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surface area of the driving region is
reduced, resulting in rotor decay:
Principles of Flight 37
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). 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
hovering, because it is not rotating
around the C of G, but the rollover
point, you have less chance to get
out of trouble, and the only effective
control is through the collective (do
not raise it). In other words, the lift
from the rotor disc that should be
vertical is inclined and converted
38 Canadian Private Pilot Studies
into thrust, above the enter of
gravity, so trying to use the cyclic to
level, and the collective to get you
off the ground is wrong!
Dynamic rollover is worst with the
right skid on the ground (counter
clockwise main rotor) and with a
crosswind from the left, with left
pedal applied and with thrust about
equal to the weight (i.e. hovering). It
is possible for the machine to roll
upslope if you apply too much cyclic
into the slope, or downslope if you
apply too much collective, enough to
make the upslope skid rise too much
for the cyclic to control. Avoid it by
keeping away from tail winds, and
land and take off vertically.
Ground Effect
In the hover, downwash is stopped
by any surface within about 1 rotor
length. Because downwash velocity
is reduced, so is the lift vector, which
becomes more vertical (which itself
increases thrust a little more and
reduces drag), resulting in less
induced drag and a reduction in the
　
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