Variable Geometry Chevrons for Jet Noise Reduction

Transcription

1 12th AIAA/CEAS Aeroacoustics Conference (27th AIAA Aeroacoustics Conference) 8-10 May 2006, Cambridge, Massachusetts AIAA Variable Geometry Chevrons for Jet Noise Reduction Frederick T. Calkins * and George W. Butler. Boeing Commercial Airplanes, Seattle WA James H. Mabe Boeing Phantom Works, Seattle WA Boeing is applying cutting edge smart material actuators to the next generation morphing technologies for aircraft. This effort has led to the Variable Geometry Chevrons (VGC), which utilize compact, light weight, and robust shape memory alloy (SMA) actuators. These actuators morph the shape of chevrons on a jet engine fan nozzle trailing edge in order to optimize acoustic and performance objectives at multiple flight conditions. We have completed a flight test of the VGC system on a Boeing ER with GE-115B engines. In this paper we describe the VGC design, development and performance during flight test. We demonstrated autonomous operation of the VGCs, which did not require a control system or aircraft power. The VGC concept demonstrated an exciting capability to optimize jet nozzle performance at multiple flight conditions. The VGC system provided a robust test vehicle to explore chevron configurations for community and shock-cell noise reduction. This capability was demonstrated with two examples of a parametric study which showed the influence of VGC configurations on community noise reduction and shock-cell generated cabin noise reduction during cruise. I. Background Reducing the level of jet noise continues to be a significant environmental problem for the aviation industry. Through the years many methods to reduce noise have been employed resulting in a gradual decrease since the first commercial jets. 1 Chevrons, also known as tabs or serrated edges, are one noise reduction technology that has been explored extensively by The Boeing Company. Chevrons are serrated aerodynamic devices along the trailing edge of a jet engine primary and/or secondary exhaust nozzle. 2 They have been shown to manipulate the engine exhaust flow and reduce both community and shock-cell noise. One source of noise from commercial high-bypass ratio turbofan engines is the turbulent mixing of the hot jet exhaust, fan stream, and ambient air. Chevrons have been shown to greatly reduce jet noise by encouraging advantageous mixing of the streams. Measurements of Overall Sound Pressure Level (OASPL) have shown 2-5 db reduction in the far field noise. 3 To achieve the noise reduction, the secondary exhaust nozzle chevrons are typically immersed into the fan flow. However, this immersion also results in drag, or thrust losses. Since these losses can be a considerable penalty for flights with long cruise times, present state-of-the-art fixed chevron designs are pushed toward little or no immersion. During long flights the interior aircraft noise is an important issue for flight crews and passengers. Shock- cell noise is responsible for a major component of the aft cabin interior noise. It is generated by the supersonic fan and core streams and their interaction with the free stream during climb and cruise. 4 Chevrons reduce shock-cell noise by manipulating the engine exhaust flows at the source of the shock-cell noise. In order to meet contradictory chevron shape requirements, Boeing has applied morphing technology to the fan chevrons. Morphing technologies increase a system s performance by manipulating characteristics to better match the system state to the operating conditions defined by the environment and task. 5 Such a capability allows the chevrons to immerse into the flow to lower noise at takeoff and retract to reduce thrust losses at cruise. In addition, a controllable chevron could be used as a design tool to rapidly and efficiently compare chevron configurations for noise reduction, operability, and performance. Figure 1 shows a schematic of a jet engine with chevrons on the trailing edge of the fan nozzle (thrust reverser translating sleeve). The take-off and cruise conditions are shown as * Senior Engineer, Boeing Commercial Aeroacoustics, Seattle WA 98124, 67-ML, and AIAA Member. Project Engineer, Boeing Commercial Aeroacoustics, Seattle WA 98124, 67-ML, and Senior AIAA Member. Senior Engineer, Boeing Phantom Works Flight Technology, Seattle WA 98124, 4A-51, and AIAA Member. 1 Copyright 2006 by The Boeing Company. Published by the, Inc., with permission.

