Symmetry and dissymmetry of lift are shown in Figure 2-6 . The blade speed varies from 300 MPH at the tip station to 0 at the center of blade rotation on the hub. When a helicopter is hovering in a no-wind condition, there is symmetry of lift. The lift is equal on advancing and retreating halves of the rotor disc area because speed is the same on both halves. Dissymmetry of lift is the difference in lift that exists between the advancing half of a rotor disc and the retreating half.
Dissymmetry is created by forward movement of the helicopter.
When the helicopter is moving forward, the speed of the advancing blade is the sum of the indicated airspeed of the helicopter plus the rotational speed of the blade. The speed of the retreating blade is the rotational speed of the blade minus the forward speed of the helicopter. The total speed squared is 160,000. The retreating half of the disc has a blade tip speed of 300 MPH minus the 100 MPH indicated forward speed of 200 MPH, and velocity squared is 40,000. In this example the advancing blade creates four times as much lift as the retreating blade.