angle of attack stability and CG
 

May someone please tell me what is the relation between angle of attack stability and CG? I know a more forward CG gives better stability and less sensitive control, but why? Is it related to the difference of AOA of the wing and the tail?
I remember learnt somewhere that the larger the AOA differnce of the wing and tail, the higher the AOA stability, is it true?
Thanks in advance.
Tim.

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.

'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.'

Not necessarily. Think about decalage/longitudinal dihydral. Check:

http://www.av8n.com/how/htm/aoastab.html

That's all well & good and looks nice on paper, but placing the CG behind the mainplane center of lift implies three things:

1. The horizontal tail lift is positive, and

2. The aircraft is seriously lacking natural stability.

3. It also means that Cg movement (fuel burnoff) does not substantially affect cruise drag. (Cg movement would only shift the load share between main & tail planes)

Thus I have a problem accepting this configuration - unless artificial stability / FBW are controlling things.

In real airliners we know that Cg movement affects stability AND ALSO cruise drag - and the reason is that the stab has negative lift, to counter the nose-down moment of the Cg-Cp couple.

Further evidence is found in the Lusaka 707 freighter accident in the 70's in which half the horizontal tail failed on approach. Did the aircraft pitch nose-up? No, it immediately went nose-down due to the loss of half the nose-up moment of the tail.

If we can resolve these pieces of evidence with a lifting tail, then I might be convinced otherwise...