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Welcome to the world of R/C Helicopters. The
helicopter is probably the most challenging form of
radio control model, being mechanically complex in
nature, and requiring 100% concentration while
operating.
Flying a model chopper is not unlike balancing a
metal bearing or a marble on a piece of glass. If
the mechanics of the chopper have been adjusted and
aligned very well, it is similar to having a flat
piece of glass. If the chopper is not set up just
right, it is like having a convex piece of glass
where the bearing wants to keep rolling off to one
side.
The first thing the budding helicopter pilot must
realize is that the model works on the very same
principles as the full-size and controlling the
chopper is just as difficult, if not more-so due to
size and orientation. It is not simply a matter of
pushing one button for up, and another for forward
flight, etc. Flying a helicopter, just like flying a
model aircraft, is a skill that must be learned and
that can only happen with practice . . . now that I
have you thoroughly discouraged! Although building
and flying a model chopper can be complex, it is
also extremely satisfying. Being able to accurately
control a vehicle which you can hover, fly forward,
backward, sideways, and do all kinds of interesting
maneuvers and aerobatics, as well as land at your
feet, is very exciting.
Unlike learning to fly a model airplane where
flying with an instructor is a must, you basically
learn to fly helicopters by yourself. Before you
start flying, however, some time with an experienced
helicopter pilot will be invaluable. He can help you
set up your helicopter (it is extremely important to
have the mechanics set up accurately for safe and
easy flying) as well as give you some tips on
flying; what to expect from your model and how to
operate the controls.
How does a Chopper Work?
There are basically two different types of
helicopters, those that have collective pitch and
those that do not. Collective pitch is where the
pitch of the main rotor blades may be simultaneously
increased or decreased to change the amount of lift.
This gives a quicker response to changes in vertical
thrust as controlled by the pilot. On helicopters
without collective pitch, the amount of lift is
controlled entirely by the speed of the rotor blades
or in other words the speed of the engine (throttle
control). The reaction time is longer and thus the
control is less responsive.
There is a trade-off, however, and that is the
cost and complexity of the rotor head. There are a
great deal more moving parts in a collective pitch
rotor head and thus, they are more expensive. Most
current choppers are of the collective pitch
variety.
On a standard, collective pitch helicopter there
are four controls and these are operated by five
channels of your radio system. These controls are
the collective pitch, the fore and aft cyclic pitch,
the side to side cyclic pitch, and the tail rotor
pitch. The collective pitch must also be coupled
with the throttle of the engine so that when more
load is put on the main rotor blades by increasing
the pitch, more throttle is applied to help overcome
the additional drag.
Helicopter flight is governed by the pitch, or
angle, of its rotor blades as the sweep through the
air. When climbing or descending, the pitch of each
blade is changed simultaneously and to the same
degree. To climb, the angle or pitch of the blades
is increased. To descend, the pitch of the blade is
decreased. Because all blades are acting
simultaneously, or collectively, this is known as
collective pitch. For forward, backward and sideways
flight, an additional change of pitch is provided.
By the means the pitch of each blade is increased at
the same selected point in its circular pathway.
This is known as cyclic pitch.
When a helicopter is started up and the rotors
begin to turn they are maintained in flat pitch,
with no angle, or bite on the air. As the engine
warms up and the rotors turn faster, the collective
pitch is increased and the helicopter lifts
vertically. To make the aircraft fly forward, the
collective pitch is retained, keeping the aircraft
in the air, while the cyclic pitch is adjusted to
enable each blade to have more bite as it passes
over the tail. To stop the helicopter and hover, the
cyclic pitch is returned to neutral, causing the
rotor blades to have the same pitch thought-out
their cycle, allowing the collective pitch to retain
the helicopter hovering in the air.
The left stick of your radio transmitter controls
the collective and throttle in the vertical
direction and the tail rotor pitch in the side to
side direction. Your right stick controls both
cyclic operations; up and down for fore and aft
control and side to side for the cyclic side to side
control. There are also mixing functions which mix
the throttle and collective functions, and the
throttle/collective and tail rotor functions.
Operation
The engine of a helicopter drives both the main
rotor shaft and the tail rotor via a series of gears
and a clutch. As the motor comes to speed, the
clutch engages and begins to turn both rotor
systems. Generally, at this point, there is no pitch
on the main rotor blades and thus no lift. The
throttle is increased until the main rotor blades
are brought up to speed. To lift the helicopter
collective pitch is applied.
Because, for every action there is an equal and
opposite reaction, when the engine is forcing the
rotor blades to turn in one direction, the body of
the helicopter will want to rotate in the opposite
direction. The function of the tail rotor is to
correct this tendency. The tail rotor blades provide
enough thrust to the side to keep the helicopter
pointing in one direction. By increasing or
decreasing the pitch of the tail rotor blades the
direction the helicopter is pointing can be changed.
The cyclic control permits the main rotor blades to
be varied independently making the helicopter move
in a horizontal direction. If one of the rotor
blades increases pitch as it approaches the rear
while the opposite blade decreases in pitch while
approaching the front during its rotation, more lift
will be produced in the rear, tilting the helicopter
forward, and thus moving the helicopter in a forward
direction. The same principle applies for side to
side and rearward, allowing the helicopter to fly in
any direction.
The control of the cyclic and collective pitch is
transferred from the radio servos to the rotor
blades via the swash plate. Part of the swash plate
is stationary while the other part is allowed to
rotate with the rotor head. Control linkage is
connected from the servos to the stationary part of
the swash plate as well as from the rotating part of
the swash plate to the rotor head.
When flying a chopper, small control inputs are
continually required by the pilot to correct for
deviations in the flight path. That is why 100%
concentration is required in chopper operation. The
more accurately the chopper is set-up, the fewer the
number of corrections that are required by the
pilot.
What Happens if the Engine Stops?
Auto-rotation is a way for helicopters to land
successfully after a loss of power from the engine
to the rotor drive systems.
This is accomplished with the aid of a special
device known as an auto-rotation clutch which allows
the rotor blades to free-wheel. As soon as power has
been cut, the throttle/collective control is brought
back all the way.
This will usually bring the main rotor blades to
have slightly negative pitch. As the helicopter
starts to descend, the air moving through the blades
will keep them spinning. The spinning blades will
act like a parachute in reducing the helicopters
decent. When the helicopter nears the ground, the
pilot increases the collective pitch making the
pitch of the blades again positive.
The momentum of the blades is converted to lift,
slowing the descent of the helicopter down further,
enabling it to land softly. |
