Interesting point about the slip ball in SE work.
As I understand it the certification authorities allow up to 5 deg bank toward the live engine in addition to the rudder input. I think that this is because it allows further performance to be squeezed out of the a/c at low weight t/o where you may be near VMCA. You could be running out of rudder, but the bank allows you to maintain centreline tracking on climb out. I think it's 5deg because any more would raise spoilers and degrade lift.
The Airbus flight director and 'slip' indicator work together to achieve this.
The slip indicator becomes a 'beta target' when an EO is sensed, and if it is centred the a/c will still turn toward the dead engine (with wings level). The F/D commands a small (5deg) bank toward the live engine to correct this.
The resultant flight control inputs also correspond to a minimum drag config.
If you think about it, it's actually impossible to completely centre a normal slip indicator in assymetric flight on a twin (or more) type. If you do you'll probably have to apply cross control on aileron in order to track straight. Undesireable due to unecessary drag.
I'd need a diagram to explain that point more clearly, but think about the way the displaced thrust line acts relative to the yaw axis of the a/c. If you trim the rudder to allow the a/c to track straight there will still be a 'slip' indicated. Conversly if you 'dial out' the slip with the rudder trim you won't track straight without some bank.
I seem to recall the B737 manual states the same, and recommends allowing the ball to be slightly off centre (i.e. slipping) if the a/c is trimmed to track straight with wings level.
Clear as mud?
[ 03 December 2001: Message edited by: maxalt ]