Atmospheric Flight

Control Surfaces

An airplane has three control surfaces: ailerons, elevators, and a rudder. The control stick controls the ailerons and elevators. The rudder pedals control the rudders.

The ailerons are flap-like structures on the trailing edge of the wings - one on each side. Ailerons work in opposition. When the pilot moves the control stick to the right, the right aileron will tilt up and the left aileron will tilt down. This will cause the airplane to roll to the right. When the pilot moves the control stick to the left, the left aileron tilts up, the right aileron tilts down and the airplane rolls to the left. This happens because as the aileron tilts downward (effectively increasing camber) more lift is created and the wing rises. As it tilts upward, less lift will be created and the wing will fall. If the wing of one side of the airplane rises and the other descends, the airplane will roll towards the side of the decrease in lift.

The elevators are also flap-like structures that are mounted on each side of the horizontal stabilizer. Flying an airplane in its proper orientation and level to the horizon, when the pilot pushes the control stick forward, the elevators tilt downward. When the pilot pulls the control stick back, the elevators tilt upward. When the elevator tilts downward more lift is created (like the ailerons) and the tail rises and this lowers the nose. This is called pitching downward. When the elevator tilts upward, less lift is created and the tail descends so the fuselage pitches nose-up.

The rudder is located on the vertical fin. The two rudder pedals are located at the pilot's feet. When the pilot pushes on the right rudder pedal, the rudder tilts to the right and the airplane yaws nose-right. When the pilot pushes on the left rudder pedal, the rudder tilts to the left and the airplane yaws nose-left. Again this is due to lift. However, the direction of this lift force is different than the lift force that causes the airplane to fly. When the rudder tilts to the right, lift is created, which "lifts" or pushes the vertical stabilizer to the left. This, in turn, causes the airplane to yaw nose-right. The opposite motion occurs when the rudder tilts to the left.

Airplane control systems rely on moments caused by lift forces. They can be made more effective by changing the force or the moment arm. The controls are usually put far away from the center of gravity to maximize the moment arm. Think about how the amount of force can be varied. Do you think control surfaces move more when an airplane is flying fast or when it is flying slowly? How about when it is high in the atmosphere or close to the ground?

Table of Contents