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6. I believe that my wings are now stronger than they were.
Joe Dallas
Joe, I have several things for you to consider about your design. I am not trying to ?rain on your parade? here, but in my opinion what you are doing here is not a good idea without the ?blessing? of the designers who know the critical design loads and load paths in the airframe and I am reasonably sure they will not discuss this with you for reasons of liability. My opinion here is based on my many years of airframe structural analysis experience with the military. I am aware that there is at least two RV-12 variant aircraft now flying with fuel in the wings, but that does not make it a good design mod.
First the wing spar caps were never designed to take these increased bending loads. Remember the potential max 3g load factor has to be included on the increased mass you have put into the wing. On top of that the designer has to add a factor of 1.5 to assure no structural rupture at ultimate load. Say you are carrying 10 gallons in one tank. Do the math. That would be, just for the fuel, 10 x 6 lb/gal x 3 = 180 lbs for limit load and 180 lbs x 1.5 = 270 lbs for ultimate load. I imagine your spar caps are now undersized.
How about the added shear and bending moments you are now introducing into the attachment fittings for the wings into the fuselage. Again they could be now undersized.
How about the two shear webs (wing ribs) you have now dead ended into your fuel cell rather than into an intercostal (inboard to outboard) connecting the two wing ribs that bound your fuel cell. If any portion of the flight control loads flow through these stub ribs, where do these loads go now? You have broken the rib cap continuity.
The clips you show abutting the fuel cell show no attachment to the cell. Are they containment barriers or do they attach to the fuel cell? What holds this heavy mass, when filled, in place in the wing - the physical straps that appear to go over the tank tops in the assembled picture?
Wing skins are thin. Can they take the localized increased wing skin shear flow with the same rivet size and pitch of the original design for the life of the airframe?
I am surprised to see that you are still using the light weight ribs at the two vertical ends of the fuel cell. I would expect thicker gage webs and perhaps now a full T cap with no scalloping to take an increased shear and bending moment around the contained cell.
So far I have only discussed the bending stiffness of the wing, or the EI related design issues. There are also wing torsional stiffness concerns or GJ impacts which also need to be considered. You have probably changed the torsional stiffness of an inboard portion of both wings. How do you consider these impacts? I will not go into a discussion of these design issues.
In short, you and others that have or are modifying or adding structure that has significant primary structure and critical load path impacts without consideration of at least these basic design concerns are inviting serious degradation of airframe structural strength and airframe life. The airplane could fly safely for years without suffering a visible structural failure but you could, and I am speculating here, end up with internal (buried) fatigue cracks from repeated cycles beyond the natural design capabilities of the one-of-a-kind configuration you have built into the aircraft. Only if you understand the critical design loads, load paths, and run a detailed stress analysis will you really know what you have.
George