Originally Posted by
JamesNoBrakes
That's just my point, the engines are mounted away from the fuselage in my "conventional twin" scenario, how does the spiraling slipstream from the left engine "jump" backwards and right and "hit the tail"? Why doesn't it go "straight back"?
I cannot say with any degree of certainty. I will make note that most of the books I've been reading do mention slipstream in a twin. However, it is not expanded upon since asymmetrical thrust is nearly always the critical design consideration.
Do recall, though, that it's not about air from the slipstream striking the tail and pushing it. It's about the slipstream creating a sideways component relative to the free stream flow. Thereby causing a slight change to the effective relative wind interacting with the vertical tail.
With that said, some thoughts/questions come to mind: Is the slipstream in a twin still close enough to change the rudders local AOA? Does the slipstream hold a uniform diameter, or does it widen as it travels backwards? Consider that the both engine in a critical engine configuration will move air to the right at the top of the slip stream.