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17. Some basic terms used in model helicopter flying
The term „rotary wing machine" indicates that the helicopter’s lift is derived from rotating „wings"
which take the form of rotor blades. As a result, a helicopter does not require a minimum
forward speed in order to fly, i.e. it can hover.
Cyclic pitch
Cyclic pitch variation is used to steer the machine around the roll and pitch axes. Changing
cyclic pitch has the effect of altering blade pitch depending on its position in the circle. The
effect is caused by tilting the swashplate, which then effectively tilts the helicopter in the
required direction.
Collective pitch
Collective pitch provides control over vertical movement, i.e. for climb and descent. The pitch of
both rotor blades is altered simultaneously.
Torque compensation
The spinning rotor produces a moment which tends to turn the whole helicopter in the opposite
direction. This effect must be accurately neutralised, and this is the task of the tail rotor. Tail
rotor blade pitch is altered to vary torque compensation. The tail rotor is also used to control the
model around the vertical (yaw) axis.
Hovering
This is the state in which the helicopter flies in a fixed position in the air, without moving in any
direction.
Ground effect
This occurs only when the machine is close to the ground, and it falls off as altitude rises. At an
altitude of about 1 - 1.5 times the rotor diameter, ground effect is completely absent. Normally
the revolving airflow from the main rotor is able to flow away freely, but in ground effect the air
strikes an obstacle (the ground) and forms an „air cushion". In ground effect a helicopter can lift
a greater weight, but its positional stability is reduced, with the result that it tends to „break
away" in an unpredictable direction.
Climb
Any excess power above that required for hovering can be exploited to make the helicopter
climb. Note that a vertical climb requires more energy than an angled climb which includes
forward motion. For this reason a model with a given amount of motor power will climb more
rapidly at an angle than vertically.
Level flight
A helicopter absorbs least power when flying straight and level at about half-power. If you have
trimmed the machine carefully for a steady hover, it will tend to turn to one side when flown
forward. The reason for this phenomenon is that the rotor blade which is moving forward
encounters an increased airflow caused by the wind, and this increases its upthrust compared
with the blade which is moving downwind, where the same airflow has to be subtracted. The net
result is a lateral inclination of the helicopter.
Descent
If the helicopter’s rotor speed is relatively low and you place the helicopter in a fast vertical
descent, the result can be that insufficient air flows through the rotor. This can cause what is
known as a „turbulent ring", when the airflow over the blade airfoil breaks away. The helicopter
is then uncontrollable and will usually crash. A high-speed descent is therefore only possible if
the helicopter is moving forward, or if the rotor is spinning at high speed. For the same reason
care should be exercised when turning the model helicopter downwind after flying into wind.
Flapping motion of the rotor blades
As we have already seen, the forward-moving blade produces greater upthrust than the trailing
blade. This effect can be minimised by allowing the leading blade to rise and the trailing blade to
fall. The rotor head is fitted with what is known as a flapping hinge to allow this movement, and
this prevents the rotor plane tilting excessively in forward flight. In model helicopters a single
hinge shared by both blades has proved an effective solution to the problem.
