Originally Posted by Superpilot
In the study of static longitudinal stability, why is it accepted that lift always acts at the Aerodynamic Centre? What are the reasons for this?
I'm sure I'll discover soon but it would aid my learning if I found out now!
Thanks
Basically because "aerodynamic centre" is defined that way.
The "center of lift" does move chordwise with angle of attack. Center of lift is defined such that there is a force but no turning moment about that point.
Because center of lift moves, it is confusing for engineers. Instead of center of lift therefore it is convienent to define a fixed point and consider there to be both a force and a moment acting at this point. It turns out that it is possible to find a point at which the force varies with angle of attack but the moment is nearly constant. This is the Aerodynamic centre.
http://www.grc.nasa.gov/WWW/K-12/airplane/ac.html