曝光台 注意防骗
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could be damaged, or a particularly
flexible blade could hit the tail
boom. At certain critical speeds (50-
100 RPM), blades will pass in and
out of the stall. Holding the cyclic in
the direction of the wind will keep
the pitch of the advancing blade to a
minimum and stop it lifting in the
first place.
Other ways of minimising the effect
include parking the helicopter away
from the downwind side of
obstructions or the downwash or
slipstream of other machines,
keeping the collective down, or
accelerating and decelerating the
blades as quickly as possible. In
addition, point the nose out of wind,
so that the lowest deflection is away
from the tail boom:
Having the wind from the rear helps
you keep an eye on the low blade at
the front, but this means landing
downwind in the first place.
Do not use pitch on the collective or pedals
to slow the blades down –droop stops
depend on friction for proper
operation, and all you will be doing
is lightening the load where it ought
not to be.
Icing
Ice adversely affects performance,
not only by adding weight, but also
altering the shape of lift producing
surfaces, which changes your stalling
speed – autorotation could therefore
be a lot more interesting than
normal (the US Army found that
half an inch on the leading edge
reduces your lifting capacity by up to
50%, and increases drag by the same
amount) – if your engine stops, you
could really fall out of the sky!
On top of that, fuel could freeze in
wing tanks, as could control
surfaces, and slush picked up on
take-off could stop the landing gear
from operating, as well as flight
instruments.
Zero degrees is actually the point at
which water becomes supercooled
and capable of freezing. Airframe
icing happens when supercooled
water droplets strike an airframe
below that. Some of the droplet
freezes on impact, releasing latent
heat and warming the remainder
which then flows back, turning into
clear ice, which can gather without
noticeable vibration. On the ground
this can mean ground resonance, and
bits of ice flying off rotor blades. In
flight, the extra weight and drag
could cause descent and improper
operation of flying controls. So—it's
a good idea to avoid icing conditions
108 The Helicopter Pilot’s Handbook
but, in any case, you shouldn't go if
you haven't got the equipment,
which naturally must be serviceable.
The "clean aircraft concept" means
that nothing should be on the
outside that should not be there,
except, perhaps, for deicing fluid.
All ice should be removed from
critical areas before take-off,
including hoar frost on the fuselage,
because even a bad paint job will
increase drag, which is relevant if
you're heavy, and hoar frost will
have a similar effect. Deicing details
should be entered in the relevant
part of the Tech Log, including
start/end times, etc. The critical
areas include control surfaces,
rotors, stabilisers and the like.
The ability of an object to
accumulate ice is known as its catch
efficiency; a sharp-edged object is
better at it than a blunt-edged one,
due to its lesser deflection of air.
Speed is also a factor. Due to the
speed and geometry of a helicopter's
main rotor blades, their catch
efficiency is greater than that of the
fuselage, so ice on the outside of the
cabin doesn't relate to what you
might have on the blades. In fact,
Canadian Armed Forces tests show
that you can pick up a lethal load of
ice on a Kiowa (206) rotor blade
inside 1-6 minutes, although it’s true
to say that 206 blades, being fairly
crude, don’t catch as much as more
sophisticated ones, such as those
found on the 407. It’s also true that
some helicopters, such as the
Sikorsky S61, will not take ice on the
main or tail rotor blades down to
about –1 Centigrade, due to friction.
The rate of accretion is important,
not the characteristics of the icing,
although clear ice is definitely worse
than rime ice, since the latter
contains air bubbles and is much
lighter and slower to build—it also
builds forward from the leading edge
as opposed to spreading backwards.
Variations on clear ice are freezing
rain and freezing drizzle, both of
which have larger droplets and are
caused by rain, snow or ice crystals
falling through a layer of warmer air
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本文链接地址：
The Helicopter Pilot’s Handbook(72)
