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55 Figure 7-6. Cellular glass foam material. reinforced turf layer. The loose fill approach offered the poten- become harder at deeper levels could achieve a degree of per- tial advantage of reduced manufacturing and installation cost, formance leveling between large and small aircraft (Figure 7-8). as compared with pre-fabricated foam blocks. Further, this concept is fairly independent of the material cho- Chapter 11 discusses the aggregate foam concept evaluation sen. Many crushable materials exist, and depth-varying layups in detail. for most could be achieved, including cellular cement. This concept was evaluated as a parallel graduate-level 7.5.8. Additional Study: research study; Chapter 12 summarizes the relevant find- Depth-Varying Foam ings from the study. The depth-varying foam concept was developed by the 7.6. Displaceable Material Systems research team. It involves the use of non-homogeneous crush- able foam that becomes firmer as the bed depth increases. Several displaceable material systems are listed in Table 7-3, The cellular cement used in existing EMAS arrestors is homo- but only one of these systems was selected for detailed evalu- geneous, having the same density and strength throughout ation: loose engineered aggregate. This material is much like each block of material. Changing the density and strength to normal gravel in that the individual aggregate pieces do not Figure 7-7. Aggregate foam: close-up of microstructure (left) and pile (right).

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56 steeper than the angle of repose, the material runs freely, act- ing similarly to a fluid (39). The loose sand or gravel that pours on top slides down the surface, returning the sides once again to the angle of repose. At slopes shallower than the angle of repose, the material does not behave like a fluid, but Material Becomes a solid: a fluid would eventually flatten out into a pancake Harder shape of uniform thickness, as a puddle. Mechanically, the angle of repose is determined by the internal angle of friction Figure 7-8. Depth-varying material concept. for the material. This angle of friction divides the different regimes of behavior. For a tire rolling through an aggregate, the physical behav- ior follows the illustration of Figure 7-9. The material below (in general) break or compress. They can be compacted a certain level behaves in a solid-like fashion and undergoes by removing the void spaces between them, or they can be compaction beneath the weight of the tire. Above this level, displaced. However, after a tire overrun, they can simply fluid-like behavior dominates, and the material is pushed out be raked back into the ruts with little change in the effec- of the way of the tire, similar to water being pushed by the tiveness of the material. Gravel systems are most often used hull of a boat. The uppermost part of the aggregate can spray for arresting large trucks that lose effective braking on to the sides at high speed when the tire cuts through the ma- down-hill roads, but they have been used for aircraft in the terial quickly. In the "wake," a rut is left in the gravel. U.K. (4, 13). Altogether, there are three major components to the energy The engineered aggregate differs from normal gravel in dissipation: that the particulates are all roughly spherical, instead of angu- lar or elongated shapes. This has the principal advantage of 1. Compaction of the lower layer, preventing settling of the material over time, which is typical 2. Friction between pieces of aggregate moving around the for normal gravels. Similarly, since the particulates are made tire, and of the same material, the friction between individual pieces is 3. Momentum transfer in projecting the aggregate away from more consistent. the tire. 7.6.1. Drag Load Dynamics Unlike the crushable material systems, the drag load from the aggregate is highly rate dependent. The momentum Although a gravel-type arrestor bed appears to load the components are proportional to the tire speed squared (2, landing gear in a similar fashion as the current EMAS design, p. 8). This means that the drag load decreases as the aircraft the dynamics involved are quite different. The tiregravel slows, progressively exerting less stopping force. It has also interaction is highly complex because aggregates have more been found that at higher rates, the tires skim the surface, than one mode of behavior. not penetrating as deeply and generating lower drag loads Consider a pile of sand or gravel, created by pouring the (2, p. 16). material from a spout above it. The pile forms in a cone shape Low inter-particle friction allows for more fluid-like behav- whose sides are at an inclined "angle of repose." At an angle ior and permits the tires to sink more deeply into the aggregate Vertical Load Imparted Upper Surface Can Drag Spray at High Speed Load Imparted Upper Region has Pseudo-Fluid-Like Behavior Direction of Travel Rut Created Material Compressed Height Lower Region Compacted, Solid- Pavement Height Like Behavior Figure 7-9. Physical performance of gravel/aggregate materials.

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57 (13). This can be either desirable or undesirable, depending on 7.6.3. Overall Aircraft Dynamics the vehicle to be stopped. For truck arresting, allowing the vehi- The overall aircraft dynamics are the same as for the crush- cle to sink to the axles is positive, as the gravel also engages the able system (Section 7.5.4). undercarriage of the truck. For an airplane, however, there is no undercarriage to halt the descent into the medium, and sinking too deeply could result in drag loads high enough to cause land- 7.6.4. Summary of Mechanical Factors ing gear to fail. To summarize the dynamics of aggregate/gravel arrestor Another factor related to internal friction is the impact of beds, the following factors have a direct impact on the over- environmental variables such as moisture, dust, or ice in the all performance: arrestor bed. Dry, clean gravel generally has lower inter-particle friction than damp, dusty gravel. Friction, or the resistance Material properties of sliding between the particles, is in contrast to cohesion, or Particle size/gradation the sticking of the particles together. Dust, moisture, and ice Particle shape can all generate cohesion between the particles as well. In Particle density severe winter environments, testing has revealed that an Inter-particulate properties open bed of gravel can form a deep ice crust layer, rendering Internal angle of friction it ineffective (13). Inter-particle friction A variant to be considered in this research effort involves Inter-particle cohesion applying a protective cover layer to the aggregate bed (Fig- Speed of travel ure 7-10). Open arrestor beds produce a spray from the Material depth uppermost layer of the aggregate, which could be ingested by Landing gear configuration an aircraft engine; this cover layer will protect against inges- Pneumatic tire pressure tion. It could also mitigate or eliminate the development of Limiting landing gear design loads, with special consider- ice crusts in cold environments. However, the layer could ation for the nose gear alter the dynamic response of the aggregate bed because the Vertical limit load aggregate would be confined. Thin protective cover layers Longitudinal (drag-direction) limit load added to crushable systems do not induce a fundamental change in dynamic behavior, so this effect (if it exists) would be unique to covered aggregate beds. 7.6.5. Candidate 3: Engineered Aggregate Altogether, the aggregate behavior is highly complex and An engineered aggregate arrestor concept is called the must be treated as an entirely different dynamic response from Engineered Root-zone Arresting System (ERAS). It uses a that of the crushable material systems. manufactured aggregate composed of nearly spherical parti- cles (Figure 7-11) and is topped with a reinforced turf cover layer. 7.6.2. Effect of Landing Gear Configuration The effect of the landing gear configuration is essentially the same as for the crushable system (Section 7.5.3). Vertical Load Top Cover Layer Imparted Results In Material Drag Confinement Load Imparted Figure 7-10. Physical performance of gravel/ aggregate materials with confining top layer. Figure 7-11. Engineered aggregate.