Dissymmetry of lift is the unequal rotor thrust, or lift, produced by the rotor disc due to forward flight or wind.
With forward flight, one blade is advancing into the wind while the other blade is retreating, or going with the wind. Uncorrected, the advancing blade produces more lift than the retreating blade, as the airflow over the advancing blade is greater. If left uncorrected, the helicopter would be difficult to fly and would roll to the left due to the increased lift from the right side of the rotor disc. The lift is equalized across the rotor disc through a process called flapping. With flapping, the rotor blades are able to move vertically to increase or decrease their angle of attack and thus increase or decrease the lift produced by an individual blade.
Example: Calculate lift at 100 knots indicated airspeed for the advancing and retreating blade using the lift formula CL*½p*V2*S.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-18 Principles of Helicopter Flight, 2nd Edition, pg. 91 FM 3-04.203-2007 Fundamentals of Flight pg. 1-39
Flapping is the vertical movement of a blade up or down to increase or decrease lift in order to compensate for dissymmetry of lift.
To equalize lift across the rotor disc, the advancing blade flaps up and the retreating blade flaps down. Flapping modifies the resultant relative wind by moving with or against the induced flow, which changes a blade’s angle of attack. The effect of the advancing blade flapping up is the same as increasing the induced flow. The increased induced flow will decrease the blade’s angle of attack as the resultant relative wind is influenced more by the induced flow and less by the (rotational) relative wind. With a lower angle of attack and the same rotational speed, the advancing blade produces less lift than without flapping. The opposite is true for the retreating blade. When the retreating blade flaps down, it moves with the induced flow. This movement reduces the induced flow and increases the retreating blade’s angle of attack as the resultant relative wind is influenced less by the induced flow and more (rotational) relative wind.
As a blade’s angle of attack changes, so does the blade’s inflow angle. The inflow angle is the angle between the rotational relative wind and the resultant relative wind. Other factors removed, there is an inverse relationship between the inflow angle and the blade’s angle of attack. If the inflow angle increases, the angle of attack decreases, producing less lift. If the inflow angle decreases, the angle of attack increases, producing more lift.
No Wind Hover: No Flapping
Induced Flow (IF): The downwash. Rotational Relative Wind (RW): from rotation of the blade Resultant Relative Wind (RRW): combination of induced flow and (rotational) relative wind Inflow Angle (IA): RW – RRW Blade Angle (BA): Physical angle of the blade Angle of Attack (AOA): BA – RRW
Advancing Blade: Flaps Up Decreases AOA, less lift
Moving the blade up is the same as increasing the induced flow, like walking into the wind verses with the wind. The AOA decreases as the RRW is influenced more the induced flow and less by the (rotational) relative wind.
Retreating Blade: Flaps Down Increases AOA, more lift
Moving the blade down is the same as decreasing the induced flow, like walking with the wind verses into the wind. The AOA increases as the RRW is influenced more by the (rotational) relative wind and less by the induced flow.
There are several methods of flapping. In a fully articulated rotor system, like the 300CB, each blade flaps individually. In a semi-ridged rotor system, like the Robinson R22/R44, the blades flap as a unit, when one flaps up, the other flaps down.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-19 Principles of Helicopter Flight, 2nd Edition, pg. 92 FM 3-04.203-2007 Fundamentals of Flight pg. 1-13, 1-40