Analysis
Envisage a viewpoint above a single-rotor helicopter in still air. For a stationary (hovering) helicopter, whose blades of length of r metres are rotating at ω radians per second, the blade tip is moving at a speed rω meters per second. At all points around the disc mapped out by the blade-tips, the speed of the blade-tip relative to the air is the same: everything is balanced.
Now imagine the helicopter in forward flight at, say, v meters per second. The speed of the blade-tip at point A in the diagram relative to the air is the sum of the blade-tip speed and the helicopter forward-flight speed: rω+v. But the blade-tip speed at point B, relative to the air, is the difference of its rotational speed and the forward-flight speed: rω-v.
Since the lift generated by an aerofoil increases as its relative airspeed increases, on a forward-moving helicopter the blade-tip at position A produces more lift than that at point B. So the rotor disc produces more lift on the right hand side than on the left hand side. This imbalance is the "dissymetry of lift".
Read more about this topic: Dissymmetry Of Lift
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