# Is there a difference in stability between a tail downforce (cg ahead of ac) and a tail upforce (cg behind)?

So from what I understand, having a -

1.cg ahead of the neutral point(ac) with a tail downforce

and

2.cg behind ac with a tail upforce

are both stable configurations.(Please correct me if I'm wrong.)

Will the first configuration be more stable due to less lift at the tail? What are the advantages and disadvantages of both these configuration?

Also if I have a configuration with cg between cp and neutral point, neutral point being the most aft point, will it be stable?

No, cg ahead of neutral point is stable, cg behind it is unstable.

Neutral point is the centre of pressure when both main wing and horizontal stabilizer fly at the same $$C_L$$. If the cg is just a bit ahead of neutral point, the tail still flies with positive lift. If it is ahead of the centre of pressure of the main wing, tail flies with negative lift.

The further forward the centre of gravity is, the bigger the difference between the coefficients of lift and therefore the stronger restoring force. But note that since it is second order negative feedback, the restoring force creates phugoid oscillation and if that is too fast, it will be difficult for the pilot to stop. So the stability has to be just right.

This is still true in canard configuration, but there the forward surface that flies at higher $$C_L$$ in stable configuration is the canard.

• When you say neutral point do you mean the aerodynamic center of the entire aircraft? – Zaber Aug 6 '18 at 20:20
• @Zaber, I mean aerodynamic center of the entire aircraft at a very specific condition when both main wing and horizontal stabilizer fly at the same coefficient of lift—which is the point where putting the cg will result in neutral stability. Because aerodynamic center of the entire aircraft moves with the elevator position otherwise. – Jan Hudec Aug 6 '18 at 20:31
• (@Zaber, and wikipedia confirms that that is indeed the correct terminology) – Jan Hudec Aug 6 '18 at 20:34
• Am I correct in saying that the tail will fly with a negative lift when cg is ahead of the wing aerodynamic center? – Zaber Aug 6 '18 at 20:42
• @Zaber, yes, you are. But it is mostly irrelevant for longitudinal stability, which requires that the aft airfoil flies at lower coefficient of lift than the forward airfoil, but it does not have to be negative (of course when it is negative, the difference is bigger and that means the restoring force is stronger). – Jan Hudec Aug 6 '18 at 20:46