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This video states that the c.g of most non tapered airplane is 25%-30% of wing chord REAR of the leading edge.The aerodynamic center(a.c) of most wing is 25% rear of the leading edge ....Does this mean the a.c can coincide with the c.g in a tailplane?

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  • $\begingroup$ Your title and body ask different questions. Also it’s recommended to use full words (not abbreviations) at least once. $\endgroup$ – Notts90 supports Monica Aug 16 '18 at 11:56
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For longitudinal stability in a conventional wing-and-tail layout, the CG needs to be ahead of the wing's AC. This causes a nose-down moment that is countered by downward lift at the stabilizer, so that the stabilizer will raise the nose as speed builds up. Thus, if the aircraft is disturbed into a dive, the nose will automatically rise as speed builds; conversely, if disturbed nose up, as speed falls off the nose will tend to drop.

Aircraft with active control (especially computer control), as well as some alternative layouts (such as a classic canard) can alter this conventional stability setup -- for instance, in a canard, the stabilizer at the nose lifts, and the CG will be ahead of the leading edge of the wing in most cases.

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    $\begingroup$ Actually, for stability the CG needs to be ahead of the overall neutral point, which is a bit behind the wing aerodynamic centre. But since stability increases as the CG moves forward, usually it ends up ahead of wing AC. $\endgroup$ – Jan Hudec Aug 15 '18 at 21:01
  • $\begingroup$ It's simpler: CG needs to be ahead of the whole aircraft AC (aka neutral point), regardless of the aerodynamic configuration (canard or classic etc.) Whether the tail produces negative lift is immaterial; it is easily possible to make a stable classic design with positive tail lift: you just need to ensure that the tail flies at a lower lift coefficient than the wing. Also, longitudinal stability per se is the short-period AoA stability (the airplane tends to restore AoA when it's disturbed); speed stability is a derived concept. $\endgroup$ – Zeus Aug 17 '18 at 5:45
  • $\begingroup$ @Zeus I agree, but IMO that's more answer than this question asked for. $\endgroup$ – Zeiss Ikon Aug 17 '18 at 11:06
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Yes, the centre of gravity can coincide with the aerodynamic centre of the main wing, particularly because that point is rather unremarkable. For stability, the centre of gravity needs to be ahead of the neutral point, but that is the overall aerodynamic centre if both main wing and horizontal stabilizer are flying at the same coefficient of lift, which means it is behind that of the main wing itself.

Aft limit of 30% of mean aerodynamic chord seems pretty typical, though IIRC the forward limit is usually further forward than 25%.

The aerodynamic center(a.c) of most wing is 25% rear of the leading edge

Yes, but keep in mind that this is only a typical value. In particular supercritical airfoils tend to have it quite a bit further aft. But you rarely see those on straight-wing aircraft; usually on swept-wing ones.

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  • $\begingroup$ According to this en.m.wikipedia.org/wiki/Longitudinal_static_stability , doesnt this mean that an airplane with c.g at the aerodynamic centre will have neutral stability? $\endgroup$ – David Teahay Aug 21 '18 at 12:32
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    $\begingroup$ @DavidTeahay, that wikipedia article uses the term “aerodynamic centre” to mean the “neutral point”. Unfortunately the term is used to mean other things in other contexts (especially aerodynamic centre of the airfoil vs. aerodynamic centre of the aircraft); the term “neutral point” is clearer. $\endgroup$ – Jan Hudec Aug 21 '18 at 21:55

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