The location of the aerodynamic centre can be tricky to find in an existing aeroplane, it will shift as a function of speed, Angle of Attack, flap deflection etc. In the pre-design phase the aerodynamic centre is often taken at 25% of Mean Aerodynamic Chord, and since you draw a plan view of the aircraft, we can use this to get an approximation.
If the wing has straight taper over the planform, the MAC can be constructed like in the picture below. Then take the point at 25%: this approximates the location of the aerodynamic centre for one wing, the other side is identical of course, and aerodynamic centre of both wings is halfway the connection line of the two.
However, like many transport jets, the A320 has a trailing edge sweep of zero next to the fuselage to accommodate the main gear. So the aerodynamic centre shifts a bit aft. You can construct it or estimate it, aerodynamicists may frown upon this. There are many sites that show the graphical method of the picture above (here's mine), because it is nice and simple. To illustrate the complexity of finding the MAC in other cases, here is a graph from the same source.
The above method gets us the MAC, which is the first step in determining the AC of the aircraft. A further method for finding AC in the pre-design phase is presented in Appendix E of Torenbeek:
- Find wing AC as above.
- Wing/Fuselage pitching moment, Appendix E.7 This is sensitive to the pressure distribution and is therefore hard to predict, except for:
- Forward shift due to fuselage sections fwd & aft of the wing, particularly the nose. Based on the ratio of fuselage lengths.
- Correction for lift loss in the wing/fuselage lift carry-over according to DATCOM.
- Fuselage pitching moment. Equation obtained from Munk's theory for near circular cross sections.
- Nacelle contributions. Appendix E.8, cannot be predicted reliably other than by wind tunnel tests, only a statistical correction is given.
The above is used for determining the AC of the aircraft. The contribution of the tailplane is included in the lift equations since it is dependent upon the Angle of Attack of the aircraft.
The complete method including graphs and equations is too lengthy to present in this forum, and can be consulted directly in the referenced Appendix.