[Courtesy of Mark Drela, drela "at" mit.edu, March 2000]
The issue of V-tail size and angle came up several times at the last meeting. Below is a short blurb on converting a conventional tail to an equivalent V-tail and vice versa.
To convert a conventional tail into a V-tail:
A_vtail = A_vertical + A_horizontal
angle = arctan[ sqrt( A_vertical / A_horizontal ) ]
Notes: "A_vtail" is the area of both halves together, rotated flat. "angle" is the V-tail's dihedral angle from the horizontal.
To convert a V-tail into a conventional tail, use the reciprocal formulas:
A_horizontal = A_vtail * [ cos(angle) ]^2
A_vertical = A_vtail * [ sin(angle) ]^2
These formulas are strictly correct only for large tail aspect ratios, since they do not account for the local interference and lift cancellation at the V-tail roots during yaw or "rudder" application. But they are much better than guessing.