Department of Mechanical. Engineering, Wichita State. University, Kansas, USA. Rasoul Moradi. TASS Americas,. Livonia, Michigan,. USA. Hamid M. Lankarani.
Proceedings of IMECE2012 2012 ASME International Mechanical Engineering Congress and Exposition November 9-15, 2012, Houston, Texas
IMECE2012-89024 EVALUATION OF DYNAMIC PERFORMANE OF AIRCRAFT SEATS FOR LARGER PASSENGER POPULATION USING FINITE ELEMENT ANALYSIS Prasannakumar S. Bhonge Department of Mechanical Engineering, Wichita State University, Kansas, USA
Rasoul Moradi TASS Americas, Livonia, Michigan, USA
ABSTRACT Dynamic aircraft seat regulations are identified in the Code of Federal Regulations (CFR), 14 CFR Parts § XX.562 for crashworthy evaluation of a seat in dynamic crash environment. The regulations specify full-scale dynamic testing on production seats. The dynamic tests are designed to demonstrate the structural integrity of the seat to withstand an emergency landing event and occupant safety. These tests are carried out on a 50th percentile Hybrid II Anthropomorphic Test Device (ATD) representing average 50 percent of human population. In this study, the dynamic performance of seats are evaluated for larger passenger population for both transport and general aviation seats. For this, Finite Element Analysis (FEA) of an aircraft seat model is analyzed by utilizing a 50th percentile e-ATD and validated with a 50th percentile ATD sled test results. Then the effect of a 95th percentile standard ATD in an aircraft passenger seat is investigated using FEA. Comparison of the 50th percentile and the 95th percentile electronic ATD models (e-ATDs) is carried out on the test parameters. This includes the restraint loads, the floor reactions and the head paths. Based on the comparison it is concluded that the seat loads go up in the range of 20 to 30 % if designed for larger passenger population.
Hamid M. Lankarani Department of Mechanical Engineering, Wichita State University, Kansas, USA
using crash dynamics computer programs, actual accident data analysis, and dynamic test programs of aircraft seats [1]. These performance standards take the form of two dynamic test conditions, as shown in Figure 1 for aircraft category Part 23, 25, 27, 29 (2-5). For a typical seat test program conducted at US Civil Aero Medical Institute (CAMI). Test condition 1 is a combined vertical-longitudinal velocity change dynamic test condition, denoted as XX.562 (b) (1). The purpose of this test is to evaluate the structural integrity and spinal injury due to vertical loading in emergency landing condition, as the predominant impact is along the spinal column of an occupant [2-5]. Test condition 2 is a longitudinal velocity change dynamic test condition, denoted as XX.562 (b) (2). The purpose of this test is to evaluate the structural integrity of the seating system, restraint system, occupant safety and seat deformations for horizontal loading in case of emergency landing. In this scenario, the predominant impact is along the longitudinal axis [2-5]. Details of these tests are provided in Table 1.
INTRODUTION In the early 1970’s from the existing crash studies, the General Aviation Safety Panel (GASP) established pass/fail performance criteria on aircraft seat and restraint system for preventing or minimizing injuries from primary impacts, secondary impacts and occupant skeleton loads. The aim was for the passengers in aircraft fatal crash situation, such as emergency landing, to still be able to free themselves independently and have reasonable chance of survival. These dynamic performance standards were based on comprehensive analysis of full-scale aircraft impact tests, parametric studies
Figure 1: Federal Aviation Regulations (FAR) dynamic seat test conditions 1 and 2
Table 1: Aircraft seat regulations [2, 3]
to investigate the effect of expanding coverage of the human population (95 percent) on loading an aircraft seating system. Finite element modeling and analysis were carried out to demonstrate or to compare the effect of using of a 95th percentile e-ATD on the important test parameters such as the restraint loads, the floor reactions and the head paths. A validated seating system methodology has been utilized and then modified for 50th and 95th percentile male occupants and for different seating configurations. Livermore Software Technology Corporation (LSTC), LSDYNA-Explicit [8] was used to solve the nonlinear transient FEA of seating systems and 50th and 95th percentile e-ATDs were used to simulate ATDs.
FEA METHODOLOGY FOR AIRCRAFT SEATS
Both tests must be conducted with an occupant simulated by a 170-pound Anthropomorphic Test Device (ATD) as defined by 49 CFR Part 572, Subpart B [6] or its equivalent, sitting in the normal upright position. The Society of Automotive Engineers (SAE) standard, AS8049 B [7], supports detailed information on the dynamic seat testing procedures and acceptance criteria. The Head Injury Criteria (HIC) in many aircraft seating system is substantiated using the head path analysis. In such scenarios, side walls, row seats are not installed as a part of seating system when tested. The head path (of 50th percentile male ATD) from the photometric data is applied to determine if the head contact with the interior is interacted. Additional 3 inches (76.2 mm) of head path is then added to accommodate larger male occupant (95 percentile) dimensions. On the other hand, in the automotive crashworthiness, regulations demand for a range of ATDs to accommodate larger population (95 percent). Some of the important Federal Motor Vehicle Safety Standards (FMVSS), and the European Economic Commision of Europe (ECE) regulations are as follows
Finite Element Analysis (FEA) has become an integral part of product design to substantiate the certification test/s or possibly to replace the certification test/s. In aircraft seat industry, the Advisory Circular (AC) 20-146 [9] outlines a methodology for the dynamic seat “Certification By Analysis” for use in Parts 23, 25, 27 and 29 airplanes and rotorcraft. The case of a certification by substantiation test/s increases the necessity of validation. The AC also demands correlation be made between +/- 10% of the sled test results and FEA results.
