Good tip Blagard. I think most players don't even know about trim set. It doesn't stay set with analog trim but when you hold the period key it will move the trim indicators and show you what the analog trim position needs to be to get the same result.
You can trim for a dive with auto-angle (shift x) and it will also trim aileron as well as elevator but trim set will let you set a descending, level, or climbing turn. It would be handy if jumping to the gunner position didn't turn trim set off.