C4 AIRCRAFT DIRECTIONAL CONTROL

The previous section analyzes the impact of the reduction of the friction forces on aircraft braking performance. The reduction of friction forces also significantly reduces aircraft directional control.
One should also consider the effect of the crosswind component on a slippery runway.
C4.1 Influence of slip ratio
When a rolling wheel is yawed, the force on the wheel can be resolved in two directions:
One in the direction of wheel motion, the other perpendicular to the motion.
The force in direction of the motion is the well-known braking force.
The force perpendicular to the motion is known as the .side-friction force. or .cornering
force..

Steering capability is obtained via the cornering force.

Yaw angle

Cornering force
Friction force

Figure C12
Maximum cornering effect is obtained from a free-rolling wheel, whereas a locked wheel produces zero cornering effect. With respect to braking performance, we can recall that a free-rolling wheel produces no braking. In other words, maximum steering control is obtained when brakes are not applied. One realizes that there must be some compromise between cornering and braking.
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The following figure illustrates this principle:

Figure C13
The above figure shows that when maximum braking efficiency is reached (i.e. 12%
slippage), a significant part of the steering capability is lost.
This is not a problem on a dry runway, where the total friction force, split in braking and
cornering, is high enough. It may however be a problem on a slippery runway, where the
total friction force is significantly reduced. In some critical situations, the pilot may have to
choose between braking or controlling the aircraft. It may not have both at the same time.

Yaw angle Yaw angle Aircraft motion Aircraft motion

Cornering force
Cornering force
Friction force

Braking force
Friction force
Braking force

DRY RUNWAY CONTAMINATED RUNWAY
Figure C14

C4.2 Influence of wheel yaw angle
The cornering force also depends on the wheel yaw angle. The wheel yaw angle is defined as the angle between the wheel and its direction of motion. The cornering force increases with the yaw angle, however if the wheel is yawed too much, the cornering force rapidly decreases. The wheel yaw angle providing the maximum cornering force depends on the runway condition and diminishes when the runway is very slippery. It is around 8° on a dry runway, 5° on a slippery runway and 3° on an icy runway.
C4.3 Ground controllability
During a crosswind landing, or aborted takeoff, cornering force is the primary way of
maintaining the aircraft within the runway width.
In a crosswind situation, the aircraft crabs. That is, the aircraft’s nose is not aligned with
　
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