曝光台 注意防骗
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height/speed and RPMs, plus
ENGINE OUT audio/visual
warnings (if fitted), because there's
so much noise you can't tell whether
the engine's still going anyway. While
speed is of the essence, there is
usually time enough to verify actual
engine failure by looking at the
instruments while you're reducing to
Techie Stuff 253
autorotation speed to maintain
height, certainly in a Bell, unless
you’re very heavy in a high hover
situation, such as long-lining, where
you have no time to do anything
other than dump the pole.
For all practical purposes, your
gliding distance is about equal to
your height or, put simply, what you
can see slightly above the bottom of
the windscreen. If you keep your
landing spot in the same place in that
area, your speed watch needn’t be so
critical (remember sight picture
approaches?). In fact, once you've
set your speed, keeping a mental
note of the attitude will enable you
to look out more. Loss of RPM at
the entry into autorotation is more
important—a higher angle of attack
from the new relative airflow as air
rushes up through the rotors will
cause enough drag to slow the rotors
drastically, especially if your weight is
high or air density low, meaning that
your blades will be at a higher pitch
angle anyway. Get that collective down,
and bring the airspeed back to autorotation
speed. Then accept the inevitable, that
you may hit something, so your
primary focus now is to ensure you
and your passengers' survival, that is,
to protect the cabin area as much as
possible. Of course, it would be nice
to save the whole ship, but don't
stretch the glide, for example,
towards a clear area and risk losing
the RPM, or having less control over
the rate of sink. Clear areas should
really be within a normal glide.
On this point, remember that the
helicopter is better able to cope with
a vertical rather than a horizontal
crash (oops, sorry, landing), since the
gear can usually take some
punishment, as is proven daily by
student pilots. You can use the tail
boom and main rotors, too,
especially in trees, mentioned below.
To ensure the horizontal element is
reduced, the best tactic will be to
land in a decelerating attitude, which
will mean making sure that the rear
skids hit first (this will also help you
keep straight. The reason a Jetranger
requires to be levelled during normal
autorotative landings is to preserve
the gearbox mountings, but this is
less of a consideration right now). A
couple of good reasons for avoiding
run-on landings are obstacles, and
soft ground, which would increase
your chances of nosing over, due to
the inertia of the gearbox, engine
and rotors, etc. against the drag of
the skids.
Reducing collective to compensate
for the extra drag will, of course,
increase the rate of descent, at which
point the inner 25% of each blade is
stalled, and the outer 30% is
providing a small drag force. In
other words, it is being driven:
The right hand view above is what
happens to the lifting area if you
vary the ideal speed – it moves
towards the retreating blade side,
and when it reaches the edge, you
get your VNE for autos. The best
254 Operational Flying
lift/drag ratio in autorotation is
obtained at best endurance speed,
whatever that is (check the manual,
but most helicopters are designed
for a speed of about 45 kts).
Try to establish the cause of engine
failure—if it's fire, close the throttle,
but, if not, consider initially closing it
only to idle speed (or not closing it
at all) as the engine may be able to
provide enough power to help you
out a little, but this point is
controversial as many people think
you should secure the engine and
fuel in case the landing gets hashed
up, which may actually be caused by
a playful engine, or a fire caused by a
hot one (turbines cool down quite
fast after they are turned off). Also,
if the main drive shaft breaks in a
206 or a 407 (and others), you will
need the engine to drive the tail
rotor. However, the discussion
below will consider it closed.
Several factors may affect your rate
of descent, such as gross weight, air
density, airspeed and rotor RPM.
Changing airspeed, though, is about
the only one you have direct control
over that gives you some flexibility,
as the RPM must remain in a small
speed band to be effective—
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