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would have used before.
All changes in velocity from cyclic
movements are known as transitions.
Autorotations
Note: This section deals with exam
requirements only. For a more
detailed discussion of what the
working pilot may need to consider,
refer to The Helicopter Pilot's
Handbook, by yours truly.
Loss of RPM at the entry into
autorotation is the most significant
problem—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 the air density low, meaning
your blades will be at a higher pitch
angle anyway.
Reducing pitch to compensate 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 best lift/drag ratio in
autorotation comes at best
endurance speed (check the manual,
but most helicopters use about 45
kts). This is when the driving region
of the disc is exactly centred. As you
increase speed, it moves towards the
retreating blade side until it touches
the edge, which is your power-off
VNE. If it goes beyond the edge, the
surface area of the driving region is
reduced, resulting in rotor decay.
When about 70 feet from the
ground, (depending on whether you
think you are descending or moving
across the ground too fast), use
rearward cyclic to slow down
vertically and horizontally, in a
manoeuvre called the flare. 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).
176 Canadian Professional Pilot Studies
It also increases the rotor RPM.
Continue the flare progressively (and
sharply), to be at the correct speed
for landing at 10 feet, applying
collective as flare effect decreases to
check the descent more positively,
watching for drift (the “check” is the
application of some collective to
brake the descent – in the 206 it can
be a positive movement; in the 407
and Astar, it can just be a pause). As
the flare ends, and the kinetic energy
of the rotors is used when the
collective is raised, the airflow
through the rotors is reversed,
assisting you to level (the amount
depending on the model), ready to
cushion the landing as you apply
collective pitch. This is where
correct use of airspeed during the
descent will have had the most
beneficial effects—as the kinetic
energy stored in the blades is what
slows you down, it follows that any
you have to use to slow an
unnecessarily fast rate of descent is
not available for the final stages of
touching down.
Ground Resonance
In flight, most parts of a helicopter
vibrate at their own natural
frequency. On the ground, they
collect through the landing gear - if
its natural vibration matches that of
the main rotor, every time a blade
rotates, the present vibrations
receive another reflected pulse to
increase their amplitude, and which
can cause the aircraft to tip over and
be destroyed. Peculiar to some
helicopters, with fully articulated
rotors, because they have dragging
hinges (they are there to counteract
vibration caused by movement of
the blade's centre of mass), this is
indicated by an uncontrollable lateral
oscillation increasing rapidly in
sympathy with rotor RPM. It could
also be caused by blades not being in
balance, unequal tyre pressures or
finger trouble, but will only occur if
the gear is in contact with the
ground. It's best avoided by landing
or taking off as cleanly as possible,
but, if it does occur, you must either
lift off or lower the collective and
close the throttle.
Dynamic Rollover
This occurs when your helicopter
has a tilted thrust vector with respect
to the C of G, commonly
encountered with some side drift
when you have one skid or wheel on
the ground acting as a pivot point,
but you can also get a problem when
your lateral C of G falls outside the
width of the skids or wheels. Every
object has a static rollover angle, to
which it must be tilted for the C of
G to be over the roll point, for most
helicopters being 30-35°. As your
lateral cyclic control at that point is a
lot less effective than if you were
　
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