relative windA
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. Relative wind is defined as the airflow relative to an
airfoil:
Relative wind is created by
movement of an airfoil through the air. 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. The 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).
This figure illustrates how
still air is changed to a column of descending air by rotor blade action:
This flow of air is called an
induced flow (downwash). 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.
Airflow from rotation, modified by induced flow, produces the resultant
relative wind. In this illustration, angle of attack is reduced by induced
flow, causing the airfoil to produce less lift:
When the helicopter has
horizontal motion, the resultant relative wind discussed above 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 the 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. Generally the downward
velocity of induced flow is reduced. The helicopter moves continually into an
undisturbed air mass, resulting in less time to develop a vertical airflow
pattern. As a result, additional lift is produced from a given blade pitch
setting.
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