Doug,

 

The ventral fin was recommened for the 235 airframe in order to take the wiggle out of the airplane in flight without having to do major surgery to the airframe.  The 320/360 airframe has an overall longer length, larger vertical stab and rudder than the 235 airframe.  The changes were among several made as a part of the ever evolving improvements to the Lancair two-seat model.  Following through with the addition of the ventral fin by also adding it to the bottom of the rudder, increased the rudder area by about 20 percent.  The addition of the ventral fin was a suggestion/recommendation and was entirely voluntary.  So, some 235 airframes have it and some don't.  It is not related in any way to the change to a larger horizontal stab.

 

Gary Edwards

LNC2