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When you raise the collective, the
front portion of the disc creates
more lift, which actually takes effect
over the retreating side, causing a
roll towards the advancing blades
(right, in a 206).
The reason why the disc produces
more lift at the front is because there
is more induced velocity at the rear,
and less angle of attack, and less lift.
Principles of Flight 171
Tail Rotor Drift
In the hover, the tail rotor provides
more of a force in the relevant
direction than is actually required to
counteract the torque from the main
rotor. In other words, it's doing
more work because it is impractical
to place antitorque thrust at the
front of the machine. In the picture
below, the blades rotating around
point O at points A are
counterbalanced with a double force
BB, as you would get with a typical
tail rotor. If you cancel out one each
of A and B, you are left with a side
loading that causes movement:
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. It is also why, when
slinging, you need a clear space on
that side so you can go there safely,
and the ground crew need to be
taught to go the opposite way.
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. Instead, the 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.
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,
172 Canadian Professional Pilot Studies
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
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