I have heard some people concern themselves with the stiffness of the rear fuselage of the ES and IV, specifically the joint to the tail section, which is in a location close to that of minimum section modulus.  Some have added a bulkhead to add stiffness, the idea being that with downward tail loads the top deflects upward, essentially trying to straighten out.  To accommodate this the sides deflect inward.  I set up an experiment to measure this effect.  I temporarily glued hardpoints inside the fuselage just behind the joint and at the height of the longitudinal joint.  I then applied a torque along the lateral axis of approximately 450 ft-lb and measured the change in width (distance between the hardpoints).  I think this load is something close to half of a normal load, which I estimated at 330 pounds of downforce at the horizontal stabilizer applied 3 feet behind the joint.  The change in width was somewhere between .002 and .005, not very much.  My conclusion is that adding structure to reduce this deflection isn't going to buy much.  Any opinions on this?

Gary Casey