DYNAMIC LONGITUDINAL STABILITY
Longitudinal dynamic stability consists of two basic modes, one of which you have already met, phugoid (Figure 2.6). Phugoid motion consists of long period oscillations that involve noticeable changes in pitch attitude, aircraft altitude and airspeed. The pitching rate is low and because only very small changes in AOA occur, damping is weak and sometimes negative.
The second mode involves short period motion of relatively high frequency that involves negligible changes in aircraft velocity. During this type of motion, static longitudinal stability restores the aircraft to equilibrium and the amplitude of the oscillation is reduced by the pitch damping contributed by the tail plane (horizontal stabilizer). If instability was to exist in this mode of oscillation, porpoising of the aircraft would occur and because of the relative high frequency of oscillation, the amplitude could reach dangerously high proportions with severe flight loads being imposed on the structure.
DIRECTIONAL STABILITY
As you already know, directional stability of an aircraft is its inherent (built-in) ability to recover from a disturbance in the yawing plane, i.e. about the normal axis. However, unlike longitudinal stability, it is not independent in its influence on aircraft behaviour because as a result of what is known as aerodynamic coupling, yaw displacement moments also produce roll displacement moments about the longitudi-nal axis. As a consequence of this aerodynamic coupling aircraft directional motions have an effect on lateral motions and vice versa.
The nature of these motions is yawing, rolling and sideslip, or any combination of the three.
With respect to yawing motion only, the primary influence on directional stability is provided by the fin (or vertical stabilizer). As the aircraft is disturbed from its straight and level path by the nose or tail being pushed sideways (yawed), then due to its inertia the aircraft will continue to move in the direction created by the disturbance. This will expose the keel surface to the on-coming airflow. Now the fin, acting as a vertical aerofoil, will generate a sideways lift force which tends to swing the fin back towards its original position, straightening the nose as it does so.
It is thus the powerful turning moment created by the vertical fin, due to its large area and distance from the aircraft CG, which restores the aircraft nose back to its original position (Figure 2.7). The greater the keel surface area (which includes the area of the fin) behind the CG, and the greater the moment arm, then the greater will be the directional stability of the aircraft. Knowing this, it can be seen that a forward CG is preferable to an aft CG, since it provides a longer moment arm for the fin.
We finish our study of basic aerodynamics by looking briefly at the way in which aircraft are controlled. This introduction to the subject is provided here for the sake of completeness and in order to better understand the interactions between stability and control.
Longitudinal dynamic stability consists of two basic modes, one of which you have already met, phugoid (Figure 2.6). Phugoid motion consists of long period oscillations that involve noticeable changes in pitch attitude, aircraft altitude and airspeed. The pitching rate is low and because only very small changes in AOA occur, damping is weak and sometimes negative.
The second mode involves short period motion of relatively high frequency that involves negligible changes in aircraft velocity. During this type of motion, static longitudinal stability restores the aircraft to equilibrium and the amplitude of the oscillation is reduced by the pitch damping contributed by the tail plane (horizontal stabilizer). If instability was to exist in this mode of oscillation, porpoising of the aircraft would occur and because of the relative high frequency of oscillation, the amplitude could reach dangerously high proportions with severe flight loads being imposed on the structure.
DIRECTIONAL STABILITY
As you already know, directional stability of an aircraft is its inherent (built-in) ability to recover from a disturbance in the yawing plane, i.e. about the normal axis. However, unlike longitudinal stability, it is not independent in its influence on aircraft behaviour because as a result of what is known as aerodynamic coupling, yaw displacement moments also produce roll displacement moments about the longitudi-nal axis. As a consequence of this aerodynamic coupling aircraft directional motions have an effect on lateral motions and vice versa.
The nature of these motions is yawing, rolling and sideslip, or any combination of the three.
With respect to yawing motion only, the primary influence on directional stability is provided by the fin (or vertical stabilizer). As the aircraft is disturbed from its straight and level path by the nose or tail being pushed sideways (yawed), then due to its inertia the aircraft will continue to move in the direction created by the disturbance. This will expose the keel surface to the on-coming airflow. Now the fin, acting as a vertical aerofoil, will generate a sideways lift force which tends to swing the fin back towards its original position, straightening the nose as it does so.
It is thus the powerful turning moment created by the vertical fin, due to its large area and distance from the aircraft CG, which restores the aircraft nose back to its original position (Figure 2.7). The greater the keel surface area (which includes the area of the fin) behind the CG, and the greater the moment arm, then the greater will be the directional stability of the aircraft. Knowing this, it can be seen that a forward CG is preferable to an aft CG, since it provides a longer moment arm for the fin.
We finish our study of basic aerodynamics by looking briefly at the way in which aircraft are controlled. This introduction to the subject is provided here for the sake of completeness and in order to better understand the interactions between stability and control.
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