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. This is due to that air being accelerated for a longer period of time as it travels to the rear of the rotor system.

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.

So, what does this mean to us pilot's? Well, the result is a tendancy for the helicopter to roll slightly to the Right as it accelerates through approximately 20 knots or if the headwind is approximately 20 knots. (Assuming a counterclockwise main rotor rotation, reverse for a clockwise rotation).

You can recognize transverse flow effect because of increased vibrations of the helicopter at airspeeds just below effective translational lift (ETL) on takeoff and just passing through ETL during landing.

To counteract transverse flow effect, a cyclic input will be needed to correct the rolling tendancy.

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Page Last Updated on: Nov-06-2017