At the moment flight-mechanics has 14 questions. Do we need it?
It might be misused, but flight mechanics and aerodynamics are very different fields.
Aerodynamics deals with the application of fluid dynamics (the origins of lift and drag, 3D wing theory, etc) while flight mechanics deals with the combination of forces on an airframe as a solid body (effects of lift and drag depending on their location, endurance and range, rates of turn, etc)
If anything I'd rather merge the most of the aircraft-physics questions into flight-mechanics, since most of the aircraft-physics questions look to me like mechanics questions (stability, control, performance). I would volunteer to expand the usage guidelines to clear up the misunderstandings.
The "flight-mechanics tag" is arguably redundant with the more widely-used "flight-dynamics" tag. It seems hard to make an argument that the appropriate subject for one tag is merely a subset of the appropriate subject for another tag; they seem essentially the same.
The lesser-used one, which is the "flight-mechanics" tag (31 uses at present), should be deleted, and most questions with this tag should be given the "flight-dynamics" tag (68 uses at present). If possible (i.e. if they don't already have 5 tags by that point), these questions should also be given the broader (less-specific) "aircraft-physics" tag, but this tag is arguably less useful because of its broadness.
The "aircraft-physics" tag is arguably much too broad to justify deleting BOTH the "flight-mechanics" AND the "flight-dynamics" tag.
Note that the "flight-mechanics" tag has NO usage guidelines at present.
From the Wikipedia entry on dynamics: (https://en.wikipedia.org/wiki/Dynamics_(mechanics))
Dynamics is the branch of classical mechanics concerned with the study of forces and their effects on motion. Isaac Newton defined the fundamental physical laws which govern dynamics in physics, especially his second law of motion.
and from the Wikipedia entry on mechanics (https://en.wikipedia.org/wiki/Mechanics) (accessed April 21 2020)
Mechanics (Greek μηχανική) is the area of physics concerned with the motions of macroscopic objects. Forces applied to objects result in displacements, or changes of an object's position relative to its environment.
While dynamics seems to be a subset of mechanics, it is hard to come up with examples in the aviation context of questions that would be appropriate for the "flight-mechanics" tag but not for the "flight-dynamics" tag.
Perhaps one could make the case that questions that deal with the forces in a steady-state climb, descent, etc would better described as relating to "flight-mechanics" than "flight-dynamics", strictly speaking, but "flight-dynamics" seems an adequate description of the latter as well.
Would questions about things such as the load paths through structural elements of an aircraft in steady-state flight best described as fitting within the "flight-mechanics" tag but not the "flight-dynamics" tag? Possibly. Nonetheless, given the little usage the "flight-mechanics" tag has received, it would seem adequate to tag such questions with both the "aircraft-physics" and "flight-dynamics" tags. Or better yet, use the "aircraft-structures" tag in addition to the "aircraft-physics" tag for these specific cases.
Also relevant-- on October 31 2019 I posted an answer to this Meta question that stated:
"Yet I see that a user has recently edited three questions to remove the flight-mechanics tag. Is this wise? The questions are: What is an acceptable phugoid oscillation duration? , Does lift equal weight in a climb? , Is excess lift or excess power needed for a climb? . These sure seem like "flight mechanics" questions to me, and when I click on the "flight-mechanics" tab I get lots of other related questions. "
I now also now notice that the "flight-dynamics" tag was not added to the three questions noted above when the "flight-mechanics" tag was deleted. This seems to me to represent an unfortunate loss of specificity. Again, the "aircraft-physics" tag is arguably too broad to be a good substitute for the "flight-mechanics" tag; the "flight-dynamics" tag is a much better choice. Certainly it does not seem inappropriate to also add the "aircraft-physics" tag to such questions as well, if they still have an unfilled "tag slot" after adding the "flight-dynamics" tag.
As long as the "flight-mechanics" tag exists and there is no community consensus to discontinue it, I may or may not continue using it (in addition to the "flight-dynamics" tag) for questions such as these. But I would now argue that "flight-dynamics" would be a perfectly acceptable substitute for "flight-mechanics" for most of these questions.
So in conclusion, I would suggest that the "flight-mechanics" tag should be deleted, and most questions with this tag should be given the "flight-dynamics" tag if they do not have it already.
If we decide not to do this, we should come up with some usage guidelines for the "flight-mechanics" tag, so people don't use it to describe in-flight mechanical problems (as has happened at least once) or people who maintain aircraft. Also, unless we clearly decide to delete the "flight-mechanics" tag, it seems inappropriate to go removing it from questions for which it is well-suited.