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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
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
120 The Helicopter Pilot’s Handbook
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—
remember that only the portions of
the blades between 25-70% of their
length provide any lift. In any case,
every 1% reduction in rotor RPM
results in a 2% loss of thrust, which
will be the same as if somebody
threw that weight in the back of your
machine. The other significant point
about keeping your RPM up
concerns the tail rotor, which runs at
a fixed speed relative to the main
rotors – if they go slower, the tail
rotor does too, and loses some of its
effectiveness.
Changes in airspeed can have
dramatic effects on the rate of
descent. If the recommended IAS is
60 knots (fairly common), for
instance, speeds of either 30 or 100
could increase RoD to as much as
3000 fpm, because the lift vector is
reduced when you alter a relatively
horizontal rotor disc, so if you want
to change your angle of approach,
don't forget to use collective to
compensate (when going for range,
you must use both collective and
speed to get the full effect—you can
pull collective until you get nasty
noises in your ear). As forward
airspeed increases, the driving region
of the autorotating blades moves
towards the retreating blade side –
the point where it meets the edge of
the rotor disc is where power off Vne
is found. If you go beyond it, your
driving region gets smaller, so your
rotor RPM will decay.
Turns will have a similar effect, but
the results will be worse if pedals are
used. Steep turns are good ways of
losing height if you find yourself
overshooting—if you are seated on
the right, turn right first, then go left,
so you have the best possible view
through the windscreen, and you
don't get the instrument bulkhead in
the way on finals. If you are on the
left, go the other way. However,
some pilots alternately overshoot
and undershoot the area they want
to land in, rather than doing S-turns.
What you do is bracket the spot with
smaller and smaller movements until
you hit (get used to what your
machine does in straight-in autos).
Each helicopter will have an
optimum rate of descent for the
longest range, but most light
helicopters can stretch their glide at
10-20 mph faster than best lift/drag
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本文链接地址：
The Helicopter Pilot’s Handbook(80)
