Aerodynamically the X tail is most efficient because no surfaces work "against" each other...
Well, that's debatable, especially when you consider the cases of combined control inputs of simultaneous rudder and elevator.
However, the case for intersection drag is a little more clear cut. "Inside" corners generate intersection drag. A V-tail has only one inside corner, a T-tail and a Conventional tail each have two, and a Cruciform tail has four. The case for an X-tail will depend on the specific details of the particular tail design in question, but could be as bad or even worse than a Cruciform tail's.
As someone who has designed with all major types of tails, including considerable experience with V-tails, and with models that are available with the builder's option of V-tail or conventional tail, I can say with confidence that the aerodynamic differences between the tail types are extremely small, if the tails in question have been designed properly. The main advantages of the V-tails over other types in actual practice are in the areas of structural efficiency and durability. For models, those advantages can be considerable, if the tail is designed to take advantage of them.
This subject has been thoroughly thrashed over many times in the past, almost as much as the Downwind Turn and the Skeeter threads. Some of the results of those discussions can be found in the "Ask Joe and Don" section of our website.
Don Stackhouse @ DJ Aerotech [EMAIL PROTECTED] http://www.djaerotech.com
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