hoveringHovering is the term applied
when a helicopter maintains a constant position at a selected point, usually a
few feet above the ground (but not always, helicopters can hover high in the
air, given sufficient power). For a helicopter to hover, the main rotor must
supply lift equal to the total weight of the helicopter. With the blades
rotating at high velocity, an increase of blade pitch (angle of attack) would
induce the necessary lift for a hover. The forces of lift and weight reach a
state of balance during a stationary hover.
Hovering is actually an element
of vertical flight. Assuming a no-wind condition, the tip-path plane of the
blades will remain horizontal. If the angle of attack of the blades is increased
while their velocity remains constant, additional vertical thrust is obtained.
Thus, by upsetting the vertical balance of forces, helicopters can climb or
descend vertically.
Airflow during hovering
At a hover, the rotor tip vortex
(air swirl at the tip of the rotor blades) reduces the effectiveness of the
outer blade portions. Also, the vortexes of the preceding blade severely affect
the lift of the following blades. If the vortex made by one passing blade
remains a vicious swirl for some number of seconds, then two blades operating at
350 RPM create 700 longlasting vortex patterns per minute. This continuous
creation of new vortexes and ingestion of existing vortexes is a primary cause
of high power requirements for hovering.
During hover, the rotor blades
move large volumes of air in a downward direction. This pumping process uses
lots of horsepower and accelerates the air to relatively high velocities. Air
velocity under the helicopter may reach 60 to 100 knots, depending on the size
of the rotor and the gross weight of the helicopter. The air flow pattern of a
hovering helicopter is illustrated here:
Note how the downwash (induced
flow) of air has introduced another element into the relative wind which alters
the angle of attack of the airfoil. When there is no induced flow, the relative
wind is opposite and parallel to the flight path of the airfoil. In the hovering
case, the downward airflow alters the relative wind and changes the angle of
attack so less aerodynamic force is produced. This condition requires the pilot
to increase collective pitch to produce enough aerodynamic force to sustain a
hover. Although this does increase the lift, it also increases the induced drag,
and so total power required is higher.
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