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125 11.4.5.3. Parameter Sensitivities be a rate-independent process, and that could affect the over- all rate sensitivity of the medium. This trait requires confir- Using the metamodels, it is possible to review how sensi- mation through larger-scale testing. tive the landing gear loads are to the different variables of speed, bed depth, and penetration percentage. Figure 11-21 shows a surface plot for the 44.5-in. main-gear tire of the 11.4.5.4. Data Transformation B737 in an 18-in. deep aggregate foam arrestor bed. The final metamodel data for each tire was converted for Stronger drag loads (shown as the lower, more negative val- use by the APC. LS-OPT was used to extract nominally 9,000 ues) occur when the penetration ratio increases, up to the max- data points from each metamodel and export it into tabular imum of 1.0 (or 100%). The loading is, therefore, strongly form. A MATLAB conversion program was written to map dependent on the depth of penetration into the bed, as would this data into multi-dimensional matrix form that could be be expected. However, unlike with crushable block foam, the quickly accessed by the APC. increase is non-linear, as shown by the downward concavity of the surface. 11.5. Arrestor Performance By contrast, the variation with speed shows very little change Predictions between 10 and 70 knots. The loading is fairly insensitive to speed, reflecting the low rate-sensitivity that would be expected 11.5.1. Scope of Simulations from crushable glass foam. Practically speaking, this means Using the APC, a separate optimal arrestor was designed that the aggregate foam system will exert nearly the same decel- for each of the three trial aircraft: CRJ-200, B737-800, and eration load on an aircraft travelling at high or low speed. This B747-400. Subsequently, an optimal mixed-fleet arrestor was behavior is desirable for an arrestor and is consistent with the designed as a compromise best-fit for all three aircraft. general behavior of the current cellular cement material. All arrestment predictions assumed the following: While overall rate insensitivity is expected for a glass foam material, the aggregate foam glass presents an additional layer 50-ft setback distance; of complexity. The inter-particle void compaction may not 50-ft gradual decline to the maximum bed depth; Drag Force (lbf) Speed (knots) Penetration Ratio Figure 11-21. Metamodel drag load surface plot for 44.5-in. tire in an 18-in. deep aggregate foam bed.

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126 Table 11-6. Individual aircraft 70-knot arrestor beds for aggregate foam arrestor system. Nose-Gear Limit Nose-Gear Ultimate Current EMAS, Load Criterion Load Criterion Optimal Designs Aircraft Strength Depth Bed Strength Depth Bed Depth (in.) Bed (%) (in.) Length (%) (in.) Length Length (ft) (ft) (ft) CRJ-200 68% 19.0 326 59% 21.1 309 22 258 B737-800 103% 29.3 356 72% 36.0 315 22 287 B747-400 61% 36.0 580 61% 36.0 580 28 495 70-knot starting speed for the aircraft; Compared with the similar EMAS design cases on the right, No reverse thrust; the distances are all somewhat longer. Braking factor of 0.25 before and within the bed; and Material strength in the table is given as a percentage of the Arrestor bed loads based on interaction with tires, neglect- tested material (original gradation) since there is not a simple ing strut and axle components. plateau strength as with normal crushable foams. The scaling assumes that a 75% maximum compression is retained and Arrestor beds were designed for two different nose-gear that the gradation remains the same. However, the composi- loading criteria: tion of the aggregate foam material itself is altered in density to effect a stronger or weaker foam aggregate. Generally, a 1. Limit Load Criterion, where the drag load applied to the range of acceptable strength and depth combinations was nose strut cannot exceed the limit load for the nose gear available, so a different material strength could be chosen than (FAR Part 25.509); the one given. 2. Ultimate Load Criterion, where the drag load applied to Table 11-7 shows the compromise design case with the best the nose strut cannot exceed the ultimate load for the arrestor design for all three aircraft. With the material strength nose gear; and depth as specified, the B747 would require 592 feet to decelerate from 70 knots. Per typical practice for EMAS design, Since the ultimate loading criterion permits higher loads on the bed length may be specified such that all aircraft satisfy the the strut, deeper beds and shorter stopping distances resulted minimum 40-knot exit speed requirement. The bed designs in from those cases. the table are fixed with a 400-ft length for comparison with the It was determined through experimentation that the aggre- other alternatives. At this length, the B747-400 would have a gate foam arrestor design functioned equally well for a par- maximum exit speed of approximately 56 knots, which satis- tially or fully recessed bed. In practice, a partially recessed bed fies the requirements of AC 150-5220-22a. would require a shallow basin, and a raised, flat mound of Interestingly, this multi-aircraft design is the best one-size- arrestor material covered with turf. The simulations conducted fits-all arrestor bed from among the three candidate systems. in this section, however, assume a simpler, fully recessed bed, Some of the other candidate design cases provided better where the top of the arrestor is level with the runway (Fig- single-plane performance, but they lagged comparatively ure 11-3). Two design variables were considered for the arrestor: the Table 11-7. Fleet design arrestor bed for bed thickness and the material strength. The material strength aggregate foam arrestor system. was adjusted by applying a scale factor to the metamodel load- ing data in the APC during a simulation. It was considered an Nose-Gear Ultimate Load Criterion open variable because the aggregate foam can be manufactured Bed Dimensions Bed Depth: 36.0 in. at a variety of strength levels (varying density). Bed Length: 400 ft Material Strength Scaling: 86% 11.5.2. Performance for Test Aircraft Aircraft Exit Speed (knot) Stopping Distance (ft) CRJ-200 70+ 367 Table 11-6 lists best-case arrestor designs for each aircraft B737-800 70+ 327 taken individually. Each arrestor bed listed uses a different material strength and depth optimized for the design aircraft. B747-400 56 400

