National Academies Press: OpenBook

Developing Improved Civil Aircraft Arresting Systems (2009)

Chapter: Chapter 12 - Depth-Varying Foam Material

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Suggested Citation:"Chapter 12 - Depth-Varying Foam Material." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 12 - Depth-Varying Foam Material." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 12 - Depth-Varying Foam Material." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 12 - Depth-Varying Foam Material." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 12 - Depth-Varying Foam Material." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 12 - Depth-Varying Foam Material." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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Suggested Citation:"Chapter 12 - Depth-Varying Foam Material." National Academies of Sciences, Engineering, and Medicine. 2009. Developing Improved Civil Aircraft Arresting Systems. Washington, DC: The National Academies Press. doi: 10.17226/14340.
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137 As a companion effort to the ACRP research, additional graduate-level research was undertaken to explore a depth- varying foam material concept. A summary of that research is given in this chapter. For an in-depth examination, the original research document should be consulted (33). The goal of the study was different from that of the full system concept evaluations in Chapters 9 through 11. The evaluation of the depth-varying concept did not include full aircraft arrestment simulations. Instead, it was con- fined to a narrower study of the effects of the material on two different tires: the main-gear tires of the CRJ-200 and the B737-800. While the likely arresting performance can be inferred based on the results, APC simulations were not conducted. 12.1. Depth-Varying Foam Concept By its nature, an arrestor bed installed at an airport is a sin- gle, static system. It must arrest any and all aircraft that could overrun the runway end, from large B747s down to small regional jets. Problematically, these aircraft differ in ways that have a significant effect on arresting efficiency; the arrestor slows some aircraft more quickly than others. To improve the one-size-fits-all performance of an arrestor bed, a depth-varying foam arrestor material was investigated. The cellular cement used in existing EMAS arrestors is homo- geneous, having the same density and strength throughout each block of material. Changing the density and strength to become harder at deeper levels could potentially achieve a degree of performance leveling between large and small air- craft (Figure 12-1). This concept was evaluated using an idealized foam material model rather than a particular type of crushable foam (cement, polymer, glass, etc.). Many crushable foam options exist, and depth-varying layups for many could be achieved. 12.2. Depth-Varying Profiles Considered Multiple depth profiles for the material were possible. Fig- ure 12-2 illustrates three simple profile types that were con- sidered, as well as a depth-invariant material, which represents the current homogeneous arrestors. The curves have been normalized to compare overall trends. Viewing the profiles shows that the linear and quadratic cases sit astride the exponential function, given the same start- ing and ending points. As such, the exponential case did not seem to offer novel content, and the evaluation was narrowed to include three profiles: 1. Constant (baseline), 2. Linear profile, and 3. Quadratic profile. 12.3. Modeling Approach The modeling approach for the depth-varying material eval- uation was essentially the same as for the glass foam material (Section 9.4). Only points of contrast in the methods will be discussed in this section. 12.3.1. Calibration of Material Model Material calibration was not undertaken because the eval- uation involved idealized materials. All three material options assumed zero Poisson ratios, which is consistent with typical crushable foam behaviors. Figure 12-3 illustrates the idealized compression load curve for the material model. 12.3.2. Tire and Arrestor Simulations As with the glass foam material evaluation, the arrestor models were constructed in LS-DYNA using the deformable FEM tire models and SPH arrestor beds (Figure 12-4). C H A P T E R 1 2 Depth-Varying Foam Material

