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Otherwise, you might find a power
and speed combination that will
maintain height until you find a
suitable landing area, then you've got
as much time as your fuel lasts to
solve the problem. Don't forget that
the cyclic can be useful for changing
direction and enabling you to fly
sideways to create drag from the tail
boom and vertical stabiliser, for
example. It's the sort of situation
where it pays to be creative
sometimes. After all, the aim is to
walk away, not necessarily to
preserve the machine. Two other
things you can try if you finally make
the hover—stirring the cyclic so as
to dump lift, and pumping the
collective to produce a similar effect.
Both will serve to confuse the
machine enough so it forgets which
way to turn! With a jammed power
pedal (left, in a 206), what also works
is to crab in the way the machine
wants to, come to a high hover
sideways and let the machine settle
by itself. You will find very little
input is required by you.
If you want to run-on for landing,
get the wind and/or nose off to the
retreating blade side, so the fuselage
is crabbing, and control your
(shallow) descent with a
combination of throttle and
collective, applying more of the
latter as the throttle is closed just
before touchdown so you run on
straight. Note that some helicopters
(such as twins, or the AStar) won’t
let you use the throttle as precisely as
that. Not only that, you may well be
so busy that worrying about minor
details like the wind’s exact quarter
will be the last thing on your mind.
For a running landing, on most
machines, about 30% torque at 30
kts will put you in a good position
for landing at 30 ft, and a little
power at the last minute will put
your nose nicely straight. For the
non-power pedal, keeping straight
involves either more speed or less
power, and you have to accept more
of a run-on.
In an AStar (or TwinStar), the
recommendation in the book is to
come in with some left sideslip (i.e.
crabbing right). Slow down until the
nose starts to move to the left, and
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
Principles of Flight 35
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
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. This,
however, adds complexity and
expense to the design.
A semi rigid rotor has the blades fixed
with regard to feathering, but they
can flap up and down because the
whole head can 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, the lift vector
is reduced, because some of it is
diverted to the direction selected:
The resultant (i.e. the diagonal line
drawn across the two vectors) is
where the main force finally ends up.
　
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