My 1988 235/320 has a ventral fin starting just below the TE of the wing to 10" of height at the rudder.   I had a 7" vertical crack in the paint on both sides of the fin just below the point of minimum girth on the fuselage (a point just below  the FE of the horiz Stabilizer)  The cracks did not go into the fuselage. Careful sanding revealed only the paint was cracked, not the fin or fuselage. Put a 2 bid application on either side 6"for and aft of this area and will monitor.  The paint Sterling Polyurethane is very brittle in my opinion, but there was some type movement in the tail assy.  Perhaps the larger rudder and fin contributed to the cracks.. I'm considering the  addition of a Horizontal Prepreg bulkhead thru this area.. Something to look for   Regards.   Don SKeele  (850) 626-4946

Don,

 

How interesting that your message arrived on the very day I installed a tail cone stiffener in my 1989 (first flown in 1996) Lancair 320.

 

First, a description of the structure.  Remembering that these Lancairs are monocoque design, where the skin participates in the strength of the structure, note the structure starting at the seats.  The seat back (joined to the rear spar) and the rollover provide the last full circle of rigidity until the aft leaning bulkhead attached to the LE of the horizontal stabilizer and extending up into the vertical stabilizer (call this point B), the partial baggage bulkhead certainly provides some rigidity even though it is only half-way up the fuselage (call this point A). 

 

From "B" aft, there is another bulkhead in the lower fuselage, the tail post, the horizontal stab and the ribs in the vertical stab, when bonded together, combine to make this section very rigid with respect to movement of and between the fuselage and the stabilizer components.  Between A and B, the tail cone "O" becomes somewhat discontinuous at the "strake" that rises out of the fuselage to begin the vertical stabilizer.  That is, the fuselage looks more like a "U" topped by an weak inverted "V".  Any twisting movement can put stress on the side walls of the V and at the point itself.  Remember that the midpoint joint of the vertical stabilizer is not an overlapping joggle but just BID over the joint.

 

This is not to say the tail is structurally weak, none have come off that I know about.  But there can be unwanted movement in this area (between A and B) that can lead to what Don has seen.  Others have seen horizontal cracks in the sides of the inverted V that were thru the glass and the last straw that motivated me to add the stiffener was finding barely perceptible cracks (easier to feel than to see) in the clear coat on the joint at the apex of the inverted V.

 

There can be different reasons for the tail stress, aggressive aerobatics, unusual fuselage harmonics set up by the prop and others.  Since changing my prop 3 years ago, I noticed outboard elevator hinge wear, especially in the right hinge.  Recently, during a prop balancing session, an observer noted a lot of tail movement at high RPM.  These items, along with the cracks, indicated something undesirable was going on in the back. 

 

There have been a variety of fixes for the lack of stiffness in this area - Such as just more glass, a flat prepreg panel bridging the V/U joint, a foam filler floxed in the V with glass added to complete converting the U to an O and maybe others.  Be advised that odd shaped people, peculiar tools and uncommon methods are required to accomplish work inside the tail cone after the plane has been built - but it can be done.  If the plane is currently being built and the final half of the vertical stab has not been glued on yet, it would be easier to complete the tail cone O by working from the outside.

 

Do you, the owner of an LNC2 need to do this?  I don't know.  Different people have had difference experiences and perhaps some will add to this topic (Empennage Flexing - Voodoo or Deep Doodoo?).