A large-scale arrestor model was created in LS-DYNA to simulate overruns by aircraft tires. No protective cover layer for the bed was included in the model, and it assumed a con- tinuous material without seams. 12.3.2.1. Arrestor Bed Models The arrestor bed models were constructed using half- symmetry to reduce computation time. They varied in size depending on the aircraft tire being used. The bed length was determined by the distance required for the tire to make a certain number of rotations such that the loading settled to a steady-state condition. The bed width was determined by the tire width such that artificial boundary effects were minimal and the response approximated that of a wide bed of the material. The beds were 36 in. wide, 300 in. long, and 30 in. deep. However, the effective depth of the bed was adjusted by use of a movable rigid plane. For the 44.5-in. tire, the SPH particles were sized at 2 in., but the smaller 29-in. tire used a bed with 1.5-in. particles to maintain a low discretization error. The depth-varying arrestor bed featured a division of the SPH material into several parts, forming stratified layers. Each part/layer had its own material definition based on its depth. Experiments showed that each layer required at least two rows of SPH particles in order to initialize properly. The 30-in. deep arrestor, with 2-in. particles, had eight separate layers (Figure 12-5). 12.3.2.2. Tire Models The tire models were fully deformable FEM, as discussed in Appendix F. However, for this limited study, only the main-gear tires for the B737 and CRJ-200 were used. The goal was to observe the behavioral trends, from which likely arresting performance was inferred. Table 12-1 summarizes the tires that were used in the evaluation. 12.3.2.3. Sequencing of Simulations The sequencing method for this evaluation differed from that of the foam glass evaluation. Because a full metamodel of the design space was not required, the simulations were con- ducted using a constant vertical load rather than a prescribed vertical penetration depth. The load for each tire was chosen as the static vertical load when the aircraft was decelerating at 10 ft/s2. Because the load was prescribed, the tire penetrated to a steady-state depth that varied depending on the arrestor bed material properties. After experimenting with several methods, the sequence illustrated by Figure 12-6 proved superior on the basis of pro- viding the most consistent results with the shortest settling time. The settling time refers to the second time stage, lead- ing up to the steady-state rolling behavior when the load measurements on the tire are recorded. Motion damping became one of the main problems with the combined model, tending to control how long the simu- lation had to run before the model approached steady-state. Two types of motion required damping: (1) oscillations in the 138 Material Becomes Harder Figure 12-1. Depth-varying material concept. Figure 12-2. Depth-varying profiles considered. Compression Stress (σ) Compression Strain (ε) Energy Absorbed in Compression Plateau Slope Figure 12-3. Idealized crushable foam stress–strain compression curve.

tire itself as it rebounded outward and inward from the axle line, and (2) vertical oscillation of the axle. Properly imple- mented damping prevented over- and under-damping the solution, leading to the shortest available run time while not altering the overall response of the system. The final method involved a vertical damping value that produced slight overshoot behavior. Using this approach ensured that the full depth had been reached, while keeping simulation times short. Depending on the strength of the arrestor material, the simulation time required for settling could still vary considerably. 12.3.3. Batch Simulations Using the arrestor bed model, large batches of simulations were conducted to generate metamodels for studying the effects of depth-varying material properties. For each tire, the run sets had three open variables: • Bed depth, in incremental depths from 18 to 30 in.; • Material Initial Strength, which defined the compressive strength at the surface of the bed; and • Material Strength Gradient, which defined how rapidly the strength of the material increased with depth (the quadratic or linear coefficient of the material profile curve). The large batch simulations were conducted using LS-OPT. Based on the initial model files, LS-OPT generated 139 30 in. 36 in. 8 Layers 2-In. Particles Top-Most Layer Figure 12-5. Sectional view of eight-layer arrestor bed model. SPH Arrestor Bed Constructed with Stratified Layering Deeper Material Becomes Harder Deformable FEM Main Gear Tires for B737-800 and CRJ-200 Models Constructed Using Half- Symmetry Figure 12-4. Depth-varying arrestor and tire model. Aircraft Landing Gear Tire Designation Included in Evaluation Main Gear H29x9.0-15 Included CRJ-200 Nose Gear R18x4.4 - Main Gear H44.5x16.5-21 Included B737-800 Nose Gear H27x7.7-15 - Main Gear H49x19-22 - B747-400 Nose Gear H49x19-22 - Table 12-1. FEM tire library for glass foam arrestor models.

