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3.8.  DISSYMMETRY OF LIFT

The area within the circle made by the rotating blade tips of a helicopter is known as the disc area or rotor disc. When hovering in still air, lift created by the rotor blades at all parts of the disc area is equal. Dissymmetry of lift is the difference in lift that exists between the advancing half of the disc area and the retreating half, and it is created by horizontal flight or wind.

When a helicopter is hovering in still air, the tip speed of the advancing blade is about 600 feet per second. The tip speed of the retreating blade is the same. Dissymmetry of lift is created by the horizontal movement of the helicopter in forward flight, and the advancing blade has the combined speed of blade velocity plus speed of the helicopter. The retreating blade loses speed in proportion to the forward speed of the helicopter.

Figure 3.7 illustrates the dissymmetry of lift and shows the arithmetic involved in calculating the differences between the velocities of the advancing and retreating blades. In the figure, the helicopter is moving forward at a speed of 100 knots, the velocity of the rotor disc is equal to approximately 355 knots, and the advancing rotor speed is 455 knots. The speed of the retreating blade is 255 knots. This speed is obtained by subtracting the speed of the helicopter (100 knots) from the tip speed of 355 knots. As can be seen from the difference between the advancing and retreating blade velocities, a large speed and lift variation exists.

(ROTATIONAL VELOCITY) ± (HEL FORWARD SPEED) = (AIRSPEED OF BLADE)
Figure 3.7. Dissymmetry of Lift.

Cyclic pitch control, a design feature that permits changes in the angle of attack during each revolution of the rotor, compensates for the dissymmetry of lift. As the forward speed of the helicopter is increased, the aviator must apply more and more cyclic to hold a given rotor disc attitude. The mechanical addition of more pitch to the retreating blade and less to the advancing blade is continued throughout the helicopter's range.

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