Improved High Frequency Dynamic Airframe Loads and Stress Prediction

Transcription

1 Improved High Frequency Dynamic Airframe Loads and Stress Prediction Phillip Lang Technical Fellow The Boeing Company, Philadelphia, PA Dr. Louis Centolanza Aerospace Engineer US Army Aviation Applied Technology Directorate (AATD), Ft Eustis VA Abstract As part of the Aviation Applied Technology Directorate (AATD) Light, Affordable and Durable Structures Program, a new high cycle internal loads method has been developed that increases the accuracy of predictive methods over existing techniques. Using the new methodology, critical airframe components are identified and highly detailed break-out models of these sub-structures are created. Motions at the points on these critical structural elements where they join the larger airframe are calculated using a new method that couples the full aircraft NASTRAN model and the comprehensive rotor analysis model (RCAS) in a unique fashion and eliminates necessary, but inaccurate, assumptions. The motions at the points of these critical structural elements are then applied to the break-out models as boundary conditions for calculation of the internal loads and stress. In addition, methodology has been developed to help guide the development of newly designed aircraft by using the full airframe model to identify local sub-structures that are candidates for break-out analysis. Introduction The ability to accurately predict high cycle stresses in primary structure during airframe development is critical for accurate evaluation of the structural integrity of all rotorcraft. Current tools and methodologies lack the required level of fidelity and validation to allow their results to be confidently relied on for certification of new and modified airframe structures. As a result, much time and money is invested in certification testing, both on the ground and in the air. Increasing the accuracy of the predicted internal loads and the reliability of the fatigue methods is highly desirable as a means of reducing the certification testing required. enough fashion to support design. The level of fidelity of a typical airframe internal loads NASTRAN model has increased greatly in recent years, resulting in improved predictions of static internal loads. However, the models are still not sufficiently detailed to accurately predict dynamic internal loads at frequencies of n/rev and higher (where n is the number of blades). Figure 1 shows a comparison of the existing method of calculating n/rev airframe loads with an internal loads NASTRAN model to flight test data. Notice that both the magnitude and the airspeed trend are not properly captured. Analytical capabilities have significantly improved with the advancements in computing performance and finite element modeling capability. These improvements have allowed for increased fidelity analysis models that are yielding more accurate analytical predictions in a timely Presented at the American Helicopter Society 62nd Annual Forum, Phoenix, AZ, May 9-11, Copyright 2006 by the American Helicopter Society International, Inc. All rights reserved. Figure 1: Compared To Flight Test

2 An increased level of model fidelity is required to capture the local structural details that are important for the accurate prediction of high frequency internal loads and the resulting stress concentrations. This requirement to provide a significantly increased level of modeling detail and still provide timely design support is the focus of the new method. Global to local finite element modeling methods are coupled to comprehensive rotor analysis codes in a new way and improved internal load and stress predictions result. Methodology The new method to calculate high frequency airframe loads and stresses uses global/local techniques typically applied to static loads analysis, modified for use with dynamic analysis models. Figure 2 shows an overview of the process. The method has four major parts as follows. 1. Calculate airframe natural frequencies and mode shapes 2. Calculate sub-structure boundary motion 3. Create detailed sub-structure finite element model 4. Calculate internal loads and stresses Calculate Airframe Natural Frequencies and Mode Shapes The first step of the process involves using an airframe NASTRAN model to calculate natural frequencies and mode shapes that will be used to calculate the substructure boundary motions. The airframe model is configured to the gross weight configuration desired, and mode shapes and frequencies are calculated at the rotor hubs and at the boundary locations of the substructure to be analyzed. The rotor mass properties must not be included in the airframe NASTRAN model because of rotor / fuselage coupling considerations described in the next section. The model used for validation purposes is the Chinook model shown in figure 3. This NASTRAN model was originally developed as part of the NASA Design Analysis Methods for Vibrations (DAMVIBS) Program in the mid 1980 s (Reference 1) and has been continually updated over the years. Calculate airframe natural frequencies Extract sub-structure mode shapes RCAS Rotorcraft Comprehensive Analysis System Calculate substructure boundary motions Enforce boundary conditions on break-out model Figure 2: Methodology flow Calculate internal loads

