Stek79,

Gap closed on slipstream. Area distribution is the question. Slip stream does not center on stab line. Slipstream “is what it is” by the time it reaches the back end of the plane. I do not think the spiral necessarily centers on the thrust line either. I think the airflow might tend to use the fuselage centerline as a core to revolve around. I have not seen wind tunnel tests or similar on this. The difference points to where to distribute side area and your original question. I suspect that fuselage side area forward works like stator vein to straighten out flow. This is the reason for my disagreement with your suggestion of a weighted sum calculation. Look at it like the fuse modifies the spiral flow, and the vertical tail reacts to the result. Your idea of high thrust line splitting flow about existing vertical fin is actually correct with no fuselage in-between. I have flown many pusher designs with tail mounted on a small dowel located very low, usually out of the prop flow. When the motor is high compared to the fin they do yaw right instead of left. The H-stab straightens flow also, but I think too far aft to matter.

For a tractor prop on a normal fuselage, I think high mounted thrust leaves little fuse area above to straighten prop wash across top of fuselage, resulting in larger cross flow hitting the vertical fin. The implication of this reasoning is a lower mounted thrust line will leave more fuselage area above to straighten flow over fuse top, lessoning the cross impact on the vertical tail. Running an enclosed exhaust above the fuselage within a long cowling would increase upper fuselage area even further. Additionally, the small fuselage area below would leave greater sideways spiral underneath. A counterbalancing sub fin would become more effective and not need to be as large. Interestingly, what I have just described is the milestone pattern design lineage begun by a guy named Joe Bridi with his Dirty Birdy. Compare to his earlier Chaos series. What do you suppose influenced these changes?

It would be good to experiment with shapes in the way dick Hanson mentioned above. Cut out various slab/profile fuselage test shapes, mount an engine in front, and hold them while running to evaluate. Results should be very telling. Essentially this is a wind tunnel test outside of a tunnel.