ORIGINAL: Moerig
If 90 deg bank was possible without departing level flight pitch would be yaw and yaw would be pitch in world coordinates?
In "world co-ordinates", yes. But it gets confusing trying to switch back and fore between the aircraft and the world, so we stick to using the aircraft's frame of reference. As mentioned earlier, the overall change of heading is broken into the two components in yaw and pitch, the component in the yaw axis is cosine angle of bank and the component in the pitch axis is sine angle of bank. The rate of angular motion seen by the yaw gyro is therefore rate of turn times cosine angle of bank. At 60 degrees of bank cosine is 0.5 so the gyro sees a rate of rotation only half of the rate of the aircraft's rate of turn. This in effect gives a change in the ratio of fishtailing to turning, since any fishtailing continues in the yaw axis but the rate of turn seems to the gyro to be less and less as bank increases, so it maintains its full fishtail sensing but very little turn sensing.
The point at which the rudder gyro will oppose you the most is in small angles of bank, with high turn rate which means slow speed. In other words times like turning finals are most likely to encounter the rudder gyro trying to counteract the turn with opposite rudder.