# Does the aerodynamic center move when flaps and slats are deployed?

Does the aerodynamic center of an aircraft move when flaps and slats are deployed?

I am trying to figure out if the AC of an aicraft moves with landing and take-off configurations. I know Cl and Cm change a lot, but I can't find a reference where the movement of the AC is discussed.

I've checked Roskam, Anderson and Aerodynamic Characteristics of Wing Sections and it seems like the AC position doesn't change only with flaps but they don't show examples of wings with flaps and slats.

• Better read Fluid Dynamic Lift, chapter 5, section III. This should answer all your questions. Commented Dec 12, 2017 at 21:58

The aerodynamic center does not move when plain or camber flaps are used. A plain flap will only change the camber of the wing and, therefore, will only affect its zero-lift pitching moment. Flaps have their own aerodynamic center, and their lift change should be summed up in the flap's aerodynamic center.

Note that the aerodynamic center of the aircraft is used for the lift force change resulting from a change in the angle of attack while the flap aerodynamic center is used for the lift force change from a change in the flap's deflection angle. With flaps deployed, a change in the angle of attack will still cause a lift change which acts on the aircraft's aerodynamic center! Only the constant pitching moment around the aerodynamic center will change with flap deflection, and in order to calculate this moment, assume the lift change from flap deflection to act in the flap aerodynamic center.

The location of the flap aerodynamic center changes with the relative chord of the flap. For a flap approaching a relative chord of 0, the theoretical flap aerodynamic center is at 50% of the wing's chord. With increasing flap chord the flap aerodynamic center moves forward proportionally with the flap's chord increase until it reaches the 25% chord point for a flap of a relative chord of 1 (a full-flying control surface, in other words). See Figure 46a from chapter 5 of Sighard Hörner's Fluid Dynamic Lift for proof.

Flaps which increase the wing's area will of course shift the neutral point in proportion to the change in the wing's chord effected by the flap.

Slats may be viewed like a slat-sized wing followed by a wing-sized flap and an incidence change equal to the slat's deflection angle.

• Thank you for your answer! It seems like I'll need time to understand it completely
– JGG
Commented Dec 13, 2017 at 18:11

Yes the AC changes when flaps are deployed. Torenbeek has a section on change of pitching moment as a function of flap deflection in Appendix G, section G-3.3.b. The methods described are empirical, based on statistical information and on Glauert's linear thin airfoil theory.

The shift in pitching moment is of course highly dependent on the type of flaps used, the deflection, trailing edge flap span, wing sweep, and change in wing camber and chord extension.

The author has a chapter on leading edge devices as well, for which he signals a lack of empirical data. There is no section for AC change as a function of leading edge flap deflection.

• Yes, it seems like everyone estimates the change of pitching moment but neglect the changes in the AC position. That is what I've done but I like to be sure about the real AC position. Thank you for your answer :)
– JGG
Commented Dec 12, 2017 at 16:15
• Um - a change in pitching moment is a change in AC. Commented Dec 12, 2017 at 21:45