As bank is increased into the operating engine, Vmca decreases ... and as bank is increased away from the operating engine, Vmca increases. This is to do with the effects of sideslip and directional stability on yawing moments and, consequentially, the rudder deflection required to make up the difference if one is to achieve a straight flight path.
Any chance of a little more detail here?
I can see that the optimum (zero-slip) straight-and level situation is (say) 5 degrees into the operating engine with corresponding rudder input. If I bank 15 degrees (to 10 degrees into dead engine, or 20 degrees into operating engine) without changing the rudder input, don't I turn just as if I were in a 15 degree bank?
For the same rate of turn, why should I run out of control authority (is it rudder or aileron?) in one direction before the other?