1-18 Directional Control about the Vertical Axis (YAW)

Directional control about the vertical axis of the airplane is obtained through the use of the rudder. The rudder is a movable surface hinged to the trailing edge of the vertical stabilizer (fin) and attached by mechanical linkage to the rudder pedals located in the cockpit. By pressing the right rudder pedal, the rudder is deflected to the right, which causes the relative wind to deflect the tail to the left and the nose to the right. If left rudder pressure is applied, the reverse action occurs and the nose is deflected to the left. It should be understood that the purpose of the rudder during flight is to control yaw and not to turn the airplane. [Figure 1-31]
Figure 1-31.—Effect of rudder.
Some airplanes are equipped with a rudder trim tab, which reacts in a similar manner on the rudder as does the aileron trim tab on the aileron and the elevator trim tab on the elevator.

 The amount of control which the pilot has over the airplane is dependent upon the speed of the airflow striking the control surfaces. Effective airplane stability also depends upon speed of the airplane through the air. The greater the airspeed the greater the effect of stability as a restoring force.


An airplane is designed and certificated for a certain maximum weight during flight. This weight is referred to as the maximum certificated gross weight.  It is important that the airplane be loaded within the specified weight limits before flight, because certain flight maneuvers will impose an extra load on the airplane structure which may, particularly if the airplane is overloaded, impose stresses which will exceed the design capabilities of the airplane. Overstressing the airplane can also occur if the pilot engages in maneuvers creating high loads, regardless of how the airplane is loaded.  These maneuvers not only increase the load that the airplane structure must support, but also increase the airplane’s stalling speed.

The following will explain how extra load is imposed upon the airplane during flight. During flight, the wings of an airplane will support the maximum allowable gross weight of the airplane. So long as the airplane is moving at a steady rate of speed and in a straight line, the load imposed upon the wings will remain constant. A change in speed during straight flight will not produce any appreciable change in load, but when a change is made in the airplane’s flightpath, an additional load is imposed upon the airplane structure. This is particularly true if a change in direction is made at high speeds with rapid forceful control movements.
 According to certain laws of physics, a mass (airplane in this case) will continue to move in a straight line unless some force intervenes, causing the mass (airplane) to assume a curved path. During the time the airplane is in a curved flightpath, it still attempts, because of inertia, to force itself to follow straight flight. This tendency to follow straight flight, rather than curved flight, generates a force known as centrifugal force which acts toward the outside of the curve.

 Any time the airplane is flying in a curved flightpath with a positive load, the load the wings must support will be equal to the weight of the airplane plus the load imposed by centrifugal force. A positive load occurs when back pressure is applied to the elevator, causing centrifugal force to act in the same direction as the force of weight. A negative load occurs when forward pressure is applied to the elevator control, causing centrifugal force to act in a direction opposite to that of the force of weight.
 Curved flight producing a positive load is a result of increasing the angle of attack and consequently the lift. Increased lift always increases the positive load imposed upon the wings. However, the load is increased only at the time the angle of attack is being increased. Once the angle of attack is established, the load remains constant. The loads imposed on the wings in flight are stated in terms of load factor.