The rudder fitted to P-51 series aircraft is noticably thicker at its
hinge line than the vertical fin section just in front of it. Several
writers have characterized the benefits of this "bulged" configuration
as 1. reduced form drag at the juncture of fin and rudder, and 2.
improved airflow characteristics over the rudder, with resultant
increased control effectiveness and less tendency for the surface to
exhibit a "dead band" around the neutral point.

If this design element performs as described, why is it not more
widely employed in elevators and ailerons as well as rudder
applications, particularly in the faster homebuilts? I believe that
it has been used on the ailerons of some of the after-market Pitts
wings as well as some of the midwing acro designs. Have any downside
effects been encountered?

Hawkeye (Skyote NX8XX)