To: Winnie
The upward flapping of the advancing blade you are addressing is also accompanied by downward flapping of the retreating blade. This occurs at about 20-25-knots during translational lift and is referred to as flap back or blow back depending on your religious persuasion. The technical term is Transverse Flow Effect.
If Frank Robinson is your religious leader then it is called "We-Wa".
To answer another post the movement of the blade is mechanical and has nothing to do with profile drag. The advancing blade leads where drag is at its' greatest.