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127 with respect to the multi-plane case. The reason for the supe- 12 in. deep in the material. For a normal crushable block rior performance appears to be the depth-varying nature of foam system, the rut would tend to cut near the bottom of the the material. Deeper beds were feasible for the aggregate bed at the bottoming compression depth of the material. In foam, which performed better in arresting the B747, with its this case, however, the depth-varying properties of the foam large-diameter tires. Yet, because of the depth-varying prop- aggregate (exponential curve of Figure 11-7) allow the tires to erties, the smaller aircraft each found their own equilibrium find their own natural equilibrium depth in the material, rut depths without being overloaded. The equilibrium depth which is at a much shallower penetration. The tires are, in a effect is further discussed in Section 11.5.3.2. sense, floating atop a layer of partially compacted material. Because the material has not been fully compressed, oscilla- tions can result as the tire bounces above and below the equi- 11.5.3. General Observations librium rut depth. 11.5.3.1. Deceleration and Nose-Gear Loading Because of the rut depth effect, large oscillations were observed in a number of design cases attempted. New design The overall deceleration and loading trends on the three air- practices were required to stabilize the ruts. In general, two craft showed several common characteristics. Figure 11-22 and main principles emerged: Figure 11-23 illustrate sample deceleration and nose-gear load plots generated by the APC for the B737 aircraft. Figure 11-22 1. Oscillations are more likely for beds that are deep relative represents the arrestor bed using the limit load design criterion, to the tire diameter, and while Figure 11-23 is based on the ultimate load criterion. The 2. Oscillations are more likely where the decline distance is overall bed lengths and depths reflect the values of Table 11-6. not substantially longer than the wheel base of the aircraft The upper plot in each figure shows the aircraft speed (discussed further in the next section). decrease relative to the nose wheel location in the arrestor bed. The lower surface line of the arrestor is depressed by the The B747 was not observed to form any lasting oscillations bed thickness, as discussed in Section 11.5.1. in any design case. Any bouncing quickly settled to a steady- The middle plot in each figure shows the deceleration of the state condition. It was determined that this was because all aircraft in g's. The deceleration first increases when the nose tires on the B747 are 49-in. diameter, which is substantially wheel penetrates the bed at zero feet, while the main-gear tires larger than the maximum 36-in. bed depth. are still on pavement. The deceleration then increases strongly when the main-gear tires enter the arrestor, at a nose-wheel location of 50 to 100 ft. After the initial transition, the deceler- 11.5.3.3. Decline Distance Effects ation oscillates in both cases, at about 0.65 g for the limit design The decline distance is the length of the depth-tapering and about 0.8 g for the ultimate design. The oscillations are region of the arrestor bed. For the two other candidate sys- substantially more severe in the ultimate design case due to tems, a 50-ft decline distance was sufficient to produce stable the combined effects of the deeper bed and the depth-varying arrestors. However, it was found that a 100-ft decline was material properties. Among the three arrestor candidates, this considerably more stable for the aggregate foam concept due oscillatory tendency is unique to the foam aggregate concept. to its relative size with respect to the aircraft wheelbase. The reason for the oscillations becomes clearer when analyzing A shorter 50-ft decline produced sometimes drastic oscil- the wheel rut characteristics in the next section. lations, as shown in Figure 11-25 and Figure 11-26. This The lower plot in each figure shows the nose-wheel drag design case for the B737 is identical to that of Figure 11-22 loading, which proved to be the limiting load for the arrestor except for the decline distance, which was 100 ft. The cause bed design. The loading is highest between 100 and 120 ft, of the oscillations appears to be the initial pitch rate caused which is after the main-gear tires have entered the bed. When by the sudden drop of the nose wheel, which set up a "por- the main-gear tires enter, the deceleration increases sub- poising" behavior in the aircraft. Since the rut bottom was stantially, and this causes the aircraft to pitch forward and floating, rather than bottomed, the motion damped out presses the nose wheel deeper into the material. The deeper slowly. penetrations lead to higher drag loads on the nose wheel. 11.5.4. Braking Effects 11.5.3.2. Rut Depth Effects In the APC simulations, braking loads for the main-gear Figure 11-24 gives the corresponding main and nose-wheel tires while in the arrestor bed were added to the drag loads of rut depths for the limit case of Figure 11-22. As shown by the the tirearrestor metamodel. Thus, the net drag load on the lower dashed line in each figure, the rut is only about 10 to main-gear tires was due to the braking plus the arrestor

