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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
actually prevented from escaping
properly, as when hovering close to
a building, or in a tight confined
area, the effect will be to tilt you in a
direction 90° from where
recirculation was introduced, or even
pull you down if all sides are affected
(as when landing in a courtyard).
Thus, if you are hovering a 206 next
to a building in front of you, the
recirculation occurs at the front, but
the disc will tilt to the left and make
the left skid hover lower than usual
which, if it catches you unawares,
might cause dynamic rollover (see
above). If you are closer than a third
of your disc diameter, the advancing
blade is also affected, in the above
example, pulling you towards the
building.
Be particularly careful within 1 rotor
diameter of another helicopter.
Vortex Ring
This occurs when you encounter
your own downwash, and you don't
need a high rate of descent to do it
Principles of Flight 39
(in essence, the vortices that should
trail after you in the cruise remain
around the machine at low speeds
and interfere with your lift). The
symptoms are random vibration,
buffeting, pitching, yawing, rolling,
an accelerated rate of decent and
momentary loss of cyclic control. It
is caused in a similar way to
recirculation, since the airflow
caused by the descent increases the
blade vortices, which reduces the
angle of attack as they share the
same space. With a rate of descent
matching the speed of the
downwash, there is no angle of
attack, and therefore no lift (the root
area will be stalled). If you like,
imagine the outer part of the main
rotors encased in a large doughnut
of recirculating air:
You are most likely to encounter it
during a low or zero forward speed
descent at a medium rate (500-1500
fpm) and a high power setting,
typically found in a steep approach,
where the column of air remains
underneath your helicopter. To get
out of it, reduce power, enter
forward flight or autorotation, but
bear in mind that you will lose a lot
of height anyway. Better still, keep
out of it with positive forward speed
or by descending more gently, so the
doughnut is below the machine.
Power
The types of power required are to
overcome the various sorts of drag,
namely parasite power, rotor profile power
and induced power, which accounts for
about 60% of the power needed to
hover (rotor profile power takes up
the other 40%).
Here is a graph that shows the
relationship of power required
against various forward speeds:
You can see that a little power is
needed at first to prevent the
machine sinking as the lift vector is
tilted and reduced, and you
transition into forward flight out of
the ground cushion, then reduces
drastically until the effects of parasite
drag come into force and require
much more power for forward speed
(it increases as the square of speed).
The lowest point of the curve is the
speed at which the least power is
needed, and is therefore the best for
endurance. The maximum range speed
occurs slightly later, when the curve
starts to rise more sharply (where the
projected line from the origin cuts
　
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