Okay, although this is mainly a physics question, it has an aerodynamic aspect to it so I will post it here.
I believe the argument that the "downwind turn" phenomona is bogus but am always troubled by the inertia issue that is raised in the hypothetical case where a model is going the same airspeed as a headwind, thus has no groundspeed. After a 180 turn it will be going the same airspeed as before but will now have a groundspeed twice the airspeed (or it will have fallen out of the sky...)
Its inertia or momentum relative to the earth's frame of reference has changed without question. The forces involved have to be aerodynamically generated during the turn but it is not obvious to me how this happens.
Allan