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with similar blade rotation). If you
use the other one, not only will the
fuel governor ensure that the aircraft
will settle after a short time (using
the power pedal by itself makes it
climb), but a large bootful of the
power pedal in a fast turn the other
way will create a torque spike.
Rotor Systems
Three or more blades require a fully
articulated rotor, which essentially
allows all of them to move in their
various planes independently. This,
however, adds complexity and
expense to the design.
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.
In flight
In the hover, other things
being equal, the lift vector
acts directly upwards:
When you tilt the disc, the lift vector
is reduced, because some of it is
diverted to 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):
36 Canadian Private Pilot Studies
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
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
behind, your lift vector becomes
more vertical, for more thrust with
less drag. The reason you have to
lower the collective to maintain
height at this point is because the
angle of attack has increased against
the new relative airflow. This also
means less engine power is required.
Of course, all the while rotor
efficiency is increased with forward
flight, at some point you need to
increase power to overcome drag
from the fuselage, which is
increasing at a faster rate. This is
why you should not reduce power at
the end of a climb until you have
both the speed and height you want
(if you reduced power at, say, 1500
feet and 60 knots, but you really
wanted 100 knots, you wouldn't be
able to accelerate beyond a certain
point without applying more power
than you would have used before.
All changes in velocity from cyclic
movements are known as transitions.
Autorotations
Loss of RPM at the entry into
autorotation is the most significant
problem—a higher angle of attack
from the new relative airflow as air
rushes up through the rotors will
cause enough drag to slow the rotors
drastically, especially if your weight is
high or the air density low, meaning
that your blades will be at a higher
pitch angle anyway, to meet the
conditions of flight.
Reducing pitch to compensate will,
of course, increase the rate of
descent, at which point the inner
25% of each blade is stalled, and the
outer 30% is providing a small drag
force (in other words, it is being
driven). The best lift/drag ratio in
autorotation is at best endurance
speed (check the manual, but most
helicopters use about 45 kts). This is
when the driving region of the disc is
exactly centred. As you increase
speed, it moves toward the retreating
blade side of the disc until it touches
the edge, which is your power-off
VNE. If it goes beyond the edge, the
　
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