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cyclic controls (see Airframes &
Engines, next), and is not the same as
the angle of attack, between the chord
and the relative airflow.
Rotor Profile Drag comes from rotor
blades at zero pitch, occurring purely
because the blades are rotating. Air
flowing through the disc at positive
angles of attack suffers from induced
drag, which is highest in the hover.
The downwards motion of air
through the blades is called the
induced flow.
As the helicopter moves, the blades
will move above and below the plane
of rotation, in a process called
flapping. A flapping hinge allows this
movement to happen, to cope with
different angles of attack around the
disc, and equalise the lift around it.
They are needed when you have
more than two blades, which would
use a teetering head, and work like a
seesaw for the same effect, where
the two blades will flap as a unit.
When the helicopter moves forward,
the blade going forward will develop
more lift because of the added speed
(from the helicopter's forward
movement and that of the blade), so
it will fly higher. As it does so, the
angle of attack reduces because of
the change in relative airflow.
On the other side, the blade going
backwards will generate a lot less lift
because of its reduced speed, in
some areas producing a reverse
effect, which will cause the blade tip
to stall if it gets large enough - on a
Bell 206 at 100 kts, the non-lift
producing area of the retreating
blade is about 25%. This will make it
170 Canadian Professional Pilot Studies
fly down to increase its angle of
attack, to create more lift (needing
more forward cyclic to compensate).
Disymmetry of lift, therefore, is the
difference in lift between the
advancing and retreating blades,
compensated for by flapping, which,
unfortunately, causes the centre of
mass of the blades to move, making
them speed up or slow down relative
to each other. Limited movement
horizontally is provided with dragging
hinges - dragging is the movement of a
rotor blade forward or backward in
its mounting. However, such hinges
are only found in articulated heads
(when a blade is ahead of its normal
position, it is leading, and when
behind, it is lagging). Semi-articulated
heads (as with the AStar) may have a
flexible coupling that allows foreand-
aft movement, but with no
flapping hinges – instead, the blades
flex when compensating for lift. The
pitch angle of the blades is changed
by feathering, i.e. allowing them to
rotate around their axes.
The speed at which the retreating
blade tip stalls depends on the total
pitch of the blade, that is, whatever
is set by the combination of
collective and cyclic. The cyclic input
will increase with speed, and the
outer part of the blade will stall first,
the maximum effect being felt just
aft of the trailing edge. In the cabin,
you will detect a rolling tendency
(usually towards the advancing
blade) and a rearward tilt, together
with stick and aircraft vibration and
reverse cyclic behaviour.
Thus, the helicopter stalls as a
function of going too fast, rather
than too slowly, as with an aeroplane
– the retreating blade flapping down
to increase its lift gets a very high
angle of attack, which announces
itself with a lot of vibration. Try to
avoid the problem if possible, by
watching your airspeed and keeping
away from VNE in gusty conditions.
Recover by lowering the collective.
The advancing blade can stall, too,
but from compressibility and high
speed buffeting near the speed of
sound, which limits forward speed.
The rotor disc behaves like a
gyroscope, and is subject to precession,
meaning that an input doesn't have
an effect until 90° later in the
direction of rotation (see Instruments
for more on this). Thus, if you
pushed the cyclic forward, and the
controls were not corrected, you
would actually move left or right,
according to which way round the
blades were going. To cater for this,
control inputs are applied in advance
of the blades' movement. Their
delayed response is phase lag.
The effects of this can be seen when
increasing the collective in forward
flight (say when taking off)– there
will be a roll towards the advancing
blade because the front portion of
the disc is always more efficient than
the rear, due to Transverse Flow, which
is a fore and aft disymmetry of lift.
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