Aircraft Yaw Control

OPERATION AND EFFECT OF YAW CONTROL

RUDDERS

The rudder is a vertical control surface that is usually hinged to the tail post aft of the vertical stabilizer and designed to apply yawing moments to the airplane, that is, to make it turn to the right or the left about the vertical axis.

The movement of the rudder is controlled by pedals operated by the feet of the pilot. The pedals are interconnected so that when one is pressed the other one move in the opposite direction. When the right pedal is pressed, the rudder swings to the right, thus bringing an increase of dynamic air pressure on its right side. This increased pressure causes the tail of the airplane to swing to the left and the nose to turn to the right.

Although it appears that the rudder causes the airplane to turn, it must be pointed out that the rudder itself cannot cause the airplane to make a good turn. Newton's first law of motion states that a moving body tends to continue moving in a straight line unless some outside force changes its direction.

When rudder is applied to an airplane in flight, the airplane will turn, but it will continue to travel in the same direction as before unless a correcting force is applied.

Thus, with rudder only, the airplane will turn sideways and skid. In order to prevent this skid in a turn, the ailerons are used to bank the airplane. In a car, a banked turn is much easier to negotiate at comparatively high speeds than a flat turn. It is the same with an airplane. To prevent skidding in a turn, the airplane must be banked.

Too much of a bank without sufficient rudder in a turn will cause slipping; that is, the airplane will slide down toward the inside of the turn. It is therefore necessary that the proper amount of rudder and aileron be applied when entering a turn in order to produce what is termed a coordinated turn. Usually, after the airplane is placed in a turn, the rudder pressure is almost neutralized to hold the turn. Likewise, it is necessary to reduce the amount of aileron used to place the airplane in turn.

Another factor to note concerning turns is that the steeper the turn, the more the elevator will have to be used. Thus, a properly executed turn requires the use of all three of the primary controls.

Rudder is also used for correcting asymmetry caused by engine failure of a multi engine aircraft and for control (steering) of aircraft on ground at high speeds. For these reasons rudder is made large and placed at the tail so that it has a large moment arm.

RUDDER LIMITER

But because of its size and placement, at high speeds a little movement of the rudder will cause the aircraft to yaw dangerously. Therefore, as the speed of the aircraft increases the rudder travel needs to be limited. This is achieved by progressively mechanically limiting rudder travel when aircraft speed is increased beyond a certain speed sometimes pedal travel too may get limited. Some manufacturers even go to the extent of limiting the rate of deflection of rudder with increasing

aircraft speed.