A fundamental aircraft motion is a banking turn. This
maneuver is used to change the aircraft heading.
The turn is initiated by using the
roll, or bank, the aircraft to
one side. On the figure, the airliner is banked to the right
by lowering the left aileron and raising the right aileron. The
lift of the wings of the aircraft
which is always directed perpendicular to the
flight path and perpendicular to the wings generating the lift.
As the aircraft is rolled, the lift vector is tilted in the direction
of the roll. We can break the lift vector into two
One component is vertical and opposed to the weight which is always directed
towards the center of the earth. The other component is an unopposed side
force which is in the direction of the roll, and perpendicular to the
As long as the aircraft is banked, the side force is a constant, unopposed
force on the aircraft. The resulting motion of the
center of gravity of the aircraft is a
circular arc. When the wings are brought level
by an opposing motion of the ailerons, the side force is eliminated
and the aircraft continues to fly in a straight line along a new heading.
Notice that the
is not used to turn the aircraft. The aircraft is turned through the action of
the side component of the lift force. The rudder is used during the turn
to coordinate the turn, i.e. to keep the nose of the aircraft pointed
along the flight path. If the rudder is not used, one can encounter an
adverse yaw in which the drag on the outer wing pulls the aircraft
nose away from the flight path.
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