2 the two limits of the chevron tip motion. Boeing s efforts to realize such a capability resulted in the Variable Geometry Chevron (VGC). The VGC program goals were to raise the technology readiness level of the VGC technology and validate the VGC system concept for noise reduction through a flight test. Boeing employed a multi-disciplinary team to rapidly move a smart material based morphing structure application from concept to flight test in three-and-a-half years. 6 Actuator testing and development led to a wind tunnel test in 2002, which provided proof-of-concept validation. 7 The program then focused on reaching a technical readiness level (TRL) 7 with a flight test as part of the Boeing led Quiet Technology Demonstrator 2 program. Key components of the program included testing and development of a new Nickel-Titanium alloy, 8 smart material and composite modeling and design, composite integration, actuator design and integration, actuator fabrication and processing, control system development, 9 shape sensing, 10 and system qualification for flight test. A second wind tunnel test in 2004 validated the VGC design and control system, 11 paving the way for a flight test in August In section 2 of this paper we present the VGC concept, control system, and operations. Section 3 discusses the calibration testing, autonomous and powered flight test performance, and two test cases demonstrating the VGC s utility for exploring chevron configurations for community noise and shock-cell generated interior noise reduction. Cruise Variable Immersion Take-off Free Stream Fan Stream Primary Flow Figure 1. Jet engine with Variable Geometry Chevron mounted on the trailing edge of the fan nozzle. II. Variable Geometry Chevron A. Overview Variable Geometry Chevrons utilize compact, light-weight, robust, thermally-activated shape memory alloy actuators to morph a chevron to achieve acoustic and performance objectives. Shape Memory Alloys (SMA) convert thermal energy into mechanical energy by way of a thermally induced micro-structural change in the material. 12 Previous work demonstrated that thermally activated shape memory alloy actuators were ideal for the VGC concept. SMAs have high energy density and can produce excellent dynamic strain capability under large working stresses which allows them to deform a stiff aerodynamic surface. This program pioneered the use of monolithic 60-Nitinol, a nickel-rich Nitinol alloy with 60% Ni, 40% Ti by weight, rather then the more common binary NiTinol (55- Nitinol), as actuators for aerospace applications. 8,13 While in the past a majority of the SMA based concepts and designs for morphing structures have used wire actuators, the simplicity of the monolithic flexure actuator is a great advantage. It has a very small part count and provides a simple, low-profile method of connecting to and deforming the substrate structure. Most importantly SMA actuators can be thermally activated by the change in ambient temperature between take-off and cruise, thus enabling autonomous operation. 2

3 Actuator + Spring Assembly Cover plate Attach Fasteners 60-Nitinol Actuator with heater Base Chevron Composite Laminate NiTinol SMA Composite substrate Free Stream Figure 2. Assembly of VGC showing key components. Fan Stream The basic design of the VGC embedded low profile SMA actuators within a conventionally shaped chevron, with the same form factor as current fixed chevrons of interest. The SMA actuators are mated with the stiff chevronshaped substrate forming the functional VGC as shown in Figure 2. The chevrons had to be individually controlled and able to change immersion to produce a variety of configurations. According to preliminary specifications, the VGCs were required to produce a dynamic tip motion of over 0.9, from 0.3 into free stream to 0.6 into fan stream. We needed to monitor all tip immersions to ensure no failures and to gauge performance of the configurations. Key features of the VGC design effort included SMA actuator design, substrate design, system component integration, and feedback control. An individual chevron contained actuators, heaters, temperature and strain sensors, a cover, and associated wiring. Each VGC included three SMA actuators fastened to a stiff carbon fiber composite substrate integral to and extending from the production thrust reverser. The SMA actuators were attached to the substrate with bolts through two threaded holes along the centerline of the length. Thin film heaters were mounted on each actuator to control the temperature and hence the shape of the chevron. A flexible cover protected the actuators and wiring on the free stream side. The cover was free to move in-plane relative to the substrate via sliding connectors and a conformable rubber seal. The substrate s surface strain was measured at 3 locations and the strain measurements were correlated with the chevron s shape to provide real time tip position data. Three thermocouples monitored the temperature of the actuators and substrate. The full flight design included 14 chevrons integrated into the trailing edge of a GE115B thrust reverser s acoustic panel as shown in Figure 3. The thrust reverser was identical to production versions with the additional serrated shapes extending from the trailing edge. It was fabricated using production tooling. The sensor wires passed through a conduit in the acoustic panel to the fan case and were then routed to a data acquisition system. Heater wires were passed down the same conduit, through the fan case, wing, and into the cabin. A control computer, health monitoring computer, and heater power supplies were located in the cabin. The data acquisition and control system communicated with the data acquisition unit on the engine, the heater power supplies, and the airplanes flight data system. 3

