Something to do with the high angle of attack of the wing creating such a volume of turbulent airflow behind it that the stabilizer and elevator are not effective enough aerodynamically to push the nose down. The CAA required a stick push system on Tee tail aircraft but the FAA didn't. So our three holers at Dans had a pneumatic stick pusher but over here in the USA they dont have them. I believe Mike Lithgow was the test pilot in the Salisbury 1-11, and as I recall in a most exemplary demonstration of the test pilots art, was trying different configurations and reporting on their effect all the way down. Per Ardua..........