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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
38 JAR Private Pilot Studies
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
into thrust, above the centre of
gravity, so using 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).
The machine could 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. The way to avoid it
is to keep 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
power needed to hover (a Bell 206
typically will need 15% less in the
ground cushion). In addition, the
accelerated air, having slowed down,
increases the pressure underneath
the rotor disc.
The effect is even more pronounced
when you lower the collective to
stop climbing, and will be more
apparent closer to the ground.
Factors that will reduce this are the
surface you are hovering over (the
harder and smoother the better, and
the more level), and the wind, which
will vary the direction of the
downwash from under the blades.
Any above about 10 kts, of course,
will produce translational lift.
Recirculation
When hovering near the ground,
some downwash comes back on
itself and goes through the rotor disc
twice, which reduces the lift
whenever this happens because it
does so at a higher speed and
reduces the space available for the
angle of attack in the resulting
vector. Vortices are present at the
rotor tips all the time, because they
are caused by centrifugal force, but
they are usually more than offset by
ground effect.
If your ground effect is reduced for
any reason (see above), the chances
of recirculation increase, requiring
more collective and cyclic to
compensate. Where the downflow is
　
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