A knowledge of relative wind is particularly essential for an understanding of aerodynamics of rotary-wing flight because relative wind may be composed of multiple components. As an example, consider a person sitting in an automobile on a no-wind day with a hand extended out the window. There is no airflow about the hand since the automobile is not moving. However, if the automobile is driven at 50 miles per hour, the air will flow under and over the hand at 50 miles per hour. A relative wind has been created by moving the hand through the air.
Relative wind flows in the opposite direction that the hand is moving.
The velocity of airflow around the hand in motion is the hand's airspeed. When the helicopter is stationary on a no-wind day, rotational relative wind is produced by rotation of the rotor blades. Since the rotor is moving horizontally, the effect is to displace some of the air downward. It is most predominant at a hover under still wind conditions. Because the rotor system circulates the airflow down through the rotor disk, the rotational relative wind is modified by the induced flow.