曝光台 注意防骗
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you have your landing speed.
Loss of Tail Rotor Effectiveness
This is sometimes known as tail rotor
breakaway, or a stall, which is not
strictly correct, as thrust is still being
produced – it’s just not enough for
the task in hand. It shows up as a
sudden, uncommanded right yaw (with
North American rotation), and has
amongst its causes high density
altitudes and power settings, low
airspeeds and altitudes, and vortex
ring. Your helicopter will be more
susceptible to it if the tail rotor is
masked by a tail surface, like a
vertical fin, and it can be especially
triggered by tail and side winds (this
is actually a significant reason for
maintaining main rotor RPM – as
the tail rotor runs at a fixed speed in
relation to it, lower NR will reduce
tail rotor effectiveness in
proportion). Recovery in this case
comes from a combination of full
power pedal, forward cyclic and
reduction in collective, or
autorotation. Prevention lies in
keeping into wind and always using
the power pedal (left in a 206 or one
with similar blade rotation). If you
174 Canadian Professional Pilot Studies
use the other one, not only will the
fuel governor ensure that the aircraft
will settle after a short time (using
the power pedal by itself makes it
climb), but a large bootful of the
power pedal in a fast turn the other
way will create a torque spike.
Rotor Systems
Three or more blades require a fully
articulated rotor, which essentially
allows all of them to move in their
various planes independently of each
other. This adds complexity and
expense to the design, however.
A semi rigid rotor has the blades fixed
with regard to feathering, but they
can flap up and down because the
whole head is allowed to teeter, like
a seesaw.
A rigid rotor only allows feathering,
but the blades are more flexible
towards their ends, so they bend
when absorbing the forces of flight,
producing the same effect as
flapping and dragging hinges, but
removed from the root.
In flight
In the hover, other things being
equal, the lift vector acts directly
upwards:
When you tilt the disc forward, it is
reduced, because some of its power
is diverted to the selected direction:
The resultant (i.e. the diagonal line
drawn across the two vectors) is
where the main force finally ends up.
The tangential velocity is the speed of
the blades' rotation. It increases with
distance away from the hub, until it
finally becomes a tangent to the edge
of the disc, hence the name.
Combined with the downwash velocity,
you end up with a resultant
corresponding with the blade's actual
speed and path, or the relative wind
(although its name suggests
otherwise, the downwash
component moves upwards):
In autorotation, the function of
downwash velocity is replaced by air
going up through the rotors and
creating a larger angle of attack.
The flight velocity is the reciprocal of
the relative airflow, made up of
downwash and tangential velocity
and movement of the machine
through the air.
Translation is the conversion from
hover to forward movement, where
the helicopter is supported by other
means than its own power, that is,
relative airflow. Translational lift is the
extra thrust you get from forward
Principles of Flight 175
movement, when the new airflow
enters the disc. The helicopter flies
better because you get more air
through per unit of time, which has
a lower induced velocity because it
hasn't had a chance to speed up just
before going through the rotor. As
tip vortices are also being left
behind, your lift vector becomes
more vertical, for more thrust with
less drag. The reason you have to
lower the collective to maintain
height at this point is because the
angle of attack has increased against
the new relative airflow. This also
means less engine power is required.
Of course, all the while rotor
efficiency is increased with forward
flight, at some point you need to
increase power to overcome drag
from the fuselage, which is
increasing at a faster rate.
This is why you should not reduce
power at the end of a climb until you
have both the speed and height you
want (if you reduced power at, say,
1500 feet and 60 knots, but you
really wanted 100 knots, you
wouldn't be able to accelerate
beyond a certain point without
applying more power than you
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