The reference surface S and length l are arbitrary, to the point that in some
cases a surface not existing physically, like a power 2/3 of the displacement for
airships, is used.These references simply express the dependence of aerodynamic
forces on the square of the dimensions of the body and that of the moments on
their cube. It is, however, clear that the numerical values of the coefficients
depend on the choice ofSandl, which must be clearly defined. In the case
of road vehicles, the surface is that of the cross section, withsome uncertainty
about whether the frontal projected area or that of the maximumcross section
has been used (Fig. 21.4).