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Some sources state that AC must be located behind CG, so that AC is stable. There is a source, which throughout the study of stability and control consider AC located ahead of CG and positive cambered tailplane, stating that such airplane will be stable. I understand that there is not only one right decision and it's possible to design aircraft with AC located both behind CG and in front of. The question is, what is typical arrangement of CG and AC for most modern large jet transport aircraft? (especially interested in b737 and a320)

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  • $\begingroup$ The aerodynamic centre of a wing is different from the AC of the whole airplane (its called N.P).@Nikita $\endgroup$ – David Teahay Sep 24 '18 at 12:34
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AC of the whole aircraft must be located behind CG, so that AC was naturally statically stable [in pitch, or more accurately, by angle of attack]. This pretty much follows from the definition of AC, and there is no way around it.

However, this is not the whole story.

First, it is possible to design an airplane with 'positive cambered tailplane'. As such, there is no requirement that the tailplane produced negative lift (for a classic design); there is merely a requirement that the tailplane had a lower lift coefficient than the wing.

Second, a statically unstable airframe can still be made stable by using computer control. From the point of view of the pilot, F-16 or Su-27 are stable, even though their airframes are not. In some marginal cases, when the aircraft has a lot of pitch damping, even average pilot can cope with mild instability.

As for airliners, all of them have AC behind CG, but I don't have specific numbers.

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  • $\begingroup$ Are there airliners, which have AC of the wing located ahead of the CG? (mostly interested in typical airliners, not marginal cases) $\endgroup$ – Nikita Sep 24 '18 at 7:51
  • $\begingroup$ @Nikita, AC of the wing alone matters little for the whole aircraft stability analysis. For classic airfoils, it may well be forward of CG (of the aircraft), but so what? The tail is there to fix it. But again, I don't have any particular examples at hand. $\endgroup$ – Zeus Sep 24 '18 at 8:08
  • $\begingroup$ Now I got it. Thank you $\endgroup$ – Nikita Sep 24 '18 at 12:59

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