Sikorsky-Boeing’s SB-1 Defiant Helicopter

Putting it simply, at a stable hover with no ambient wind, a helicopter’s top blade generates an equal amount of lift throughout its whole rotation around the central axis. But when you start moving forward through the air, the blade starts developing extra lift on the side where the blades are rushing forward into the wind, and less lift on the opposite side where the blades are going backwards with the wind.