3
$\begingroup$

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.

$\endgroup$
3
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ so to clarify, if the coefficient say is -0.50, it stabilizes more than a -0.01 $\endgroup$ – Kino Jul 28 at 9:44
  • 1
    $\begingroup$ Yes indeed, -0.01 is almost neutral. $\endgroup$ – Koyovis Jul 28 at 11:52
  • $\begingroup$ 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. $\endgroup$ – Zeus Jul 29 at 6:59
  • $\begingroup$ @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. $\endgroup$ – Koyovis Jul 29 at 7:27
  • $\begingroup$ 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. $\endgroup$ – Zeus Jul 29 at 7:59

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.