Definition
The unequal lift produced across the rotor disc of a helicopter in forward flight, caused by the difference in airspeed between the advancing blade and the retreating blade. The advancing blade moves into the relative wind and produces more lift, while the retreating blade moves with the relative wind and produces less. If left uncorrected, this imbalance would roll the helicopter toward the retreating side. It is compensated for by blade flapping, which allows each blade to rise and fall as it travels around the disc, equalising lift across the rotor.
Plain English
When a helicopter flies forward, one side of the spinning rotor cuts into the wind faster than the other side, so it makes more lift than the slower side. If nothing fixed this, the helicopter would tip over. The blades are designed to rise and fall as they spin around so the lift evens out.
Context Anchor
Seen in helicopter and other rotorcraft discussions of forward flight, rotor behavior, and control.
Derivation
Dissymmetry comes from the Latin prefix dis- meaning 'apart' or 'not,' combined with symmetry, from the Greek symmetria meaning 'balanced proportion.' So dissymmetry literally means 'not balanced.' Here it describes lift that is not balanced across the two halves of the rotor disc.
Why Pilots Care
Uncompensated dissymmetry of lift creates a rolling force toward the retreating side that the rotor system must manage through blade flapping or pilot inputs to maintain level flight.
Grounding Statement
Picture a helicopter moving forward: the blade sweeping forward into the oncoming air meets faster airflow than the blade sweeping backward, so the two sides do not naturally make the same lift.
Intuition Check
Dissymmetry of lift does not mean the rotor has stopped making lift. It means the lift is uneven from one side of the rotor disk to the other.
Example Sentence 1
As the helicopter accelerated into forward flight, the instructor explained how blade flapping compensates for the dissymmetry of lift across the rotor disc.
Example Sentence 2
In forward flight the helicopter's rotor system is designed to compensate for dissymmetry of lift so the aircraft does not roll toward the retreating blade.