permutations with various bed depths, strengths, and strength gradients. It sequentially executed the simulations and extracted the load data from them. Generally, the batches were conducted with one iteration of 50 simulations. 12.4. Metamodel Analysis 12.4.1. Metamodel Method The output from the batch simulations was extracted and assembled automatically by LS-OPT, where metamodels were constructed for the drag and vertical load forces. Metamodel- ing is analogous to fitting a curve through experimental data except that it is applied to multi-dimensional data sets. These data sets were four-dimensional, including depth, strength, strength gradient, and load (either vertical or drag). The meta- models were RBF networks, which can effectively capture non-linear behaviors including multiple concavity changes across the data set. 12.4.2. Optimization Criteria and Constraints The LS-OPT simulation sought to maximize the decelera- tion for both aircraft simultaneously, assuming that the main gear were responsible for the deceleration. Therefore, the drag load on both tires was maximized. Landing gear loading limits constrained the optimization. From the tire drag load, multiplied by two tires per strut, the overall rearward strut loading was calculated. This loading was then limited to be no greater than acceptable horizontal loadings as defined by the FAR. The loading requirements of the FAR define two types of criteria: (1) limit, which the gear should withstand without suffering damage, and (2) ultimate, which the gear should withstand without collapsing. For the evaluation, the limit strength criterion was used. FAR Section 25.493 requires hor- izontal main-gear strength equivalent to a 0.8 braking factor, applied to the maximum taxi weight on the strut. Therefore, the 0.8 factor became the optimization criterion; an optimal design would achieve a main-gear drag loading factor of 0.8 for both aircraft. All nose-wheel loading was neglected, although in practice it typically limits the arrestor bed design. Since the results of the analysis would not be integrated with the APC, it was fea- sible to make this simplification. 12.4.3. Metamodel Interpretation A visual review of the metamodel proved informative for comparing the material performance. Because this optimiza- tion study had two simultaneous constraints, one for each tire, the surface plots show two constraint curves and three shaded regions. As a key for understanding the other plots in this section, Figure 12-7 gives an example of such a surface. Two curves are drawn across the surface as boundaries to the shaded regions. One curve indicates the constraint boundary for the 29.0-in. tire. The other curve indicates the constraint boundary for the 44.5-in. tire. For each case, the curve indicates the ideal case of a 0.8 strut load factor. The darkest region violates the constraints for both tires (strut overload). The two medium-shaded regions violate the constraints for one tire or the other. The light-shaded region is acceptable for both constraints. Where the region boundary curves intersect, a dual-case optimum occurs, providing an ideal load for both aircraft struts. The strut load factor would be 0.8 for both aircraft in this case. If the material was designed with the depth, strength, and strength gradient from this intersection point, the struts would be ideally loaded for maximum deceleration. 12.4.4. Linear and Quadratic Profile Comparison For either the linear or quadratic depth-varying material, the performance for the 29.0-in. tire improved. In the homo- geneous material, drastic overloading could occur if the bed 140 Acceleration/Spin-Up Constant Vertical Load Constant Forward Velocity Settled Steady-State: Take Measurements Figure 12-6. Sequencing method for depth-varying simulations.

was too deep; both depth-varying methods eliminated this drastic overloading regime. Both methods also reduced the sensitivity to the material strength. This provided a serendipitous advantage, since crushable foam materials can have a degree of scatter in their strength values based on manufacturing variation. The sensi- tivity reduction implied that a robust design can be achieved with less dependence on precision materials. The linear depth-varying approach seemed slightly better than the quadratic. While the qualitative trends are similar, the linear method appeared to offer simultaneous optimal condi- tions for both large and small tires. Hence, the linear profile was chosen for advancement to the final optimization stage. 12.4.5. Linear Gradient Material Optimum Figure 12-8 and Figure 12-9 compare 29-in. tire metamodel surfaces for homogeneous (left images) and linear (right images) depth gradient materials. Figure 12-8 compares the penetration depth of the tires, while Figure 12-9 compares the strut loading ratios. The surfaces for the 44.5-in. tire (not shown) reflected similar trends. The right hand image of Figure 12-8 shows an interesting behavior: for the linear gradient material, the response becomes insensitive to the arrestor bed depth, becoming essentially flat in the depth-wise direction. The tires settled to their own natural depth in the material and were no longer prone to 141 Dark Region: Both Struts OverloadedMedium Region: One Strut Overloaded Light Region: Loading Acceptable for Both Struts DepthMaterial Strength St ru t L oa d Fa ct or Optimum Design Point Figure 12-7. Example figure for dual-constraint surface plot. Homogeneous Strength Depth Pe ne tr at io n De pt h 100 37.5 30 18 Linear Gradient Strength Depth Pe ne tr at io n De pt h 100 37.5 30 18 -4 -22 -4 -22 Figure 12-8. Penetration depth trends for 29.0-in. tire with homogeneous (left) and linear (right) depth gradient.

