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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
fly down to increase its angle of
How Helicopters Work 13
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.
approaching the speed of sound,
which will limit 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.
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.
The point about flying controls is
that they should always be moved
smoothly. Good helicopter flying is
essentially downwash management,
which has some lag built in. If you
jerk the controls, you will get all the
14 The Helicopter Pilot’s Handbook
drag without the lift when the blades
get into position before the airflow
has a chance to catch up.
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
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The Helicopter Pilot’s Handbook(9)
