Extrapolating from Shy T.
Inertia ?
A flying machine is propelling itself due North into a 100 mph airstream at 100 mph airspeed. [GS = 0].
It flies into a space of 'magic' vacuum & falls vertically straight down under 'G'.
Same machine going due South [GS = 200 mph], ditto 'vacuum, but this time continues forwards at 200 mph in a descending arc.
IMHO a high (mass X speed) 'plane has sufficient inertia not to change its ground speed as quickly as a change of wind.
mike hallam (whose light a/c is contrary & simply does what the wind does - mostly!)