4 Figure 3. a. VGC with covers off showing 3 Ni-Ti actuators per VGC and b. mounted on GE-115B ready for flight. B. Controls Individual control of the VGC shape, quantified as the tip immersion into the fan stream, was one the system requirements. SMAs have a complex, non-linear, and double valued (hysteretic) relationship between strain (shape) and temperature. By closing the feedback control loop on tip displacement rather than temperature, the positiontemperature hysteretic non-linearities are transparent to the controller. Data from the validation tests described in section 3.2 was used to correlate measurements from three surface mounted strain gages to the tip immersion using a linear regression model. A proportional-integral (PI) feedback control system controlled the tip immersion of each VGC by minimizing the difference between the control set point and the tip immersion estimate. The control system set the input signal to the heater power supplies, which regulated the heaters on each of the SMA actuators. The controller was stable and robust and performed very well during testing. In addition, it did not require a complicated and difficult to determine VGC model. Figure 4 shows the tip immersion estimated and commanded (set point) signals versus time for one representative VGC, demonstrating control authority during changes to the set point and changes in external conditions seen as spikes in the estimated data. 0.8 Estimated Command Tip Immersion (in) Time (min) Figure 4. Feedback control system performance command and estimated tip immersion versus time. C. Operation The VGC system has autonomous and powered modes of operation. In both modes the SMA actuators are activated by thermal changes, driving a chevron shape change. The autonomous mode relies on changes in environmental and fan exhaust temperatures to heat and cool the actuators. It requires no internal heaters, wiring, control system, or sensing. By design this provided one tip immersion at take-off and another during the cooler 4

5 cruise state. For example, at take-off the chevron configuration could be optimized to reduce community noise and during cruise a second configuration could provide efficient operation, i.e. reduce specific fuel consumption. During powered mode, a control system manages internal heaters to regulate the actuator temperature and corresponding VGC shape. The control system was used to continuously vary the chevron shape between the actuation limits. This provided the ability to set and hold a chevron configuration of interest, facilitating a parametric study of the effect of chevron configurations on noise measurements. This highlights a useful feature of VGC technology. It can assist conventional component design by allowing multiple hardware configurations to be tested during a single flight test. This is a significant advantage for chevron design since testing multiple fixedchevron configurations can be problematic. In the next section the use of the VGC to compare chevron configurations for community and shock-cell noise reduction will be explored. III. VGC Performance A. Overview In August 2005 Boeing tested a number of noise reduction technologies on an All Nippon Airway (ANA) ER, including the VGC thrust reverser translating sleeve on a modified commercial GE-115B engine. The VGC system was tested on six flights during five days with a total of three different engine configurations. The controlled operation of the VGCs allowed us to examine nine different chevron configurations for both community noise and shock cell noise reduction. During the flight tests, we demonstrated autonomous operation and individual control of the 14 VGCs. The system was able to smoothly and quickly move between immersion configurations at cruise conditions, allowing us to perform a parametric study on chevron shapes for shock cell noise reduction. Test configurations included both uniform and azimuthally varying immersion configurations. All instrumentation, power, gages, sensors, and controller hardware and software worked perfectly throughout the testing. The VGC performance is analyzed by considering the VGC thermal and mechanical performance, in-flight photogrammetry of the 3D shape change, and noise measurements at different operating conditions. The VGC immersions were controlled and the system performance, strain gages and thermocouples, monitored on-board in real time. The computer system was tied to the flight computers providing access to the flight and engine conditions. VGC tip immersions, estimated from a linear regression model using three strain gage measurements, provided a performance metric. Photogrammetry measurements using a camera mounted on the belly of the airplane were recorded for three of the VGCs using a separate data acquisition system. Figure 5 shows the VGC thrust reverser mounted on the GE-115B engine operating during an engine ground test and during flight. The photogrammetry targets are mounted on the dark strips (seen in Figure 5b) and show up as bright spots when illuminated (Figure 5a). Both community noise and interior noise measurements were made for various VGC configurations. Noise measurements were taken on the ground using several different arrays and on the airplane using microphones inside the cabin and externally mounted kulites. Figure 5 a. Engine ground test with Photogrammetry targets and b. the VGC operating in flight. 5