3 NASTRAN model as a real number for normal modes analysis. This leads to further inaccuracies in the NASTRAN calculated response. The RCAS model is setup for the appropriate aircraft structural configuration and the desired aircraft trim condition. The sub-structure boundary motions are calculated for the trimmed condition and saved as complex displacements at the desired rotor harmonics, 3/rev and 6/rev in the Chinook case. Create detailed sub-structure finite element model Figure 3: CH-47D NASTRAN Model The model was used to develop the successful 3/rev and 6/rev airframe vibration-reducing stiffening that is now in place on the US Army s CH-47F, MH-47G and all newly built Chinooks (Reference 2). This NASTRAN model was also the foundation for the development of the NASTRAN mo dels used to design the CH-47F New Build and the MH-47G airframes. The model correlates well to ground shake test data, with mode shapes and frequencies showing good correlation up through the 6/rev forcing frequency (22.5 Hz). The local sub-structure finite element model is created directly from CATIA representations of the airframe. The CATIA model is imported into PATRAN to create detailed finite element models. For aircraft with sheet metal frames like the CH-47D, every part of the built up structure is included in the break-out model, with each riveted connection modeled as a point-to-point connection (NASTRAN MPC) between members. The models are created using typical 2D plate (NASTRAN CQUAD4 and CTRIA3) and 1D bar elements (NASTRAN CBAR). The model, as shown in figure 4, contains 48,712 elements (predominantly 2D shell elements approximately 0.2 inches x 0.2 inches square) and 51,403 nodes. Calculate sub-structure boundary motion Next, the mode shapes and natural frequencies are used as input to RCAS for calculation of the sub-structure boundary motions. RCAS (Rotorcraft Comprehensive Analysis System) is a comprehensive rotorcraft code and has been used extensively at Boeing for numerous aircraft level analysis of many aircraft, including the V- 22, Chinook and RAH-66. It is maintained by the U.S. Army Aeroflightdynamics Directorate and is used by the government, industry and academia. Under a previous US Army Contract, Boeing developed and correlated a Chinook RCAS model, and has used the model for numerous other analyses. As mentioned in the previous section, the rotor mass properties must not be included in the NASTRAN model. The RCAS model includes all structural, mass and aerodynamic properties of the rotor system. RCAS combines this rotor model with the NASTRAN modal data and results in a correctly coupled rotor / fuselage solution. If the rotor mass were included in the NASTRAN analysis, it would be accounted for twice and the results would be incorrect. Further discussion of the rotor/fuselage coupling problem is presented in Appendix A. In addition, the effective rotor mass is a complex term that must be input to the airframe Figure 4: Break-out Substructure Model

4 Calculate internal loads and stresses Next, the RCAS calculated boundary conditions are applied to the detailed break-out model and internal loads and stresses are calculated. As mentioned above, the RCAS results are saved as complex numbers. These data are imported into MSC/PATRAN and used as boundary conditions for static analysis. Two static load cases are setup for each aircraft trim case; one for the real part of the complex solution and one for the imaginary part of the solution. When the static solution is complete, the real and imaginary results are combined for a magnitude and phase solution that can be compared to flight test results. The boundary conditions for the detailed analysis must undergo a global to local translation. As shown in figure 5, the RCAS results are calculated using the coarsely meshed airframe model (global model) and must be expanded to match the finely meshed substructure model (local model). This is accomplished using linear interpolation and extrapolation in PATRAN using basic PATRAN functionality (reference 3). Both rotations and translations are mapped from the global model to the local model. Comparison to Flight Test The Chinook aft fuselage station 594 frame was selected as the sub-structure for modeling, analysis and methodology verification for this project. The frame, shown in figure 6, was chosen because of the availability of test data and the frame s history of inservice repairs. Harmonic strain gage data collected in CH-47D flight tests performed in 2002 was extracted from Boeing s flight test database for the gages on the station 594 frame. Figure 7 shows the location of the three strain gages on the LH station 594 frame (RH gages opposite). Note that two of the gages are on the frame outer cap and one the inner cap. A photo of the one of the strain gages and its location on the detailed finite element model is shown in figure 8. Aircraft trim data was also extracted from the CH-47D flights for use in the RCAS analysis. The detailed model of station 594 frame was loaded as described and internal 3/rev loads and stresses were calculated. A typical contour plot of the resulting stresses is shown in figure 9. Global Model Local Model Figure 5: Global to Local Boundary Condition Extrapolation