FMVSS 201: Occupant Protection in Interior Impact FMVSS 208: Occupant Frontal Crash Protection ECE R94: Frontal Impact Occupant Protection ECE R95: Side Impact Occupant Protection As a base-line, for aircraft seating system structural integrity, the evaluation is carried out using a 170- pounds 50th percentile ATD covering 50 percent of human population. From passenger safety perspective, this study makes an effort 2
Figure 2: FEA methodology for aircraft seating system
Figure 2 shows the FEA methodology for aircraft seating system composed of a main structure, bottom and a back cushions, a three-point restraint (for general aviation aircraft) and a FE 50th percentile Hybrid III dummy model [10]. Non-linear transient FEA of the seating system has been followed using following steps:
FE modeling of seat structural and non structural components, Material testing and component validation, Boundary conditions, e-ATD or dummy validation, Output discussion.
As per the defined methodology, the seat components and sub-assemblies were modeled as shells, solids or combination of shell and solid elements. Other sub-systems such as restraint and cushion were modeled as per the setup. Most commonly used metallic and non-metallic materials in the aircraft seating system such as Aluminum (Al 2024, Al 7075), Steel (AISI 4130, AISI 4340), and cushions were tested for mechanical properties such as stress -strain curve, modulus of elasticity, Poisson’s ratio and ultimate stress, as per the American Society for Testing Material (ASTM) [11-12]. Once the seat modeling of entire system is completed, a 50th percentile hybrid III dummy model was positioned as per the seat geometry and as per the guidelines provided in the SAE standard AS8049B. The database definitions has been incorporated in the final deck including graphics files, occupant output data, floor load data and belt load data and final acceleration pulse on the sled was specified. The corresponding analytical results such as belt loads, interface loads, head paths, pre-test and post-test deformations are analyzed. Final test article was prepared and the test was carried out. Once the satisfactory correlation is achieved between the FEA and test results, the model is considered to be validated, and is ready to be used to predict the effect of changes to the design. Component level testing and validation also helpful to fine tune FEA final results [13] and highly recommended when the system involves rate sensitive materials such as cushion and etc. Development of a component Head Injury Criteria (HIC) tester for aircraft seat introduced a possibility of certifying a seat without a sled test [14]. e-ATD Validation The e-ATD should meet the specifications cited in 49 CFR, Part 572 [6]. These specifications provide geometry and mass distribution parameters, the location of joints and their range of articulation, length, mass and center of gravity for each segment, assembled dimensions, and general external shape. The characteristics of the analytical dummy models (ADM) shall fall within the dimensional and mass tolerance 3
range cited in the specifications. Cited dimensions without a specified tolerance shall fall within ±0.1 inch (±2.54 mm) of the nominal value. Figure 3 shows physical ATD and the electronic ATD (e-ATD) model for 50th percentile Hybrid II. Biofidelity of the e-ATD is evaluated based on the following factors. Kinematic response, Dynamic compliance, Injury measures, Repeatability, Reproducibility, Durability, Calibration standards,
ATD
Figure 3: hybrid II test ATD and
e-ATD
e-ATD
Validation of the FE Model A case study was carried out on a forward-facing passenger aircraft seat, as shown in Figure 4. The occupant was positioned in static equilibrium over the seat cushion. Three-point restraints were modeled after the occupant was positioned. Static deformation (pitch and roll) of the seat was achieved dynamically at the same time with acceleration pulse. Validation of the forward-facing seat FEA results with the dynamic tests was done as discussed earlier in the methodology. The following parameters were studied and compared for validation purpose.
Pulse Energy balance Restraint loads Interface loads or floor reactions Seat deformations
Head trajectory
Figure 6: Kinematics comparison of Test and FEA Results Aircraft Forward
Figure 4: FE setup for aircraft seat [10]. In this study, the FEA was carried out for a higher Gs value of 16.8g. Later in the sled test the pulse was observed to be 16.2G. As a result of higher pulse in the FE analysis, most of the restraint and interface or floor loads in the simulation were observed to be at a bit higher side of the test results. In summary Table 2, the FEA results were interpolated to 16.2g to match the test results. Once the FE analysis was completed, occupant kinematics as shown in Figure 6, energy ratio and overall percentage of mass increase were verified for accuracy purpose before looking into the test parameters such as restraint loads, interface loads and the head path.
