... my intuition says that the VS+rudder will generate the minimum amount of drag for a given coordinated flight condition when they are in-trail with each other (and of course, no tab, no wedge). But I could imagine reasons why that might not be the case. (snip)
Roee, your intuition is *probably* correct. For a trimmed flight condition such as cruise, we have relatively light yaw axis trim force requirements along with high airspeeds. The VS/Rudder doesn't need to operate at a high "lift" coefficient under these conditions, so the faired VS/Rudder will work pretty efficiently. Remember, our tail surfaces have zero camber when everything's lined up.
I put the *probably* asterisks in there to answer this question because we have other coordinated flight situations where the faired VS/Rudder might not be the most aerodynamically efficient shape. In that vein, another case to consider would be a prolonged climb to altitude.
In the climb case, we have much larger trim forces required, and lower speeds. (I'm aware of the propeller-induced velocities on the tail, but those will not increase local flow speeds as much as you'd see during cruise. And the "P-factor" additives are worse while climbing) Under these conditions, we're asking the VS/Rudder system to operate at a larger lift coefficient. Adding camber to an airfoil has the effect of shifting the minimum drag point to correspond with a higher lift coefficient. Ultimately, we'd like to see C_l(required) correspond with C_d(min) to get min drag.
With the higher C_l requirement in climb, its likely that having some camber (ie, some rudder deflection) in the vertical tail would be optimum for the least drag. But, we wouldn't know that without any analysis and/or testing.
The vertical stab can be mounted with its leading edge offset to trim the airplane for one flight condition. I offset the VS in my RV4 1/2" left of centerline. This value was based on measured rudder deflection for cruise with the VS on the centerline, then calculating rudder deflection that would have the same effect. This offset was spot-on for
my 4; we'll see what my 8 requires after it flies...