Which control surface primarily influences longitudinal stability in an aircraft?

The elevator is the control surface that primarily influences longitudinal stability in an aircraft. This is because the elevator is located on the tail of the aircraft and is responsible for controlling the pitch, which dictates the nose's angle in relation to the horizon. When the elevator is deflected upward, it increases the aircraft's angle of attack, causing the nose to rise. Conversely, deflecting it downward causes the nose to lower.

Longitudinal stability refers to the aircraft's ability to maintain a steady flight path along its longitudinal axis without excessive control input. The elevator plays a crucial role in ensuring that the aircraft can achieve and maintain a desired pitch attitude, which directly impacts its stability during flight. Properly designed and positioned, the elevator can provide feedback to the pilot, helping to stabilize the aircraft against disturbances that might cause it to pitch up or down.

Other control surfaces have different primary functions. The rudder primarily influences directional stability and is responsible for controlling yaw. Ailerons are mainly used for lateral control, allowing the aircraft to roll or bank. Flaps, while they enhance lift and control during slow flight or landing, are not primarily responsible for stability. Thus, the elevator is the correct choice as it directly affects an aircraft's longitudinal stability.