When at a hover in calm, no-wind conditions, the induced flow is a significant factor affecting the resultant relative wind. As a result, the blade angles are significant and it takes more power for flight. As the aircraft increases speed, approximately 16-24 knots indicated airspeed, the impact of induced flow is reduced. Because of forward movement or wind, there is undisturbed air* meeting the front of the rotor disc. As speed increases, the portion of the disc receiving undisturbed air increases. As a result, the inflow angle is decreasing as more air is received horizontally versus vertically. As such, a lower angle of attack will produce the same lift, resulting in less power needed for flight.
The tail rotor also becomes more efficient with an increase in forward speed. There are two factors involved. First, like the main rotor, the tail rotor becomes more efficient when it operates in undisturbed air. Second, the forward movement of the aircraft reduces the amount of anti-torque thrust needed as the horizontal stabilizer or similar component, becomes more effective. As the need for anti-torque lessens, there is more power available for the main rotor.
It is a common mistake for pilots to refer to flying through ETL. An aircraft achieves ETL. Once in ETL, the aircraft is receiving the benefit of transitional lift until the relative wind is changed so the ETL is no longer achieved, such as by slowing down, flying downwind or a change in wind velocity. In general, the increase in horizontal speed or wind, the more efficient the rotor system.
* Some people will make reference to clean/dirty air instead of undisturbed/disturbed air. The use of the term clean air and dirty air should be avoided as the level of particulate matter in the air is not a factor, it is the turbulence of the air that is relevant.
FAA-H-8083-21A – Helicopter Flying Handbook pg. 2-20
Principles of Helicopter Flight, 2nd Edition, pg. 64, 99
FM 3-04.203-2007 Fundamentals of Flight pg. 1-41, 1-43