# What does a higher pitching moment mean? (ie -0.095 vs -0.0022)

I've found that as the angle of attack increases, the moment becomes more negative. I wanted to know what does that negative value say about the characteristics of the airfoil, and what is its importance.

## 1 Answer

A negative pitching moment is stabilising: it is a nose down pitching moment, which is what is required for static stability.

If the stabilising moment of the wing profile increases with AoA, the horizontal tail volume can be smaller - less resistance.

• so to clarify, if the coefficient say is -0.50, it stabilizes more than a -0.01 – Kino Jul 28 '19 at 9:44
• Yes indeed, -0.01 is almost neutral. – Koyovis Jul 28 '19 at 11:52
• Wait, no. You are talking about the pitch moment derivative per AoA ($Cm_{\alpha}$), not the moment itself. The derivative should be negative for stability, and its magnitude largely determines the 'amount' of stability; but the moment itself can be anything. Positive for negative AoA, for example. – Zeus Jul 29 '19 at 6:59
• @Zeus Yes the $C_M$/$\alpha$ of the whole plane must be negative, a result of the sum of wing, tailplane and fuselage moments. The $C_m$ of the wing profile is by definition constant in the aerodynamic centre. If the wing contributes more nose-down moment, the tailplane may contribute less. And indeed, total $C_M$ of wing, tailplane & fuselage at negative AoA will be positive. – Koyovis Jul 29 '19 at 7:27
• It is not clear what OP is really asking then. For an airfoil, "as the angle of attack increases, the moment becomes more negative" is meaningless without a reference, yet at the standard reference (AC) it is indeed const. If he's asking about the negative $Cm_0$ (which doesn't change with AoA!) of a typical camber airfoil but in the context of the whole airplane, then you can't really say $Cm_0$ is 'stabilising': it's a bias, and if you put CG between AC of the wing and NP (which is quite normal), the wing will be destabilising regardless. – Zeus Jul 29 '19 at 7:59