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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 unawares,
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
18 The Helicopter Pilot’s Handbook
example, pulling you towards the
building.
Be particularly careful within 1 rotor
diameter of another helicopter.
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,
typically found in a steep approach,
where the column of air remains
underneath your helicopter. To get
out of it, reduce power, enter
forward flight or autorotation, but
bear in mind that you will lose a lot
of height anyway. Better still, keep
out of it with positive forward speed
or by descending more gently, so the
doughnut is below the machine.
Specialised Tasks
Some of the more exotic things you
can do with helicopters include
bombing avalanches, rapelling (that
is, dropping off people to fight
forest fires, otherwise known as
dope-on-a-rope), wildlife capture,
aerial ignition, water sampling, where
you hover very low over a body of
water and a scientist dips the
equivalent of a jamjar into it (I still
haven't found out why they don't
just use a boat), or frost control,
where a large barrel of oil is lit to
provide smoke that will indicate the
level of an inversion. You then fly
with your rotors just above the
smoke to bring the warm air down
and prevent frost on crops.
Note: Some of this is dangerous!
Don’t try it without training!
External Slung Loads
A helicopter can go where cranes are
impractical or more expensive, or
you might not be able to get a load
inside the machine (maybe you
wouldn’t want it there anyway, if it’s
a dead animal or explosives), so you
try and lift it.
In theory, you can lift anything,
provided the payload is available;
I've even been asked to quote for
lowering 800 feet of unrolled
telephone cable down a mine shaft,
because the drum it was rolled on
wouldn’t take the weight. However,
more common tasks are logging,
placing air conditioning or
ventilation equipment on the roofs
of tall buildings, pulling cows out of
bogs, picking up water to put out
forest fires (water bucketing),
20 The Helicopter Pilot’s Handbook
dropping solution over forests (top
　
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本文鏈接地址：The Helicopter Pilot’s Handbook(12)