What device is primarily used to prevent Dutch roll in aircraft?

The device that primarily prevents Dutch roll in aircraft is the yaw damper. Dutch roll is a type of oscillatory motion that involves the combination of yawing and rolling movements, which can make the aircraft uncomfortable and harder to control. The yaw damper serves to stabilize the aircraft by automatically controlling the yaw moments that lead to this type of motion.

When a yaw damper is engaged, it continuously monitors the aircraft's yaw movements and makes subtle adjustments to the rudder. These adjustments help to counteract the natural tendency of the aircraft to enter into a Dutch roll, thereby enhancing stability during flight. This device is particularly important in aircraft with high aspect ratio wings, which are more susceptible to such oscillations.

The other choices play different roles in aircraft control. Auto-pilots manage overall navigation and can include functions to assist with stabilization, but they do not specifically target the prevention of Dutch roll. Ailerons are used primarily for controlling roll, and flaps alter lift characteristics during takeoff and landing but do not contribute directly to controlling yaw oscillations. Therefore, the yaw damper is the dedicated solution for addressing Dutch roll.