angle of attack stability and CG

[JABBARA]

Hi,

In fact, it is very simple, but I hope I can explain in English.

Stability is the tendency of the airplane to return to its trimmed position if somehow it is disturbed by gust or pilot input. For an airliner, stability is very desirable to increase the handling quality.

To establish this stability for pitch axis (longitudinal stability), elementary requirement is that the CG should always be in front of the aerodynamic force created by the wing. To visualize this, assume a cruising airplane is disturbed pitch up by any reason. Since AOA will momentarily increase, the aerodynamic force will also be momentarily larger than before. When this force is behind the CG, moment about the CG will increase in the direction to dampen the pitch up moment (this is the stability). Since this moment (moment=force x distance) is proportional to the distance between aerodynamic force and CG, as the CG moves forward, stability will increase or vice versa.

In the extreme case, If CG moves behind the aerodynamic force, airplane will be completely instable or unflyable. In fact, this is the desired design for fighter airplanes because the stability decreases maneuverability, however instability increase the agility. To make this airplane flyable, FBW technology is used but this is completely different issue.

I hope I could explain.

Regards.