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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.
How Helicopters Work 15
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. This is why
some helicopters have mast torque
gauges to measure the bending of
the mast.
In flight
In the hover, other things being
equal, the lift vector acts directly
upwards:
When you tilt the disc forward, the
lift vector is reduced, because some
of it is diverted towards the direction
selected:
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
movement, when the new airflow
enters the disc. The helicopter flies
better because you get more air
16 The Helicopter Pilot’s Handbook
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
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本文链接地址：
The Helicopter Pilot’s Handbook(10)
