# Calculating skin friction

{% hint style="warning" %}
An improved skin friction algorithm is currently in development.
{% endhint %}

The APM Solver uses the following expression to approximate the skin friction coefficient:

$$
Re\_{ref}=\frac{\rho\_{ref}|\mathbf{V}*{ref}|c*{ref}}{\mu\_{ref}},\\
C\_{f}=\frac{0.455}{log\_{10}(Re\_{ref})^{2.58}}
$$

In the above expression, $$Re\_{ref}$$is the Reynolds number based on the reference chord. Schlichting's relation \[1] 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.

{% hint style="info" %}
To disable the approximation of viscous loads set the dynamic viscosity $$\mu\_{ref}$$to 0. The default APM Solver value for$$\mu\_{ref}$$is $$1.7890\times10^{-5}$$.
{% endhint %}

## References

\[1] Gudmundsson, S., "General Aviation Aircraft Design Applied Methods and Procedures", Ed. 1, 2014, pp. 678-679


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://docs.aviumtechnologies.com/user-guide/the-configuration-file/calculating-skin-friction.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.