Ground Effect

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 $\xi$ be some nondimensional parameter representing the distance from the ground. This parameter might be defined as $\xi = b/h$, 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 $\xi$ can go to zero far from the ground.)

Then, to capture ground effects, there could be terms such as $C_{Z_\xi}$ and $C_{L_{\xi\Phi}}\xi\Phi$. 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 $\Phi$. (To be more exact, it depends on orientation relative to the ground. Thus, the Euler angle $\Phi$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.