I'm aware of YSFlight (used it since 2015), but it has a lot of issues and repo does not include whole commits history (it was opensourced year ago as a snapshot of latest version code base with few commits on top of it), so might be not a good example of simple flight simulator.

Physics of YSFlight described in community fork repo.[0]

Till YSFlight became open-source, its physics was a "black box" and there was a research on reverse engineering its physics using game data logs.[1]

Also, there is another fork with few fixes & enhancements, including in its flight simulator physics engine.[2]

[0] https://github.com/YSCEDC/YSCE/blob/master/Documentation/YSF...

[1] https://sites.google.com/site/ysdecaff/ysflight-scientific-r...

[2] https://github.com/pasutisu/YSFLIGHT