GRAUPNER 4449.RXN Bedienanleitung Seite 83

UNI-EXPERT mechanics
since '04 series
14.4 Hovering
This is the state in which the helicopter flies in a constant altitude in a fixed position in the air, or
moving only slowly in any direction.
14.5 Ground effect
Ground effect is a phenomenon which only occurs 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 a fixed obstacle (the ground) and forms an „air
cushion". In ground effect a helicopter can lift more weight, but its positional stability is reduced,
with the result that it tends to „break away" unpredictably in any direction.
14.6 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, i.e. one which in-
cludes forward motion. For this reason a model with a given amount of motor power will climb
more rapidly at an angle than vertically.
14.7 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 en-
counters 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 re-
sult is a lateral inclination of the helicopter.
14.8 Descent
If the helicopter's rotor speed is relatively low and you place the helicopter in a fast vertical de-
scent, the result may be that insufficient air flows through the rotor. This can cause what is
known as a „turbulence ring", i.e. 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.
14.9 Flapping motion of the rotor blades
As we have already seen, the forward-moving blade produces greater upthrust than the other
blade. This effect can be minimised by allowing the forward-moving blade to rise and the other
blade to fall. The rotor head is fitted with what is known as a flapping hinge to allow this move-
ment, and this prevents the rotor plane tilting excessively in forward flight. In model helicopters a
single hinge shared by both blades has proved a good solution to the problem.
14.10 Auto-rotation
This term refers to a helicopter flying without motor power. The rotational speed of the main
rotor can be kept high by setting both blades to negative pitch, and the airflow through the rotor
as it descends then keeps the blades turning. The rotational energy stored in the rotor by this
means can be converted into upthrust when the helicopter is close to the ground, by the pilot
applying positive collective pitch. Of course, this can only be done once, and it has to be done at
the correct moment. Auto-rotation allows a model helicopter to land safely when the motor fails,
just like a full-size machine.
However, auto-rotation places considerable demands on the pilot's judgement and reflexes; you
can only halt the machine's descent once, and you must not „flare" too early or too late. Much
practice is required to get it right.
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