6 B. Calibration and system test System validation and calibration was crucial to the flight test success. A full system test, including the VGC thrust reverser sleeves, control and health monitoring computers, power supplies, and all cabling, was performed prior to delivery of the system to the airplane. A four camera photogrammetry system, shown in Figure 6, was used to measure each VGC s shape under simulated aero-loading and thermal conditions. A cable system was used to hang weights from each VGC to simulate aerodynamic loading while in flight. One thrust reverser at a time was placed inside a chiller box, exposing the free stream side of the VGCs to temperatures down to -40 C. This simulated the thermal conditions expected during cruise operations. Data from the strain gages were taken at 70 different thermal and mechanical loading conditions and used to develop the strain-tip immersion model needed for the control system. Tip immersions from -0.3 to 0.6 relative to a line tangent to the nozzle outlet were measured, with positive values defined as into the fan stream. The VGCs were exercised from -40C to 80C, well beyond the operating temperature range (15º C to 60º C), Three thermocouples were used to monitor the temperature of the actuators and substrate for each VGC. This validation test provided the initial system calibration. A four camera photogrammetry system provided the final calibration check during the engine ground run up test with the VGC thrust reversers mounted on the engine as seen in Figure 5a. The success of the calibration plan is seen in Figure 7 which compares the time traces of one VGC s tip immersion estimate and the in-flight photogrammetry measurement. Photogrammetry Cameras Targets Chiller Figure 6. System validation and calibration test using a four camera photogrammetry system and chiller box Tip Immersion (in) Photogrametry -0.2 Estimate Time (min) Figure 7. Comparison of in flight tip estimate and photogrammetry measurement. 6

7 C. Autonomous Operation Autonomous operation was demonstrated in two parts. First, during take-off the hot fan flow heated the SMA actuators resulting in an immersed VGC tip displacement of approximately 0.3 within 500 seconds of the engine being turned on (left side Figure 8). While a future commercial system would be designed such that the VGCs are fully immersed prior to take off, the flight test thermal design enabled control during take-off. The second part of the autonomous operation was VGC tip retraction during cruise. In this case the cooler temperatures at high altitude (35,000 feet) cooled the SMA actuators below martensitic transformation temperature resulting in a second VGC shape designed to optimize cruise performance. This transition, as shown on the right side of Figure 8b, from 0.8 to immersion took approximately 600 seconds. Data from one representative VGC shows the three dimensional shape change from a maximum immersion take-off configuration to the final cruise configuration (Figure 9). A contour plot of the two shapes is also presented. Figure 8. Autonomous operation at take-off (left), cruise (right), a. engine setting, b. VGC tip immersion, c. VGC temperature, and d. Air temperature versus time. C. Controlled Operation The ability to test multiple chevron configurations during a single flight was a great benefit to the test program and provided data for future chevron design efforts. The configuration flexibility, controllability, and ability of the system to rapidly change chevron configurations were demonstrated. Both uniform and non-uniform immersion configurations were tested. During one flight the ER was flown at various altitudes with a variety of engine settings while the VGCs were individually controlled to immersion settings of interest. Data for 14 VGC tip immersions from one flight test are shown in Figure 10. The VGC tip immersions are controlled to a uniform -0.1, 0.0, 0.4, 0.6 immersion into the fan flow, and then to a prescribed non-uniform distribution. The data spread shows the VGC tip distribution with an average standard deviation across all conditions of This spread was a result of the estimate error which was corrected during post test processing of the data. Perturbations in the data correspond to an airplane maneuver or changes in the engine setting. The controller handled the disturbances caused by changes in both the system set point and engine setting. The data also shows the effect of increasing aero-loads on the VGCs resulting from increasing engine RPM. This decreased some of the VGC tip immersions when actuator authority was insufficient to overcome aero-loading at high engine settings (see point A Figure 10). The actuator authority was a function of the actuator and substrate design and not a limitation of the technology. 7

