Originally Posted by Dean Pichon
If I can accept the legacy design is almost good enough, is there a solution space that brings me to good enough using only the spar web. A one-piece, steel hinge assembly/doubler may distribute the load (even more so if bonded) over a greater portion of the web. If the ears are integral to the steel doubler, more of the bending loads will go into the entire steel doubler rather than the two small footprints of the legacy hinge brackets.
The issue is that the elevator hinge creates a point vertical load
and its associated fore/aft moment needs to get reacted into the skin with a fore/aft couple into the skin. The spar web has flanges that attach to the skin and transfer the load into the skin forming a closed cell torque box. (With bigger loads this would be helped out by a rib backing up the fitting and spar web with multiple fasteners to get that couple into the skin.) It is not a “bending” load as you describe. It is a vertical load and moment couple between upper and lower fasteners that attach the fitting. “Thicker spar web” may help with the load at the rivets attaching the fitting but then that load is transferred in the spar web through the rivets and goes around the 90 deg bend in the spar web to get to the skin attach fasteners. Going around that bend is the material thickness of the spar web and therefore a prime location for crack. A doubler may solve the current crack location in the vertical web of spar but will probably cause new cracks at edge of the doubler in the corner of the 90 deg flange that attaches the skin. If you think this through it is pretty intuitive. In my opinion that corner is the fatigue detail that Vans second iteration in the repair SB solved by having the new doubler nest and pick up skin fasteners.
Putting a large steel plate on the spar web may have another negative consequence related to horizontal tail bending. Bending due to lift on the tail is a reacted into the skin in a inbd/outbd couple (not fore/aft couple like i explain above for the point hinge fitting load) The spar is aluminum of a particular web thickness and a particular stiffness. All of a sudden the bending inbd/outbd in the spar web hit a very very high increase in stiffness of a steal doubler. Rapid increases in stiffness is prim location for cracking. This is why spar always taper in thickness changes. Think of tree branches always break at intersection to thicker branches.
No good engineering is ever presented without its caveats.
-A freebody diagram would be helpful for both of these I explain above but I am on vacation with only an iPhone to write this response.
-I am a structural design engineer so some strength engineer my give a better explanation, since they always want to prove us designers wrong.