In forward flight, air passing
through the rear portion of the rotor disk has a greater downwash angle than air
passing through the forward portion:
The downward flow at the rear of
the rotor disk causes a reduced angle of attack, resulting in less lift.
Increased angle of attack and more lift is produced at the front portion of the
disk because airflow is more horizontal. These differences between the fore and
aft parts of the rotor disk are called transverse flow effect. They cause
unequal drag in the fore and aft parts of the disk resulting in vibrations that
are easily recognizable by the pilot. The vibrations are more noticeable for
most helicopters between 10 and 20 knots.