We have been discussing couples that create rotation of a body - how do you identify the axis of that rotation? Is it along the line between (in this case as in ACs diagram) MR head and TR and, if so, where along that line?
I am ok on this one, I hope. A couple is a vector - it has direction and magnitude. The direction for a couple is parallel to the axis of rotation. It does not have a point of application as such. - it is acting on the whole body. I think that this is actually a tricky thing to conceptualise. I might also point out that this is statics, with forces and couples in equilibrium. With dynamics the situation is much more complex.
I am pretty sure that it is ok to apply statics to a hovering helicopter, but I am even having some doubts about that.
