Movement and Stability
In general an aircraft can make three kinds of movements. These movements comprise roll, pitch and yaw and can be made individually or combined. The motion of an airplane may be considered to occur about and along each of three imaginary axes (figure 1.1). The longitudinal axis runs from the nose of the airplane to the tail. The lateral axis runs across the airplane from wingtip to wingtip. The vertical axis runs vertically down.

Stability greatly depends on how an aircraft tends to return to its original attitude after a particular disruption. A stable airplane will return to its original attitude without specific pilot action. An unstable airplane will show an oscillating movement without the tendency to return to its original attitude. An airplane, which remains in the disturbed position, is neutrally stable and pilot input is required to return the aircraft into its normal flight attitude. Each type of stability can be further divided into static stability and dynamic stability. This depends on the time when stability is con-sidered.

Longitudinal axis
The longitudinal axis (1) runs through the centre of gravity from front to rear. Movement around the longitudinal axis is called rolling and is controlled by the ailerons. Stability around the longi-tudinal axis is known as lateral stability.

Lateral axis
The lateral axis (2) runs through the centre of gravity across the airplane from one side to the other. Movement around the lateral axis is called pitching and is controlled by the elevators. Stability around the lateral axis is known as longitudinal stability.

Vertical axis
The vertical axis (3) or normal axis runs vertically down through the centre of gravity and is per-pendicular to the other two axes. Movement around the vertical axis is called yawing and is con-trolled by the rudder. Stability around the vertical axis is called directional stability.

Figure 1.1 - Aircraft imaginary axes


Although lateral and directional stability are carefully balanced to complement each other in flight, the complex manner in which they interact leaves the aircraft with a certain degree of in-stability. The most common problems are Dutch Roll and Spiral Instability.
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