Sorry Scoob. I can't answer your question. But at least with the convesation that's followed, it'll keep it the subject at the top of the screen and eventually someone will be able to give you the answer you're looking for.
The concept of crossover speed relates to the situation where the rudder has displaced from centre due to a malfuntion in the actuator system and is stuck in a fixed position. I have been led to believe that no mater what the airspeed, the rudders's position is fixed (Is that incorrect?). Therefore rudder ratio devices etc don't come into it.
As the airspeed reduces, you gradually need to use more and more opposing aileron to keep the wings level. Actually you need a little bank angle otherwise the heading will be changing, a flat turn I suppose you'd call it. Eventually you reach the stage where the airspeed has reduced so much that you have full aileron and the thing just begins to roll in the direction of the rudder displacement.. .Of course this is all happening with symetrical thrust. We found that a great deal of control can be restored if you introduce asymmetrical thrust. A bit like a engine failure situation only the rudder deflection came first.
The landing would be interesting. Haven't tried that one yet.
Still wondering why it happens. I hear what you say john_tulla in your point (a) but I'd love to see it (the graph) for myself.
Cheers.