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I was reading CAE Oxford POF book 2014 edition found online, and in lesson (10-Stability and control), page239, it stated that CG must be BEHIND AC in order to maintain aircraft's stability, but I found a lot of videos and articles online stating the complete opposite, that the CG must be IN FRONT of AC. Now I don't know which is the correct answer, can anyone confirm which is true, and if so, why is the other opinion is making up this statement

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    $\begingroup$ This question is similar to: Why do most commercial aircraft have the Center of Gravity before the Aerodynamic Center?. If you believe it’s different, please edit the question, make it clear how it’s different and/or how the answers on that question are not helpful for your problem. $\endgroup$
    – sophit
    Commented Jul 13, 2024 at 8:42
  • $\begingroup$ Can you post the link(s) of the online article(s) you found stating the opposite? $\endgroup$
    – sophit
    Commented Jul 14, 2024 at 20:21
  • $\begingroup$ @sophit This appears to be one of the versions of the book. It's on page 260 (so different than OP's 239). The site requires you to enter page 266 to arrive there. The image is "Figure 10.19 Unstable wing contribution", so it appears OP misunderstood it as being the stable configuration instead. $\endgroup$
    – levant pied
    Commented Jul 15, 2024 at 18:33
  • $\begingroup$ @levantpied: thanks for the link that I read. What CAE affirms seems to be indeed correct. It would be nice to read the other sources to make a comparison. $\endgroup$
    – sophit
    Commented Jul 15, 2024 at 19:49
  • $\begingroup$ online.pubhtml5.com/aafi/cxah/#p=267 here is the book where i got this info sophit, even in the pages before this one, we can still see the CG is behind the AC $\endgroup$
    – Hassan Ashraf
    Commented Jul 17, 2024 at 4:33

2 Answers 2

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The aerodynamic center of the wing is where it has no tendency either to rotate away from or back into the wind regardless of pitch.

But let us not forget the fuselage and tail, which tends to move the aerodynamic center of the entire aircraft rearwards.

In order to have longitudinal stability, the CG must be ahead of the aerodynamic center.

Next comes the relationship of center of pressure (or lift) of the wing and the CG. In your diagram, they could be closer together and/or have a matching torque contribution from the tail.

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For static stability, the "aerodynamic center" refers to the entire aerodynamic footprint of the aircraft, wings, tail, nacelles, fuselage, that must be behind the C of G. You could say that it's a weathervane effect.

If you want the weathervane to point into the wind, the point at which the forces of the wind are focused has to be behind the pivot. The point of focus is the aircraft's aerodynamic center when factoring in all the various moments acting in the pitch axis, also known as Neutral Point.

In the case of the aircraft, as a free body in space, the pivot is the C of G. Think of it as a kind of a free floating weathervane turned on its side, that can be trimmed to align itself at an off angle to the airflow to get lift from the wings by forcing them into a positive angle to the flow (without the trimming force, it becomes, effectively, a lawn dart, and will arc into the ground with perfect static stability).

If the C of G is on the wrong side of the aerodynamic center, then all you've done is turn the weathervane backwards and it will want to switch around to point the other way.

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