over-penetration and the resulting overload behavior. This trend was not true for the homogeneous material shown on the left. This contrast implies a superior geometric coupling of the depth-varying material with the tire size and load. The strut loading plot on the right side of Figure 12-9 shows a long, dual-tire optimal region where the boundary curves overlap one another at between 24 and 30 in. of bed depth. In that region, both aircraft struts are ideally loaded at a factor of 0.8. This long overlap is superior to the crisscross intersection of the homogeneous material plot on the left. It implies that additional tires, either larger or smaller than the two subject tires, could also find optimal responses with such a design. After all, a larger tire would generally require a deeper bed; such a bed would not disturb the response of either existing tire, since neither feels the effects of the additional depth. The tires would tend to settle to their own equilibrium levels. This behavior contrasts with the homogeneous material, where the only dual-tire optimum point was a chance intersection of the two tire curves. The likelihood of a third tire happen- ing to intersect those two curves at the same junction point is minimal. Therefore, three behaviors emerge in the optimal design region of the linear depth gradient material: 1. Each tire ignores unneeded depth, and “sees” the bed as tailor-fit to its dimensions; 2. The strength sensitivity of the smaller tire is reduced, mov- ing closer to that of the larger tire; and 3. The behavior suggests the feasibility of multi-tire simulta- neous optimization due to the depth insensitivity. 12.4.6. Summary of Metamodel Analysis The metamodel analysis explored linear, quadratic, and exponential depth-varying material profiles using stratified arrestor bed models. These profiles were compared with one another and with the currently used homogeneous material approach. An initial down-selection eliminated the exponential pro- file from consideration; subsequent analysis eliminated the quadratic profile. The final analysis compared the linear pro- file material with the homogeneous approach. A dual-tire optimum design can be obtained with either homogeneous or linear depth-varying arrestor beds. However, the optimum designs for each differ in several substantial ways: • The homogeneous material strut-loading metamodels for the two tires had different shapes and magnitudes; the depth- varying material did much to bridge the response chasm, resulting in similar surfaces for the two tires. • The optimal design for the homogeneous material exhibited high sensitivity to changes in the arrestor material strength for the smaller tire; the depth-varying system reduced the sensitivity difference between the tires by 50%. • The homogeneous material did not lend itself to multiple simultaneous aircraft optimizations; the depth-varying sys- tem offered a dual-tire optimized region rather than a single intersection point, and likely could support multiple simul- taneous aircraft optimizations. • With the depth-varying material, each tire ignored unneeded bed depth, settling to its own natural equilibrium depth. 12.5. Transition to Fielded System 12.5.1. Comparison with Other Candidate Evaluations The depth-varying foam material was assessed as part of a parallel study to the overall research effort. The focal point of the investigation differed from that of the other candidate arrestor concepts, and the final evaluation is less mature. 142 2.5 0 Strength Depth St ru t L oa di ng C rit er io n 37.5 100 18 30 0 Strength Depth St ru t L oa di ng C rit er io n 37.5 100 18 30 2.5 Homogeneous Linear Gradient Figure 12-9. Strut loading trends for 29.0-in. tire with homogeneous (left) and linear (right) depth gradient.