8 Cruise Take-off Figure 9. 3D and contour plot of representative VGC take-off and cruise shape (all units in inches). Tip Immersion (in) Seconds Figure 10. In flight VGC performance and engine setting during cruise. The 14 VGC tip immersions vs time for uniform immersions controlled to -0.1, -0.0, 0.4, 0.6, and a non-uniform distribution. 8 Strain 0.6 Temperature D. Community Noise Testing The sensitivity of jet engine generated community noise reduction to chevron configurations was explored during three days of flight tests. A centerline array was used to measure the far field noise at various angles relative to the aircraft during take-off. 15 Two uniform immersion chevron configurations, of nominally 0.3 and 0.6 tip immersions, were tested in succession as shown in Figure 11a. The perturbations in the time traces of the VGC tip immersions are multiple take-off runs at various engine power settings. Figure 11b shows the relative distribution of A Engine (% RPM)

9 the chevron tips for the two configurations. Figure 12a shows the average immersion over the time of the noise flyover. Far field noise measurements were made using center microphones for the baseline (no chevrons), 0.3 uniform immersion configuration, and 0.6 uniform immersion configuration. Figure 12 shows aft arc power levels versus frequency calculated using the method described by Nesbitt et al. 14 The higher immersion configuration appears to have reduced the noise over the low frequency range compared with the baseline and lower immersion configuration. This is preliminary data and further analysis is necessary to understand the test to test variation. However, the value of testing two widely varying chevron configurations during the same test flight is significant, since day to day variations can be reduced. 0.8 Tip Immersion (in) Time (min) L1 G4 L2 G4 L3 G4 L4 G4 L5 G4 L6 G4 L7 G4 R1 G4 R2 G4 R3 G4 R4 G4 R5 G4 R6 G4 R7 G4 Immersion (in) Time Relative to Overhead Station (s) Figure 11 a. Time traces of 14 chevrons tip locations versus time during one flight test, b. Comparison of average immersion during noise flyover. Figure 12. Noise results comparing baseline, low (0.3 ), and high (0.6 ) immersion. E. Shockcell Noise Testing A parametric shock-cell noise study was completed for the uniform immersion chevron configurations shown in Figure 10. The influence of the chevron configuration on interior noise was measured by Kulite transducers mounted on the aft external surface of the airplane and with microphones inside the cabin, as shown in Figure 13. The Overall Sound Pressure Level (OASPL), integrated over 1/3 octave frequency bands, was computed. The difference between OASPL for a baseline (production engine) configuration and a given chevron configuration is 9

10 plotted versus engine setting (Nozzle Pressure Ratio, NPR) in Figure 14. The delta OASPL increases from 0.0, 0.2, 0.4, to 0.6 immersion. This means that the low frequency noise levels decrease as immersion increases. The presence of the chevrons without immersion (0.0 ) shows a large change from the baseline with relatively smaller changes between 0.2 and 0.4 immersion. There is another large OASPL reduction at 0.6 immersion. This is an excellent example of the use of the VGC as a chevron design tool. Testing multiple chevron configurations during one flight reduced some of the test variables, such as flight-to-flight or day-to-day variations in conditions. This testing capability was a great benefit to the test program and provided data for future chevron design efforts. For more information on the interior noise measurements during the flight test and the effect of chevron immersions see the references 15. Exterior Measurement Interior Measurement Shockcell Array 83 Kulites Figure 13 a. Externally mounted kulite sensors for shockcell noise measurement and b. microphones for internal cabin noise measurement OASPL Reduction from Baseline Nozzle Pressure Ratio Figure 14. Reduction in Overall Sound Pressure Level from baseline at different engine settings for four increasing uniform immersion configurations. IV. Conclusion The Variable Geometry Chevron program has been an unqualified success. Boeing has completed the first flight test of a shape memory alloy actuator used to morph a commercial aircraft structure. This VGC system demonstrated the successful use of smart materials to solve an aerospace problem and provided previously unavailable capability for a flight test, greatly enhancing the flight test program. Rapid system development allowed the VGCs to be taken from concept to flight test in three-and-one-half years. One of the keys to the VGC s successful performance was the pioneering use of a new shape memory alloy, 60-Nitinol, as actuators. The program was the first to exploit the unique shape memory properties of 60-Nitinol and model, design, and test monolithic 60- Nitinol actuators. The VGC system demonstrates a new capability for validating designs with flight tests. With variable geometry technology, a component can be tested in numerous configurations during one flight test. This 10