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128 Aircraft Velocity and Bed Profile 80 Speed [knot] and Depth [in.] 60 40 Aircraft Speed 20 Upper Surface of Arrestor Lower Surface of Arrestor 0 -20 -40 -50 0 50 100 150 200 250 300 350 400 Nose-Wheel Location [ft] Aircraft Deceleration 0.9 Aircraft Deceleration 0.8 0.7 Deceleration [g] 0.6 0.5 0.4 0.3 0.2 -50 0 50 100 150 200 250 300 350 400 Nose-Wheel Location [ft] Landing Gear Forces - NOSE STRUT 40 Nose Gear Drag 35 Nose Gear Limit Load 30 Nose Gear Ultimate Load 25 Force (kip) 20 15 10 5 0 -50 0 50 100 150 200 250 300 350 400 Location [ft] Figure 11-22. Limit criterion aggregate foam arrestor design plots for B737-800 showing speed (top), deceleration (middle) and nose-gear drag load (bottom).

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129 Aircraft Velocity and Bed Profile 80 Speed [knot] and Depth [in.] 60 Aircraft Speed 40 Upper Surface of Arrestor Lower Surface of Arrestor 20 0 -20 -40 -50 0 50 100 150 200 250 300 350 Nose-Wheel Location [ft] Aircraft Deceleration 1 Aircraft Deceleration 0.9 0.8 Deceleration [g] 0.7 0.6 0.5 0.4 0.3 0.2 -50 0 50 100 150 200 250 300 350 Nose-Wheel Location [ft] Landing Gear Forces - NOSE STRUT 40 35 30 25 Force (kip) 20 15 10 Nose Gear Drag Nose Gear Limit Load 5 Nose Gear Ultimate Load 0 -50 0 50 100 150 200 250 300 350 Location [ft] Figure 11-23. Ultimate criterion aggregate foam arrestor design plots for B737-800 showing speed (top), deceleration (middle) and nose-gear drag load (bottom).

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130 Vertical Displacements - MAIN STRUT 20 10 Displacement [in.] 0 -10 Axle Displacement -20 Rut Depth Lower Bound of Bed Upper Bound of Bed -30 -150 -100 -50 0 50 100 150 200 250 300 350 Location [ft] Vertical Displacements - NOSE STRUT 20 Axle Displacement Rut Depth 10 Lower Bound of Bed Upper Bound of Bed Displacement [in.] 0 -10 -20 -30 -50 0 50 100 150 200 250 300 350 400 Location [ft] Figure 11-24. Limit criterion aggregate foam arrestor design plots for B737-800 showing axle and rut depth for the main strut (top) and nose strut (bottom). resistance. As an approximate solution, this approach worked Additionally, the sudden application of, or increase in, air- well and was appropriate. craft braking could induce a porpoising behavior similar to the However, a small side study conducted during the metamod- short decline distance effects of Section 11.5.3.3. This brake- eling process indicated that braking applied to the main-gear induced oscillatory behavior could be studied using the APC, tires could cause the penetration depth to increase. Since the but such an investigation fell outside the scope of the current arrestor metamodel loads assumed a non-braked free-spinning effort. In actual practice, it seems doubtful that pilots in over- wheel, this depth change was not captured in the predictions. run situations would release and re-apply the brakes of the air- For a bottomed tire, little depth change is feasible and this craft. It may prove most useful to characterize the torsional effect would likely not occur. For a non-bottomed tire, how- forces associated with a fully applied brake and assume those ever, the tendency to penetrate deeper would lead to higher loads throughout the simulations. arrestor drag loads on the tire. Since this effect would only apply to the braked wheels of the main gear, it would benefit 11.5.5. Short Landings the deceleration process while not affecting the nose gear. Stopping distances could be somewhat reduced by this effect, Short landings involving an aircraft touch down inside the though it is unclear by what amount or in what limited sub- arrestor bed were not simulated. However, the potential for set of arrestor design cases. short landings presents two possible issues.

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131 Aircraft Velocity and Bed Profile 80 Speed [knot] and Depth [in.] 60 Aircraft Speed 40 Upper Surface of Arrestor Lower Surface of Arrestor 20 0 -20 -40 -50 0 50 100 150 200 250 300 350 Nose-Wheel Location [ft] Aircraft Deceleration 1.2 Aircraft Deceleration 1 Deceleration [g] 0.8 0.6 0.4 0.2 -50 0 50 100 150 200 250 300 350 Nose-Wheel Location [ft] Landing Gear Forces - NOSE STRUT 40 35 Nose Gear Drag Nose Gear Limit Load 30 Nose Gear Ultimate Load 25 Force (kip) 20 15 10 5 0 -50 0 50 100 150 200 250 300 350 Location [ft] Figure 11-25. Limit criterion aggregate foam arrestor--50-ft ramp--design plots for B737-800 showing speed (top), deceleration (middle) and nose-gear drag load (bottom).