600–2000 Hz 40 dB additional active reduction at each of multiple tones Power required: a few W per controlled tone (realistic input levels) Isolator Size: 2.5 kg and 11 cm length per isolator Lightweight controller & amplifier available About 4 kg and 18 cm for similar performance with pure passive approach Periodically-layered isolators offer a unique solution to the problem of high- frequency gearbox vibration Passively provide attenuation (300-1000) over broad frequency range Add less than 0.5 % GW in mass Accommodates axial precompression Fluidic amplification lowers the stop-band start frequency and improves control Provide broadband passive attenuation over frequency range (500–2000 Hz) Does not compromise axial stiffness Active enhancement improves performance at main disturbance tones Piezo-stack cancels passively-attenuated residual vibration Adaptive feed-forward control tracks frequency and model changes Experiments validated adaptive feedforward control approach 40 dB passive reduction in range 600–2000 Hz 40 dB additional active reduction at each of multiple tones Power required: a few W per controlled tone (realistic input levels) Isolator Size: 2.5 kg and 11 cm length per isolator Lightweight controller & amplifier available About 4 kg and 18 cm for similar performance with pure passive approach

26. Accomplishments Publications 2001 AIAA SDM paper - Developed axisymmetric Ritz-based model of layered periodically- layered isolators in compression 2002 Journal of Sound and Vibration paper - Layered Isolators 2003 AIAA SDM paper, AHS Forum paper - Preliminary fluidic layered specimens 2004 AIAA SDM paper - Design and testing of compact, lightweight fluidic layered specimen 2005 AIAA SDM paper - Design & testing of fluidic layered specimen with active augmentation Planned 2005 Accomplishments RCOE Tasks Investigate best options for active fluidic layered isolator Lightweight powering and control Best combination of passive and active approaches (number of layers, fluidic layers) Validate isolator performance under realistic dynamic and quasi-static loading conditions Axial and lateral forces, and moments Invercon Tasks (RITA project with Bell, Lord) Coordinate with Lord and Bell to pursue design of commercially-viable mount Determine detailed design constraints for Bell Model 427 helicopter Design and fabricate isolators in coordination with Lord Full scale testing at Bell or Lord facility, flight testing Possible future feature on Bell Model 429 Publications 2001 AIAA SDM paper - Developed axisymmetric Ritz-based model of layered periodically- layered isolators in compression 2002 Journal of Sound and Vibration paper - Layered Isolators 2003 AIAA SDM paper, AHS Forum paper - Preliminary fluidic layered specimens 2004 AIAA SDM paper - Design and testing of compact, lightweight fluidic layered specimen 2005 AIAA SDM paper - Design & testing of fluidic layered specimen with active augmentation Planned 2005 Accomplishments RCOE Tasks Investigate best options for active fluidic layered isolator Lightweight powering and control Best combination of passive and active approaches (number of layers, fluidic layers) Validate isolator performance under realistic dynamic and quasi-static loading conditions Axial and lateral forces, and moments Invercon Tasks (RITA project with Bell, Lord) Coordinate with Lord and Bell to pursue design of commercially-viable mount Determine detailed design constraints for Bell Model 427 helicopter Design and fabricate isolators in coordination with Lord Full scale testing at Bell or Lord facility, flight testing Possible future feature on Bell Model 429

27. Plans for 2001 - 2005 Tasks 20012002 2004 2005 STAGE ONE Axisymmetric Isolator Model Experimentally Validate STAGE TWO Characterize Strut Noise Transmission Problem Investigate Semi-Active or Active Configurations Recognized Use of Embedded Fluid Elements Completed 2003 Short Term Long Term STAGE THREE Detailed Isolator Model With Embedded Fluid Elements Experimental Validation Refined Isolator Design Experimental Validation Prestrain, Stiffness Invest. Semi-active, Active Mount Control Architecture Design Specimen Fabrication Experimental Validation INVERCON Design and Fab. Of Specimen Full Scale Testing

28. Technology Transfer and Leveraging Worked with UTRC for 3 years developing layered isolator concept Working with Lord on manufacturing of layered fluidic devices 2004 RITA Project (Invercon, Lord, Bell) Invercon is RCOE spinoff company Fosters technology transfer between RCOE and rotorcraft industry 2004 RITA grant to implement layered isolators in Bell Model 427 helicopter Invercon heading design of preliminary specimens Bell providing Transmission mounting constraints to aid design Interior noise spectrum of Model 427 Lord to fabricate and test preliminary specimens Bell to provide full-scale transmission for experimental validation of layered mount Technology Transfer and Leveraging Worked with UTRC for 3 years developing layered isolator concept Working with Lord on manufacturing of layered fluidic devices 2004 RITA Project (Invercon, Lord, Bell) Invercon is RCOE spinoff company Fosters technology transfer between RCOE and rotorcraft industry 2004 RITA grant to implement layered isolators in Bell Model 427 helicopter Invercon heading design of preliminary specimens Bell providing Transmission mounting constraints to aid design Interior noise spectrum of Model 427 Lord to fabricate and test preliminary specimens Bell to provide full-scale transmission for experimental validation of layered mount Technology Transfer

29. 1-D Model – Consider measurement accuracy If control can achieve: Between 0.25 and 1 degree error in phase Between 0.1% and 1% error in amplitude Simulation predicts 33 – 47 dB reduction at each controlled tone Consistent with experiments Potential Active Enhancement by the control algorithm Simulation of Active Enhancement

30. u Active Layered Isolator u Stop Band over 600-2000 Hz u Passive Reduction of 40 dB u Active + Passive Reduction up to 80 dB u Eurocopter & EADS Active Strut u No Stop Band u No Passive Reduction u Active 11 dB reduction at 1900 Hz (gearmeshing frequency for BK117) Active Layered Isolator Advantages - Passive and active reduction - Multiple tones controlled - Higher reduction - No static loadscarried by actuator - Low power required as the force is already passively reduced Active Layered Isolator Advantages - Passive and active reduction - Multiple tones controlled - Higher reduction - No static loads carried by actuator - Low power required as the force is already passively reduced Comparison to Other Solutions

31. Address additional tones Improve adaptation to increase convergence for imperfect models Additional features Pre-select tones for each helicopter Automatic locking to known disturbance frequencies Recommendations – Active Enhancement