Originally Posted by
john_tullamarine
With OEI if an aircraft is trimmed for zero side slip it will have some drag due to side loading
Why so ?
The minimum drag position is some slip neutralised by some bank
I can't recall an authoritative source which has minimum drag other than for zero slip.
You've gotta consider separately the drag on 2 parts of the airplane:
1. The rudder/vertical stab
2. Everything else (fuselage, nacelles, etc)
The drag on "everything else" is lowest at zero slip, obviously.
But the drag on the rudder is lowest when it is centered. And my earlier post explains why it is closer to being centered not at zero slip, but at some amount of slip with the relative wind vector coming from the good engine side.
So the minimum drag for these 2 components is at different slip conditions, and since the total parasite drag is the sum of both of the components, its lowest point will be at some compromise condition between two conditions.
I had actually never thought of this until reading this thread, or if I did, I would have concluded that the difference is miniscule, and trying to compensate for it would have been cutting with a chainsaw what you measured with a micrometer. But it seems that Airbus thinks it's enough to actually matter.