When a helicopter starts to move forward from a hover another aerodynamic condition (in addition to effective translational lift that was discussed previously) that occurs is transverse flow effect. This condition involves a differential airflow between the front and rear parts of the rotor system.
Moving forward from a hover, with no wind, the edge of the rotor system over the nose moves into clean air while the rear portion moves into air that has already been accelerated downward. This causes the angle-of-attack of the blades passing over the nose to increase, producing more lift. Because of gyroscopic precession, the maximum reaction occurs on the left side of the helicopter causing the rotor disc to tilt to the right. To continue moving straight the pilot must compensate with left cyclic.
Transverse flow effect can be recognized by an increased vibration of the helicopter at airspeeds around 12 to 15 knots and can be produced by forward flight or from the wind while in a hover. This vibration happens at an airspeed slightly lower than effective translational lift (ETL). The vibration happens close to the same airspeed as ETL because that’s when the greatest lift differential exists between the front and rear portions of the rotor system. As such, some pilots confuse the vibration felt by transverse flow effect with passing through ETL.