The countersink pilot appeared to "wander" unless it was backed up, so I used a piece of aluminum chanel underneath the flange. I also experimented with several CS depths in order to achieve a depth that would accommodate the dimpled skin. An ideal depth appears to be approximately .009" deeper than is required for the rivet itself. However, countersinking this depth in .032' aluminum will penetrate through the entire thickness of the material and result in an enlarged hole. I sent a few pics to Van's and they assured me that this will not be a problem. Particularily because the top flange of the trim tab spar will actually serve as a doubler as it is sandwiched between the elevator skin and trim tab hinge. However, From this point forward, I will only countersink a few thousandths deeper when countersinking for a dimple than is required for a rivet.