5 Figure 6: Chinook Station 594 Frame Outer Frame Cap Figure 8: Strain gage Figure 7: Station 594 Strain Gages The results (3/rev magnitude and phase) for the three strain gages on the LH station 594 frame are shown in figures 10, 11 and 12. Three sets of data are shown on each plot: 1) flight test data from CH-47D test, 2) internal stress calculated with the airframe NASTRAN model using existing methods, labeled Existing Method and 3) internal stress calculated using the new methodology, labeled New Method. The Existing Method results are calculated using the global airframe model with an effective rotor mass and vibratory hub loads calculated with RCAS. Figure 9: Typical Stress Contour

6 As shown, the new method results in significantly better correlation for two of the three strain gage locations. The magnitudes at the high airspeeds are better predicted, and the general trend vs. airspeed is more accurate for all gages. The existing method does not show the large magnitude increase at higher airspeeds. The test data shows a sharp phase angle change between 100 and 120 knots, which is not completely captured by the analysis, though somewhat better with the new method. Note that because this flight test program did not have rotating (rotor) system instrumentation, making comparisons of absolute phase angle between test and analysis is not possible. However, the relative phase trend comparison over the airspeed range is valid. Overall, the new method shows significant improvement over the existing method. Phase Datacode: New Method -180 Figure 12: LH Sta 594 Inner Cap (54102) 3P Stress Datacode: New Method Phase Note that in the airspeed range of 100 to 120 knots, the analysis shows a slight hump in the trend vs. airspeed that is not present in the test data. The RCAS model used for the analysis incorporates a prescribed wake model throughout the airspeed range. To study the impact of the wake modeling, the RCAS wake model was changed to a free wake and the boundary motions re-calculated. As shown in figure 13, the free wake model improves the correlation with test data. Imp rovement in the aerodynamic modeling in RCAS should further improve test correlation and is an area recommended for future work. Figure 10: LH Sta 594 Outer Cap (42801) 3P Stress Magntiude 180 Datacode: New Method Datacode: Prescribed Wake Free Wake Figure 13: Impact of RCAS Wake Model 90 Phase Figure 11: LH Sta 594 Outer Cap (54101) 3P Stress The right hand station 594 frame was also analyzed, and the results are simi lar to the left hand side. Figure 14 shows the results at one of the strain gages. As on the left hand side, two of the three strain gages show significant improvements in correlation with flight test data using the final methodology. In addition, the same local increase in the 100 to 120 knot range is predicted on the right hand side with the prescribed wake model.

