Your argument only holds true if you carry on going in the same direction! And that's the whole point, with models like ours we have to constantly change direction (otherwise it disappears into the distance), and it is during a relative direction change that inertia becomes significant. It takes energy to overcome inertia, you cannot accelerate from 0 MPH to 20 MPH without expending energy, it also takes time to de-accelerate which is why planes gain altitude on up-wind turns.

Using your example, sit on a carrier deck of a large carrier, 10 knot wind speed from the North and a 10 knot Southerly speed.....gives you a 0 knot wind speed relative to the carrier deck. Hold a slow stick or a HZ Commander in your hand, apply throttle bung it into the air in a Southerly direction, it should fly perfectly normally at, say 15 knots in respect to the carrier deck. That gives us a speed of 25 knots in a Southerly direction in real terms.

Now apply full right rudder and the plane will turn and change direction relative to the carrier. To do this takes energy (and a quicker direction change takes more energy). As you snap around you will find yourself facing the other way with a 20 knot head wind and loads of lift (obviously, as you are facing the other way and the wind is now in front of you). To retain the same altitude you will need your Slow stick or Commander to de-accelerate from 25 knots to 5 knots in the time it takes to snap around 180 degrees. Don't forget that acceleration and inertia are *relative* in respect to an observer.

In real terms, a Commander or a Slow stick cannot 'snap' around all that quickly, but the effects are the same only tempered by relative acceleration. In real terms, you gain height unless you can count on the use of elevators to counteract the effects of inertial gain (which the Commander does not posses).

However, all that said, I would agree with you that turbulent air is the real enemy of the RC flier. On large-winged planes the turbulence is averaged over the wings surface but on small wingspan models the turbulence will just turn your plane upside down in a heartbeat and there's not much you can do except watch it splat into the ground.