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I am building a very large(11ft span) twin engine RC airplane that is loosely based on the Beechcraft 1900D design.

I can't help but notice the extra horizontal surfaces sticking out of the aft fuselage and the vertical surfaces hanging down from the ends of the horizontal stabilizer.

Could it be that after test flying they decided that the center of pressure needed to be moved further aft, but maybe it cost less to add new small surfaces instead of altering the design of the current surfaces?

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from the wiki page

Yes that is what is the most cost effective solution after flight testing has demonstrated that the vertical tail surface has to be increased for instance. The little winglets at the end of the horizontal tail do increase aerodynamic efficiency of the stab - plus they just look cool.

from the wiki page

The BAe 125 stuck a bit of fin on top of the original fin design, the Beech 1900 added the extra vertical tail surface underneath the aft fuselage cone, in the form of 2 v-shape surfaces which also help the total horizontal tail area.

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The best guess that an outsider can make is that the designers discovered the aircraft needed more lateral stability.

This is also apparent by the fact that the 1900D has larger ventral fins (the 2 vertical surfaces that extend below the aft fuselage) than the earlier models since it has a taller fuselage (and therefore taller cabin), in order to allow passenger walk in the aisle without crouching.

Beechcraft 1900C enter image description here

Beechcraft 1900D, note the taller fuselage and larger ventral fins enter image description here

Both images from https://en.wikipedia.org/wiki/Beechcraft_1900

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