The story of YSFlight is long, but FLYBY2[0] screensaver for Windows 98 is probably one of initial implementations of YSFlight flight engine (aircraft 3D models distributed with FLYBY2 are now distributed with YSFlight in same DNM-format).

Few weeks ago Soji Yamakawa open-sourced[0,1] FLYBY2 and ported it also for FM TOWNS.

[0] https://news.ycombinator.com/item?id=30214412

[1] https://github.com/captainys/FLYBY2