Control Surfaces

A vertical stabilizer, or tail fin, keeps the airplane lined up with its direction of motion. Air presses against both its surfaces with equal force when the airplane is moving straight ahead. But if the airplane pivots to the right or left, air pressure increases on one side of the stabilizer and decreases on the other. This imbalance in pressure pushes the tail back into line.

Like the vertical stabilizer, the horizontal stabilizer helps keep the airplane aligned with its direction of motion. If the airplane tilts up or down, air pressure increases on one side of the stabilizer and decreases on the other, pushing it back to its original position. The stabilizer also holds the tail down, counteracting the tendency of the nose to tilt downward--a result of the airplane's center of gravity being forward of the wing's center of lift.

To help make turning easier, an airplane is usually less stable along its roll axis than along its pitch and yaw axes. Several factors help the pilot keep the wings level: the inclined mounting of the wings, the position of the wings above or below the fuselage, the swept-back shape of the wings, and the vertical stabilizer. As an airplane rolls, it tends to slip to the side, changing the direction of relative wind on the wings and tail. These design features help the pilot restore the airplane to its upright position.

The ailerons control roll. On the outer rear edge of each wing, the two ailerons move in opposite directions, up and down, decreasing lift on one wing while increasing it on the other. This causes the airplane to roll to the left or right. To turn the airplane, the pilot uses the ailerons to tilt the wings in the desired direction.

The elevator controls pitch. On the horizontal tail surface, the elevator tilts up or down, decreasing or increasing lift on the tail. This tilts the nose of the airplane up and down.

The rudder controls yaw. On the vertical tail fin, the rudder swivels from side to side, pushing the tail in a left or right direction. A pilot usually uses the rudder along with the ailerons to turn the airplane.

(Rev. 09/18/96)