HOW LONGITUDINAL STABILITY IS ACHIEVED
As mentioned previously, an aircraft is longitudinally statically stable if it has the tendency to return to a trimmed AOA position following a pitching disturbance. Consider an aircraft, initially without a tail plane or horizontal stabilizer (Figure 2.3), which suffers a disturbance causing the nose to pitch-up. The CG will continue to move in a straight line, so the effects will be:
• an increase in the AOA;
• the CP will move forward;
• a clockwise moment about the CG provided by the lift force.
This causes the nose to keep rising so that it will not return to the equilibrium position. The aircraft is thus unstable.
If the pitching disturbance causes a nose down attitude, the CP moves to the rear and the aircraft is again unstable (Figure 2.4).
For an aircraft to be longitudinal statically stable, it must meet two criteria:
• A nose-down pitching disturbance must produce aerodynamic forces to give a nose-up restoring moment.
• This restoring moment must be large enough to return the aircraft to the trimmed AOA position after the disturbance.
Thus, the requirements for longitudinal stability are met by the tail plane (horizontal stabilizer). Consider now, the effects of a nose-up pitching moment on an aircraft with tail plane (Figure 2.5). The CG of the aircraft will still continue to move around a vertical straight line. The effects will now be:
• an increase in AOA for both wing and tail plane;
• the CP will move forward and a lift force will be produced by the tail plane;
• the tail plane will provide an anticlockwise restoring moment (LTY), i.e greater than the clockwise moment (Lwx), produced by the wing lift force as the CP moves forward.
A similar restoring moment is produced for a nose-down disturbance except that the tail plane lift force acts downwards and the direction of the moments are reversed.
From the above argument, it can be seen that the restoring moment depends on:
• the size of the tail plane (or horizontal stabilizer);
• the distance of the tail plane behind the CG;
• the amount of elevator movement (or complete tail plane movement, in the case of aircraft with all-moving slab tail planes) which can be used to increase tail plane lift force.
All of the above factors are limited; therefore as a consequence, there will be a limit to the restoring moment that can be applied. It is therefore necessary to ensure that the disturbing moment produced by the wing lift moment about the CG is also limited. This moment is affected by movements of the CG due to differing loads and load distributions, in addition to fuel load distribution.
It is therefore vitally important that the aircraft is always loaded so that the CG stays within the limits specified in the aircraft weight and balance documentation detailed by the manufacturer. The result of failure to observe these limits may result in the aircraft becoming unstable with subsequent loss of control or worse!
As mentioned previously, an aircraft is longitudinally statically stable if it has the tendency to return to a trimmed AOA position following a pitching disturbance. Consider an aircraft, initially without a tail plane or horizontal stabilizer (Figure 2.3), which suffers a disturbance causing the nose to pitch-up. The CG will continue to move in a straight line, so the effects will be:
• an increase in the AOA;
• the CP will move forward;
• a clockwise moment about the CG provided by the lift force.
This causes the nose to keep rising so that it will not return to the equilibrium position. The aircraft is thus unstable.
If the pitching disturbance causes a nose down attitude, the CP moves to the rear and the aircraft is again unstable (Figure 2.4).
For an aircraft to be longitudinal statically stable, it must meet two criteria:
• A nose-down pitching disturbance must produce aerodynamic forces to give a nose-up restoring moment.
• This restoring moment must be large enough to return the aircraft to the trimmed AOA position after the disturbance.
Thus, the requirements for longitudinal stability are met by the tail plane (horizontal stabilizer). Consider now, the effects of a nose-up pitching moment on an aircraft with tail plane (Figure 2.5). The CG of the aircraft will still continue to move around a vertical straight line. The effects will now be:
• an increase in AOA for both wing and tail plane;
• the CP will move forward and a lift force will be produced by the tail plane;
• the tail plane will provide an anticlockwise restoring moment (LTY), i.e greater than the clockwise moment (Lwx), produced by the wing lift force as the CP moves forward.
A similar restoring moment is produced for a nose-down disturbance except that the tail plane lift force acts downwards and the direction of the moments are reversed.
From the above argument, it can be seen that the restoring moment depends on:
• the size of the tail plane (or horizontal stabilizer);
• the distance of the tail plane behind the CG;
• the amount of elevator movement (or complete tail plane movement, in the case of aircraft with all-moving slab tail planes) which can be used to increase tail plane lift force.
All of the above factors are limited; therefore as a consequence, there will be a limit to the restoring moment that can be applied. It is therefore necessary to ensure that the disturbing moment produced by the wing lift moment about the CG is also limited. This moment is affected by movements of the CG due to differing loads and load distributions, in addition to fuel load distribution.
It is therefore vitally important that the aircraft is always loaded so that the CG stays within the limits specified in the aircraft weight and balance documentation detailed by the manufacturer. The result of failure to observe these limits may result in the aircraft becoming unstable with subsequent loss of control or worse!
