曝光台 注意防骗
网曝天猫店富美金盛家居专营店坑蒙拐骗欺诈消费者
minimise collective for the hover,
and give you the most control as you
keep translational lift as long as
possible, but there’s very little up
your sleeve at the end, and you need
to be very aware of your winds, as
forward speed will mask the effects
right to the last minute, although it
does give you a good idea of the
level of your site. This assumes you
remember your training and keep
going forward and down, so the
cyclic is ahead of the game and
operating in the cleaner air in front
of the machine that helps with
translation. In other words, keep the
rotor disc forward, so the flow of air
is from front to back, especially
where snow is concerned, but you
shouldn’t use the shallow approach
with powdered snow anyway,
because you will lose sight of your
landing point at the critical moment
Specialised Tasks 73
in the resulting white cloud. The
other thing to bear in mind is that
you are trying to land at probably the
only spot available, and if you have a
problem in a shallow approach, you
aren't going to get there.
You could, on the other hand, use a
steeper angle, particularly if you're
going into a clearing surrounded by
tall trees, increasing with the wind
strength, but this requires large
handfuls of power and attitude
changes in the final stages if you
don’t get ground effect, so you
wouldn’t try this in an underpowered
piston-engined machine that really
shouldn’t be there in the first
place—the engine may be able to
cope with it, but can your tail rotor?
(leading with the pedals will help).
Anyway, since ground effect reduces
your torque requirement for the
hover by up to 15%, if you approach
in such a way that you need no more
than that amount to stop, you
should find your descent stopping
nicely in the right place, assuming
the surface is conducive to it, and
whether you have high skids or not.
You also have some potential energy
available for an escape.
I guess you could use whatever
works—I generally turn in steep
around 60 kts with the disc loaded as
much as possible, consistent with
descending at about 250 fpm – if the
blades have some tension on them,
they are less likely to be overstressed.
Not only that, the controls are more
responsive. The power used will give
you a good idea of what you need in
the hover, so you have an early
chance to abort if you are using too
much (you get to know with
experience). This works, because 250
fpm reduces the thrust required to
transition into the hover by about
15%, i.e. much the same as for
ground effect. 250 fpm is about 20
feet every 5 seconds, if you haven't
got a VSI (altimeters usually have 20-
foot segments).
Whichever you choose, if the
machine wants to weather cock, let
it—there's no point in using power
or making a lot of effort to keep
straight if you're going in the right
direction anyway. Keeping the whole
of the windward side in view over
the crest will keep you forward of
the demarcation line and in the
upflowing air. Coming at 45° will
help with escape routes and give you
a better angle.
When you make a final approach to
land, remember that you may not be
able to hover when you get there. If
you do manage it, make it low,
somewhere between 1-2 feet, and
brief, one or two seconds. No-hover
landings are not recommended.
In a confined area, there will be a
point beyond which you're
committed, so don't go beyond it
until you’re sure. Pick a point to aim
for where you know your tail will be
clear, not too far towards the end,
and fly the machine in, in as smooth
a movement as possible, going over
the lip to the clearing at around a
walking pace. As for power checks,
you will know very early on if you're
running out (keep an eye on the
torque). The size of any surrounding
trees will give you a false illusion as
to the size of the clearing, in that big
trees will make it look smaller and
vice versa. A typical clearing will have
stumps and slash all over the surface
– if you don’t have logs to land on
(and these produce their own
problems when they are slippery),
74 The Helicopter Pilot’s Handbook
take off a cleanly as possible, to
avoid your skids getting caught in
something (also be aware that tall
trees will sway from your
downwash). When landing, if there's
中国航空网 www.aero.cn
航空翻译 www.aviation.cn
本文链接地址：
The Helicopter Pilot’s Handbook(49)
