That theory holds together until you put inertia into the equation. Where you have a 10 MPH head wind and a plane that can fly at 10 MPH you will quite happily hold a stationary position in respect to the ground. Now turn the plane 180 degrees and you still have a stationary ground position but you now have a 10 MPH tail wind and no lift. In real life, no 180 degree turn is instantaneous, but you still have to accelerate from 0 MPH to 20 MPH in the time it takes to turn to achieve the same flying conditions. This is the reason why planes lose altitude on down-wind turns unless compensated for by more power, up elevator and long turns.

The opposite situation causes problems as well. Many 2-channel planes fail to fly properly in light winds due to the turn from flying downwind to upwind. The planes inertia means that as it turns into the wind, the relative wind speed increases significantly and so does the lift, causing the porpoising and looping that usually signals that the ground is going to be a factor sooner than later.

Experienced flyers using more than the beginners 2 channels have much more chance of flying successfully, given enough power and the use of elevators the head wind can be overcome and the turn downwind can be a smooth transition. Beginners with two channels will be at the mercy of the wind......in some cases you just cannot get those planes back up-wind again and they dissapear into the distance. The only way to save your 2-channel trainer is to apply full power and full turn and spiral it into the ground....it may be a bit bent but at least you've got it back....