Let’s start in a hover and go through some of the forces that you will encounter as you embark into forward flight.
During hover, the rotor blades move large volumes of air in a downward direction. This pumping process accelerates the air to relatively high velocities. Air velocity under the helicopter may reach 60 to 100 knots, depending on the size of the rotor and the gross weight of the helicopter.
The efficiency of the hovering rotor system is improved with each knot of incoming wind gained by horizontal movement or surface wind. As the incoming wind enters the rotor system, turbulence and vortexes are left behind and the flow of air becomes more horizontal. All of these changes improve the efficiency of the rotor system and improve aircraft performance.
Improved rotor efficiency resulting from directional flight is called translational lift.
Horizontal Airflow can come from wind and/or movement of the helicopter.
Transverse Flow Effect-
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 disc 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.
As the helicopter speed increases, translational lift becomes more effective and causes the nose to rise, or pitch up. This tendency is caused by the combined effects of dissymmetry of lift and transverse flow. Pilots must not allow the nose of the helicopter to pitch up.
When the helicopter transitions from hover to forward flight, the tail rotor becomes more aerodynamically efficient. Efficiency increases because the tail rotor works in progressively less turbulent air as speed increases. As tail rotor efficiency improves, more thrust is produced. This causes the aircraft nose to yaw left if the main rotor turns counterclockwise. During a takeoff where power is constant, the pilot must apply right pedal as speed increases to correct for the left yaw tendency.
ETL (Effective Translational Lift)
At about 16 to 24 knots the rotor completely outruns the recirculation of old vortexes, and begins to work in relatively clean air. The rotor system becomes more efficient with more horizontal wind (translational lift), less induced flow and vortices.
Video by Helicopter Online Ground School