Provided the wind is steady, there is absolutley NO loss of airspeed turning downwind and no gain in airspeed turning upwind. (Ignoring the increased drag experienced in all turns, that is.)
There is, however, a strong set of visual illusions (apparent slip/skid) that confuses many.
Inertia is not tied to the mass of the Earth.
[This message has been edited by Checkboard (edited 26 January 2000).]