One major factor affecting the aerodynamics is ground effect. There are two approaches to capture this effect.
One approach is to incorporate distance from the ground as a variable in the polynomials. For example, let be some nondimensional parameter representing the distance from the ground. This parameter might be defined as , where b is the wingspan and h is the distance from the wing root chord to the ground. (The reason that h is in the denominator is so can go to zero far from the ground.)
Then, to capture ground effects, there could be terms such as and . The latter term introduces something that is not seen outside of ground effect. Typically, the aerodynamic force and moment depends only on wind-relative speed; however, in ground effect the force depends on orientation as well, hence the Euler angle . (To be more exact, it depends on orientation relative to the ground. Thus, the Euler angle could not be used to simulate landing on surface that is not level.)
The other approach to ground effect is to use corrections. Reference 7 (pages 359-360) lists empirical corrections to lift and drag in ground effect. These corrections are not as accurate as incorporating ground distance into the polynomials; however, they are useful for basic, hand-calculated models.