Skin Friction Drag Formula

Equation for calculate skin friction drag is,

D_f = ( 1.328 × q_∞ × S ) / √((ρ × V_∞ × L) / µ)

where,
q - Dynamic pressure (N/m2)
S - Wing surface (m2)
ρ - Air density (kg/m3)
V - Air flow velocity (m/s)
L - Lift (N)
µ - Absolute viscosity coefficient (For normal air (T = 288K): 1.7894×10^-5kg/(ms))
D_f - Total skin friction drag