[Bonemaker]

Great circle route is shortest distance between two points on a ball. If memory serves me right

[philman_36]

Derivation and definition of a linear aircraft model

A linear aircraft model for a rigid aircraft of constant mass flying over a flat, nonrotating earth is derived and defined. The derivation makes no assumptions of reference trajectory or vehicle symmetry. The linear system equations are derived and evaluated along a general trajectory and include both aircraft dynamics and observation variables.

That link is right...https://ntrs.nasa.gov/citations/19890005752

[Bonemaker]

It is a straight line if viewed from directly above. As mentioned earlier, spherical geometry 101. Actually, as I understand it, long haul flights fly straight but make periodic adjustments to maintain the great circle. The finished flight path would appear as a jagged saw tooth line.