Micro Star 400 v2
24
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 - 1½ 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-throttle. If you have
trimmed the machine carefully for a steady hover, it will tend to turn to one side when flown for-
ward. The reason for this phenomenon is that the rotor blade which is moving forward encoun-
ters an increased airflow caused by the wind, and this increases its upthrust compared to 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 de-
scent, the result can be that insufficient air flows through the rotor. This can cause what is
known as a "turbulent ring stage", when the airflow over the blade airfoil breaks away. The heli-
copter is then uncontrollable and will usually crash. A high-speed descent is therefore only pos-
sible 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.
Auto-rotation
This term refers to a helicopter flying without motor power. The rotational speed of the main ro-
tor 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 if the pilot applies positive collective pitch when the heli-
copter is close to the ground. 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. Plenty
of practice is required to get it right.
Содержание Micro Star 400
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