Figure: Relative Wind (RW) @~copters

Relative wind:
~ is created by movement of a >blade (airfoil) through air.
~ flows in the opposite direction that blade is moving
~ the velocity of airflow around the blade in motion is the blade's airspeed

When helicopter is stationary on a no-wind day
~ rotational relative wind is produced by rotation of the rotor blades
~ since rotor is moving horizontally, the effect is to displace some of the air downward
~ blades travel along the same path and pass a given point in rapid succession
~ (a three-bladed system rotating at 320 revolutions per minute passes a given point in the tip-path plane 16 times per second)
~ still air is changed to a column of descending air by rotor blade action

Figure: Induced Flow (Downwash) @~copters

This flow of air is called an induced flow (downwash).
It is most predominant at a hover under still wind conditions.
Because rotor system circulates airflow down through rotor disk, the rotational relative wind is modified by the induced flow.
Airflow from rotation, modified by induced flow, produces the resultant relative wind.


Figure: Component of Relative Wind @~copters

When helicopter has horizontal motion
~ the resultant relative wind is further changed by the helicopter airspeed
~ airspeed component of relative wind results from the helicopter moving through the air
~ it is added to or subtracted from the rotational relative wind,
~ depending on whether blade is advancing or retreating in relation to the helicopter movement
~ induced flow is also modified by introduction of airspeed relative wind
~ the pattern of air circulation through the disk changes when the aircraft has movement
~ gnerally the downward velocity of induced flow is reduced
~ helicopter moves continually into an undisturbed airmass, resulting in less time to develop a vertical airflow pattern
~ as a result, additional lift is produced from a given blade pitch setting

catch ~>Helicopter ~>Xi ~>Formulas!