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C4 - GENERAL
does the helicopter go left and backwards on take off ?
This really is
a question that crops up all the time, because new pilots quite
reasonably expect a helicopter to lift off the ground going straight
problem is down to the main blades spinning quite fast, and the tail
rotor trying to stop the rest of the helicopter from spinning in the
opposite direction (Remember Physics at School - To every action
there is an equal and opposite reaction ?) So with a large
number of model helicopters using a clockwise rotation of the main
blades, the tail rotor is trying to force the helicopter to NOT go
anti clockwise, which is it's natural reaction.
is logical to realise that the tail rotor is actually trying to make
the helicopter go clockwise, so it is pushing the tail to the left -
with me so far ? While you are on the ground, especially until
the helicopters main blades are up to flying speed, the tail rotor
is also not up to full speed, so needs more rudder input to stop the
helicopter from rotating on the ground, especially if you are on a
As soon as the
helicopter lifts off the ground, all these forces act together with
nothing to stop them, and because the tail is pushing left, the
helicopter naturally goes left.
backwards is a little harder to describe, but it is mostly due to
precession, which is what goes on with the main blades whenever the
swash plate is not exactly level. Your first thought I am sure if
you want the heli to go forwards is to push the elevator forward,
and you would expect, if you could stop the blades instantly, to
find the forward pointing blade with downwards pitch, and the
backwards pointing blade with positive pitch to make the helicopter
nose go down ? Well, your wrong...........
In fact, due
to the laws of physics, the position of the blades would be as
described above, except that it occurs a full 90 degrees before you
would expect it to happen. Therefore, as the helicopter lifts
off the ground, this precession tends to make the helicopter come
backwards towards you.
to all of this is quite simple, and most more experienced pilots do
this without thinking about it, but just before take off, you push
the cyclic stick a little way towards the 2pm position, which
applies forward and right cyclic, correcting this tendency. Once the
helicopter is fully airborne, you re-centre the cyclic as it will no
longer be going left and back.
The best rule
of all to remember however is to always look carefully at the main
rotor disk position, and make sure it is ABSOLUTELY LEVEL with the
horizon just before lift off.
THIS EXPLANATION HELPS ... ?
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