where C fe {\displaystyle C_{\text{fe}}} is the equivalent skin friction coefficient, S wet {\displaystyle S_{\text{wet}}} is the wetted area and S ref {\displaystyle S_{\text{ref}}} is the wing reference area. The equivalent skin friction coefficient accounts for both separation drag and skin friction drag and is a fairly consistent value for aircraft types of the same class. Substituting this into the equation for maximum lift-to-drag ratio, along with the equation for aspect ratio ( b 2 / S ref {\displaystyle b^{2}/S_{\text{ref}}} ), yields the equation: