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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
Principles of Flight 39
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 the disc diameter, the advancing
blade is also affected, in the above
example, pulling 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
(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
40 JAR Private Pilot Studies
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
the curve a second time). The best
rate of climb speed gives the maximum
altitude in the shortest range, and
because the maximum power is also
available then, is the same as the
endurance speed. In other words,
the best ROC is obtained when there
is the greatest difference between the
power required for level flight and
that available from the engines.
Blade Sailing
High winds and gusts will cause the
main rotor blades of helicopters to
flap up and down and be both a
danger to people near them and the
helicopter itself, as the blade stops
could be damaged, or a particularly
flexible blade could hit the tail
boom. At certain critical speeds (50-
100 RPM), blades will pass in and
out of the stall. Holding the cyclic in
the direction of the wind will keep
the pitch of the advancing blade to a
minimum and stop it lifting in the
first place.
Other ways of minimising the effect
include parking the helicopter away
from the downwind side of
obstructions or the downwash or
slipstream of other machines,
keeping the collective down, or
accelerating and decelerating the
blades as quickly as possible. In
addition, point the nose out of wind,
so that the lowest deflection is away
　
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