Figure 5: Comparison of Test and FEA pulse.
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Figure 7: Restraint loads - Shoulder The magnitude and shape of the restraint loads in the FEA were found to be close to the test results. As shown in Figure 7 and 8, the shoulder belt was observed 15% higher and the lap belt load was 14% higher as compared to the test results.
Figure 8: Restraint loads - Lap
Figure 9: Interface or Floor reactions.
Figure 10: Interface or Floor reactions.
As mentioned earlier, in the FE analysis, the pitch and roll seat deformations were achieved dynamically, the floor loads or interface loads start from zero, whereas in case of test results, as a result of static seat deformation before the dynamic pulse is applied, preloading was observed, as shown in Figure 9 and 10. Peak floor loads in the FE analysis were found within +/-10% of the test result floor loads.
The maximum seat back deformation was observed to be 1.166” (29.62 mm) and that was close to the test result seat back deformation 1.32” (33.53 mm). Head trajectory of 32” (812.8 mm) was observed in the simulation which was 4” (101.6 mm) less as compared to the test result. Table 2 shows summary of test and FEA results. Comparisons between the FEA results and the sled test results indicate reasonable correlation, establishing confidence in the FEA methodology.
Table 2: Summary of test and FEA results
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FEA FOR LARGE POPULATIONS The methodology was then modified to seating systems including the LSTC developed 50th and 95th male e-ATDs. In
the FEA, the seat was yawed for 10 Degrees but not pitched and rolled. Comparison of 50th and 95th male e-ATDs was carried out weight, height etc. as shown in table 3.
Table 3: Comparison of 50th and 95th Male e-ATDs
Case 1: General aviation category (Part 23) aircraft seating system
th
th
Figure 11: Head Path Comparison between 95 and 50 percentile e-ATDs
In the first case study, the FEA setup was modified as per the 14 CFR 23.562 requirements (refer Table 1), on to three-point restraints, on both rigid as well as flexible seat, and analyzed using the 50th and the 95th percentile e-ATDs. Figure 11 and 12 show comparison between the 50th and 95th e-ATDs for head path and lap belt loads.
Figure 12: Belt Load Comparison between 95th and 50th percentile e-ATDs
Table 4 shows a comparison of the FEA results for rigid seating system using the 50th and the 95th percentile e-ATDs. Table 5 shows a comparison of the FEA results for flexible seating system using the 50th and the 95th percentile e-ATDs. Elastic-plastic properties were provided to the seat structural
member such as Al 2024-T3. For non structural members such as cushion, load-strain data was provided.
Table 4: Summary test and FEA results
Table 5: Summary test and FEA results
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Case 2: Transport category (Part 25) aircraft seating system The second case study was carried out on transport category aircraft seating system. The FEA setup was then modified as per the 14 CFR 25.562 requirements (refer Table
1), on to a three-point restraints, on a flexible seat, and analyzed using the 50th and the 95th percentile e-ATDs. Table 6 shows a comparison of the FEA results for flexible seating system using the 50th and the 95th percentile eATDs. Elastic-plastic properties were provided to the seat structural and non structural members.
Table 6: Summary test and FEA results
CONCLUSION As per the performance standards, the structural integrity and the occupant safety in aircraft seat tests must be conducted with an occupant simulated by a 170-pound Anthropomorphic Test Device (ATD) as defined by 49 CFR Part 572, Subpart B or its equivalent, sitting in the normal upright position covering 50 percent of human population. From passenger safety point of view, to accommodate larger human population, it is necessary to evaluate the structural integrity of aircraft seating system for a larger human population. For the purpose of this study, finite element analysis was utilized to demonstrate or to compare the effect of using the 95th percentile e-ATDs on the important test parameters such as the restraint loads, the floor reactions and the head paths. A validated FEA methodology was presented with a forward-facing passenger seat case study. Comparisons of the FEA and test results indicated reasonable correlations, establishing confidence in the FEA methodology. The methodology was then modified to a seating system including 8
the LSTC developed 50th and 95th male e-ATDs and analyzed for three seating configurations. Physical comparison shows an approximate 25% increase of weight, an approximate 20% increase of height in case of 95th occupant compared with the 50th percentile occupants. In case of rigid seat in small aircraft (Part 23), belt loads and head path were increased by approximate 20%, if designed for larger population. If analyzed for flexible seat, the lap belt load was increased by approximate 30% and interface load was observed to be increased by approximate 25%. The effect of using the 95th percentile e-ATD in case of largeer aircraft (Part 25) was observed more than compared with small aircraft. Increases in belt/restraint and interface loads were in the range of 30% to 35% for the 95th percentile male e-ATDs and approximate 24% in head path. Overall, it seems that the seat loads were increased in the range of 20 to 30%. More research needs to be done to determine effect on the seat weight when designed for larger populations.
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