Rib or Frame Stringer Support

[edbgtr]

G’Day Eng-Tippers,

A question for detail stress engineers regarding stringer beam-column compression analysis.

It has come to my notice that new forms of skin stringer construction has led to changes in stringer support conditions at the frames or ribs through which they pass. In the past, where a stringer passed through a mousehole in a frame or rib, the stringer was positively attached by fasteners to the rib or frame by means of a clip. With the loose skins no longer being attached by fasteners to the underlying airframe skeleton, but as preassembled “covers” comprising of skin and bonded-on stringers, the clips are no longer in use.

The question is this: when analysing the beam-column action of a stringer running the semi-span of, for example, a horizontal or vertical stabiliser, how are the supports at the frames or ribs considered for this analysis? In the past a correctly designed, and stabilised, clip attachment to the rib or frame could be considered a simple support for the stringer at each rib or frame station. Now, the support at the rib or frame depends on the short “overhang” beam-action of the sheet/doubler material between the stringer and the closest fastener at the edge of the mousehole in the rib or frame flange. The actual spring constant (stiffness) change from the clip support to the overhang skin support is considerable. How is this now being dealt with in analysis? Has it been determined that the overhang skin support is sufficient to still consider the stringer to be simply supported at the frame or rib?

I thank you in advance for your response.

Ed.

 

[rb1957]

ah, to cleat or not to cleat,
to save some manufacturing time,
at the cost of allowable,
that is the question.

ok, bill shakespear i ain't. uncleated stringers usually need testing to develop the allowable, to gauge the support (indirect though it may be) that the ribs/frames give the stringers. with fancy FEA you might get close, but there are some many devils in the details that i think most people would want a test to be sure.

some OEMs run the stringer through a mouse-hole and add a cleat, tying the stringer to the frame ... obviously really good support.
some grow out the stringer skin-attach flange (making a Zed stringer into a J or T) and joggle the rib/frame flange up onto both stringer flanges ... support, but not as good as a cleat. some use the skin pad-up under the rib/frame and stringer as the support ... support dependent on the pad-up. some are "lucky" enough just to run the stringer over teh skin, nail on the frame, and paint it ... ie no pad up ... clearly the simplest manufacturing, but equally clearly a sub-optimal design with low allowables and high weight.

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