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tire will not spin up (landing) or a rolling, unbraked
tire will slow in rotation and may actually stop
(takeoff). Total dynamic hydroplaning speed is represented
by the following mathematical formulas: 9 times
the squarer oot of the tire inflation pressuref or a rotating
tire (as in takeoff); 7.7 times the square root of the tire
inflation pressure for rt nonrotating tire (as in landing).
Dynamic hydroplaning is insensitive to vertical load
changes (weight), but is greatly affected by tire inflation
pressure and tire wear. Since the fluid cushion is incapable
of developinga ny appreciables hearf orce,b raking
and sideforce coefficients become almost nonexistent.
18.2.2 Viscous Hydroplaning. Viscous hydroplaning
occurs when the tires are separated from the
runway surface by a thin film. Viscous fluid pressures
in the tire-ground contact zone of rolling tires build up
with speed to the danger levels required for hydrophming
only when water-covered pavements are smooth or
smooth acting, as when contaminants considerably
more viscous than water coat the pavements. Since a tire
ORIGINAL 18.2
NAVAIR 01.F14AAD-1
operating on a surface with rubber deposits, paint, fuel,
or oil can only partially displace the napped water film,
considerablyh igher hydroplaning pressuresw ill be developed
in the tire footprint area with these more viscous
fluids. Even slight amounts of precipitation, for example,
a heavy dew that coats the pavement with a thin film
of fluid, can produce this effect. Because the tire footprint
separatest? om the runway with lessf luid deptha nd
at a lower relative groundspeed than dynamic hydroplaning
speed,v iscous hydroplaning is potentially more
dangerousth an dynamic hydroplaninga ndi snot greatly
affected by changes in vertical tire load or tire inflation
pressure. Grooved tires offer a greater advantage than
smooth tires in reducing the effects of viscous hydroplaning.
The runway pavement surface texture is also an
important factor in combating viscous hydroplaning effects.
18.2.3 Combined Dynamic and Viscous Hydroplaning.
Loss oftire hiction with increasingo r decreasing
speed on wet or flooded runway pavements can be
caused by the combined effects of viscous and dynamic
hydroplaning. Figure 18-2 shows a pneumatic tire rolling
at medium speed across a flooded pavement in a
partial hydroplaning condition. The first zone shows the
traction of the tire footprint that is supported by bulk
water (dynamic); the second zone, the fraction supported
by a thin film of water (viscous); and the third
zone, the fraction essentially in dry contact with the
peaks ofthe pavement surface texture. The length of the
first zone representst he time required for a rolling tire
in this speed condition to expel bulk water from under
the footprint; correspondingly, the length of the second
zone representsth e time required for the tire to squeeze
out the residual thin water film remaining under the
footprint after the bulk water has been removed. Since
fluids cannot develop shear forces of appreciable magnitude,
it is only in the thiid zone (essentially dry region)
that friction can be developed between the tire and the
pavement for steering, decelerating, and accelerating a
vehicle. The ratio of the dry contact area (thud zone) to
the total tire footprint area (zones 1,2, and 3) multiplied
by the coefficient the tire develops on a dry pavement,
yields the friction coefftcient the tire develops for this
flooded pavement and speed condition. As speed is
increased, a point is reached where the third zone disappears
and the entire footprint is supported by either
bulk water or a thin film. This speed condition is called
combined viscous and dynamic hydroplaning. As
speedi s further increased,a point is reachedw hereb ulk
water penetrates the entire footprint; this condition is
called dynamic hydroplaning. If the runway is not
flooded (no bulk water), such as on a runway covered
with heavy dew, it is possible for the second zone to
cover the entire footprint as speed is increased or decreased.
The pavement would have to be smooth or
Figure 18-2. Combined Viscous and Dynamic Tii
Hydroplaning
smooth acting, as in the case where contaminants are
present, for this to take place; this is called viscous hydroplaning.
18.2.4 Reverted Rubber Skids. Arevertedrubber
hydroplaning condition (also called reverted rubber
skid) takes place when a wheel skid has started on a wet
runway and enough heat is produced to turn the entrapped
water to steam. The steam in turn melts the
　
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