Causes of Transverse Flow
Air moving across the roor disk in forward flight is deflected downward because of induced flow. The greater the distance air must flow over the rotor disk, the longer the disk has to act on it and the greater the deflection. This results in a more horizontal flow of air over the forward half of the rotor disk than over the rear half; thus there is less induced flow over the front half of the rotor disk than over the rear half.
Greater induced flow velocity through the rear half of the disk decreases the angle of attack. Because the blades are operating at the same angle of incidence over the nose and tail, reducing the induced flow over the nose causes an increase in the angle of attack.
This animation demonstrates how the increased induced flow reduces the angle of attack over the tail. Increasing the induced flow changes the Rotational Relative Wind (RRW). The Angle of Attack is the angle between the RRW and the Chord Line. Therefore, increasing induced flow decreases angle of attack. The green arrow is the increased induced flow. The red arrow indicates the change in the angle of attack.
An increased angle of attack in the front half of the rotor disk increases the lift of the blade at this location. Increased lift on the blade over the nose causes the blade to flap up. Because of phase lag, the maximum upflapping blade displacement occurs over the left side of the helicopter. The decreased lift produced by the blade over the tail, combined with phase lag, results in maximum downflapping blade displacement over the right side of the helicopter. The displacement tilts the rotor disk to the right, changing the direction of the thrust vector. This change in the attitude of the rotor disk must be prevented if the helicopter is to maintain a straight flight path.
Compensating for Transverse Flow Effect
A left cyclic input decreases the pitch angle and angle of attack of the blade over the nose while increasing the pitch angle and angle of attack of the blade over the tail. These changes to blade angles of attack cause changes to lift. As the pilot senses the right tilt of the rotor, he must apply left cyclic to prevent a change in the attitude of the disk. As forward speed increases, the potential lift differential between the fore and aft portions also increases. Additional left cyclic inputs are required to prevent the right tilt of the rotor as a result of transverse flow effect. At higher airspeeds, lift differential between the fore and aft portions of the disk begins to decrease. The cyclic stick must be moved back to the right at higher cruise speeds.