When talking about T-tail configuration, I don't understand the statement that, "The vertical tail can be shorter due to the end plate effect of the horizontal tail."
What exactly is the endplate effect and how does it affect the aerodynamic characteristics of the tail?
An end plate on a lifting surface (which the vertical tail is, during a yaw maneuver) reduces tip vortex formation, which means less lifting area is needed to produce the same lift force. It also significantly reduces induced drag, though the parasitic drag of the plate is usually a net loss unless, as in the case of a horizontal tail, the "plate" serves some other function that would be required anyway.
Whitcomb and newer design winglets use careful design to give similar effects with less, or even negative drag -- but you'd need a horizontal tail anyway, so the reasons not to have a T configuration boil down to structural weight, elevator blanketing in a stall, and control routing issues.
For an example of an aircraft that makes good use of tip plates to allow a shorter wingspan, look up the Bumblebee -- at least at one time, it was the dimensionally smallest piloted aircraft ever flown, and it critically depended on tip plates on both upper and lower wings to produce enough lift to fly.
Another end plate application is putting the vertical stabilizers on the end of the H stab! This would make the elevator more effective by "trapping" higher air pressure when the elevator is deflected. Essentially a type of fence. This also showed up on the XB-70 when it's wing tips folded down to ride its own shock wave. Also seen on wing tip tanks. But, as it creates drag, the trick is to combine it with another function. Birds seem to do a great job doing this with their outer wing tip feathers, combining slats, winglets, and vertical stabilizers all in one.