In order to bring the depth-varying concept evaluation to the maturity point of the other candidates, metamodel data would need to be created for the full suite of aircraft tires. Subsequent arrestment simulations could then be conducted with the APC to quantify the overall improvements to the arrestor bed lengths and aircraft exit speeds. 12.5.2. Transition to Glass Foam System The depth-varying evaluation was carried out using a gen- eralized ideal crushable foam material. Before conducting the metamodel generation recommended in the preceding sec- tion, it would be beneficial to link the concept with actual materials and calibrate to match them. The glass foam material appears to be a strong candidate for implementing this concept (Chapter 9). It can be pro- duced in a variety of strengths, is manufactured in relatively thin blocks, and can be adhered to itself in stratified layers matching the concept layup. It also has fairly ideal crushable foam characteristics that would lead to a good performance match with the idealized material predictions. 12.5.3. Comparison with Aggregate Foam System The aggregate foam material manifested a depth-varying material property (Section 11.3.2), which should be compared with the concept investigations of this section. The aggregate foam material exhibited the best overall fleet- wide performance of the three candidate systems evaluated (Section 11.5). The fleet design arrestor bed produced the clos- est overall exit speed rating for the three test aircraft, showing strong improvements in performance for the B747, which typically lagged in the one-size-fits-all beds. This improve- ment was possible because the foam aggregate essentially had an exponential depth-varying strength profile. During the depth-varying profile investigation, it was deter- mined that the linear strength profile offered more advanta- geous behavior than the quadratic or exponential profiles. If this observation holds true during a broader assessment involv- ing the three test aircraft, it is possible that a stratified glass foam bed could perform even better than the foam aggregate. However, there are several design coefficients for either depth- varying profile, and a more extensive examination would be required to firmly establish the best fleet-wide approach. The rut depths previously shown for the aggregate foam bed tended to be fairly shallow. The penetration depth into the linear gradient arrestor bed would likely be deeper, which could reduce the amount of unused material. Yet, for the same reason, loading oscillations could potentially be more severe in the linear case since it might not prove as robust for preventing over-penetration. The loading oscillations with the aggregate foam system required control through the bed geometry design. Similar control would probably be required for a stratified glass foam arrestor bed as well. 12.5.4. Estimated System Cost and Maintenance If the depth-varying crushable foam concept were to be implemented using a glass foam material, the system cost and maintenance issues would be the same as discussed in Section 9.6.2. The nature of the glass foam concept’s layup and construction would readily permit the use of several strengths of foam material in stratified layers without altering the construction methodology. 12.6. Summary The depth-varying foam material concept was assessed as part of a parallel study to the overall research effort. The focal point of the investigation differed from the other arrestor concepts investigated, and the final concept evaluation is less mature than for the three primary arrestor candidate systems. The comparative optimization study involved a protracted set of tire and arrestor simulations, which investigated linear, quadratic, and exponential material hardening profiles. A down-selection of these alternatives found that the linear strength increase profile was the most promising, and it was compared with the incumbent homogeneous material. The results for this investigation proved positive: a linear depth-varying material offers several advantages over the current homogeneous material. The depth-varying approach helped bridge the geometry and loading gaps between large and small tires, reducing the performance disparities between them. A bed constructed using the linear depth-varying mate- rial could produce shorter arrestment distances and/or higher design exit speeds for the aircraft fleet. In practice, this methodology could be readily imple- mented using a glass foam block material, with minimal impact on anticipated cost, construction methods, or main- tenance needs. Finally, Chapter 11 demonstrated that the exponentially depth-varying aggregate foam material could achieve perfor- mance leveling between aircraft. Although the exponential profile was not ideal per the findings of this study, the aggregate foam concept illustrates the potential results when extrapolated through full aircraft arrestment simulations. 143

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TRB’s Airport Cooperative Research Program (ACRP) Report 29: Developing Improved Civil Aircraft Arresting Systems explores alternative materials that could be used for an engineered material arresting system (EMAS), as well as potential active arrestor designs for civil aircraft applications. The report examines cellular glass foam, aggregate foam, engineered aggregate, and a main-gear engagement active arrestor system.

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