Quick question on the B1900. What are those fin-like structures that extend downwards and outwards from the rear part of the fuselage? What are they called? What purpose do they serve? (keel effect?) I saw them also on several bizjets but never bothered asking one of the pilots what they were for. (I felt stupid asking the question)
Also, what are those fin-like structures that extend straight down from the horitontal stab of the B1900? What are they called? do they serve the same purpose of the fins that extend from the fuselage? Is the B1900 the only aircraft that has them? (if so, why?)
For the two bolded bits:
You may mean the "delta fins" which are fitted to the rear fuselage of Learjets. These provided a nose-down pitching moment at high angles of attack, helping stall characteristics, and also have some beneficial effects on Dutch Roll behaviour.
The CL-415 Waterbomber has "fin like structures" on the horizontal stab, too. In fact, the CL-415 and the B1900 are probably neck-and-neck for the title of "aircraft with largest number and variety of aerodynamic 'fixes' added". When you see anything that inelegant (is that a word?) it usually means that either they missed something in the design and had to fix it with an add-on, or they knew about it during design but weren't allowed to fix it for $$$ reasons.