V-tails have been fashionable in the Fourties and Fifties, and the claim was that they would cause less drag than an equally effective conventional tail. This has two reasons:
- Average chord length is longer, so the Reynolds number of the airflow is higher, causing relatively less friction drag.
- Instead of three surface-fuselage-joints, the V-tail has only two, so less interference drag is created.
Flight testing of V-tails 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, 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.