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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
Techie Stuff 255
10-20 mph faster than best lift/drag
speed and maintaining the lower
main rotor RPM limit. Best
endurance speed will give the best
lift in autorotation (i.e. minimum
sink). Where winds or density
altitudes are high, expect to increase
speed a little, but decrease it when
winds are calm or density altitudes
are low, or you need a little time to
choose a landing area or be a little
more precise (see below). If you
deliberately decrease the RPM to
increase range (and possibly lessen
the rate of descent), don't forget to
build it up again for landing.
A typical helicopter has a power
curve looking like this:
If you are aiming for a particular
spot, you will have a better chance of
hitting it if you keep your speed
below the best speed mentioned
above (60), as anything faster will
cause you to float, and force you to
anticipate the glide more (that is, the
aiming point is different from the
landing spot, and the faster you go,
the more they will be different. If
you only have one clearing, you will
have a better chance of getting there
and flaring very close to it if you
keep your speed tight). If you are on
the backside of the curve, remember
to get your full speed up by 100 feet,
or the flare will be just a change in
attitude and fail to do its proper job
of slowing you down (see below).
Having entered autorotation:
· Select landing spot
· Transmit MAYDAY
· Warn passengers
· Turn off electrics, but NOT
battery (for the intercom)
· Close throttle if required
For a clear area, when about 70 feet
from the ground (depending on
whether you think you are
descending or travelling across the
ground too quickly), use rearward
cyclic to slow down vertically and
horizontally. The amount is
proportional to your speed and
serves to increase the total lift
reaction (which stops the sink) and
shifts it to the rear (which stops
forward movement). It also increases
the rotor RPM.
Continue the flare progressively to a
nose-high attitude (in a 407, the
instrument panel should hide the
horizon), applying collective as flare
effect decreases (and the sink starts)
to allow the machine to flop forward
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