Talk:Aerodynamic center
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Challenge
[edit]There seems to be a problem with the clause: "airfoil does not vary with lift coefficient i.e. angle of attack, " Expert? — Preceding unsigned comment added by 68.121.33.199 (talk) 20:02, 15 January 2012 (UTC)
It's a strange phrase, but it is correct. Rewording the introduction wouldn't be a bad idea. — Preceding unsigned comment added by 75.142.96.151 (talk) 08:05, 30 April 2012 (UTC)
Neutral point
[edit]Is the aerodynamic center the same thing as the neutral point? cagliost (talk) 14:11, 29 June 2022 (UTC)
Answer: The aerodynamic center refers only to the wing. The neutral point refers to the whole aircraft. The neutral point is the aerodynamic center of the whole aircraft. "Instead of the centre of pressure, then, we use an alternative reference point called the aerodynamic centre or, for a whole aircraft, the neutral point." cagliost (talk) 14:12, 29 June 2022 (UTC)
- Cagliost I agree that the aerodynamic center and the neutral point are not the same; they are defined to be very different. There will be examples of certain aircraft in which the AC and the NP are at the same point, but this is a coincidence and does not indicate that the two have the same meaning. Tailless aircraft may be examples of this coincidence.
- If the CG of an aircraft is at the AC the longitudinal stability is usually satisfactory so the AC usually lies within the approved CG envelope for the aircraft.
- In stability analyses consideration must be given to the lift on the wing acting at the AC of the wing; and the lift (often downwards) on the horizontal tailplane acting at the AC of the tailplane. The neutral point is the point at which longitudinal stability is zero (neutral stability) if that is the location of the center of gravity. Neutral stability is unsatisfactory for most aircraft because positive longitudinal stability is essential for safe flying characteristics. Consequently the aft limit of the approved CG range for such aircraft must be forward of the neutral point, and the aircraft will never be flown with the CG at the NP.
- Stability analysis for aircraft with fixed horizontal stabilizer and movable elevator results in the aircraft having two neutral points - the NP with stick-fixed; and the NP with stick-free. It is most common for the NP stick-free to lie ahead of the NP stick-fixed, but both must be outside (behind) the approved CG range for the aircraft. Dolphin (t) 12:41, 5 July 2022 (UTC)
Equation for the Aerodynamic Center
[edit]Early in the article it states that at the Aerodynamic center the derivative of the Coefficient of Pitching Moment with respect to the Coefficient of Lift is zero. I believe that would only be true if the aerodynamic center was at the axis of rotation of interest (e.g. the elastic axis). It would be better to say that this is a partial derivative of the Coefficient of Pitching Moment with respect to the partial of Coefficient of Lift, and that it equals a constant that can be negative or positive or zero. Below there, the math appears correct, showing that the x-axis position of the Aerodynamic Center is the sum of a reference point (e.g. elasticit axis) plus the chord by the derivative of the Coefficient of Pitching Moment with respect to the Coefficient of Lift. If this derivative were zero, then the aerodynamic center would be at the elastic axis (which is the way helicpoter blades are designed). Old NACA papers on the behavior of airfoils called this ξ, lower-case xi. Thank you. 2600:6C40:2540:1C00:25E3:E323:E736:6336 (talk) 14:13, 9 August 2025 (UTC)