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There is another way of looking at it,
though. If you had contra-rotating
main blades, the body would stay
still, because the counteracting
forces are in line with each other.
The tail rotor, however is out on the
end of the tailboom, and therefore
has a moment arm, and enough
leverage to cause movement.
The correction can be done simply
by holding the cyclic slightly offset
from its central position. Other ways
include offsetting the mast or the
engine, rigging the controls, or
causing the disc to tilt when the
collective is raised. None, however,
eliminate it completely.
Tail rotor drift is why the helicopter
will go one way or the other
(depending on which way the blades
go round) when the engine fails in
the hover.
Disc Loading is calculated in
lbs/square foot and is obtained by
dividing thrust by the disk area. It
doesn't change by adding more
blades, or widening the existing
ones, but blade loading is lowered.
Tail Rotor Roll
If the tail rotor is below the level of
the main rotor, the drift mentioned
above will cause a couple with the
tail rotor thrust going the other way,
causing one or other of the skids to
be lower in the hover, depending on
the blade rotation (it's the left one
with North American rotation, that is,
anticlockwise as viewed from the
top). It is therefore totally normal
for one skid to be lower than the
other, unless you've left the
refuelling hose in (actually, this
characteristic is quite useful when
landing on sloping ground, as long
as the slope goes with the skids).
To combat this, you could raise the
tail rotor on a boom or lower the
rotor head, as is done with the
Brantly, but the C of G position
could screw that up anyway.
Principles of Flight 33
Tail Rotor Failure
When the tail rotor fails, it will be in
varying degrees of positive, neutral
or negative pitch, depending on what
you were doing at the time, so if you
can remember what it was, you will
have an idea of the state of the
pedals. Unless it’s a drive failure, or
you lose some of the components,
the chances are that you won’t
discover the problem until you
change your power setting, as it’s
very unlikely you’ll be flying along in
the cruise, for instance, and find a
pedal forcing itself completely over
to one side, as simulated by
instructors on test flights, unless you
have a motoring servo or similar, in
which case your problem is
hydraulics and not the tail rotor,
although the effect might be the
same. More typically, you will be in a
descent, climb, cruise or hover, with
the pedals where they should be and
won’t move when you want to do
something else. When descending,
for example, in the AS350, you will
have more left pedal (more right in
the Bell 206), both of which will aid
the natural movement of the
fuselage against the main rotors. The
pedals would be in a neutral position
if you were flying at medium to high
speeds, and the power pedal would
be forward in high-power situations,
like hovering. In any case, the spread
between the pedals is not likely to be
more than a couple of inches either
way, certainly in a 206 – try an
autorotation properly trimmed out
to see what I mean. You will notice
the same in the hover. My point is
that the situation may not be as bad
as frequently painted.
In fact, landing with a power pedal
jammed forward is relatively easy,
since the tail rotor is already in a
position to accept high power
settings (try also using a little left
forward cyclic in a 206, and pivoting
round the left forward skid), so you
may be able to come in very slowly
and even hover. If the pedals jam the
other way (right in a 206), look for
more speed because there will not be
enough antitorque thrust available.
A drive failure, on the other hand, or
loss of a component, will cause an
uncontrollable yaw, and maybe an
engine overspeed, so the immediate
reaction should be to enter
autorotation, keeping up forward
speed to maintain some directional
control (which is difficult in the
hover, so try to get one skid on the
ground at least), if you have time. If
you lose a component, the C of G
may shift as well, although an aft one
in general has been found to help
with this situation. Pilots who have
been there report that there is a
significant increase in noise with a
　
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