Settling with Power
is a condition of powered flight where the helicopter settles into its own
downwash. The condition may also be referred to as the vortex ring state.
Conditions conducive to settling
with power are a vertical or nearly vertical descent of at least 300 feet per
minute and low forward airspeed. The rotor system must also be using some of the
available engine power (from 20 to 100 percent) with insufficient power
available to retard the sink rate. These conditions occur during approaches with
a tailwind or during formation approaches when some aircraft are flying in
turbulence from other aircraft.
Under the conditions described
above, the helicopter may descend at a high rate which exceeds the normal
downward induced flow rate of the inner blade sections. As a result, the airflow
of the inner blade sections is upward relative to the disk. This produces a
secondary vortex ring in addition to the normal tip vortex system. The
secondary vortex ring is generated about the point on the blade where airflow
changes from up to down. The result is an unsteady turbulent flow over a large
area of the disk which causes loss of rotor efficiency even though power is
still supplied from the engine.
This figure shows the induced
flow along the blade span during normal hovering flight:
Downward velocity is highest at
the blade tip where blade airspeed is highest. As blade airspeed decreases
nearer the disk centre, downward velocity is less. This figure shows the induced
airflow velocity pattern along the blade span during a descent conducive to
settling with power:
The descent is so rapid that
induced flow at the inner portion of the blades is upward rather than downward.
The upflow caused by the descent has overcome the downflow produced by blade
rotation. If the helicopter descends under these conditions, with insufficient
power to slow or stop the descent, it will enter the vortex ring state:
During the vortex ring state,
roughness and loss of control is experienced because of the turbulent rotational
flow on the blades and the unsteady shifting of the flow along the blade span.
This figure shows the
relationship of horizontal speed versus vertical speed for a typical helicopter
in a descent. Straight lines emanating from the upper left corner are lines of
constant descent angle. Superimposed on this grid are flow state regions for the
typical helicopter. From this illustration, several conclusions regarding the
vortex ring state can be drawn:
The vortex ring state can be
completely avoided by descending on flightpaths shallower than about 30
degrees (at any speed).
For steeper approaches, vortex
ring state can be avoided by using a speed either faster or slower than the
area of severe turbulence and thrust variation.
At very shallow angles of
descent, the vortex ring wake is shed behind the helicopter.
At steep angles, the vortex
ring wake is below the helicopter at slow rates of descent and above the
helicopter at high rates of descent.
Power settling is an unstable
condition. If allowed to continue, the sink rate will reach sufficient
proportions for the flow to be entirely up through the rotor system. If
continued, the rate of descent will reach extremely high rates. Recovery may be
initiated during the early stages of power settling by putting on a large amount
of excess power. During the early stages of power settling, the large amount of
excess power may be sufficient to overcome the upflow near the centre of the
rotor. If the sink rate reaches a higher rate, power will not be available to
break this upflow, and thus alter the vortex ring state of flow.
Normal tendency is for pilots to
recover from a descent by application of collective pitch and power. If
insufficient power is available for recovery, this action may aggravate power
settling resulting in more turbulence and a higher rate of descent. Recovery can
be accomplished by lowering collective pitch and increasing forward speed. Both
of these methods of recovery require altitude to be successful.