Primary Flight Controls
The primary flight controls are used to move the aircraft around the three axes we discussed earlier. First of all we will explain the ailerons which are operated by control wheel rotation and control the motion of the aircraft around the longitudinal axis. The elevators are operated by forward and rearward movement of the control wheel and control the movement of the aircraft around the lateral axis and will be discussed. Last but not least, the rudder is operated by movement of the two interconnected rudder pedals and maintains directional control.


The Ailerons
The ailerons (figure 1.1) (1) are small moving surfaces usually positioned on the outboard trailing edges of the wing and controlling the rolling motion of the airplane. The ailerons act in opposite senses, if one moves up the other goes down. Deflection of the ailerons is achieved by rotating the control wheel left or right. When the control wheel is turned to the left, the right aileron moves down. Because of pressure differences, the right aileron will cause an aerodynamic force, which is lift. The left aileron moves up, and creates a decrease of lift on the left wing. This action will result in a rolling motion to the left.

In flight it is possible that control surfaces (mostly the ailerons) start oscillating as air travels over it. This effect is called flutter.


Figure 1.1 General ETOPS
The Elevators
The elevators (figure 1.2) (1) are the moving surfaces attached to each side of the horizontal stabilizer and control the pitching motion of the airplane. The elevators work in pairs. Deflection of the elevators is achieved by forward and rearward movement of the control column and moves both elevators in the same direction. When the control column is pulled backwards the elevator moves upwards and the aircraft pitches up. This is the same when the control column is pushed forwards, the elevator moves downwards and the aircraft pitches down. When the elevators change position, the airflow and aerodynamic forces change as well. If the elevators are down, the speed of the airflow above the surface increases. This creates a low pressure above the tail plane, and a high pressure beneath it.

Figure 1.2 - Elevator operation

By the differences of pressure an aerodynamic force is created. The aerodynamic force has an upward effect on the tail plane and causes a rotation around the CoG. The tail will move upwards, while the nose moves downwards. To retain safe handling characteristics during flight the CoG has to be kept within prescribed limits. Some other airplanes (smaller aircraft or jet fighters) have an all-flying tail (4). The all-flying tail is a single moving surface with no horizontal stabilizer. Both systems work according to the same principle.


ETOPS Route Segments
The rudder (figure 1.3) (1) is attached to the vertical stabilizer (2). It’s the small moving section of the vertical stabilizer, and controls the movement around the vertical axis. The vertical stabilizer prevents the yawing motion (3) of the nose and is operated by the rudder pedals. The pedals work in the opposite direction of each other, i.e. when one pedal is pushed forward, the other pedal moves backward, and vice versa. On most small airplanes the movement of the nose wheel is controlled by the rudder pedals. If the right pedal is pushed forward, the rudder will move to the right and thus turning the aircraft to the right.

The airflow on the left side of the rudder will move faster than the airflow on the right side. Because of the difference in speed, the pressure changes as well. The pressure on the right side will increase, while the pressure on the left side will decrease. The difference between pressures creates an aerodynamic force, which pushes the tail of the airplane to the left, and the nose to the right. The airplane rotates around the CoG. The rudder is used in both small and large aircraft for turn coordination. In large aircraft unwanted yaw is eliminated by the yaw damper which we will discuss later on.


Figure 1.2 - Rudder operation
Article Related

Articles

Images