Antiskid System Operation
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.