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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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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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(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.
