ChopperAddict's Hints & Tips


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   HOW 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 opposite reaction")

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 blade 
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.


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