Falngemeister, am I missing something? I got so lost in your descriptive colour, I could not figure out whether you think the "downwind turn= loss of airspeed" phenomenom exists or not!
The comparison with windshear is a spurious one- windshear, by definition, is a transient event- a turn in a particular body of air is not. And while a rotating mower blade is not operating at a constant airspeed, it is a steady state problem- we simply treat a rotational speed in the same way as we would treat straight-line speed. I daresay that it is a good analogy for helos, but I know very little about them.