7 Datacode: breakout analysis is required. The airframe NASTRAN model is proposed for this task. Phase New Method Figure 14: RH Sta 594 Inner Cap (64102) 3P Stress The analysis was repeated at 6/rev for the LH station 594 frame. The 6/rev boundary motions were applied to the break-out model of the LH frame and the stresses were calculated and compared to flight test. The results for one of the three strain gages is shown in figure 15. Overall, the correlation with test is not as good as at 3/rev, as expected, but the levels are reasonable. More work is needed to improve both the airframe and the RCAS models at these higher frequencies. Using the standard NASTRAN frequency response solution, many different output types were studied and compared to test data. Among the output quantities evaluated were: ele ment forces, mpc forces, element strain energy, element stress, element strain and grid point displacements. The output quantities all point to the same general areas of the airframe as potential candidates for break-out analysis. The one which shows the best overall match to service history is the element strain energy as shown in figures 16 and 17, where the strain energy in the primary structure at 3/rev and 6/rev is compared to field repair data collected by the US Army during a 10 year period in the 1980 s. The vertical bars represent the number of occurrences of repairs in that area of the fuselage. 3P Strain Energy Contour Plot on Airframes at 151 Kts Compared to Airframe Crack History Fwd Pylon Concentrations Cabin Concentrations Aft Fuselage Concentrations Datacode: New Method Number of Repairs Phase to to to to to to to to to to to to to to to 450 Airframe Station Number Figure 16: 3P Airframe Strain Energy Compared Field Service Data, 151 knots 451 to to to to to to P Strain Energy Contour Plot on Airframes at 151 Kts Compared to Airframe Crack History Aft Fuselage Concentrations Figure 15: LH Sta 594 Outer Cap (54101) 6P Stress Methodology for a New Aircraft From the results discussed above, it is clear that the increased detail of the breakout model, in combination with RCAS, leads to an improved vibratory stress prediction. In the case of the CH-47D, it is also clear which frames or sub-structures would require this level of detailed analysis based on the aircraft s service record. However, for a new airframe the service information needed to make this decision is not available and some other means of focusing the 5 to to to to to to to to to to to to to to to 450 Airframe Station Number Figure 17: 6P Airframe Strain Energy Compared Field Service Data, 151 knots 451 to to to to to to 630 Number of Repairs

8 As shown in the figures, the analysis predicts high concentrations of strain energy in the structures in the forward pylon, aft cabin and aft fuselage areas at 3/rev and in the aft fuselage frames at 6/rev, particularly in the station 482 and 502 frames. These concentrations correlate well to the field service data, which show a high frequency of repairs at the same locations. The strain energy concentrations also make sense when the natural frequencies of the airframe near 3/rev and 6/rev are considered. Near 3/rev, the Chinook has two modes that are mainly forward pylon modes, but also have a significant amount of engine and aft fuselage motion. Near 6/rev, the main modes of interest are engine pitching and rolling modes and drive most of the vibratory loads in the aft fuselage. Other NASTRAN output types considered, such as stress and strain, also point to the same general areas of the airframe as places where the detailed breakout analysis might be required. Using these outputs in conjunction with the element strain energy and an understanding of the modal characteristics of the airframe, critical sub-structures can be identified for detailed analysis. This analysis shows that standard NASTRAN output types can be used as a guide for analysts working on a new helicopter program. Conclusions 1. Modeling airframe sub-structures (station frames, etc.) as local detailed break-out models increases the ability to accurately predict high frequency internal loads and stresses. Using the detailed models, local deformations that are important for an accurate stress prediction are included in the calculation. 2. The coupled rotor/fuselage problem is best handled by including the rotor mass in the rotor analysis (RCAS) model only. The airframe model should include no rotor mass for airframe natural frequency calculation. 3. The frame boundary motions, used as input to the detailed sub-structure model, are most accurately calculated in the rotor analysis program (RCAS). Airframe mode shapes at the frame boundary are calculated using the airframe NASTRAN model and included in the RCAS model. 4. NASTRAN element strain energy, calculated using the rotor 3/rev and 6/rev hub loads as input, is effective to guide analysts on a new airframe project to identify candidate sub-structures for breakout analysis. Appendix A The Coupled Rotor/Fuselage Problem The body of work on this project focused on the use of detailed breakout finite element models to improve high frequency loads predictions. As discussed above, it was found that the RCAS/NASTRAN coupling method is very important in order to calculate reliable high frequency loads. It was found that using an effective hub mass in the NASTRAN forced response analysis does not produce accurate results if the effective hub mass is also included in the NASTRAN modal data used for hub loads calculation in RCAS. Figure 18 compares the results attained with and without the effective rotor mass included in the airframe model. Note that the magnitude of the response and the trend versus airspeed are much better when the effective mass is included only in the RCAS model. Therefore more of the analysis was moved to RCAS and results improved. However, RCAS is much more computationally intensive and it is desirable to do more of the analysis in NASTRAN. Datacode: No Rotor Effective Mass in NASTRAN Rotor Effective Mass in NASTRAN Figure 18: Effect of Rotor Effective Mass The typical approach for calculating airframe vibrations in NASTRAN is to include an effective hub mass and apply rotor loads calculated from one of a number of rotor codes. However, this can lead to a double bookkeeping of rotor mass and airframe dynamics if the rotor loads calculation also includes airframe modal data. In addition, the effective mass is a complex term and assumptions to change it to a real number must be made for normal modes analysis in NASTRAN. To eliminate this situation, the possibility of performing the airframe vibration analysis using an airframe model with no hub mass was evaluated. It is not intuitively obvious at first that this will work, since without hub mass in the finite element model, the airframe modes

