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hovering, because it is not rotating
around the C of G, but the rollover
point, you have less chance to get
out of trouble, and the only effective
control is through the collective (do
Principles of Flight 177
not raise it). In other words, the lift
from the rotor disc that should be
vertical is inclined and converted
into thrust, above the centre of
gravity, so trying to use the cyclic to
level, and the collective to get you
off the ground is wrong!
Dynamic rollover is worst with the
right skid on the ground (counter
clockwise main rotor) and with a
crosswind from the left, with left
pedal applied and thrust about equal
to the weight (i.e. hovering). A
machine can roll upslope if you
apply too much cyclic into it, or
downslope if you apply too much
collective, enough to make the
upslope skid rise too much for the
cyclic to control. Avoid it by keeping
away from tail winds, and landing
and taking off vertically.
Ground Effect
In the hover, downwash is stopped
by any surface within about 1 rotor
length. Because downwash velocity
is reduced, so is the lift vector, which
becomes more vertical (which itself
increases thrust a little more and
reduces drag), resulting in less
induced drag and a reduction in the
power needed to hover (a Bell 206
typically will need 15% less in the
ground cushion). In addition, the
accelerated air, having slowed down,
increases the pressure underneath
the rotor disc.
The effect is even more pronounced
when you lower the collective to
stop climbing, and will be more
apparent closer to the ground.
Factors that will reduce this are the
surface you are hovering over (the
harder and smoother the better, and
the more level), and the wind, which
will vary the direction of the
downwash from under the blades.
Any above about 10 kts, of course,
will produce translational lift.
Recirculation
When hovering near the ground,
some downwash comes back on
itself and goes through the rotor disc
twice, which reduces lift whenever
this happens because it does so at a
higher speed and reduces the space
available for the angle of attack in
the resulting vector. Vortices are
present at the rotor tips all the time,
because they are caused by
centrifugal force (fanjets enclose the
ends of the blades to stop this), but
they are usually more than offset by
ground effect.
If your ground effect is reduced for
any reason (see above), the chances
of recirculation increase, requiring
more collective and cyclic to
compensate. Where the downflow is
actually prevented from escaping
properly, as when hovering close to
a building, or in a tight confined
area, the effect will be to tilt you in a
direction 90° from where
recirculation was introduced, or even
pull you down if all sides are affected
(as when landing in a courtyard).
Thus, if you are hovering a 206 next
to a building in front of you, the
recirculation occurs at the front, but
the disc will tilt to the left and make
the left skid hover lower than usual
which, if it catches you, might cause
dynamic rollover (see above). If you
are closer than a third of your disc
diameter, the advancing blade is also
affected, in the above example,
pulling you towards the building.
Be particularly careful within 1 rotor
diameter of another helicopter.
178 Canadian Professional Pilot Studies
Vortex Ring
This occurs when you encounter
your own downwash, and you don't
need a high rate of descent to do it
(in essence, the vortices that should
trail after you in the cruise remain
around the machine at low speeds
and interfere with your lift). The
symptoms are random vibration,
buffeting, pitching, yawing, rolling,
an accelerated rate of decent and
momentary loss of cyclic control. It
is caused in a similar way to
recirculation, since the airflow
caused by the descent increases the
blade vortices, which reduces the
angle of attack as they share the
same space. With a rate of descent
matching the speed of the
downwash, there is no angle of
attack, and therefore no lift (the root
area will be stalled). If you like,
imagine the outer part of the main
rotors encased in a large doughnut
of recirculating air:
You are most likely to encounter it
during a low or zero forward speed
descent at a medium rate (500-1500
fpm) and a high power setting,
　
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