The principal cause of increased drag is the increased surface velocity (supervelocity) due to thickness. This may be computed as follows for wing-like surfaces. Consider an infinite swept wing with a perturbation due to thickness of:
Ignoring the reduction in cf due to Reynolds number and Mach number changes associated with the increased local velocity, because this cannot be computed at all well and because cf varies weakly with these:
k = 1 + 2 DU' cosL + DU'2 (1 + 5 cos2 L) / 2
Now in incompressible flow, DU' = C t/c, even for large t/c (with t/c measured in the normal direction). In 2-D subsonic flow:
DU' = C t/c cosL (1-Mn2)-0.5 = C t/c cosL / b
So: k = 1 + 2 C t/c cos2L / b + C2 cos2L t/c2 (1+5 cos2L) / 2 b2
The value of k is given in the next figure and compared with other methods and experimental data. A value for C of about 1.1 agrees best with the rather scattered data. When M cosL > 1, there is not a velocity increase due to t/c and so we take C=0.