9 may not be placed correctly. However, the hub loads are calculated using airframe flexibilities at the rotor hubs, in the form of airframe modes shapes, which are correctly coupled to the dynamics of the rotor system. It is proposed that since the resulting hub loads include the effects of properly placed airframe modes, when they are applied back onto the airframe mode, the correct aircraft dynamics result. To check these assumptions, a simple 3 degree of freedom system was analyzed as shown in figure 19. The system was modified by removing the rotor system mass, and the same constant 10,000 lb load was applied to the rotor shaft spring. As expected, without the rotor system mass the natural frequencies of the system have shifted and different airframe loads (spring forces in and ) result, as figure 21 shows. The 10,000 lb load in this case represents hub loads calculated without airframe dynamics included, applied to an airframe with no rotor mass, clearly incorrect. 1.E+06 2 Mass System : Constant 10K Force F=10,000 Rotor System Mass 1.E+05 1.E+04 F=10,000 Rotor Shaft Spring Airframe Mass 1 Airframe Spring 1 Spring Force 1.E+03 1.E+02 Airframe Mass 2 Airframe Spring 2 1.E+01 1.E+00 Frequency Figure 19: 3 DOF Spring Mass System This simple system represents the properly coupled rotor/fuselage as modeled in RCAS. The 10,000 lb load represents the vibratory loads acting on the rotor system. The shaft spring represents the interface between the RCAS and NASTRAN models where the hub loads are calculated. The other two springs and masses represent the airframe. Loads in the three springs were derived and are shown in figure 20, as a function of frequency. Note that the spring force labeled represents the rotor hub load. 3 Mass System : Constant 10K Force Figure 21: 2 Mass System, Constant Force Next, the same system (with no rotor system mass, as in figure 21) was excited with the spring force calculated with the full 3 DOF system (force, figure 20). This force represents the RCAS calculated hub load, which includes the aircraft dynamics of a fully coupled system, applied to an airframe model without a rotor system mass. As figure 22 shows, the results are identical to figure 20. The applied load (spring force ) in this case is calculated with airframe dynamics included and applied to an airframe without any rotor mass, clearly correct. 2 Mass System : Force = Celas Force from 3 Mass System Spring Force 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 F=10,000 Spring Force 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 F=F(f) 1.E+01 1.E+01 1.E+00 Frequency 1.E+00 Frequency Figure 20: 3 Mass System, Constant Force Figure 22: 2 Mass System, Force = F(f)

