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TO - How to set up the tail rotor blade position
on almost any helicopter
For some very strange reason, when building or
repairing our helicopters, most of us tend to position the tail rotor
hub about midway on the shaft as a starting point, so that there is NO
PITCH ANGLE on the tail rotor blades. PRETTY DUMB REALLY.
Think about it. What exactly are the tail rotors there
for. Yep, to control the tail, and therefore the first thing it needs to
be able to do is to hold the tail reasonably steady at a hover, which
means it always has to "fight" the reaction of the helicopter
to rotate the opposite direction to the main blade rotation. So
with a helicopter that has a clockwise rotation of the main blades, the
helicopter itself will always try to rotate ANTI CLOCKWISE against that
rotation (PHYSICS SAYS : "To every action there is an equal and
So in that case, the tail rotor blades need to be set
at around 5-6 degrees approximately in the correct direction to apply
pressure on the tail to make it move CLOCKWISE, thereby negating the
effect of the main rotors.
We can see straight away from this that a central
position of the blade hub and zero pitch on the tail rotor blades is not
what we need. We need to have the hub about 2/3rds from one end of
the throw and 1/3rd from the other end.
Now I hear many people shouting, that will give us a
biased tail throw. Well, yes, it will, but that is still fine as
we are only ever going to move the tail pitch pretty well the same
amount in either direction. In other words, our NON CENTRAL CENTRE
POINT is the CORRECT PLACE, and if you try it out on your helicopter,
you will soon see that you only move from that point about the same
distance in either direction.
Think about it briefly, to turn AGAINST the rotors
blade rotation, we will need even more tail rotor pitch - OK so
far. To turn WITH the rotor blade rotation, we just need to return
to our "central" position, and then reduce it a bit further to
let the main blade inertia push the tail around with the main
rotors. This is much easier for it to do, so we do not need as
much to achieve a turn WITH the main rotors as we do AGAINST them.
The picture above is for a clockwise
main rotor aircraft, so hub is moved INWARDS
from the centre point of the shaft to provide the correct tail
pressure to hold the tail against the main rotor rotation.
In the picure above you can clearly see the offset of
the hub, but PLEASE NOTE the bell crank arm position, which is dead
perpendicular, as should the servo horn be with the tail set like this.
PROOF - Sure - watch your heli in a hover, and do
pirouettes to the left and right, which way does it turn more rapidly
(With or Without the main rotor direction.) Yep, correct, when you
remove the pitch form the tail blades, the helicopter will piro much
faster, and will also probably climb little, whereas doping a piro
against the main blades will be slower, and cause the helicopter to
descend a bit. Its all about BASIC PHYSICS REALLY.
This leads me on neatly to setting the maximum travel.
In line with the above, most helicopters rarely need
to be able to push the tail blades to the maximum distance with the main
rotors, but do need to be able to push them a little further than doing
piros in the opposite direction. Therefore it is totally
reasonable to set up endpoints that are pretty well the same distance in
either direction, even though this means that the rudder control will
not move the hub all the way in both directions.