Why are the aerodynamic characteristics of a finite wing different from the properties of its airfoil section?
Because the wing is finite, while the airfoil is assumed to stretch out sideways indefinitely. The consequence is that on a wing you get spanwise flow outside of the plane of symmetry, while on the airfoil all flow speeds are in the airfoil's plane only.
Spanwise flow is caused by the pressure difference between lower and upper side which can equalise around the wingtips. This lets the pressure difference diminish towards the tips and especially in the rear part of the wingtips. The consequences are:
- Lift is smaller on a "real" wing relative to a wing of the same airfoil with no such sideways flow.
- The center of pressure of a "real" wing is ahead of the one of the airfoil.
- On a swept wing the air transverses the surface differently than it transverses the airfoil. Basically, a sideways speed component is added to the speed around the airfoil.