Which part of the aircraft does the rudder affect?

The rudder is a crucial component of an aircraft's control surface located on the vertical stabilizer (tail fin) and primarily affects the aircraft's directional stability. By changing the angle of the rudder, pilots can control the yaw of the aircraft, which is the movement around the vertical axis. This allows the aircraft to turn left or right and helps to counteract any unwanted yawing motion caused by factors like wind or asymmetric thrust.

Directional stability refers to how well the aircraft maintains its intended direction without excessive pilot inputs. When the aircraft yaws to one side, the rudder can be used to bring it back on course, thereby enhancing directional stability.

The other concepts, such as longitudinal stability, vertical stability, and rotational stability, pertain to different aspects of flight. Longitudinal stability relates to pitch control (movement around the lateral axis), vertical stability pertains to the ability of the aircraft to maintain a steady heading, and rotational stability is a broader term that can refer to how the aircraft responds to inputs in any flight direction. However, these factors do not directly describe the unique role of the rudder, making directional stability the accurate answer in this context.