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80 Drag Force (lbf) Speed (knots) Penetration Ratio Figure 9-25. Metamodel drag load surface plot for 44.5-in. tire in an 18-in. deep arrestor/turf bed. A MATLAB conversion program was written to map this Arrestor beds were designed for two different nose-gear data into multi-dimensional matrix form that could be quickly loading criteria: accessed by the APC. 1. Limit Load Criterion, where the drag load applied to the nose strut cannot exceed the limit load for the nose gear 9.5. Arrestor Performance (FAR Part 25.509); Predictions 2. Ultimate Load Criterion, where the drag load applied 9.5.1. Scope of Simulations to the nose strut cannot exceed the ultimate load for the nose gear. Using the APC, a separate optimal arrestor was designed for each of the three trial aircraft: CRJ-200, B737-800, and Since the ultimate loading criterion permits higher loads on B747-400. Subsequently, an optimal mixed-fleet arrestor was the strut, deeper beds and shorter stopping distances resulted designed as a compromise best-fit for all three aircraft. from those cases. All arrestment predictions assumed the following: It was determined through experimentation that the glass foam arrestor design functioned best as a partially recessed · 50-ft setback distance, bed, such that the fully crushed material depth was level with · 50-ft gradual decline to the maximum bed depth, the runway. This left 85% of the bed thickness above grade, · 70-knot starting speed for the aircraft, and 15% below grade. This approach produced the smoothest · No reverse thrust, landing gear loads by limiting the effective step-up or step- · Braking factor of 0.25 before and within the bed, down that the aircraft experienced upon entering the bed. · Cover layer had negligible effect, Two design variables were considered for the aircraft: the · Material had no seams (Section 9.4.3), and bed depth and the material strength. The material strength was · Arrestor bed loads based on interaction with tires, neglecting adjusted by applying a scale factor to the metamodel loading strut and axle components. data in the APC during a simulation. It was considered as an
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81 Table 9-6. Individual aircraft 70-knot arrestor beds for glass 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 (psi) (in.) Length (psi) (in.) Length Length (ft) (ft) (ft) CRJ-200 17 20.0 294 25 19.9 243 22 258 B737-800 15 25.6 388 20 30.0 302 22 287 B747-400 47 36.0 409 53 36.0 406 28 495 open variable because the glass foam can be manufactured at 9.5.3. General Observations a variety of strength levels (varying density). The overall deceleration and loading trends on the three aircraft showed several common characteristics. Figure 9-26 and 9.5.2. Performance for Test Aircraft Figure 9-27 illustrate sample deceleration and nose-gear load plots generated by the APC for the B737 aircraft. Figure 9-26 Table 9-6 lists best-case arrestor designs for each aircraft represents the arrestor bed using the limit load design criterion, taken individually. Each arrestor bed listed uses a different while Figure 9-27 is based on the ultimate load criterion. The material strength and depth that are optimized for the design overall bed lengths and depths reflect the values of Table 9-6. aircraft. Generally, a range of acceptable strength and depth The upper plot in each figure shows the aircraft speed combinations was available. Compared with the similar EMAS decrease relative to the nose wheel location in the arrestor design cases on the right (provided by ESCO), the distances bed. The lower surface line of the arrestor is depressed by 15% are comparable if the ultimate loading criterion is used. of the bed thickness, as discussed in Section 9.5.1. Table 9-7 shows the compromise design case with the best The middle plot in each figure shows the deceleration of arrestor design for all three aircraft. The CRJ-200 limits the the aircraft in g's. The deceleration first increases when the bed depth in this case, while the B737 controls the bed length. nose wheel penetrates the bed at zero feet, while the main-gear With the material strength and depth as specified, the B747 tires are still on pavement. The deceleration then increases would require 840 ft to decelerate from 70 knots. Since this is strongly when the main-gear tires enter the arrestor, at a longer than a standard RSA, it is highly impractical. Per typical nose-wheel location of 50 to 70 feet. After the initial transition, practice for EMAS design, the bed length may be specified the deceleration settles to a fairly constant value in both cases, such that all aircraft satisfy the minimum 40-knot exit speed at about 0.6 g for the limit design and about 0.8 g for the ulti- requirement. The bed designs in the table assume a 400 ft mate design. The relatively constant deceleration value is pos- length, which is sufficient to arrest the two smaller aircraft with sible because the glass foam material shows only mild rate 70-knot exit speeds. At this length, the B747-400 would have dependence. An ideal arrestor bed would provide a perfectly a maximum exit speed of approximately 46 knots, which constant deceleration; the glass foam bed performance approx- satisfies the requirements of AC 150-5220-22a. imates the ideal bed fairly well. The lower plot in each figure shows the nose-wheel drag loading, which proved to be the limiting load for the arrestor bed design. The loading is highest between 50 and 125 ft, which Table 9-7. Fleet design arrestor bed for glass foam arrestor system. is after the main-gear tires have entered the bed. When the main-gear tires enter, the deceleration increases substantially, Nose-Gear Ultimate Load Criterion and this causes the aircraft to pitch forward and presses the Bed Dimensions 26 psi material nose wheel deeper into the material. The deeper penetrations 19.1 in. depth lead to higher drag loads on the nose wheel. 400 ft long Aircraft Exit Speed (knot) Stopping Distance (ft) 9.5.4. Braking Effects CRJ-200 70+ 244 In the APC simulations, braking loads for the main-gear B737-800 70+ 338 tires while in the arrestor bed were added to the drag loads of B747-400 46 400 the tirearrestor metamodel. Thus, the net drag load on the
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82 Aircraft Velocity and Bed Profile 70 Aircraft Speed 60 Upper Surface of Arrestor Speed [knot] and Depth [in.] Lower Surface of Arrestor 50 40 30 20 10 0 -10 -50 0 50 100 150 200 250 300 350 400 Nose-Wheel Location [ft] Aircraft Decleration 0.8 Aircraft Deceleration 0.7 0.6 Deceleration [g] 0.5 0.4 0.3 0.2 0.1 -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 9-26. Limit criterion glass foam arrestor design plots for B737-800 showing speed (top), deceleration (middle) and nose-gear drag load (bottom).
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83 Aircraft Velocity and Bed Profile 70 Aircraft Speed 60 Upper Surface of Arrestor Speed [knot] and Depth [in.] Lower Surface of Arrestor 50 40 30 20 10 0 -10 -50 0 50 100 150 200 250 300 350 Nose-Wheel Location [ft] Aircraft Decleration 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 9-27. Ultimate criterion glass foam arrestor design plots for B737-800 showing speed (top), deceleration (middle) and nose-gear drag load (bottom).