According to our Australian Bell rep., who tends to be fairly clued up, the 206L originally had no end plates, but had gihugeous fairings on the aft cross tubes. The requirements had to do with stability for IFR, and was then met by adding the end plates to the horizontal stab and removing the fairings. The angles on the end plates are optimised for cruise conditions, and are predicated on the rotor downwash in cruise.
The 205 differential in incidence angles on the horizontal stab intrigued me, since the reply given was that it is optimised to give stability of airflow and rotor downwash in REARWARD flight!! The 205/UH1 is certificated for 35 knots backwards, hence the differential stab incidence. Cf the 212, which is NOT certified for such speeds backwards, and has equal angles on the stab.
If anyone has a different explanation, I'm only repeating what I got from the Bell rep......
