V-tails have been [fashionable][1] in the [Fourties][2] and [Fifties][3], and the claim was that they would cause less drag than an equally effective conventional tail. This has two reasons:

 1. Average chord length is longer, so the Reynolds number of the airflow is higher, causing relatively less friction drag.
 2. Instead of three surface-fuselage-joints, the V-tail has only two, so less interference drag is created.

[Flight testing of V-tails][4] showed only marginal advantages, and in damping they are less effective than a conventional tail which is dimensioned for the same control effectiveness. The control effectiveness of a control surface is proportional to the cosine of the V-angle, but the damping characteristics go down with the square of the cosine.

@nimbusgb mentions the tendency of the V-tailed Bonanza to fishtail: That is a consequence of too little lateral damping.

Also, when a combined elevator-rudder input is commanded, the V-tail will produce a very high deflection on one side, while the two commands will cancel out on the other side. This will reduce overall control power in cases where a combined elevator-rudder input is commanded over what is possible with a conventional tail.

Even the glider designers, who had enthusiastically [tried out V-tails][5], have now returned to conventional tails (more precisely, T-tails) because the expected benefits did not materialize. And glider designers will try anything to improve the aerodynamic quality of their designs.


  [1]: http://en.wikipedia.org/wiki/HKS-1
  [2]: http://en.wikipedia.org/wiki/Fouga_CM.170_Magister
  [3]: http://en.wikipedia.org/wiki/Akaflieg_Braunschweig_SB-5_Danzig
  [4]: http://www.wwiiaircraftperformance.org/me109/me109g-14003.html
  [5]: http://en.wikipedia.org/wiki/Glasfl%C3%BCgel_H-101