Particularly during the after-landing
roll, special attention must be given to maintaining
directional control by the use of rudder or nose-wheel
steering, while keeping the upwind wing from rising
by the use of aileron. When an airplane is airborne,
it moves with the air mass in which it is flying regardless
of the airplane's heading and speed. When an airplane
is on the ground, it is unable to move with the air
mass (crosswind) because of the resistance created
by ground friction on the wheels.
Characteristically, an airplane has a greater profile
or side area behind the main landing gear than forward
of the gear. With the main wheels acting as a pivot
point and the greater surface area exposed to the
crosswind behind that pivot point, the airplane tends
to turn or weathervane into the wind.
Wind acting on an airplane during crosswind landings
is the result of two factors. One is the natural wind,
which acts in the direction the air mass is traveling,
while the other is induced by the forward movement
of the airplane and acts parallel to the direction
of movement. Consequently, a crosswind has a headwind
component acting along the airplane's ground track
and a crosswind component acting 90° to its track.
The resultant or relative wind is somewhere between
the two components. As the airplane's forward speed
decreases during the after landing roll, the headwind
component decreases and the relative wind has more
of a crosswind component. The greater the crosswind
component, the more difficult it is to prevent weathervaning.
Maintaining control on the ground is a critical part
of the after-landing roll because of the weathervaning
effect of the wind on the airplane. Additionally,
tire side load from runway contact while drifting
frequently generates roll-overs in tricycle-geared
airplanes. The basic factors involved are cornering
angle and side load.
Cornering angle is the angular difference between
the heading of a tire and its path. Whenever a load
bearing tire's path and heading diverge, a side load
is created. It is accompanied by tire distortion.
Although side load differs in varying tires and air
pressures, it is completely independent of speed,
and through a considerable range, is directly proportional
to the cornering angle and the weight supported by
the tire. As little as 10° of cornering angle
creates a side load equal to half the supported weight;
after 20°, the side load does not increase with
increasing cornering angle. For each high-wing, tricycle-geared
airplane, there is a cornering angle at which roll-over
is inevitable. The roll-over axis is the line linking
the nose and main wheels. At lesser angles, the roll-over
may be avoided by use of ailerons, rudder, or steerable
nose wheel but not brakes.
While the airplane is decelerating during the after-landing
roll, more and more aileron is applied to keep the
upwind wing from rising. Since the airplane is slowing
down, there is less airflow around the ailerons and
they become less effective. At the same time, the
relative wind becomes more of a crosswind and exerting
a greater lifting force on the upwind wing. When the
airplane is coming to a stop, the aileron control
must be held fully toward the wind.
