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central point in the glider called the CG. This point is the center of the glider’s total weight and varies with the loading of the glider. 

Yaw is movement around the vertical axis, which can be represented by an imaginary straight line drawn vertically through the CG. Moving the rudder left or right causes the glider to yaw the nose to the left or right. Moving the ailerons left or right to bank moves the glider around the longitudinal axis. This axis would appear if a line were drawn through the center of the fuselage from nose to tail. Pulling the stick back or pushing it forward, raising or lowering the nose, controls the pitch of the glider or its movement around the lateral axis. The lateral axis could be seen if a line were drawn from one side of the fuselage to the other through the CG. [Figure 3-20] 

## Stability 

A glider is in equilibrium when all of its forces are in balance. Stability is defined as the glider’s ability to maintain a uniform flight condition and return to that condition after being disturbed. Often during flight, gliders encounter equilibrium-changing pitch disturbances. These can occur in the form of vertical gusts, a sudden shift in CG, or deflection of the controls by the pilot. For example, a stable glider would display a tendency to return to equilibrium after encountering a force that causes the nose to pitch up. 

Static stability and dynamic stability are two types of stability a glider displays in flight. Static stability is the initial tendency to return to a state of equilibrium when disturbed from that state. The three types of static stability are positive, negative, and neutral. When a glider demonstrates positive static stability, it tends to return to equilibrium. A glider demonstrating negative static stability displays a tendency to increase its displacement. Gliders that demonstrate 

neutral static stability have neither the tendency to return to equilibrium nor the tendency to continue displacement. 

Dynamic stability describes a glider’s motion and time required for a response to static stability. In other words, dynamic stability describes the manner in which a glider oscillates when responding to static stability. A glider that displays positive dynamic and static stability reduces its oscillations with time. A glider demonstrating negative dynamic stability is the opposite situation; its oscillations increase in amplitude with time following a displacement. A glider displaying neutral dynamic stability experiences oscillations, which remain at the same amplitude without increasing or decreasing over time. Figure 3-21  illustrates the various types of dynamic stability. 

Both static and dynamic stability are particularly important for pitch control about the lateral axis. Measurement of stability about this axis is known as longitudinal stability. Gliders are designed to be slightly nose heavy in order to improve their longitudinal stability. This causes the glider to tend to nose down during normal flight. The horizontal stabilizer on the tail is mounted at a slightly negative AOA to offset this tendency. When a dynamically stable glider oscillates, the amplitude of the oscillations should reduce through each cycle and eventually settle down to a speed at which the downward force on the tail exactly offsets the tendency to dive. [Figure 3-22] 

Adjusting the trim assists in maintaining a desired pitch attitude. A glider with positive static and dynamic longitudinal stability tends to return to the trimmed pitch attitude when the force that displaced it is removed. If a glider displays negative stability, oscillations increase over time. If uncorrected, negative stability can induce loads exceeding the design limitations of the glider. 

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