Calculating skin friction
An improved skin friction algorithm is currently in development.
The APM Solver uses the following expression to approximate the skin friction coefficient:
In the above expression,
is the Reynolds number based on the reference chord. Schlichting's relation  is used to obtain the skin friction coefficient from the Reynolds number. The skin friction coefficient is then multiplied by the surface velocity vector at the centre of each panel to approximate the loads resulting from viscosity.
To disable the approximation of viscous loads set the dynamic viscosity
to 0. The default APM Solver value for
 Gudmundsson, S., "General Aviation Aircraft Design Applied Methods and Procedures", Ed. 1, 2014, pp. 678-679