When calculating rudder control power using the equation (from Perkins and Hage)
CNδr = -av(τ)(Sv/Sw)(lv/b)ηv
Where av is the vertical tail lift curve slope, τ is the rudder effectiveness parameter, Sv and Sw are the vertical tail area and wing area, respectively, lv is the longitudinal distance from the CG to the VT aerodynamic center, b is the wingspan, and ηv is the vertical tail efficiency, how does one determine ηv? I have an equation for the horizontal tail efficiency, but I'm not sure that applies to vertical tails as well. Unless I overlooked something, no book I own (Perkins and Hage, Raymer, McCormick, or Pamadi) mentions how to determine the vertical tail efficiency. In an example in my college stability and control class notes, 0.9 was used for the Navion example that were were doing. I am going to tentatively use that value for my scale T-28 until I can be steered in the right direction, since the Navion's overall configuration is pretty similar to a T-28. Any thoughts?