tandem rotor helicopters
with thanks to the
helicopterpage
Tandem rotor helicopters operate a
little differently than do the
single rotor variety. In a tandem
rotor helicopter, you have no tail
rotor, so there is no translating
tendency to deal with, but you
still have pedals for directional
control at a hover. Your cyclic
control, which is used as it
always has been in single rotor
helicopters, has not changed
either. The only thing different
in terminology for tandem rotor
aircraft is the term "Thrust
control", which is used to
describe the collective pitch
control lever. It is used in the
same way as any other collective,
but the tandem guys use the term
thrust control.
Rotor Position for Level Hovering
Tandem rotor helicopters operate
in forward flight by using
"Differential Collective Pitch" or
"DCP". DCP is basically just
increasing more pitch in one rotor
system then the other to make the
aircrafts attitude change. By
increasing the pitch in the aft
system more than the forward
system, the aircraft will tilt
nose low, and accelerate forward.
To climb without changing
airspeed, more pitch is placed in
both systems simultaneously. It is
really a matter of aircraft
attitude more than anything else.
If the aircraft is in a nose high
attitude, it will climb and bleed
off airspeed. If it is too nose
low, it will dive and increase
airspeed. The amount of pitch put
in each rotor system will dictate
airspeed and altitude. The pilot
will fly the aircraft just like
any other, but the rotor systems
will act in a way peculiar to
tandem helicopter flight. The
picture here is of a tandem
helicopter in level hovering
flight. Notice both rotor systems
are depicted as level. The actual
blades will "Cone" a bit. What
this means is they will bend
upwards to the tip. The more the
weight on the aircraft, the more
the blades will cone. For our
descriptive purposes, the pictures
will not depict coning.
Here's a Chinook doing slope
operations.
Rotor
Position for Manoeuvring
You
will notice in the first of these
two pictures both of the rotor
systems are tilted to one side.
This is what allows the aircraft
to fly sideways. This is
accomplished by using lateral
cyclic, and a corresponding pedal
input to maintain directional
control. What the cyclic and
pedals do is actually put inputs
to the rotor systems to make them
both tilt the same direction, as
if there were two cyclics both
putting in duplicate inputs to the
two rotor systems. In the second
picture, you see the front rotor
system level, and the aft system
tilted, allowing the helicopter to
pivot around the forward mast.
This can be a handy manoeuvre when
there is little room to move the
nose, but plenty of room to swing
the tail of the aircraft. This is
accomplished by using large pedal
inputs and small cyclic inputs to
make the aircraft pivot around its
nose. The manoeuvres just
happen mainly by feeling your way
through it while you judge your
distances, make sure your engine,
or engines are working properly,
and that you are not going beyond
the aircrafts limitations. You are
so busy doing other things, you do
not actually think about the
actual control inputs to induce
the manoeuvre.
More Rotor Position for
Manoeuvring
In the first of these two
pictures, you see both rotor
systems are tilted in opposite
directions. This is accomplished
by pedal only inputs. By
depressing one pedal over the
other, cyclic inputs are put in
both systems in opposite
directions to pivot about the
centre of the aircraft. Both rotor
systems receive equal cyclic
inputs, and the helicopter just
spins nicely at its centre without
the pilot having to move his
cyclic control at all. In the
second picture, a pivot around the
tail is depicted. This is
accomplished by heavy cyclic
inputs by the pilot, and little or
no pedal inputs. This will make
the tail stay in one place, and
the nose of the aircraft moves
laterally until it spins about the
aft mast. (Also known as the aft
vertical shaft, due to its
height). This is accomplished by a complicated
system that requires a lot of
linkages, a lot of control tubes,
and a pilot who trusts his
maintenance crew to make sure it
all was put together properly.
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