Antiskid System Operation Tutorial
When
brake force is applied to a vehicle wheel that is in normal
contact with the pavement, the rubber of the tire begins to
stretch in response to friction heating and the force applied to
the tire-pavement interface. This has the effect of making the
tire circumference significantly larger than it is without the
brakes applied.
When
brake force is applied, the angular velocity of the braked wheel
drops by several percent. Early researchers thought that this
slowing down was the result of the tire slipping against the
pavement and coined the term "slip velocity" to express the
difference between the circumferential speed of the braked and
un-braked wheels. If the level of braking is increased until the
co-efficient of friction, mu, can no longer support the force
being applied to the rubber, then true slip begins and the
available stopping force begins to diminish.
Operation at the peak of the mu-slip curve gives the highest
braking efficiency. Research suggests that a small level of true
slip may increase mu and that the peak of the curve actually
occurs after true slip has begun. Operation just beyond the peak
of the curve results in increased tire wear and if the brake
force is further increased, a skid develops that may lock the
wheel and blow the tire if unchecked. Aircraft tires can blow in
as little as 300 milliseconds at high speeds if the wheel is
locked.
Modern
Hydro-Aire brake control systems work by measuring the speed of
the wheel to determine slip and developing a correction signal
to adjust brake pressure to keep the tire operating at the peak
of the mu-slip curve. A rotary transducer, which is usually
mounted in the aircraft axle, measures wheelspeed and provides a
signal to an electronic brake system control unit (BSCU). The
control unit derives where the tire is operating on the mu-slip
curve for the prevailing runway conditions and sends a
correction signal to the antiskid valve to reduce applied brake
pressure.
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