10 To illustrate the effect of the double bookkeeping the rotor effective mass and the airframe dynamics, the rotor system mass was placed back into the system and excited with the spring force calculated with the full 3 DOF system (same force used in figure 22). This configuration represents the incorrect coupling method of including the rotor effective mass in the NASTRAN analysis and in the rotor analysis. As figure 23 shows, the results are incorrect and different than those in figure 20. But note that the system frequencies are correct. Spring Force 1.E+06 1.E+05 1.E+04 1.E+03 1.E+02 1.E+01 1.E+00 3 Mass System : Force = Celas Force from 3 Mass System Frequency Figure 23: 3 Mass System, Force = F(f) F=F(f) The spring force for element 2002 calculated using the correct coupling (figure 20) and the incorrect coupling (figure 23) are compared, as shown in figure 24. The results show that the incorrect coupling can lead to either an over-prediction or under-prediction of the load, depending on the frequency of excitation. NASTRAN frequency response was checked using four different hub load calculations as input to the NASTRAN analysis. Loads were also calculated using the Boeing TECH-02 rotor analysis code (reference 4) and were extracted from a previous CH-47 flight test program using two different techniques. This CH-47 test program had a number of forward and aft rotor shaft bending gages and a number of blade bending gages on one blade on each rotor head. A Boeing program called GENCOR (reference 5) was used to calculate vertical hub shears and hub moments using measured blade bending gage harmonics data and TECH-02 analytical blade mode shapes. The in-plane hub shears were calculated using simple loads summation of the rotor shaft bending mo ments. In addition to GENCOR, hub moments were also calculated using flight test loads summation. Two different sets of hub loads were created from this data; one set containing vertical shears and moments calculated using GENCOR and in-plane hub shear calculated using shaft bending moments (labeled GENCOR Moments in the following plots) and the other set containing vertical shears from GENCOR and in-plane hub shears and moments from the shaft bending moments (labeled as SBM Moments in the plots). Each of these load sets was applied to the NASTRAN airframe with no hub mass and airframe loads were calculated. The results are shown in figures 25 and 26. For reference, the calculation using RCAS hub loads applied to the airframe model that includes an effective rotor mass is also shown (labeled RCAS with Hub Mass ). STATION 594 UPPER CROWN OUTER T CAP LEFT SIDE 3P STRESS 1.E+06 1.E+05 Force in Spring Effect of Incorrect Coupling Correct Coupling Incorrect Coupling Flight Data RCAS Tech-02 SBM Moments Gencor Moments RCAS - with Hub Mass Strain Gage Spring Force 1.E+04 1.E+03 1.E+02 1.E+01 TRUE AIRSPEED: KNOTS 1.E+00 Frequency Figure 25: LH Station 594 Stress Figure 24: Effect of Incorrect Coupling Based on the results of this simple analysis, the premise that the rotor hub mass should not be included in the

11 RH STATION 594 UPPER CROWN OUTER T CAP RIGHT SIDE 3P STRESS Flight Data RCAS Tech-02 SBM Moments Gencor Moments RCAS with Hub Mass DATA CODE: Capabilities and Status, Presented at AHS Aeromechanics Specialists Conference, San Francisco, CA., Jan. 1994, pp. PS.4-1 PS Staley, J., Mathew M., Tarzanin, F., Wind Tunnel Modeling of High Order Rotor Vibration, American Helicopter Society 49th Annual Forum, May 1993 TRUE AIRSPEED: KNOTS Figure 26: RH Station 594 Stress For this location, the results are mixed. For the LHS (figure 25), results with the RCAS hub loads best match the measured strain data, while on the RHS all load sets give about the same results. Also, the LHS shows a large difference when the rotor hub mass is included in the NASTRAN analysis, while the RHS does not show as much difference. With the loads applied to the NASTRAN model with no hub mass, each of the load sets do show reasonable trends with airspeed compared to the test data. More analysis is needed to further develop and validate this process. Acknowledgments This material is based upon work supported by the Aviation Applied Technology Directorate (RDECOM) under Contract No. DAAH10-03-C-0053 References 1. Gabel, R., Lang, P., and Reed, D., The NASA/Industry Design Analysis Methods for Vibrations (DAMVIBS) Program Boeing Helicopters Airframe Finite Element Modeling,, AIAA 33rd Structures, Structural Dynamics and Materials Conference, Dallas, Texas, April 13-15, Gabel, R., and Lang, P., CH-47D Airframe Vibration Reduction Through Airframe Stiffening, American Helicopter Society 54th Annual Forum, May MSC Software, MSC.PATRAN for Advanced Users (PAT 302), Example Exercises, Lesson Shultz, L. A., Panda, B., Tarzanin, F. J., Derham, R. C., Oh, B. K., and Dadone, L., Interdisciplinary Analysis for Advanced Rotors - Approach,