Tail cracks in a 360?  Interesting..

 

If you search Marv's web site you will see a story about tail flutter in the large tail 320/360.  This sparked a significant debate, maybe even a feud, between Martin Hollmann and Lance back in the early 90's.  Martin used GVT testing to identify a flutter mode involving tailcone bending (like a swimming dolphin) and tailcone torsion at moderate airspeeds (150 kts or so).  He recommended stiffening the tailcone in order to suppress this mode with a large three-bid layup on the top and bottom of the tailcone, all the way from the baggage bulkhead (or was it the canopy) to the vertical tail.

 

Lance, on the other hand, told us that Martin was wrong and that independent flutter analysis did not reveal this problem.  And in fact, we haven't seen a bunch of Lancairs crash with their tails detached.

 

On the other hand, if the mode is real but highly damped, maybe that's what's causing the cracks.  We would not expect to see those in the beginning of service life, but later on.  And I tend to believe test data over analysis, so...

 

Myself, I did not add the large layups.  I did, however, place the equivalent amount of fiberglass into the fuselage mating joggle on the inside of the airplane.  This should have the effect of raising the frequency of the tail flapping mode, whil doing nothing for the tail torsion mode.  Spreading the frequencies apart should stop the problem.  I called Martin about this and he agreed (in principle) but would not give it his seal of approval without an expensive analysis.  (I can't blame him -- he is, after all, in the business.)

 

No cracks in my (airplane's) tail -- but then, I'm not flying it yet, so don't listen to me.

 

- Rob Wolf