Three Axes of Rotation
The glider is maneuvered around three axes of rotation: yaw (vertical), lateral, and longitudinal. They rotate around one 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]
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.
Another factor that is critical to the longitudinal stability of a glider is its loading in relation to the CG. The CG of the glider is the point at which the total force of gravity is considered to act. When the glider is improperly loaded so it exceeds the aft CG limit, it loses longitudinal stability. As airspeed decreases, the nose of a glider rises. To recover, control inputs must be applied to force the nose down to return to a level flight attitude. It is possible that the glider could be loaded so far aft of the approved limits that control inputs are not sufficient to stop the nose from pitching up. If this were the case, the glider could enter a spin from which recovery would be impossible. Loading a glider with the CG too far forward is also hazardous. In extreme cases, the glider may not have enough pitch control to hold the nose up during an approach to a landing. For these reasons, it is important to ensure that the glider is within weight and balance limits prior to each flight. Proper loading of a glider and the importance of CG is discussed further in Chapter 5, Performance Limitations.
Another factor that can affect the ability to control the glider is flutter. Flutter occurs when rapid vibrations are induced through the control surfaces while the glider is traveling at high speeds. Looseness in the control surfaces can result in flutter while flying near maximum speed. Another factor that can reduce the airspeed at which flutter can occur is a disturbance to the balance of the control surfaces. If vibrations are felt in the control surfaces, reduce the airspeed.
Another type of stability that describes the glider’s tendency to return to wings-level flight following a displacement is lateral stability. When a glider is rolled into a bank, it has a tendency to sideslip in the direction of the bank. For example, due to a gust of wind, the glider wing is lifted and the glider starts to roll. The angle of attack on the downward going wing is increased because the wing is moving down and now the air is moving up past it. This causes the lift on this wing to increase. On the upward going wing, the opposite is occurring. The angle of attack is reduced because the wing is moving up and the air is moving down past it. Lift on this wing is therefore reduced. This does produce a countertorque that damps out the rolling motion, but does not roll the glider back to wings level as the effect stops when the glider stops. [Figure 3-23] To obtain lateral stability, dihedral is designed into the wings.
Dihedral is the upward angle of the wings from a horizontal (front/rear view) axis of the plane. As a glider flies along and encounters turbulence, the dihedral provides positive lateral stability by providing more lift for the lower wing and reducing the lift on the raised wing. As one wing lowers, it becomes closer to perpendicular to the surface and level. Because it is closer to level and perpendicular to the weight force, the lift produced directly opposes the force of weight. This must be instantly compared to the higher and now more canted wing referenced to the force of weight. The higher wing’s lift relative to the force of weight is now less because of the vector angle. This imbalance of lift causes the lower wing to rise as the higher descends until lift equalizes, resulting in level flight. [Figure 3-24]