## What is dissymmetry of lift?

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.

Reference(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

Other Helicopter Aerodynamic Principles

## What is flapping?

Flapping is the vertical movement of a blade up or down to increase or decrease lift in order to compensate for dissymmetry of lift.

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
Angle of Attack (AOA): BA – RRW

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.