Originally Posted by
Intruder
For the glider example, it likely has to do with the basic design philosophy of the builder, as well as nomenclature.
If a glider is actually a high-performance sailplane, focused on competitive long-distance soaring, then the wing will be designed as much for speed as for glide ratio (L/D). A "1 degree" flap setting or similar may be incorporated to get better L/D at lower speeds for thermalling. OTOH, other sailplane mfgrs will instead incorporate a "negative flap" setting for high-speed dashes, while the normal 0 deg setting is used for thermalling.
You actually go for minimum sink (Min drag) during thermalling, and thus often use a positive flap setting.
This will be at a lower speed and thus higher deck angle than best L/D (which isn't always the most efficient inter-thermal speed) in a glider with no flaps. Thus, if the gliders incidence is set such that it produces minimum profile drag at best L/D, you will present the bottom of the fuselage to the airflow, increasing profile drag.
A positive flap setting will give approximately the same wing performance, while putting the deck-angle back to optimum.
You are correct that then a reflex, negative flap setting will do the same thing at speeds greater than best L/D.
I've often thought variable incidence would be a better solution than flaps, though obviously mechanically more complex.