In flight, any aircraft will rotate about its center of gravity, a point which is the average location of the mass of the aircraft. We can define a three dimensional coordinate system through the center of gravity with each axis of this coordinate system perpendicular to the other two axes. We can then define the orientation of the aircraft by the amount of rotation of the parts of the aircraft along these principal axes. The yaw axis is perpendicular to the wings and lies in the plane of the aircraft centerline. A yaw motion is a side to side movement of the nose of the aircraft as shown in the animation.
The yawing motion is being caused by the deflection of the rudder of this aircraft. The rudder is a hinged section at the rear of the vertical stabilizer.
As described on the shape effects slide, changing the angle of deflection at the rear of an airfoil changes the amount of lift generated by the foil. For the vertical stabilizer and rudder, the orientation of the airfoil causes a side force to be generated. With greater deflection of the rudder to the left, the side force increases to the right. With greater deflection to the right, the side force increases to the left. The change in side force created by deflecting the rudder causes the airplane to rotate about its center of gravity. The pilot uses this ability to keep the nose of the aircraft pointed in the direction of travel.
On all aircraft, the vertical stabilizer and rudder create a symmetric airfoil. This produces no side force when the rudder is aligned with the stabilizer and allows the combination to produce either positive or negative side force, depending on the deflection of the rudder. Some fighter planes have two vertical stabilizers and rudders because of the need to control the plane with multiple, very powerful engines
A still slide of this animation is also available. It looks like this:
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