A hovering helicopter can require a lot of power. However, as it moves forward the horizontal flow of air across the rotor system improves the efficiency by changing the induced flow, and therefore the relative wind, which increases the blades’ angle of attack. This added efficiency is called translational lift. The forward motion also causes other aerodynamic issues with the rotor system, like dissymmetry of lift and transverse flow effect (a later discussion).
Wind can also create translational lift. Trying to hover at a constant altitude in gusty winds requires the pilot to constantly add or reduce power to compensate. Gusty winds can affect the tail rotor and power changes require pedal input as well. Holding a precise hover in these conditions is challenging.
With no wind, translational lift starts with any amount of airspeed and continues to develop as the helicopter’s speed increases. However, somewhere around 50 knots (it varies between different helicopters) induced drag increases to the point where it overtakes the gain in efficiency from translational lift.
Effective translational lift (commonly referred to as ETL) is a term used to describe the airspeed at which the entire rotor system realizes the benefit of the horizontal air flow. This happens when the helicopter’s rotor disc moves completely out of its own downwash and into undisturbed air. Depending on the helicopter this occurs between 12 and 18 knots of airspeed. The pilot will recognize effective translational lift on departure when the helicopter begins to have a noticeable tendency to climb and on approach when the helicopter starts to sink as the airspeed drops below ETL.