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air subsides suddenly, you will be
going down faster than you can
apply it. Do not fight it, but guide
the aircraft towards a lifting slope, or
try for a cleaner column of air. You
might get help from the ground
cushion, but the effect will be less on
a slope or grass. When valley flying,
upslopes or slopes exposed to the
sun can produce updraughts, so
place yourself on a converging
course to the line of the ridge and
positioned to obtain a straight flight
path two thirds up the slope and one
across, which is generally the area of
smoothest flight. However, local
conditions could vary this.
You could climb on a lee slope (that
is, the other side from where the
wind is coming from), taking
advantage of the updraught formed
by stronger wind returning on itself
(i.e. riding the backlash, which tends
to occur with abrupt surfaces).
Also, there is so little room to
manoeuvre if something goes wrong,
or you meet someone coming the
other way. If you have to do this,
converging on the ridge line at 45o
gives you the best chance of an
escape route.
Similarly, try and avoid flight along
lee slopes, but if you need to
(because life's sometimes like that),
smoothest flight will be obtained by
flying as close as possible to the
ground, say about six inches, so
you’re in the boundary layer, which
is a steady movement of air close to
the surface, with a vertical element.
This gives even less room for error,
though. A good illustration of the
boundary layer comes from your car
after it's just been washed—water
left on the bodywork will not be
affected by the air flowing over it,
because it's in a layer all to itself. Air
next to mountains behaves in the
same way and, when landing at least,
will be mostly what you work with.
Winds
Winds can increase your operational
ceiling, payload, rate of climb, range
and cruise speed. They can also do
the opposite, and be very difficult to
predict, with formidable up and
downdraughts associated with them.
When cruising downwind, along a
lee slope or not, sudden wind
reversals could make you exceed
Specialised Tasks 69
VNE or even take away your airspeed
completely.
There are several types of wind,
loosely be grouped into prevailing or
local, with the latter subdivided into
other types, such as anabatic,
katabatic, etc., and which are
infinitely variable (glacier winds
descend 24 hours a day). The
prevailing wind is steady and fairly
reliable, and starts to affect you from
about 6000 feet upwards. Smoke
from local fires may be used to
detect direction, as can water, but
this may only give half the story. For
instance, it's not uncommon for the
windsocks at each end of Banff
airstrip in the Rockies to be 180° out
with each other! Indeed, upper
winds can come in many directions
at different levels, and are usually the
opposite of lower winds. Where
mountains are concerned, they also
acquire a vertical element, which is
actually where the boundary layer
comes from.
As a guide to speed, whitecaps on
water foam at 10 mph. Dark
depressed puddles on water are
called bearpaws (or catpaws) and are
caused by downbursts. The most
important thing to watch out for is
the funnelling of wind as it
progresses down a valley, so
although the mean windspeed may
be reported as 5 knots or so, you
may find it as high as 30 in some
places, and not necessarily coming
from the expected direction.
In fact, understanding how air
moves around terrain is one of the
keys to good mountain flying,
particularly the demarcation lines
between smooth and turbulent air.
In general, that moving up is
smooth, and that moving down is
turbulent. You can visualise the
difference if you think of a waterfall,
and the state of the water before and
after dropping over the edge. Close
to the ground, the air moves in
laminar fashion (the boundary layer),
but the depth of the layer and the
gust spread will vary considerably,
depending on the nature of the
surface and its heating. The flow will
be broken if the ground becomes
rough, or there are trees, and the
wind is strong. Turbulence will occur
on both sides, resulting in an
updraught close to the leeward side
and a downdraught close to the
windward side as air is made to curl.
The movement of air over a crest
line has a venturi effect, giving an
　
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本文鏈接地址：The Helicopter Pilot’s Handbook(46)