11 was demonstrated community noise and interior noise tests comparing the noise reduction for different chevron configurations. Finally variable geometry technology demonstrates new and as yet unexplored capability to optimize aircraft performance for multiple flight conditions. Designers can use this technology to optimize an aircraft component for different flight conditions rather than being limited to a compromise design point. We see this program as an important step in the utilization of smart material-based morphing structures for commercial aircraft. Acknowledgments The authors would like to thank Eric Nesbitt, Michael Czech, and Eric Bultemeier for their contributions to the VGC performance analysis. An extremely large team made this test possible including Mike Lallement and his nacelle team from Spirit Aerospace (formerly Boeing Wichita), Dan Outlaw, Kevin Arent, and Darin Welsh and their Boeing Commercial Test Flight team, and Darin Arbogast, Bob Ruggeri, Mark Herrin and the Boeing Phantom Works Flight Technology team. References 1 C. W. Tam, Jet Noise: Since 1952, Theoretical and Computational Fluid Dynamics, Vol 10, pp , J. Bridges, M. Wernet, C. Brown, Control of jet noise through mixing enhancement, NASA/TM , O. Rask, E. Gutmark, S. Martens, Acoustic investigation of a high bypass ratio separate gloe exhaust system, AIAA , 42nd AIAA Aerospace Sciences Meeting, Reno, NV, Jan T. R. Bhat, U. W. Ganz, A. Guthrie, Acoustic and flow field characteristics of shock-cell noise from dual flow nozzles, AIAA , 11th AIAA/CEAS Aeroacoustics Conference, Monterey, CA, May J. Bowman, B. Sanders, T. Weisshaar, Evaluating the impact of morphing technologies on aircraft performance, AIAA , 43rd AIAA Structures, Structural Dynamics, and Materials Conference, Denver CO, April E. Nesbitt, G. Butler, D. Reed, The Boeing Company, Deployable segmented exhaust nozzle for a jet engine, US Patent 6,718,752, April F.T. Calkins and G.W. Butler, Subsonic Jet Noise Reduction Variable Geometry Chevron, 42nd AIAA Aerospace Sciences Meeting and Exhibit, AIAA , Reno NV, January D.J. Clingman, F.T. Calkins, and J.P. Smith, Thermomechanical Properties of 60-Nitinol, SPIE Smart Structures and Materials 2003, SPIE-5053 p , San Diego CA, March R. Cabell, N. Schiller, J. H. Mabe, R.T. Ruggeri, and G.W. Butler, Feedback Control of a Morphing Chevron for Takeoff and Cruise Noise Reduction, Active 2004, Williamsburg VA, September S. Klute, R. Duncan, R. Fielder, G. Butler, J. Mabe, A. Sang, R. Seeley, M. Raum, Fiber-optic shape sensing and distributed strain measurements on a morphing chevron, AIAA , 44th AIAA Aerospace Sciences Meeting, Reno, Nevada, January J. Mabe, R. Cabell, G. Butler, Design and control of a morphing chevrons for takeoff and cruise noise reduction, AIAA , 11th AIAA/CEAS Aeroacoustics Conference, Monterey CA, May R. C. Smith, Smart Material Systems: Model Development, Chapter 5, Frontiers in Applied Mathematics 32, SIAM, Philadelphia, G. Julien, Nitinol Technologies Inc., Manufacturing of Nitinol parts and forms, US Patent 6,422,010, July 23, E. Nesbitt, V. Mengle, B. Callender, R. Thomas, M. Czech, Flight Test Results for Uniquely Tailored Propulsion- Airframe Aeroacoustic Chevrons: Community Noise, AIAA , 12th AIAA/CEAS Aeroacoustics Conference, Cambridge MA, May E. Bultemeier, U. Ganz, E. Nesbitt and J. Premo, Effect of Uniform Chevrons on Cruise Shockcell Noise, AIAA , 12th AIAA/CEAS Aeroacoustics Conference, Cambridge MA, May