Back in 74--I built some pusher -twin boom pattern models - they were dreadful flops at that task.
The thrust seemed puny on both - (little 40 powered and larger 60 powered designs)
The design was a swept constant chord --with the prop on the CG- and twin booms and a tail group which was of relatively small area.


I tried some tests -as I did not have a wind tunnel .
These were tests to see what the air going to the prop and from the prop was actually doing.
I used an old setup - a simple piece of string -held in various positions to see what the airflow looked like.
The strange thing was that the string -held aft the prop --showed the air stream rapidly became a very small cone !
It was shaped almost like a water whirlpool , going down a drain.
So - are there any bits of knowledge that explain this?