When is rotor downwash the greatest for helicopters?

Rotor downwash is greatest when the helicopter is in a hovering flight. In this state, the rotors produce a significant amount of lift by displacing a large volume of air downward. As the rotor blades spin, they create an area of low pressure above the blades and increase the air pressure beneath them, resulting in a concentrated column of downward-moving air directly beneath the helicopter.

In hovering flight, the helicopter's weight is fully supported by this downwash, causing the air to be pushed straight down as it expands outward. This intense downwash can affect the helicopter's performance and handling characteristics, as well as create challenges for nearby objects or personnel due to the strong airflow.

In contrast, during forward flight, the rotor system becomes more streamlined, and some of the downwash is directed to the rear, which results in a decrease in the intensity of the downwash felt directly below the helicopter. When descending, although the helicopter is generating downwash, it is also subtracting lift, which means the rotor’s contribution to downwash is less than in a hover. Similarly, during takeoff, while there is significant downwash, it is not as concentrated as in a hover because the helicopter is still transitioning from a stationary to a moving condition. Thus,